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basement insulation question

Posted by nohandle (My Page) on
Thu, Feb 4, 10 at 14:49

I have a 3-level split home, with concrete block foundation. In the lower level, the foundation goes up about 4 feet to grade level and has a 3 foot stud wall (pony wall some call it) on top of the block. I'm sure many of you are familiar with this design and many times it offers itself to building a ledge around the room at grade level upon finishing.
However, I plan on building a straight wall in front of the foundation and continuing this straight up to the floor joists of the second level (no ledge). I will put batt insulation against a moisture barrier on the foundation part, and in between the original studs on the top part.

My question is, do I have to fill the resulting air gap between the original stud wall and the new wall with more insulation?...or can I leave an air gap between the top wall's insulation and the subsequent vapour barrier.
In other words, does it matter if I leave the top part of the new wall with no insulation (between the warm side of the insulation of the original wall and the vapour barrier), or should I double insulate the top part of the wall to fill this gap?
Hopefully I've explained myself well enough.
Thanks for any comments.


Follow-Up Postings:

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RE: basement insulation question

If you're using fg in a void it is imperative to fill the entire area with fg. Otherwise, you are setting up convection currents in the void, which will render the fg much less effective. Even installed correctly, fg insulation is leakier than other common insulating materials. (Scroll down to page 10 in the link.)
For the section that is below grade, forget the fg altogether and use extruded polystyrene (XPS) or expanded polystyrene (EPS) boards and no separate vapour barrier.


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RE: basement insulation question

Thanks worthy, your comments on convection currents is exactly what I needed to hear...something I had forgotten about. However, in order to run electrical receptacles at the proper height on that wall, I have to build a stud wall in front of the block below grade. I think it would be easier and cheaper to do moisture barrier, followed by batts (roxul rock wool, not fg) and vapour barrier, than it would be to put 3+ inches of XPS or EPS in this part of the wall.
That is unless you or someone else can suggest another strategy?


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RE: basement insulation question

This is why you don't put fg in the basement.*

Basement FG mould
Plastic vapor barrier prevents inward drying.
Common outcomes are odor, mold, decay and corrosion.
Photo: Building Science Corp.

See page 7 of BSD-103.

*The exception is behind a permeability variable barrier, the only one at this time being MemBrain.


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RE: basement insulation question

You'll have the same problem using anything other than non-solid insulation materials. In other words, with the one exception of MemBrain, stick to foam boards or sprayed foam. However, you can put fg etc. in the wall as long as there is at least 1" (or equivalent) of XPS attached to the inside foundation wall.


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RE: basement insulation question

Thanks again,
But if I may, I would like a little more clarification on this.
First of all, although I realize all basements are going to have some moisture issues, relatively speaking, I have a very dry basement. So, from your last statement, if I attached 1" of XPS to the lower half of the wall (the cement block part) and built my new stud wall on top of that, I could confidently fill the remaining gaps with batt insulation to the ceiling?
But, from what I've read in the building science articles, I should NOT put a vapour barrier on top of the insulation to allow for inward drying, unless it is MemBrain?
I have no problems with this, it just seems counterintuitive. What happens at the top half of the wall (wood stud) that is built on top of the block wall above grade? It is brick veneer on the exterior if that matters.


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RE: basement insulation question

Above-grade, Building Science Corp. calls for only vapour retarders in a cold climate. However, this advice may clash with prescriptive building codes, so you may be required to install a vapour barrier.

I have had similar problems here, with some inspectors insisting on vapour barriers in the basement where there is XPS while another inspector allowed closed-cell sprayed polyurethane foam (ccSPF) to be installed sans an (unneeded) vapour barrier.


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RE: basement followup

I know it's hard sometimes to keep up with the science on this!

Note, too, that the Building Science Digests etc. are constantly upgraded.


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