Insulating basement

Old 04-30-13, 08:34 PM
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Insulating basement

First I will apologize as there are many threads with a similar title.

I have been search / reading for days and a few specific questions have come up that I am have a hard time getting an exact answer to.


New house [4 years old], looking to finish basement. Finished spaced will be ballpark 700sq ft.

Currently there is nothing at all done in the basement other then blanket insulation on the walls.


1. Considered spray foam, but then have been reading about all the off gassing. While I get that most people will never really have an issue w/ this [smell more than anything else] after 24 hours I have 2 young kids and the idea of having something sprayed that needs everyone to be out of the house for 24 hours. I have a hard time believing that this gas won't permeate the soft fabrics all over the house which could cause a sensitivity in the kids. At this point, it would take a lot for me to go back to this route.

2. Insulate walls w/ rigid foam + roxul batts once the walls are up. This foam insulation would be Owen's Corning, 2" thick rigid foam [R10], PL300ed directly to the concrete walls [obv. would be taking down the current bag insulation]. Once this is all up, frame the walls [sill gasket underneath]. After framing is done, Roxul the walls w/ their R14 product [I am up in Canada, R15 is US]. After Roxul is up, vapour barrier the walls. total R rating 24.

What popped into my head today is why even bother w/ the foam. If the walls were framed w/ 2x6's instead of 2x4's, I could then just use the Roxul R22 product for a very similar overall R rating.

In terms of space, I would not be losing any more vs the 2" foam + 2x4's. This then would also allow me to offset my wall 1" to 1.5", giving the foundation walls some room to breathe.

Any thoughts out there, this has been killing me for days now.

Also, while we all maintain the costs in our head, the cost of 1 solution vs the other is not my driving factor here at all.

I am simply looking to make the best decision possible for a space that will hopefully be used for years to come.
Old 05-01-13, 02:19 AM
Join Date: Oct 2008
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These, I would say no.
"After Roxul is up, vapour barrier the walls."
The objective is to allow the very small amount of moisture vapor to pass all the way through into your basement. Any full vapor barrier will stop it and force the moisture level to accumulate until everything is as wet as the ground outside.

"If the walls were framed w/ 2x6's instead of 2x4's"
This would place the wood near or next to the concrete where it would be food for mold.

"This then would also allow me to offset my wall 1" to 1.5", giving the foundation walls some room to breathe."
Air circulation is not the type of breathing you want, drying to the inside via some vapor diffusion is. Air circulation provides an inside source of moisture, either from inside the home to a cool wall or from the bottom of the wall convecting up to the cold top and taking the moisture with it.

This thinking is not my invention, but from groups like Building Science Corp and some very talented folks up in Canada. Your research people are usually ahead of ours. I can dig out the specific related links if needed.

The part that concerns most people is allowing any moisture to pass into the basement. However, the tiny amount that would be flowing through 2" rigid insulation is dwarfed by the concrete floow and all of the other sources. People will generate more moisture. Once a basement is sealed around the house to foundation to reduce the huge path for heat loss, basement humidity will need to be controlled. This can be with a dehumidifier or by conditioning the basement with heating and cooling. If cooling is not available or necessary, then the dehumidifier would be.

Sure nice to be able to discuss these issues before construction is 90% done .
Have you read any of the Building Science links?


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