Basement Insulation Options – Vapour (Vapor) Barrier
Hi Everyone,
I am currently building an addition on the back of a 95 year old house in Toronto, Canada.
-My new basement walls are 10” CMUs, of which about feet are above grade and five feet below grade. The exterior of the below grade portion is covered by a semi-flexible hard plastic membrane for waterproofing.
-The carpenters have framed my basement walls with 2×4” lumber, and on the interior face of the concrete walls have placed Typar housewrap – wrapped over the top plate and under the bottom plate of the interior lumber walls to act as a bulk water barrier.
– My contractor is now planning to add R12 fiberglass batts between the 2×4”s and then cover with a 6-mil poly vapour barrier. Over that will be 1/2” gypsum drywall.
Everything I have read that is published by the Canadian government or Canadian energy boards recommends an interior vapour barrier.
Everything I have read on this site or in the in-depth articles put out by BSC strongly recommend against interior vapour barriers, especially below grade.
Can anyone offer any insight? Since, where I live, we heat our home for about 7-8 months a year and use A/C for about 2-3 months a year, my primary concerns are
a) if I do not have a vapour barrier on the warm side of the insulation, will the resulting/potential wintertime condensation on the cold basement walls result in significant mold and mildew on the lumber and drywall?
b) if I do have a vapour barrier, will I have issues with water (vapour or liquid) migtrating in through the floors and walls being trapped in the wood framed walls, and resulting in mold and mildew?
p.s., The 2×4 walls were built only about 1/2” off the concrete walls, so there is no room for XPS or EPS panels, so short of paying through the nose for spray foam, fiberglass batts seem to be my only option
Replies
Everything I have read that is published by the Canadian government or Canadian energy boards recommends an interior vapour barrier.
Is this for below-grade walls?
Generally for below grade an interior vapor barrier is seen as undesirable, since "moisture drive" is mostly from the outside in.
You don't say how much of the wall is above grade.
References
Hi. Thanks for the replies.
The following links are two of many recomending an interior vapour barrier in below grade walls: http://oee.nrcan.gc.ca/residential/personal/new-home-improvement/choosing/insulation-sealing/basement/khi-inbasement.cfm?attr=4 - about 3/4 of the way down
http://www.hydro.mb.ca/your_home/home_comfort/2_basement_crawlspace.pdf - PAGE 25
As mentioned, the basement walls are roughtly three feet above grade and five feet is below grade.
Thanks again,
Mike
The first link you gave says:
Cover the basement walls with a polyethylene moisture barrier that extends from grade level only to the bottom of the wall allowing extra at the bottom – about 30 cm (1 ft.) – to lie under the new frame wall. This will protect the insulation, studding and wall finish from possible water damage.
To me, this recommends poly behind the studs and batt insullation, not on the face of the studs. This makes sense because it prevents moisture from entering the stud wall assembly, and still allows drying potential from the stud assembly to the inside of the room.
I agree - but below, in the finishing section, it reads as follows:
Finishing
Install a polyethylene air and vapour barrier over the studs and insulation. Leave enough extra polyethylene at the top to connect to the air barrier in the joist space as discussed below. Seal all edges, seams and penetrations in the barrier with acoustical sealant. All joints should overlap over a stud and be sealed with a continuous bead of sealant that is run between the layers of polyethylene at the lapped joints. Staple the polyethylene to the stud through the bead of sealant.
In theory, I suppose it could work--you would be creating an impervious wall assembly if you're able to seal everything so tightly that no vapor could enter the assembly from inside or out. But I doubt that it's worth the effort compared to doing it the BCS way.
As mentioned, the basement walls are roughtly three feet above grade and five feet is below grade.
Well, you've mentioned it now. Didn't in your original post (though I suspect you think you did).
Correct
Dan H - my apologies - you are correct, I left out a word in my original post.
That being said, since the housewrap on one side of the wall is vapour permeable, I don't think I will have a double sealed wall.
I think the question boils down to - will vapour drive into the wall during the heating season be significant enough to outweigh the risks of having a wall assembly that won't dry out if it gets wet from the other side?
First off, there's not really such a thing as "vapor drive". Water vapor (like all other gases) naturally migrates from areas of high partial pressure to areas of lower partial pressure. That is to say, humidity attempts to equalize across a barrier (and how well it can equalize depends on the effectiveness of the barrier and how rapidly humidity is being added on one side and/or removed on the other.
But when humidity "equalizes" it does so in terms of "absolute humidity" (ie, dew point), not relative humidity. So what's 40% relative humidity at room temp will be 100% relative humidity when the temp in the lower 40s. (40% at 70F = 44.6F dewpoint.)
It's generally said that the earth maintains a constant 50F temperature. Though this is obviously at best a crude approximation, it does suggest that if humidity is reasonably well-controlled (below 35% or so) then the dewpoint is unlikely to be reached at the basement wall if the wall is several feet below grade. The difficulty lies with that portion of the wall that is above or near soil level. If it is insulated somewhat on the outside then staying above the 45F mark is reasonably possible in most climates, and condensation is unlikely to occur.