basement insulation question

nohandleFebruary 4, 2010

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.

Thank you for reporting this comment. Undo

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.

    Bookmark   February 4, 2010 at 9:51PM
Thank you for reporting this comment. Undo

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?

    Bookmark   February 9, 2010 at 11:22AM
Thank you for reporting this comment. Undo

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

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.

    Bookmark   February 9, 2010 at 1:53PM
Thank you for reporting this comment. Undo

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.

    Bookmark   February 9, 2010 at 2:08PM
Thank you for reporting this comment. Undo

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.

    Bookmark   February 10, 2010 at 12:13PM
Thank you for reporting this comment. Undo

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.

    Bookmark   February 10, 2010 at 3:01PM
Thank you for reporting this comment. Undo

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.

    Bookmark   February 10, 2010 at 3:03PM
Sign Up to comment
More Discussions
What kind of ceiling to do?
We are wanting to finish out basement rustic/lodge...
water in unfinished basement
Well its in the 50s here in NJ and the snow is melting....
Easier way to burst up concrete?
Since it seem I have to do some concrete busting work...
ideas for basement?
Hoping some of you more creative people can help me...
Michael Oslosky
How to re-do the stairs for my basement remodel
So I'm undergoing a simple remodel of a small basement....
Sponsored Products
Thumprints Juicy Lime & Linen Table Lamp
$403.00 | LuxeDecor
Triumph Sports 6 ft. 4-in-1 Rotating Game Table - 45-6065
$324.99 | Hayneedle
Crosley 5-Piece Pub Dining Set with Cabriole Leg and Shield Back Stools - KD5200
$509.00 | Hayneedle
Casablanca Earth Shower Curtain
Jamie Young St Croix Mercury Glass Pendant Chandelier
Lamps Plus
Westinghouse 4-cubic-foot Refrigerator
Thumprints Spiral Slate Gray & White Table Lamp
$326.00 | LuxeDecor
Go Pet Club Beige Cat Scratcher Furniture - 52 in. - F207
$65.92 | Hayneedle
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™