Basement insulation

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Posted by: from Angus
11/3/2015 at 9:03:07 PM

I have started looking at finishing our basement in our new home. The builder has from what I see put tyvek against the foundation wall then used pink baton insulation and then Poly over the pink insulation. It is nailed along the center of the wall to the foundation and there is a 10-11 inch space from the concrete floor to the bottom of the insulation.

My question is - I would like to stud and drywall the basement and was looking at leaving this up and building in front of it to save some money. Not too worried about losing the few inches. The insulation has a 20 stamped on it. Would that be r20? How should I go about this?

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Date/Time11/4/2015 at 10:16:35 AM

Hi Cory,

This question gets asked a lot on Ask the Pros!

The insulation under the vapour barrier nailed to the concrete wall is minimum code requirement for an unfinished basement and is required to extend a minimum of 2' below exterior grade.

If you are going to frame and finish your basement, this insulation and vapour barrier must be removed. Some people will slit the vapour barrier however this leaves areas that have "flaps" of the old vapour barrier sealed in the wall cavity which will, once additional insulation is added above it (on the warm on winter side) allow moisture in the wall cavity to condense to liquid water in the wall.

A good method to finish a basement is to remove entirely the vapour barrier and insulation installed by the Builder, inspect the walls for any water penetration or structural issues (better to find them now than after everything is done), and then frame the new exterior walls 1" away from the masonry and spray the cavity with closed cell spray foam insulation.

An alternative is to use 1" - 2" rigid foam insulation against the foundation walls and then frame against that. This insulation acts as a "Thermal Break" reducing or eliminating the "bridging" effect the wood or steel framing would create by having one side in contact or close proximity to the cold foundation wall and the other side in close proximity to the heated finished space, essentially bypassing any insulation you have between the studs.

If you go the rigid insulation route, try "Polyisocyanurate" board. It actually has the same R-value or resistance to heat transfer as closed cell spray foam (about 6.7/inch).

In either case you will need to add additional insulation between the studs to get to R-24 at least.

Lastly, you will be required to cover the entire framed assembly with a minimum of one layer of 6mil polyethelene vapour barrier (this is not the same as plastic tarping). You must take care to ensure that seams are overlapped and taped or caulked with accoustical sealant to prevent warm moist air from migrating into the wall cavity and potentially condensing into liquid water later.

An exception would be with the closed cell spray foam which acts as its own vapour barrier, however, check with your Municipality when applying for the permit, because some Inspectors require special treatment for closed cell spray foam on wood framing.

Best of luck with your project!

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