What kind of vapor barrier to put up, before insulation and drywall? - AVS Forum
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post #1 of 12 Old 09-08-2007, 11:54 PM - Thread Starter
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I've pulled the old drywall and weak batts of insulation off the walls of my detached garage. I'll prime the wood, soon, to help seal it against decay. But there are still some gaps where one can see light come in from the outside, so putting up some kind of vapor barrier after priming but before insulating seems like a good idea.

Photos and more info are here:

http://www.avsforum.com/avs-vb/showt...8#post11565958


I used some "Great Stuff" foam to seal some areas on one wall already, but I think that something with more coverage is a good idea, on the interior of every exterior wall, while it's all exposed.

What's the right stuff?

And can I really use 2x the regular amount of insulation, since I'm building a whole second frame and wall?
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post #2 of 12 Old 09-09-2007, 05:25 AM
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Usually the vapor barrier goes toward the "warm" side of the wall. So it's insulation, vapor barrier then the drywall.

If you are using batt insulation a lot of them come with a built in vapor barrier.
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post #3 of 12 Old 09-09-2007, 05:25 AM
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Quote:
Originally Posted by nathan_h View Post

so putting up some kind of vapor barrier after priming but before insulating seems like a good idea.

I researched the heck out of this issue and found that the current best current information shows that using rigid foam is better than any kind of vapor barrier. Although fiberglass and wood framing sealed with a vapor barrier against foundation walls is the worst and can be a recipe for mold: even for walls that appear to be dry -- there is still moisture vapor that comes through even apparently dry foundation walls and if it is trapped by the barrier and condenses in the fiberglass, or wood studs, can often encourage mold growth.

The rigid foam won't support mold growth and acts as a moderate vapor retardant to allow any vapor to slowly dissipate to the room.

Some good sources I used are:

http://www.buildingscience.com/docum...commendations/ (very good!!)
http://www.buildingscience.com/docum...dations2014air ("One of the worst assemblies for basement walls from the perspective of mold and moisture problems is a foundation wall that is internally framed and insulated with fiberglass cavity insulation and covered with a plastic vapor barrier."
http://www.eere.energy.gov/buildings...s/db/35398.pdf
http://www.eere.energy.gov/buildings...s/db/35017.pdf
http://www.buildingscience.com/bsc/r...oundations.pdf
http://www.buildingscience.com/bsc/r...on_systems.pdf
http://www.buildingfoundation.umn.ed...jectReview.htm
http://ths.gardenweb.com/forums/load...4110282.html?4
http://www.newsday.com/features/home...,452831.column
http://www.housingzone.com/proremode...CA6358797.html
http://www.huduser.org/Publications/...sturehomes.pdf (p. 54, 84)
http://www.housingzone.com/topics/pr.../pr04ca007.asp
http://www.housingzone.com/proremode...CA6404480.html
http://www.buildingscienceseminars.c..._Basements.pdf

also see articles in Fine Homebuilding magazine
No. 169 March 2005 p. 78
and No. 162, May 2004 p. 52


If there are serious water issues in the basement it might make sense to put a vapor barrier right up against the foundation wall making sure that any water can drain to drain that goes all along the foundation perimeter. Then frame and insulate after that, although that seem like a bit of a bandaid although probably necessary in places with things like stone foundation where there is no choice.
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post #4 of 12 Old 09-09-2007, 07:09 AM
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zmisst,

The OP seems to be asking about an above ground garage, which is completely different to the basement situation you've described.

nathan,

BIG got it right when he said that the vapour barrier goes on the warm side of the insulation. Put simply the purpose of a vapour barrier is to stop warm moist air reaching a cold surface where the moisture will condense. In North America and Europe this means the inside surface of the insulation. In the tropics and where I am now, the situation is reversed and we use a vapour barrier on the outside, or sometimes on both sides.

The only true vapour "barrier" is a sheet of metal. All other materials are vapour "retarders". Many materials which are waterproof are not vapour barriers, meaning they stop liquid water but not water vapour.

The best and most practical vapour barrier is aluminium foil, which for building purposes is normally used as a double layer with fibreglass reinforcing between the layers. I don't know what brands you have in the US but it should be very easy to find. The joints in the foil are sealed with self adhesive aluminium foil tape.

I hope this helps ... and please forgive my spelling of vapour and aluminium ... I can't help myself.
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post #5 of 12 Old 09-09-2007, 08:26 AM
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Quote:
Originally Posted by Peter M View Post

zmisst,

The OP seems to be asking about an above ground garage, which is completely different to the basement situation you've described.

oh yes, sorry. I'm so used to seeing vapor barrier issues in basements I didn't notice that. Yes, in above ground situations vapor barrier (yes really a retarder) usually goes on the warm side although see below.

Here are some relevant code provisions (and some of the links above discuss above ground insulation also.)

N1102.5 IRC 2006: Moisture control. The building design shall not create conditions of accelerated deterioration from moisture condensation. Above-grade frame walls, floors and ceilings not ventilated to allow moisture to escape shall be provided with an approved vapor retarder. The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation.

IRC ยง 322.1: Moisture Vapor Retarders/Moisture Control. In all framed walls, floors, roofs, and ceilings making up the building thermal envelope, a vapor retarder must be installed on the warm-in-winter side of the insulation, unless the framed area is ventilated to allow moisture to escape. (Vapor retarders are designed to prevent the movement of moisture-laden air from the warm side of the wall to the cool side. In temperate climates, vapor retarders are placed on the interior (warm in winter) side of the wall cavity; in hot, humid climates, they are placed on the exterior (warm) side of the wall cavity. According to the Asthma Regional Coordinating Council of New England, walls should be designed to dry to both the interior and the exterior and basements should be designed to dry to the interior. Installing vapor barriers interferes with the ability of walls to dry in both directions, so their use should be limited to severely cold climates. Paper-faced cavity insulation should be used in place of plastic interior vapor barriers.)
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post #6 of 12 Old 09-09-2007, 08:44 AM - Thread Starter
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Interesting. Okay, well, then I'll rely on the vapor barrier that is part of the insulation batts.

But that raises the question of what to do with the cracks between the boards that will, if un-treated, allow fresh air/daylight/moisture/plants to get into the wall and interface with the batts....

Should I put up some thin plywood of FSB?
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post #7 of 12 Old 09-09-2007, 10:38 AM
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Quote:
Originally Posted by nathan_h View Post

But that raises the question of what to do with the cracks between the boards that will, if un-treated, allow fresh air/daylight/moisture/plants to get into the wall and interface with the batts....

spray foam if big cracks, caulk if small. tyvek house wrap is a water (not really vapor) barrier that is used under siding on the outside.
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post #8 of 12 Old 09-09-2007, 10:41 AM
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What is the outside of the garage covered with? (never mind I looked at the pictures).

You might be able to cut some pieces of house wrap and fill the area from stud to stud.

http://www2.dupont.com/Tyvek_Constru.../homewrap.html

After looking at the pictures, have you given any thought to re-siding the whole structure ? Put up a house wrap then cover the hole thing with a vinyl siding.
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post #9 of 12 Old 09-09-2007, 11:30 AM - Thread Starter
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Hmmm. Residing is more than I want to bite off, especially since two sides are on neighbors' property lines and require that extra level of "what are you doing?" in addition more work and $. So if I can get it going nicely without that, that's ideal

The Tyvek, after using foam, sounds like the kind of solution I need. And I think I have learned that "vapor barrier" is not exactly what I meant!

Something to better seal the outside away from the inside, including the insulation, should do the trick.

Thanks, guys.
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post #10 of 12 Old 09-10-2007, 05:20 AM
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Using icynene (spray in foam) - no vapor barrier - no mold issues.db
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post #11 of 12 Old 09-10-2007, 10:47 AM - Thread Starter
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Thanks for the suggestion. icynene looks cool.

But it's not really a DIY solution, and I've got zero budget for trades, for now.

So I think I'm going to use the "Great Stuff" foam on any small gaps, paint (spray) a sealing latex primer over everything, put up something like tyvek, and then staple batts of insulation. From what I'm reading here and elsewhere, this is probably the best DIY solution available to me.

Then I'll built the second wall, run power etc through it, and drywall.
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post #12 of 12 Old 09-11-2007, 08:41 PM
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Quote:
Originally Posted by nathan_h View Post

Thanks for the suggestion. icynene looks cool.

But it's not really a DIY solution

I just hired someone and it looked very easy. you can buy the stuff at efi.org (the place that did mine buys their stuff from them too. The efi.org commericial website sells the larger quantities. It's very easy to use.
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