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Old 02-01-2011, 12:31 PM   #1
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How to frame/insulate wall paralell to joists in basements


Okay, so I'm almost done the eco-energy retrofit on my basement. All header spaces are filled with two layers of c-200 XPS foam board for R-20. Walls are 2 of c-200 XPS glued to the poured concrete, with 2x4 framed walls with Roxul, then covered in drywall.

Framing on the walls that are 90 degrees to the joist is easy of course. I attach the top plate to the joists, the wall stops when I hit the joists and the headers spaces are broken up into little rectangles of space that is still accessible (for final sealing/checking/more insulation) even after the framing is done.

My problem is along the opposite walls. The headers on my south walls are not as accessible due to joist location. In one case the header space is made completely inaccessible by a joist. It's got a gap of 3" that I can't get at. The other part of the south wall there is a very small gap, just big enough for me to sneak the 2" foam board in and do the sealing work, BUT not that I've put the foam board on the wall below it and covered the whole sill the gap is now so small that I can't get my hands and forearms into the space.

I was going to leave it somewhat open, I am feeling fairly small air leaks that I just can't reach anymore, despite having worked hard to seal the space with silicon, etc before and with each layer of XPS.

My options as I see it are:
1. Completely seal this section of header with only the XPS in place, leaving an air gap inside but no way for air from inside the basement to enter the header space.
2. Put up the top plate leaving a small gap allowing some very modest air to circulate behind the wall, which I would then be framing and installed Roxul in front of.


Please let me know if this topic is useless without a picture. I can snap one when I get home. Sorry if I haven't explained well, am at work


Last edited by wilsonstark; 02-01-2011 at 12:38 PM. Reason: made an error making a poll
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Old 02-01-2011, 01:18 PM   #2
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How to frame/insulate wall paralell to joists in basements


Okay, I admit it is useless without images. I've sketched (somewhat amateurishly) a cross section of the wall as it currently stands. Note that at this point I can't do much *inside* that air space without cutting down or otherwise damaging the glued-in-place XPS.
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Old 02-01-2011, 07:23 PM   #3
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How to frame/insulate wall paralell to joists in basements


great foam away.
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Old 02-01-2011, 07:42 PM   #4
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How to frame/insulate wall paralell to joists in basements


Is this what you're looking for?

Spray the whole space in with polyurethane foam? Never done something like that before, but it should work if you don't overspray it (leave room for it to expand)

Then shave and frame.
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Old 02-01-2011, 07:46 PM   #5
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How to frame/insulate wall paralell to joists in basements


I'd also add some blocking in the adjacent open bay just to keep that joist from rolling
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Old 02-01-2011, 08:10 PM   #6
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How to frame/insulate wall paralell to joists in basements


Use the great foam made for doors and windows. It does not expand like regular foam.
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Old 02-01-2011, 08:22 PM   #7
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How to frame/insulate wall paralell to joists in basements


Quote:
Originally Posted by proremodel View Post
Use the great foam made for doors and windows. It does not expand like regular foam.
i'm pretty sure even with that stuff you fill 50% and it expands the rest of the way.

either way you're gonna want to watch what you're doing
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Old 02-01-2011, 08:33 PM   #8
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How to frame/insulate wall paralell to joists in basements


Quote:
Originally Posted by fungku View Post
i'm pretty sure even with that stuff you fill 50% and it expands the rest of the way.

either way you're gonna want to watch what you're doing
yea I agree just don't go foam happy lol
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Old 02-01-2011, 11:16 PM   #9
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How to frame/insulate wall paralell to joists in basements


Great advice, thanks! Two clarifying questions if I may....

1. The existing foam board is already R-20. If I'm sealing it up completely, do I need to fill in the remaining space with still more insulation or could I leave it as dead air?

Am planing on blocking to support the top plate with leftover chunks of 2x4, parallel with the joist and flush with the bottom of it. Does that sound okay?
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Old 02-01-2011, 11:20 PM   #10
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How to frame/insulate wall paralell to joists in basements


Just leave it as dead air. You have insulated that more then enough!
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Old 02-01-2011, 11:26 PM   #11
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How to frame/insulate wall paralell to joists in basements


You have 5 inches of foam in your basement? Id agree use spray foam start at the back and work your way up and out. Go halfway let it do its thing then if more is needed add it. If you over fill it its no biggy you just cut it back. If you dont care Id leave it.

Last edited by ubenhad4; 02-01-2011 at 11:30 PM.
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Old 02-01-2011, 11:49 PM   #12
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How to frame/insulate wall paralell to joists in basements


The main problem with leaving a dead air space there is that convective loops will form and transfer the heat produced in the room to the framing above the wall. http://www.diychatroom.com/f98/how-b...ulation-90438/

Unless you air seal the drywall, use a foam sill sealer under the bottom p.t. plate for a thermal/capillary break, and fill the void with medium density insulation, you will lose heat from it (at a faster rate than if using fiberglass or another). http://www.buildingscience.com/docum...wall-approach/

http://www.greenbuildingadvisor.com/...-building-code

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Old 02-02-2011, 06:35 AM   #13
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How to frame/insulate wall paralell to joists in basements


Great answers thanks. From what you folks are saying I'll make enough space so I can sneak the Roxul in. Re. Number of inches of foam, yes I have 4" of c-200 rigid extruded pink board already in the header spac, each 2" layer is marked as R-10.

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