Are vapor barriers necessary?

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  #1  
Old 10-01-15, 05:01 PM
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Are vapor barriers necessary?

I keep finding links suggesting they're not. I'm in Canada in a heating dominated climate (it is possible however that we'd have a few days with the dewpoint outside higher than the inside of the AC'd house, but very rare).


Vapor Retarders and Vapor Barriers | GreenBuildingAdvisor.com

and

Why Did Painters Refuse to Paint Insulated Houses in the 1930s?

and this one suggests that attics are special

Insulation for Homes - Vapor Barriers and Installation Tips - Part 2

what's the deal?
 
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  #2  
Old 10-01-15, 05:07 PM
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Not only are they necessary, they are required by code. A properly installed vapour barrier insures the moisture inside your house does not migrate into the wall cavity where it can (and will) condense and result in mold growth. Some will say all that is needed is an air barrier, but moisture can still propagate through air barrier rated materials. Only a vapour barrier will stop both air and vapour movement within the wall cavity.

You will hear some people say vapour barriers don't work, and actually cause more problems, but the reason for that is they were not installed properly. This is especially true with houses in the 70's and 80's.

The excess moisture in a house due to vapour barriers is dealt with by means of mechanical air exchangers, which are now required in all new construction.
 
  #3  
Old 10-01-15, 05:17 PM
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Vapor barriers are one of those items that no one can really put their finger on as to how they got started. But once it became part of the building process, they were included in every home. But as often happens, time eventually points out where that can be a problem. Thus the more modern approach:

Today they like to say, "use them only in the deep south and far north". The primary exception is, you also need them where codes still require them.

They have also defined several levels of vapor restriction and now prefer the term "vapor diffusion retarder". The idea of slowing the vapor movement as opposed to blocking it relates to your concern about some conditions where even in Canada you can have that ac on. In essence, a little condensation is ok, as long as it has a direction to eventually dry.

Being in Canada, if you are in one of the colder regions, then apply your vapor barrier to the inside under the drywall.

The other change that has taken precedence over vapor barriers is air sealing. Since far more moisture moves through an air leak than diffuses through a piece of painted drywall, being aggressive with air sealing is far more beneficial.

Bud
 
  #4  
Old 10-03-15, 06:08 PM
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In thinking about how the attic is setup however, i have some concerns - with latex paint on the ceiling and plaster above there will be some but not a huge amount of vapour going out. Im more concerned with any sort of water or condensstation getting into the ceiling ABOVE the paint into the plaster and lathe or the joists, and then having nowhere to go - barrier above it in attic, and then latex paint layer below - it has nowhere to dry out.

Perhaps faced insulation would give some vapour reduction instead of full barrier, so while there's escape of vapour, it's reduced, but ultimately also a path to drying in case of any issues.

Thoughts? (Such a damned if you do damned if you dont setup!)

(If anyone wants to nerd out, page 13 of this paper has a good table for vapour permeability and absorption:

http://www.ecotimberframe.ie/pdf/Bre...ildingsNBT.pdf )
 
  #5  
Old 10-03-15, 06:54 PM
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There is no perfect combination that works for summer, winter, and a variety of household conditions. As long as we don't trap the moisture between two vapor barriers it should have some drying ability in at least one direction. Now, that direction may change with the seasons and thus many climates are not requiring a real vb. In other cases I'm hearing the pros recommend a "smart" vapor barrier. Other than the title I have not looked into it, but if the cost is reasonable it does offer another option.
Smart Vapor Retarders | GreenBuildingAdvisor.com

Bud
 
  #6  
Old 10-03-15, 11:25 PM
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awesome info. MemBrain from CertainTeed looks pretty ideal.

in fact: "The most common use of MemBrain is in jurisdictions where stubborn building inspectors want to see a sheet membrane "vapor retarder" on the interior of the wall, because of a misguided interpretation of the building code, and who stubbornly refuse to learn building science principles." LOL.

That gives me comfort in the idea that if there is a pooling/concentration of wetness from any source (roof leak, condensation, etc), then it will be able to dry out nicely.

Now my problem is finding where the heck I can get this stuff in Canada!! There was a product-shill review in a major Toronto newspaper (Toronto Sun), which listed LOWE's Canada, but their website has almost nothing (except "sell quality products such as from manufacturers like CertainTeed"...) - CT has a seller search which gives me ONE place in Southern Ontario (population nearly 10 million). Ridiculous!

CertainTeed Where to Buy - Search Results

Ok found it by using google's search instead of Lowe's own. Sigh.

https://www.lowes.ca/radiant-barrier..._g1568192.html
 

Last edited by mathx; 10-03-15 at 11:36 PM. Reason: found canadian source
  #7  
Old 10-04-15, 05:09 AM
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Keith is from Canada, maybe he has a source to recommend. I checked here in Maine and it is everywhere. Do you have any drywall supply stores, or try a regular lumber yard?

Now you have me interested, I'll have to check this stuff out and see how much the cost difference is.

Bud

Ok, I answered before reading to the bottom.
 

Last edited by Bud9051; 10-04-15 at 05:10 AM. Reason: correction
  #8  
Old 10-04-15, 07:12 AM
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When it comes to vapour barriers, you either do it as tight as possible, or not at all. There is no "letting some escape but not all".

The biggest problem when people talk about vapour barriers is that they disregard the fact that they are also an air barrier. Disregarding a VB and instead focusing on Air sealing is redudent, as a properly installed VB with also give you a 100% air barrier.

The science behind VB installation is correct, it is the practice in the field that has caused all the issues people talk about. The building codes in Canada are very clear about when and where vapour barriers are required and what types of matierials are permitted to be used.

In your case, what I would do is focus on sealing all the wall and ceiling penetrations (light fixtures, outlets etc) using accustical sealent to the best of your ability. If you want to go one step farther, two coats of oil paint is classsified as a vapour barrier. If you want, use two coats of oil primer, and then top coat with latex. Next do the best you can at sealing all the seams around the baseboards and other trim with caulking. This is about the best method yuou can use when trying to retrofit an old space to be air/vapour tight, wiothout removing any of the interior finish.
 
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