This page is from the original Code Check website. To see the latest version choose "Home Page New" Conditioning Habitable Space All information that follows are opinions of individuals. The "Authority Having Jurisdiction" (local building department) has the final say in whether an installation is "to code".
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| Plumbing | Mechanical | Electrical | |
For a well illustrated explaination of the concepts behind vapor pressure, vapor diffusion etc. see "Controlling Moisture in Mixed Climates" in the August issue of Journal of Light Construction, [To contact JLC- RR#2, Box 146, Richmond, VT 05477; or call 800/552-1951]
murhpy01 <murhpy01@concentric.net> writes:
>IMHO, the use of poly is considered only if the water table is
extremely
>high and you have permanent dampness at the surface...(usually a wetland
>& you shouldn't build there anyway). I have observed its use as
an
>inexpensive alternative to doing the job right!
Not to offend, but I do disagree. Even in desert areas, there is moisture
in
the ground. This can be shown by building a solar still. Just dig a hole,
suspend a clear plastic film over the hole with a rock in the middle for
a
weight and sit back. In the desert, you can get about a quart or two of
water from a 3 foot system. This is ground water, being wicked up from
way below. This happens even in a dry crawl. To prove it to yourself,
find the dryest dirt crawl you know of. Cover the floor with a poly barrier
and wait a day. Let me know if there is condensation on the underside.
There will be. The reason the top of the soils is normally dry is that the
moisture is drying out as fast as it is wicking up. There is still a huge
amount of water coming up & this causes high moisture levels in the
crawl and elsewhere in the house. IMHO, A solid vapor barrier should
be required in all crawls. (Except in waterfront where there are different
problems)
Of course all of your observations are also true. A poly barrier on the
floor doesn't help if it is covered with water from plumbing, exterior
bad drainage, etc. To keep a crawl properly dry, we've got to control
water from the top and the bottom.
My advice is free, you get what you pay for.
Pete Engle, PE
Almost Home
Rumson, NJ
I am converting an old (small) barn into a home office. We live in just
outside of Phila.
I need to know about "VAPOR BARRIERS" ... I am insulating the
interior
but really dont have a clue regarding what I should do about a vapor
barrier. Is there a "rule of thumb" or something? Are there better
and
worse quality products that I should be aware of ?
Should the VAPOR BARRIER be appiled directly to the underside of roof
or
to the drop ceiling ?
PLEASE HELP!
Respond to frazierg@erols.com
**
In cold climates - PA is one - the vapor barrier goes on the heated side
of
the insulation - No Exceptions. Water vapor is not steam - it is a
molecular gas which flows from areas of high pressure (indoors in winter)
through the wall to an area of low pressure (outdoors in winter). The
purpose of the vapor barrier is to prevent water vapor from entering the
wall - when it travels mostly through the insulation the vapor condenses
(turns into water) and freezes. In very cold climates this ice can
actually build up all winter and run out on your floor in the spring .
Worse yet, it blisters paint, rots sheathing, destroys the insulating value
of batts. And you want that humidity inside during winter - it seems
warmer ("latent heat"), you'll breathe easier, etc.
Remember - a vapor barrier is for stopping molecules under pressure,
not
little drops of water. Keep joints to a minimum - most people use 6 mil
polyethylene stretched over the inside face of the studs, with joints taped
with duct-tape. The brown kraft-paper-faced batt insulation is not a good
vapor barrier (although better than nothing).
One thing most people don't get - the vapor barrier on the inside must
be
*at least* as vapor-tight as the cladding on the outside. Once the vapor
enters your wall you're sunk if it can't get out again - it freezes. If
you have a Dryvit house or a Tyvek-wrapped house on the outside pay
particular attention to a good vapor barrier On the Heated Side of all
Walls, Floors, and Ceilings - no exceptions. Probably your barn doesn't
have this problem.
You will be advised that ceiling vapor barriers go on the top of the
insulation. This is not true. You will be advised that the vapor barrier
above a crawl space goes on the bottom of the insulation. This is not
true. In Pennsylvania, the vapor barrier goes on the heated side of the
insulation - always.
Vapor barriers are installed on the ground in crawl spaces and under
concrete slabs, but that's to keep moisture in the ground out of the space
above - a different circumstance entirely.
As for a dropped ceiling - where's the insulation? The barrier can go
anywhere as long as its on the heated side of the insulation blah blah
blah. Good Luck.
Subject: Re:
Vapor Barrier Theory
Date: Mon, 01 Sep 1997 07:51:20 -0700
From: Michael Brown <mdbrown2@uiuc.edu>
Organization: University of Illinois at Urbana-Champaign
Newsgroups: alt.home.repair
I must take issue with one of the articles, though.
*******************************************************
Subject: Re: -Vapor Barrier Between Roof and Ceiling?
From: "Doug Julien AIA" <dfjulien@stlnet.com>
Newsgroups: alt.architecture
Date: 6 Apr 1997
Organization: Julien Associates Architecture St. Louis
*snip*
Water vapor is not steam - it is a
molecular gas which flows from areas of high pressure (indoors in winter)
through the wall to an area of low pressure (outdoors in winter). The
purpose of the vapor barrier is to prevent water vapor from entering the
wall - when it travels mostly through the insulation the vapor condenses
(turns into water) and freezes
*snip*
Remember - a vapor barrier is for stopping molecules under pressure,
not
little drops of water.
*****************************************************
I'm sure Doug is a fine architect, but he has apparently
never studied mass transport. Although it is possible to
have small pressure differentials between the interior
and
exterior of the house (if the barometric pressure
suddenly drops or raises from the weather), that is not
the driving force that moves the water molecules through
the walls.
The pressure theory implies that if the house was at the
same pressure as the atmosphere (usually the case), then
there would be no condensation.
The true driving force is a concentration gradient, there
is a higher concentration on the inside that wants to
equilibrate with the outside. Try it at home by putting
a drop of food coloring in a bowl of water. Without
stirring, the color will completely mix with the water,
how fast depends on how hot the water is. That is mass
transport due to a concentration gradient.
Brownian motion describes the path that the particles
take
as they diffuse to a position that lowers their energy.
Higher
concentration means higher energy, lower concentration
means
lower energy, hence the concentration gradient is the
driving force.
-Mike
Carpenter turned Engineer
Subject: Re: Vapor Barrier Potpouri
Date: 2 Sep 1997 17:30:11 GMT
From: gof@jules.chem-eng.nwu.edu (Jerry Fountain)
Organization: Northwestern University, Dept. of Chemical Engineering,
Evanston,
IL
Newsgroups: alt.home.repair
In article <340C133F.2AD8@uiuc.edu>, Michael Brown <mdbrown2@uiuc.edu>
wrote:
>danhicks@millcomm.com wrote:
>>
>> Concentration gradient, vapor pressure difference -- what's the
>> difference??
>
>I would call it a big difference.
Except we are not talking liquid solutions here, but rather gas mixtures.
In gases, the concentration is often described by the _partial pressures_
of
the components. Therefore the original author is indeed on the correct
track.
The bulk pressure will be equalized, but the total composition will vary.
| Jerry Fountain | | Laboratory for Fluid Mechanics, Chaos, and Mixing | |
| gof@chem-eng.nwu.edu | | Northwestern University | |
| (847) 491-3555 (Office) | Department of Chemical Engineering | ||
| (847) 491-3728 (FAX) | | 2145 Sheridan Road, Evanston, IL 60208 | |
Subject: Re: Vapor Barrier Theory
Date: 3 Sep 1997 14:14:16 GMT
From: gof@jules.chem-eng.nwu.edu (Jerry Fountain)
Organization: Northwestern University, Dept. of Chemical Engineering,
Evanston,
IL
Newsgroups: alt.home.repair ,
alt.architecture
In article <340CC406.7250@uiuc.edu>, Michael Brown <mdbrown2@uiuc.edu>
wrote:
>Jerry Fountain wrote:
>>
>> Except we are not talking liquid solutions here, but rather gas
mixtures.
>> In gases, the concentration is often described by the _partial
pressures_ of
>> the components.
>
>I'm not talking about liquid solutions either. I don't
>usually split hairs over term usage, but vapor pressure
>and partial pressure are not generally used interchangably.
>
>The pressure is an effect of the presence of the mass.
>The amount of mass is the the concentration and determines
>the pressure. Since concentration determines pressure, it
>is simple minded to say that pressure is the driving force
>of the diffusion. It is more appropriate to attribute it
>to the mass (concentration), hence the driving force is
>the concentration gradient.
They should _never_ be used interchangeably. Here is your original *snip*
|*******************************************************
|
|Subject: Re: -Vapor Barrier Between Roof and Ceiling?
|From: "Doug Julien AIA" <dfjulien@stlnet.com>
|Newsgroups: alt.architecture
|Date: 6 Apr 1997
|Organization: Julien Associates Architecture St. Louis
|
|*snip*
|Water vapor is not steam - it is a
|molecular gas which flows from areas of high pressure (indoors in winter)
|through the wall to an area of low pressure (outdoors in winter). The
|purpose of the vapor barrier is to prevent water vapor from entering the
|wall - when it travels mostly through the insulation the vapor
condenses
|(turns into water) and freezes
|
|*snip*
|
|Remember - a vapor barrier is for stopping molecules under pressure, not
|little drops of water.
|
|*****************************************************
Note that the term "vapor pressure" is never used, only "pressure".
In
winter, the _partial_ pressure is indeed higher inside the house. This was
my point. For those still following this, a little background to understand
what Mike and I are splitting hairs about.
Partial Pressure: The amount of pressure exerted by a given component
of a
gas (for an ideal gas it is also the _molar_ fraction
times the total pressure). For example, air is (approx)
78% Nitrogen, 21% Oxygen, 1% Other (mostly Argon) All are
molar percentages, i.e., for every 78 molecules of
nitrogen there are 21 molecules of oxygen and 1 of argon.
They are not weight/mass fractions. At sea level where the
total pressure is 1 atmosphere the partial pressure of
oxygen is 0.21 atmospheres.
Vapor Pressure: The pressure exerted by a pure gas in equilibrium over
pure liquid at a given temperature.
There are cases where the partial pressure and the vapor pressure are
the
same. *Just* at the surface of a pot of boiling water, the partial pressure
of water vapor is 1 atmosphere. But as you move further away it drops off
due to dilution. A point to be made is that the TOTAL pressure remains 1
atmosphere. The composition of the gas changes (more water near the
surface, more nitrogen/oxygen further away) but the total pressure does
not.
Now, back to our discussion.
Ok, the driving force for diffusion is the concentration. Mike and I
agree
on this point. For gases, the molar, not mass, concentration is the proper
term. Molar concentrations in gases are typically measured with respect
to
their partial pressures. The partial pressure of water vapor inside a house
during winter is higher than outside due to the temperature difference
(which leads to humidity and saturation details left out for clarity). Since
we have a difference, there is a driving force for a flux (movement from
one
side to the other). The vapor barrier is put in the way to prevent this
flux. So it can be said that a difference in the partial pressure
of water
between the inside and the outside requires the vapor barrier. It cannot
be
said that the vapor pressure of water does (since the term is not being
used
properly).
Mike is correct, I am correct, and the original post is (mostly) correct.
The confusion comes in because the original post is not clear about what
pressure he is implying. Partial pressure is the correct term, total
pressure and vapor pressure are not. It would be nice if the author
Is everyone sufficiently confused now??? :-)
P.S.: There are simplifications in the above, so I would suggest any
*small*
things be discussed in e-mail. Large bo-bos are fair game though.
| Jerry Fountain | | Laboratory for Fluid Mechanics, Chaos, and Mixing | |
| gof@chem-eng.nwu.edu | | Northwestern University | |
| (847) 491-3555 (Office) | Department of Chemical Engineering | ||
| (847) 491-3728 (FAX) | | 2145 Sheridan Road, Evanston, IL 60208 | |
(Mr. Joe J. Jakubiak) wrote:
> Iíve insulated the walls and ceiling around an interior,
1st floor,
> bathroom I added with unfaced insulation. It is a 2 story home. Now
> I'm wondering if I should put up a plastic vapor barrier in the ceiling
> and/or walls? Will that trap too much moisture or will it help keep
> the insulation dry? Finished surface will be painted green board.
> JJJ
You need a vapour barrier between hot and cold -- always on the warm
side
and never on the cold side; otherwise, insulation will become wet from
water condensation as it cools inside the walls. In northern climates (is
this different in Florida or Texas where almost no heating occurs ?), you
should place the plastic just outside of gypsum boards. You would have
something like this:
| ----------- | brick or other outside clad |
| air space | |
| ........... | Tyveck, black panels, black paper over plywood... |
| ::::::::::: ::::::::::: ::::::::::: | Insulation |
| ----------- | Plastic sheet, with taped seems and tape around outlets |
| ;;;;;;;;;;; | Gypsum board |
You won't have any vapour barrier in that ceiling, because the room above
that bathroom is a heated one. If you were building a second story
bathroom, you would need a vapour barrier in the ceiling.
Now, for the humidity you create with showers, etc. Do yourself and your
walls a favour. Invest in a good silent fan. For a decent-size bathroom
(5
x 8, 6 x 10 ft...), buy something like 140-150 cfm, and for an oversize
bathroom or one with a two or three heads shower stall, get a 200 or 300
cfm fan. A silent one would be valued at 1.5 or 2 sones , going up to 2.5
sones for the 300 cfm fan (1 sone is equal to the noise of an average size
fridge).
Wire the fan and lights independently. I find that a 30 minutes
spring-loaded timer works great for the shower. It makes the fan work for
10-15 minutes after the shower ends, and you don[LC-PI]t start the noise
for a
simple hand wash.
Hokey McKinley wrote:
>A recent conversation with a insulator has me confused on two
fronts.
>I live in northern NY and in houses we build with fiberglass
insulation;
>use propa-vent in the roof [ventilation] and 6 mil. poly as a vapor
>barrier on all exterior surfaces. I know about the air-tight dry wall
>system and believe it works as well if not better than poly. This
>insulator told me that he doesn't use ventilation in his cathedral
>cielings or vapor barrier anywhere. The material is cellulose. His
>claim was that the density of the material makes both unnecessary, and
>in the case of poly harmful. He also was of the opinion that using a
>larger cavity than 8 " was a waste of money in that 99.9% of your
>insulating capacity will be in that outside 8", and
thickening the
>wall/roof is only affecting capacity in the 1/100 % range. Is there
merit
>to his contentions?
>Remove the first 's' in address.
>Hokey McKinley
Assuming you're talking about a cathedral ceiling here, using cellulose
to dense pack the cavity is a good way to go. Cellulose has a higher
R-value per inch than fiberglass and, because it does not depend on
trapped air for it's R Value, can be installed in a dense manner where
it also stops air infiltration.
The purpose of ventilation in your area is to remove moisture. The
cellulose alone is not going to do the job, and I'd encourage you to
consider a vapor retarder underneath the cellulose. At a minimum, make
sure that ALL penetrations thorugh the ceiling as well as through top
plates, etc., adjacent to the ceiling, are well sealed. This includes
all hvac, electrical and plumbing penetrations. Air transported
moisture is far more critical than vapor diffusion, so sealing holes is
more important than the vapor retarder.
I'm a little confused about what you're saying (or your contractor is
saying) regarding the "8" of cavity. If he's talking to you about
just
how much insulation is needed, cellulose has an R Value of about 3.5
per inch. In an 8" cavity, that would give you R28. For your climate,
that may or may not be sufficient. Certainly, as the insulation depth
increases, there's a diminishing return on investment. However, most
climates see R30 as a minimum, though that number is often lower in
cathedral celings (R19 where I live).
I don't see where having a poly vapor retarder is "harmful"
to the
cellulose...that makes no sense to me (unless he's spraying wet
cellulose, than he must make sure it's VERY DRY before he puts up the
poly).
Hope this helps.
Frank
Want to learn more about Advanced Energy's Applied Building Science
Center? Visit our Web Site at: http://www.aec.ncsu.edu
Look@my.sig (Mark Atanovich) writes:
>ljberkow@aol.com (Ljberkow) writes:
>
>> We are building a new house in a subdivision. The builder doesn't
use
>> "house wrap". On new houses and additions elsewhere,
we notice wrap by
>> Tyvek all around the exteriors. Is wrap needed and why? As for
>> insulation, our builder uses pink fiberglass inside. By the way,
we live
>> in New England.
>> Should we ask for a house wrap?
>> Advice and comments are welcomed.
>>
>> Thanks,
>> lisa
>
>Some type of vapor barrier is required. If not Tyvek (a brand name)
>than a dark colored sheathing. Ask your builder.
Actually, this isn't a "vapor barrier" -- the vapor barrier
goes on the
inside of the house in northern climates. The Tyvek is an infiltration
barrier and also prevents any rain or condensation that gets behind the
siding from getting into the insulation and framing.
Dan Hicks
Hey!! My advice is free -- take it for what it's worth!
http://www.millcomm.com/~danhicks
jmccanta@hotmail.com (Jay T. McCanta) wrote:
>I am insulating an addition on my house. I am using Kraft paper
>backed insultation and I thought that would be an adequate vapor
>barrier. I am not beign told that I should put a plastic vapor
>barrier over the kraft paper. I live in Seattle, and it does get
>humid (dank) in the winter.
The kraft paper has a permeability rating of ~1.0
Poly has a permeability rating of .5 or less.
The vapor barrier is designed to keep moisture in the house and not
lose it to an arrid exterior. Northern climates are very arrid in the
winter. I don't think that the addition of poly will help any moisture
migration problems that you will encounter in Seattle.
>Do I put the plastic vapor barrier (4 mil. poly sheet?) over the
>paper? Does it matter? Should I slash the paper before I put on the
>plastic (as one person sugested)?
If you still want to put the plastic on the wall, then face staple it
to the studs, don't slash the paper. You will have a difficult time
glueing you drywall, if that is what you intended to do.
>Also, if I am to cover the paper (and I am willing if it is a smart
>thing to do), can use the leftover Tyvek house wrap I have?
Tyvek is designed for an exterior application. It will shed water and
reduce the air infiltration. Don't use it on the interior.
Harry
>thanks for the thoughts.
representing Sunshine Insulation Co.
harry@sunshine.com
Frank Vigil <fvigil@aec.ncsu.edu> wrote in article
> I'm not so sure that I haven't heard about a lot of mold/mildew
problems i
n attics in your part of the country....and
I'm going to check with some of
my colleagues in
Portland about this.
We do have problems but they are inside the house or in unproperly vented
attics and crawlspaces. I've seen crawl spaces over high water
table or
natural springs for many years without any mold
or dry rot because they
were vented.
>You say code is "1/150" I've seen this code and I seem
to recall that it's
1/300 th [Code
information] there is a vapor retarder in the celing and 1/150 if
there is no vapor
retarder. Could I be mistaken on this code
issue?? The Super Good
Cents Construction manual (This is
different from the "Good Cents" used
elsewhere
in the country), written for your area,and fairly well
regarded
concerning it's recognition of basic building science, states
that for flat celings, "when vents are placed both near the ridge
and
at the eaves, 1 sq.ft net free area of vent is required
for every 300 sq. ft.
of ceiling area. When all vents are
places at the same level, 1 saq. ft. net
free area of vent
is required for every 150 sq. ft. of ceiling area."
I was being general. Our code (UBC) does have similar language.
In section 3.14 of the manual, concerning "recommendedpractice
for
moisture control," it states: "...[to
control vapor diffusion], Install vapor
retarders
in all floors, walls and ceilings,on the warm side of the insulation."
When the Super Good Cents code first came out in this area, it had people
wraping plastic vapor barriers like you say. Many people
ended up ripping
them out soon after they moved in
because of mold and bad air in the house.
Since then
there have been a lot of complicated designs trying to properly
ventilate the house. Because of that, the preferred practice (IMHO)
in this area
is to not install the vapor barrier
on the ceiling.
>An important point to remember here is, we're talking about controlling
vapor
diffusion, which, in the order of importance regarding
moisture transport
mechanisms, is last (least) on the list.
More important to control is bulk
moisture (rain and snow),
capillary action ("wicking"), air transported moisture
and last, vapor diffusion. However, it's still important and we shouldn't
disregard it, and we certianly should not intentionally
encourage it to move
into our attics. And, although ventilation
helps to remove any moisture that
does get there, we shouldn't
count on it to take care of all of it. My point about
"accidental
vs intentional" to which you referred, still stands, I believe. You
say
that if we add the proper ventilation to the attic, that
will be sufficient. The problem
with your theory is, we can
only have adequate ventilation if we have the driving
force.
This means wind or stack, in this case. But there are a *lot* of days when
we
have no wind and little if any stack, so where's the "proper
ventilation" then? In the
winter, if we have a very
cold attic due to a great job of insulation (no house heat
radiating
into the attic), but no wind to move it out, house moisture will migrate
to
the colder attic above and condensate.
If this is true, I have not seen it in our area.
>Remember, codes are never considered "optimal," they're
considered *minimal,*
meaning, at the VERY LEAST, you should
do <such and such. We're all familiar
with antiquated
codes that are either flat wrong or no longer valid, but they're still on
the books and we're still dealing with them. Just because
it's "code" or "standard practice"
in
NO way means it's right! You say: I agree and it's more important to take
extra steps
to mitigate local conditions.
The theory I was referring to was that of "recommending" not
placing vapor retarders in
attics. As for codes, nope,
they don't require it here either, but, as I said above, that does
not make it right.
>You made a good point about asking what climate zone the original
post was referring to.
I've mentioned this many times when
I've seen repsonses that I realized were climate
dependent
and should not be an across the board practice. We're all happy to share
and help
out when someone here asks a question, but we need
to be careful about the fact that we
usually don't have
all the facts to give answers without a lot of disclaimers attached. Still,
I
think what I'm referring to here is a good practice in
your climate (which, btw, is very similar
to NC..check out
the HDD and CDH's).
>Putting in my own disclaimer here, I would mention that not having
a vapor retarder in
the ceiling is no guarantee of problems
to come, nor is HAVING a vapor retarder going to
offer a
guarantee of not having a problem. I am simply stating that it's considered
sound buidling
science to control your moisture and I don't
view dumpting the moisture into my attic as a
good "control"
strategy.
Obviously this is a controversial issue. I'm sure there our times when
your method will
perform better and I know there are
times when my method will wok better. But as long
as
there is proper ventilation above, inside and below the house, both methods
will work. I
admit, I am prejudice because of the
problems I have seen first hand.
> Thanks for your response though, I've enjoyed the discussion.
Thank you!!
Subject: Re: Attic Ventilation Question
Date: 27 Aug 97 03:10:08 GMT
From: danhicks@millcomm.com
Newsgroups: alt.home.repair
>Hi all:
>
> I've asked this question at various places locally and have received
> conflicting advice, so I thought I'd try the newsgroup.
>
> My house currently has no attic ventilation. Since I'm re-doing the
> roof (complete tear-off) this is the time to add some. HOWEVER,
> there is no lower eaves ventilation, and no practical way to add any.
> (There's 2" x 4" blocking between the roof rafters at the
top of the
> outside wall, and the exterior of the house is stucco'd all the way
> to the roof sheathing. To add inlet ventilation I'd have to go
> through an inch of stucco and a 2" x 4". The roof is all
hips, no
> gable ends.)
>
> So, does putting on ONLY exhaust vents make any sense in this
> situation? My instinct says "NO" since there's realistically
no way
> of drawing in cool air, except from the house itself.
>
> Anybody have a contrary or confirming opinion? If you think that
> adding only exhaust ventilation gains me something, any
> recommendations?
You could drill holes through the stucco and blocking and then either
install round vent plugs or cover the soffits with an aluminum or plastic
soffit system. About three 1.5" holes per rafter bay would probably
give
you acceptable ventillation.
Otherwise, I'd consider using the mat style ridge venting material on
the
hips to allow some air in, then conventional vents or a rigid ridge vent
along the top ridge.
You want to get as much air FLOW as possible, preferably without leaving
significant dead spaces.
Dan Hicks
Hey!! My advice is free -- take it for what it's worth!
http://www.millcomm.com/~danhicks
Subject: Re: Warm Humid Climates?
Date: Wed, 13 Aug 1997 20:19:19 -0700
From: Michael Brown <"mdbrown2 at"@at uiuc.edu>
Organization: University of Illinois at Urbana-Champaign
Newsgroups: alt.home.repair
Ed Hatcher wrote:
>
> But what happens in summer, when hot heavy air comes in contact with
the
> nice cool vapor barrier up against the sheet rock of the air conditioned
> room? Doesn't the moisture precip out and make the insulation soggy
and
> ineffective, not to mention mouldy?
Good question, the reason is not readily obvious.
The cold air in the summer (inside) is much
warmer than the cold air in the winter (outside).
That leads to a greater inside/outside temperature
differential in the winter than in summer.
The lower the temperature difference, the lower the
amount of condensation. Remember that the building
materials can handle some moisture.
Consider the Great Lakes region. An average summer
inside/outside differential might be 70/90, and an
average winter might be 70/0. Let's assume about 70%
relative humidity in all cases. These are not
extreme conditions, just rough averages.
The summer air will cool to 78 before water begins
to condense. Since 0 deg is off the chart I have,
I'll estimate the amount of moisture in the cold air
and say that there is twice as much water that will
condense from the inside in the winter, than would
from the outside in the summer. (Assuming I remember
how to read a psychrometric chart).
-Mike
Carpenter turned Engineer
Subject: Re: Air Transort
vs Vapor Pressure?
Date: 19 Aug 1997 16:29:56 GMT
From: fvigil@aec.ncsu.edu (Frank Vigil)
Organization: Advanced Energy: Applied Building Science Center
Newsgroups: alt.architecture , alt.building.architecture
, alt.building.construction
Well, I'd have to agree with Jeff, with certain qualifications.
If you
look at Moisture Transport Mechanisms, and thier order of importance,
you'll find that it starts with Bulk moisture. In terms of a
*magnitude* of order difference, it then goes down to capillarity, air
transported moisture, and finally, vapor diffusion. Thus, the focus
should be, as Jeff said, on chases, by-passes, and any other
penetrations through the ceilling. Over the past 15 years, I've seen
hundreds and hundreds of attics with no vapor retarder in the ceiling
(in this part of the country, they don't ever build them any other way)
and they had no moisture problems. Now, this being said, that's no
guarantee that there never will be a problem. Depending on the climate
zone, inside vs outside temperatures and R/H, there is a possibility
that such a thing could happen from vapor diffusion alone. It's just
that it's less likely than from some other forms of moisture transport.
I'd focus more on air transported moisture as a potential problem than
vapor diffusion (unless I"m in the business of SELLING vapor retarders,
which many folks are so that's what they recommend). If you're really
all that worried about vapor diffusion, just paint the sealing with a
low perm paint.
Frank
+++ Want to know more about Advanced Energy's Applied Building Science
Center? Visit our Web site at: http://www.aec.ncsu.edu
+++
>> Sally,
I have inspected thousands of houses and have never seen a problemwith a
house where there was insulation and no vapor barrier. 99+% of water vapor
in any attic comes from the chases:" spaces around plumbing vents and
chimneys, etc. There may also be air flow around ceiling fixtures.
>> Be sure that air flow around these is limited and you will not
have a moisture problem.
>>
>> Have a look at some of my photomicrographs and SEM at
>> < http:www.cybercom.net/~jmhi>
>>
>> Jeff
>>
>Jeff,
>
>You are correct about the airflow around ceiling fixtures and spaces
around plumbing vents, chimney, etc. However vapor can penetrate
walls, ceilings, and roofs by diffusion. Vapor passes through construction
materials due to the permeability of the construction materials. Vapor is
not a problem until it reaches its dew point and condenses into moisture
and deteriorates the building materials of wall, roof, and floor assemblies.
>
>I have also inspected thousands of roofs. When inspecting the roof,
I also inspect the underside of the roof deck, the roof rafters, the ceiling
joist, the insulation and all other related building components. I have
seen that deteriorated decking and rafters due to the lack of a vapor barrier
or an inadequate vapor barrier.
>
>Many parts of the country require you to install a vapor barrier on
the warm side of the insulation. Please don't tell people you don't need
a vapor barrier until you have all of the facts.
>
>Michael H. Lichy
>RC Lichy & Associates
>m_lichy@nauticom.net
> http://www.nauticom.net/www/roofing
Subject: Re: Thermal barriers
in attic
Date: Thu, 21 Aug 1997 14:01:17 -0700
From: Michael Brown <mdbrown2@uiuc.edu>
Organization: University of Illinois at Urbana-Champaign
Newsgroups: alt.home.repair , misc.consumers.house
David Harper wrote:
>
> The roof is pitched very high and there is an enormous amount of
> roof facing the afternoon sun -- about 1500 sq./ft.
Sounds like you need some solar panels to generate
your own electricity.
> First, will the thermal barrier, by reflecting back heat, cause
my
> roof any damage and/or cause the asphalt shingles to age quicker?
Let's see. The shingles act as grey bodies (inefficient
black bodies) adsorbing most of the radiant heat that
hits them and then radiating the heat back out just as
efficiently.
The reflectors will point that heat back to the shingles,
which now are being heated on both sides, not just one.
(Actually, they now have only one direction in which to
radiate heat away from themselves).
I believe the shingles will die an untimely death.
A better use of resources would be to set up water
lines that will provide hot water for household use
and will also remove a lot of heat from the roof.
Water has a very high heat capacity.
Or install some photoelectric cells. Too much
trouble? Then just install a gable end fan to
draw air through the attic and remove the excess
heat.
-Mike
Carpenter turned Engineer
Subject: Re: Radiant barrier insulation
Date: Fri, 22 Aug 1997 17:01:09 GMT
From: sober@back.com (sober)
Organization: back.com
Newsgroups: misc.consumers.house
On 22 Aug 1997 06:36:54 GMT, rice@kcomputing.com (Brian Rice) wrote:
>I live in a cottage near San Jose, California--lots of sunshine--with
an
>uninsulated roof, although the ceiling has a layer of pink fiberglass
>batting (no R-value marked). The cottage gets up to 10 degrees hotter
>than outdoors on sunny afternoons. I've been considering Reflectix,
>a radiant-barrier type insulating material (it's essentially bubblewrap
>made out of Mylar).
>
>I understand that one is supposed to put it on the bottoms of the
>rafters. But that suggests to me a lot of heat is going to wind up being
>transferred to the air between the Reflectix and the roof. Unfortunately,
>my attic is poorly vented: there are no soffit or rooftop vents. Not
>even one of those side louvers.
This is your problem. As the mass heats up in the attic, all that
thermal radiation heats up the living space. If you ventilated your
attic properly, there are standards based on the square footage of the
attic, you wouldn't need the radiant barrier. You sure could use it, but
you wouldn't NEED it.
sober
Subject: Re: Radiant barrier insulation
Date: Fri, 22 Aug 1997 17:01:09 GMT
From: sober@back.com (sober)
Organization: back.com
Newsgroups: misc.consumers.house
On 22 Aug 1997 06:36:54 GMT, rice@kcomputing.com (Brian Rice) wrote:
>I live in a cottage near San Jose, California--lots of sunshine--with
an
>uninsulated roof, although the ceiling has a layer of pink fiberglass
>batting (no R-value marked). The cottage gets up to 10 degrees hotter
>than outdoors on sunny afternoons. I've been considering Reflectix,
>a radiant-barrier type insulating material (it's essentially bubblewrap
>made out of Mylar).
>
>I understand that one is supposed to put it on the bottoms of the
>rafters. But that suggests to me a lot of heat is going to wind up being
>transferred to the air between the Reflectix and the roof. Unfortunately,
>my attic is poorly vented: there are no soffit or rooftop vents. Not
>even one of those side louvers.
This is your problem. As the mass heats up in the attic, all that
thermal radiation heats up the living space. If you ventilated your
attic properly, there are standards based on the square footage of the
attic, you wouldn't need the radiant barrier. You sure could use it, but
you wouldn't NEED it.
sober
Subject: Re: Is styrofoam a
good exterior insulation?
Date: 21 Aug 1997 13:02:00 GMT
From: fvigil@NoSPAM.aec.ncsu.edu (Frank Vigil)
Organization: Advanced Energy: Applied Building Science Center
Newsgroups: sci.engr.heat-vent-ac , alt.home.repair
Dan:
There are many benfits to exterior foam insulation. Among them are
that by adding foam insulation, the studs are then insulated, as
opposed to just the area between the studs. Considering all of the
framing that is in a house, this can add up to a significant amount of
energy loss. Dow, in an add that ran for years, used to say "35% of
your heat loss through the walls is through the framing." I've never
sat down to verify or dispute that number, but I can assure you it is a
lot.
If you live in a cold climate, foam insulation will also keep your
exterior siding colder, thus reducing the opportunity for condensation
from warm inside air striking it. Also, if properly installed and
sealed, foam can serve as protection against windwashing, which can
increase energy loss.
It's not NECCESSARY to have foam insulation, but there can be benefits.
Careful evaluation of cost vs benefit should be made. I don't know
what climate zone the person who originated the thread is from, but
there's a lot of areas in the country where foam can be cost effective.
Frank
+++ Want to learn more about Advanced Energy's Applied Building Science
Center? Visit out Web site at: http://www.aec.ncsu.edu
+++
In article <33fb99d6.0@nntp1.nac.net>, danhicks@millcomm.com says...
>
>>Ask yourself why you want styrofoam vs just more fiberglass (or
>cellulose) insulation. The foam has slightly higher R value per inch,
>but it's more expensive than fiberglass for the same R value.
>
>Dan Hicks
>Hey!! My advice is free -- take it for what it's worth!
> http://www.millcomm.com/~danhicks
Subject: Re: Is styrofoam a good exterior insulation?
Date: 21 Aug 1997 17:47:15 GMT
From: johno@vcd.hp.com (John Ongtooguk)
Reply-To: johno@hp-vcd.vcd.hp.com
Organization: Hewlett-Packard
Newsgroups: sci.engr.heat-vent-ac , alt.home.repair
Followup-To: sci.engr.heat-vent-ac , alt.home.repair
nospam (bdesind@rt66.com) wrote:
: How can styrofoam be used as siding or exterior insulation, applied
to the
: outside of the framing? Since styrofoam is an excellent vapor barrier,
: that would seem to put the vapor barrier on the "outside". I've
seen lots of
: references to styrofoam used in this way.
Having what is effectively another vapor barrier on the outside of
a wall when foam board insulation is used presents some problems
of VB theory, but it's no different than that presented by the use
of OSB or CDX plywood sheathing on walls. In my opinion either
wood sheathing or foam could be a problem if wet, green framing is
closed up and maybe with poorly sealed interior walls, but otherwise
it doesn't seem to cause big problems as sheathing with no venting
in exterior walls is common building practice.
John Ongtooguk (johno@vcd.hp.com)
Subject: Re: Is styrofoam a good exterior insulation?
Date: Thu, 21 Aug 1997 22:38:00 -0400
From: sscott@lightlink.com (Steve Scott)
Organization: Art Matrix - Lightlink
Newsgroups: sci.engr.heat-vent-ac , alt.home.repair
On Thu, 21 Aug 1997 19:27:31 -0700, Danny Gough
<housedoc@netunlimited.net> wrote:
>Cellulose and Icynene foam are better insulators and reduces air
>leakage rates of the house.
I could never, in good faith, recommend cellulose in exterior walls in
upstate NY. Attics, fine. I have seen far too many houses that have
blown cellulose have their paint blown off in huge sheets.
Subject: Re: Is styrofoam a good exterior insulation?
Date: Sun, 24 Aug 97 18:17:25 GMT
From: bdesind"nospam"@rt66.com
Organization: Rt66.COM, New Mexico #1 ISP
Newsgroups: sci.engr.heat-vent-ac , alt.home.repair
In article <5tn2a2$5e5@winter.news.erols.com>, hdveatch*@erols.com
(Harry) wrote:
>bdesind"nospam"@rt66.com wrote:
>
>>I've read enough about insulation to be thoroughly confused. Everyone
(?)
>>knows that a frame house must have a vapor barrier on the "warm"
side of the
>>insulation (i.e., inside, for most areas). Otherwise, you get moisture
in the
>>insulation and framing (very bad).
>
>>How can styrofoam be used as siding or exterior insulation, applied
to the
>>outside of the framing? Since styrofoam is an excellent vapor barrier,
>>that would seem to put the vapor barrier on the "outside".
I've seen lots of
>>references to styrofoam used in this way.
>
>>I'd like to use fiberglass batts in my new house, with an extra
inch of
>>styrofoam on the outside. Is this a good idea? How can I avoid trapping
>>moisture in the framing and fiberglass insulation (I'm assuming
that the
>>fiberglass would have a vapor barrier to the "inside").
>
>>Would styrofoam on the inside walls be a better idea?
>
>>Can anyone help me figure this out? Thanks a lot in advance!
>
>>Brian.
>
>Although the exterior styrofoam violates a cardinal rule in
>insulation, it is possible to effectively use foil faced styrofoam
>board on the exterior. You must use as good or better vapor barrier
on
>the interior side, i.e. poly over unfaced fiberglass. It is also a
>good idea to use a low porosity interior paint. Most oil based paints
>are low porosity and certain water based paints also qualify. Talk to
>your paint supplier and get the low porosity paint.
>The styrofoam board is usually used in place of exterior sheathing.
It
>doesn't have a lot of strenght to resist lateral or vertical motion,
>so I wouldn't recommend it in areas prone to high winds (hurricanes)
>or in areas where foundation settling is common.
>Harry
>representing Sunshine Insulation Co.
>harry@sunshine.com
>
What about putting the styrofoam on the INSIDE of the wall, between the
framing and the drywall? Is that a better idea. That way, the styrofoam
- along with the standard fiberglas vapor barrier backing - would be part
of the 'inside' vapor barrier. The only problem I can think of would be
difficulty in nailing the drywall, or nailing the electrical outlet boxes
to
the studs, particularly if I use another 3/4" of plaster on the interior
of
the drywall.
By the way, the house will be near Albuquerque, NM, which has a moderate,
but
relatively dry climate. Winter lows are normally around 10-20 degrees, but
can reach 10 or 20 below. The house will have a stucco exterior finish.
Thanks again for your help.
Brian.
Subject: Re: Is styrofoam a good exterior insulation?
Date: Sun, 24 Aug 1997 19:25:33 -0700
From: Danny Gough <housedoc@netunlimited.net>
Organization: NetUnlimited, Inc.
To: bdesindnospam@rt66.com
Newsgroups: sci.engr.heat-vent-ac
, alt.home.repair
References: 1
, 2 ,
3
bdesindnospam@rt66.com wrote:
>
> In article <5tn2a2$5e5@winter.news.erols.com>, hdveatch*@erols.com
(Harry) wrote:
> >bdesind"nospam"@rt66.com wrote:
> >
> >>I've read enough about insulation to be thoroughly confused.
Everyone (?)
> >>knows that a frame house must have a vapor barrier on the "warm"
side of the
> >>insulation (i.e., inside, for most areas). Otherwise, you get
moisture in the
> >>insulation and framing (very bad).
> >
> >>How can styrofoam be used as siding or exterior insulation,
applied to the
> >>outside of the framing? Since styrofoam is an excellent vapor
barrier,
> >>that would seem to put the vapor barrier on the "outside".
I've seen lots of
> >>references to styrofoam used in this way.
> >
> >>I'd like to use fiberglass batts in my new house, with an extra
inch of
> >>styrofoam on the outside. Is this a good idea? How can I avoid
trapping
> >>moisture in the framing and fiberglass insulation (I'm assuming
that the
> >>fiberglass would have a vapor barrier to the "inside").
> >
> >>Would styrofoam on the inside walls be a better idea?
> >
> >>Can anyone help me figure this out? Thanks a lot in advance!
> >
> >>Brian.
> >
> >Although the exterior styrofoam violates a cardinal rule in
> >insulation, it is possible to effectively use foil faced styrofoam
> >board on the exterior. You must use as good or better vapor barrier
on
> >the interior side, i.e. poly over unfaced fiberglass. It is also
a
> >good idea to use a low porosity interior paint. Most oil based
paints
> >are low porosity and certain water based paints also qualify. Talk
to
> >your paint supplier and get the low porosity paint.
> >The styrofoam board is usually used in place of exterior sheathing.
It
> >doesn't have a lot of strenght to resist lateral or vertical motion,
> >so I wouldn't recommend it in areas prone to high
winds (hurricanes)
> >or in areas where foundation settling is common.
> >
> >Harry
> >representing Sunshine Insulation Co.
> >harry@sunshine.com
> >
>
> What about putting the styrofoam on the INSIDE of the wall, between
the
> framing and the drywall? Is that a better idea. That way, the styrofoam
> - along with the standard fiberglas vapor barrier backing - would be
part
> of the 'inside' vapor barrier. The only problem I can think of would
be
> difficulty in nailing the drywall, or nailing the electrical outlet
boxes to
> the studs, particularly if I use another 3/4" of plaster on the
interior of
> the drywall.
>
> By the way, the house will be near Albuquerque, NM, which has a moderate,
but
> relatively dry climate. Winter lows are normally around 10-20 degrees,
but
> can reach 10 or 20 below. The house will have a stucco exterior finish.
>
> Thanks again for your help.
> Brian.
Brian,
There are four moisture transport mechanisms you need to be concerned
with in buidlings. The are:
The first two mostly deal with moisture coming into the house from the
outside ie rain, hydrostatic pressure driven groudn water. The methods
to control these are usually simple and straight foward.
The latter two typically move moisture from inside the building to
outside and from outside to inside.
Possibly one of the more distructive and misunderstood mositure movement
mechansims in homes is air transported moisture. Depending on the
temperature, air can hold varying amounts of moisture. If air containing
moisture comes in contact with a surface that is colder than the
dewpoint temperature, condensation will occur. Here lies the potential
damage if this moisture is not controlled.
Assume a 2000 sq ft home at 70 degrees F and the relative humidity is
50%. This air will contain approximately 9.5 lbs of water. As air is
cooled, it's ability to hold moisture decreases. If this same air drops
to 51 degrees F, it cannot hold anymore moisture and is said to be
saturated. (100% relative humidity) If it sees a surface 51 degrees or
less, it will give up moisture or condense. This is why single pane
windows condense in some areas in cold weather.
All of this is important to understand, because if you build a wall or
ceiling in your home that has a surface below the dewpoint of indoor air
and this air gets into the cavity it will condense on that surface,
whether you have a vapor barrier or not. Then, all kinds of bad things
could happen.(structural damage, microbial growth etc)
You can predict if moisture will condense in your wall or ceilings by
calculating the temperatures inside the wall. Then you can use a
psychometric chart to find out if those temperatures will support
condensation. This will give you the best answer to the question about
what insulation and where to put it is best. It's much more reliable
than asking opinions from everybody.
If you know the potential problem,it's good to know the way to solve
it.
There are three conditions must be satisfied for moisture movement to
occur
Contolling mositure movement is as simple as eliminating one of the
three. In most residential applications, the most recommended is method
is to seal the holes and contol abnormal driving forces. By using a 100%
continuous air barrier (not insulation, vapor barrier or house wrap),
you decrease the air movement into walls and ceiling cavities where they
can do damage. Advanced air sealing techniques are simple and
inexpensive but are most effective at construciton. Stop abnormal
driving forces in the home by making sure with testing that the duct
system is airtight, with returns in each room that are pressure
balanced.
The fourth moisture tranport mechanism is vapor diffusion. The same
conditions as above still apply. Vapor diffusion is a function
of the
vapor permeability of the material, the driving force called vapor
pressure (more precisely, the difference in vapor pressure across the
material). All materials are permable to vapor. They are not able to
stop diffusion, hence the term vapor diffusion retarder, not vapor
barrier. Since vapor diffusion is a function of the surface area, if 80%
of the building envelope surfaces are covered with a vapor diffusion
retarder, that retarder would be 80% effective. A continuous vapor
diffusion retarder is typically not the number one necessity in
controlling moisture. A good low perm paint primer could act as a very
effective vapor diffusion retarder.
Building envelopes very rarely fail as a result of moisture movement
by
vapor diffusion, although, they are commonly misdiagnosed as having done
so. Vapor diffusion in the order of magnitude has less effect than air
tranported moisture, buld moisture and capilarity.
It has been calculated that the movement of water vapr through a 3/4
inch hole with a 10 pascal pressure difference is 100 times greater than
movement of water vapor as a result of diffusion through 10 sq ft of
drywall. So, don't be loosin sleep over vapor barriers
Don't conclude that vapor diffusion should be ignored. Rather it should
not be your major concern. Over the last few years, over zealous concern
about vapor barriers, has been the tail wagging the dog, with respect to
the real problems of moisture control in houses.
As to your wall construction, your latitude is about the same as mine
(34 deg). Use 1 inch polyisocyanurate exterior sheathing (about R-7)
with either stabilized (wet spray) cellulose or icynene foam insulation.
Get info on how to air seal your home and do as much as you can. Use
airtight electrical boxes, and recessed cans if you use them. There are
great sucesses in Canada using a new method that uses seal seal gaskets
or use EPDM gaskets making drywall airtight.
Build your home as airtight as is practical, insulate it right and
ventilate it for the health of you and your family. The home will be
healthier, more comfortable, economical and last a long, long time.
Danny Gough
Air Comfort Diagnostics
Check ©1998
by Redwood Kardon
