# Batts over foam



## tech14 (Sep 23, 2011)

Hi guys, I have a studded basement wall with 5/8 foam behind it glued to the block. I am adding roxul batts over the foam. So, first question, is this ok? and second, would i need a vapor barrier on before i put the paneling back on top?


----------



## AGWhitehouse (Jul 1, 2011)

tech14 said:


> So, first question, is this ok?


Yes



tech14 said:


> and second, would i need a vapor barrier on before i put the paneling back on top?


No, the only location a vapor retarder should be placed in a basement wall system is directly against the concrete. The foam is serving as your vapor retarder already.


----------



## maxwell_smart00 (Dec 15, 2011)

In Canada, a vapour barrier goes on the warm side of the house. In the south, I think it goes on the opposite. 

So knowing where you're from might help clear it up a bit...


----------



## AGWhitehouse (Jul 1, 2011)

maxwell_smart00 said:


> In Canada, a vapour barrier goes on the warm side of the house. In the south, I think it goes on the opposite.
> 
> So knowing where you're from might help clear it up a bit...


Doesn't matter where you are when you're talking basements because the earth is a constant temperature and vapor drive, below grade, is always from the exterior inward. NEVER put a vapor barrier on the interior side of a wall when the exterior side of that wall is below grade.


----------



## tech14 (Sep 23, 2011)

Well, Ontario Canada. 

Good to know, as just about everyone i know puts vapor barrier then drywall in their basements... so thats wrong?


----------



## teststrips (Sep 21, 2011)

The foam does act as a vapor barrier, and you wouldn't want to have a double vapor barrier which causes even more issues. I'd personally seal the seams on the foam side as best as possible with a caulk, spray foam, or a tape that's rated for the foam you used. Its probably too late since you already have things studded, but 5/8 inch foam may not be thick enough to prevent condensation within your roxul. You can read this - http://www.greenbuildingadvisor.com...lating-minimum-thickness-rigid-foam-sheathing, for an explanation, but the basics are that if the foam isn't thick enough, the temp point where condensation is likely to occur will happen in the roxul (where water can actually form) instead of inside the foam (where a closed cell foam makes it impossible for water to condense). All of my reading has been for a normal wall and not a basement situation, so you might be OK... not sure what complexities a basement wall throws into the foam thickness equation.


----------



## Windows on Wash (Aug 30, 2011)

AGWhitehouse said:


> Doesn't matter where you are when you're talking basements because the earth is a constant temperature and vapor drive, below grade, is always from the exterior inward. NEVER put a vapor barrier on the interior side of a wall when the exterior side of that wall is below grade.


+1

This is the reason that PSK fiberglass is such a terrible option.


----------



## AGWhitehouse (Jul 1, 2011)

Since the earth is typically always very moist (rain, snow, etc.) and your home usually not so much, the vapor drive is almost always from the exterior to the interior. Hence why you want the barrier on the outside. 5/8" XPS foam will only give you a Class III vapor retarder rating while 5/8" of polyisocyanurate MAY give you a Class II (can someone verify?). If you put a layer of plastic on the inside (Class I retarder), then you will be trapping the moisture driving in from the exterior. Eventually mold will begin to grow and a bad musty smell will creep in. The only way mold won't grow is if your foundation was properly water proofed and the drainage system is in proper working order. Since it is rare for these systems to be 100% operational over the duration of a foundation system, it is generally assumed that the vapor drive is present.


----------



## tech14 (Sep 23, 2011)

this foam is just the white, i think that's the expanded type? It had a bit of smell, i tore all the 3 sheets off one of the walls as it wasn't studded like the rest, just glued on foam and paneling. behind it was a ton of dirt, bugs and in some areas it was sweating. I try to keep up with the humidity in the summer just to keep things dry. also any benefit to paint the block wall? I ve read good and bad things


----------



## AGWhitehouse (Jul 1, 2011)

tech14 said:


> also any benefit to paint the block wall?


If you're putting 2" of XPS foam on it...Nope...it's just a waste of money & time. The paint does exactly what 2" of XPS foam does...retards the moisture drive.


----------



## tech14 (Sep 23, 2011)

I ll be using 5/8 white (like packing material, not dense like pink) and one wall of 1/2" of the same. Still up in the air as to a solid yes or no. One site http://www.homeconstructionimprovement.com/vapor-barriers-for-basement-insulation/
states "Typically we are not in favor of a vapor barrier if you’ve installed at least 1-1/2″ of rigid foam (approx. R9). It is possible if you install a thinner layer of foam that the surface of the foam could be cool enough to promote condensation if water vapor moves from the conditioned room and hits the surface of the foam board. For that reason we recommend a vapor barrier if you’ve used less than the 1-1/2″ of foam" so that sort of puts me in that situation. Also, roxul doesnt mould as far as i know.
http://www.homeconstructionimprovement.com/vapor-barriers-for-basement-insulation/


----------



## AGWhitehouse (Jul 1, 2011)

Sounds like you've got EPS, which won't retard any moisture drive. I would recommend going with at least 1-1/2" of XPS foam sheathing against the concrete. I can't recommend putting a vapor barrier on the interior side of the studs.


----------



## Gary in WA (Mar 11, 2009)

Page #9 for thickness of foam board; http://www.eere.energy.gov/buildings/building_america/pdfs/db/35017.pdf

Your ESP is not worth much, too thin and wrong material. Some test results for your location, pp.15, 19, 49, 54, 64; ftp://ftp.cmhc-schl.gc.ca/chic-ccdh...ngual/Vapour_Permeance_Volume_1_Web_sept5.pdf

You’ll need to pick your system for a permit to finish; http://www.nrc-cnrc.gc.ca/eng/ibp/irc/rr/rr199/part2.html

http://www.nrc-cnrc.gc.ca/eng/ibp/irc/rr/rr199/part3.html


Gary


----------

