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pasichnyk 12-18-2010 01:13 PM

Insulate split framed/poured basement wall, prior to framing...
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I'm planning to insulate my basement wall with XPS (glued with PL300 and sealed with sprayfoam) edge to edge, and then frame over the XPS giving room for more XPS and electrical and some plumbing in the framed cavity. The question I have is around the unframed upper section of the wall. Is it best to just fill this up with XPS prior to covering the whole wall with XPS, or should I fill with something else?

What thickness of XPS would you recommend on the cement portion before framing? What about on the framed portion?

House is located in Seattle.

See attached pictures.


What about the cavities on the framing that I do over the top of the XPS? Can i fill this with Cellulose/Cotton 3.5" batts safely? Or do i need to stick with XPS all the way? Figured some batts would be easier to encapsulate the wiring/receptacles/plumbing that will go into the 2x4 walls. Thanks!

jklingel 12-19-2010 01:49 AM

It appears that most of the concrete is against dirt. Check w/ local code folks, but I suspect 2" (R10) is OK in Seattle below grade; 4" is even better, of course. Cotton or cellulose, treated w/ borates, should be fine. Mineral wool is another option. Air seal via the airtight drywall approach, and no vapor barrier (ie, 6 mil poly) needed, esp if you will ever run an air conditioner. Closed cell spray foam is another option, but perhaps more spendy. Make sure you keep water away from the house; grading, gutters, drainage, etc.

pasichnyk 12-19-2010 02:26 AM

Yeah, most concrete is below ground, just the top foot and a half or so is above. roof slope and gutters run to the left wall and other side of the house, with drainage into the sewer. House is a few steps above street level, with the back side on a hill, so hopefully drainage won't be an issue. Its a 1940 house and appears to stay very dry, so hopefully that continues.

So what I was thinking was 2"xps over the entire wall, and then cotton UltraTouch r13 batts in the interior framing. From your comments, sounds like that should be good but I'll check with an inspector on Monday to confirm local codes/recomendations.

The one outstanding question is what should I put in the existing framing that surrounds the windows (they will be replaced too...). Pack it full of xps? Seems that the resulting R-value would be overkill, but not sure what else(if anything) i should use to fill the space.


jklingel 12-19-2010 12:44 PM


Originally Posted by pasichnyk (Post 553512)
The one outstanding question.... but not sure what else(if anything) i should use to fill the space.


Why not cott or cell batts? Apparently both are cut a tad wide, so that they fit snug. Rigid foam is good, but a PITA to fit, then spray foam all the gaps.

pasichnyk 12-19-2010 01:56 PM

Oh, I'm absolutely fine putting some cotton/cellulose bat in there but just wasn't sure about whether that would cause moisture retention issues. I suppose if i had cotton in the existing cavities, then continuous xps over the whole wall, then cotton in the interior framing... Exterior cotton could dry outwards, and interior could dry inwards, so I should be ok right?

jklingel 12-19-2010 07:36 PM

Oh. 2 layers of batts with xps in between? on the block, i'd be inclined to put the xps against the block, fur out w/ 2x4s and then fill the stud cavities w/ batts. That is what I gathered you were planning on doing. Above grade, I THINK you'll be OK with the xps sandwiched between two layers of batts. 1" of xps has perm rating of 1.2, and two inches will have less (exactly how much I am not sure). For comparison, poly has a perm of 0.06, ie almost impermeable to vapor. So above grade, I would be inclined to have nothing but batts, just because I am not 100% sure about the xps in the middle impeding vapor too much. Vapor BARRIERS (like poly) need to be less than 1/3 of the way through the wall's overall R value, if that makes sense. XPS in the middle may be a concern; I don't know for sure. Were it mine, (above grade) I'd fill the outer wall w/ batts, split batts so they are roughly 2" thick, and stick them between the outer wall and inner wall, then fill the inner wall w/ more batts. That will give you 9" of batts, and even if it is not perfectly fit, with a good air barrier system, dense insulation, and such a small area, for all practical purposes you'll have it schnockered. j

pasichnyk 12-19-2010 07:54 PM

Yeah, on the cement, I had planned on doing exactly what you stated (xps on the cement, 2x4 framed up against the xps, cotton batts in the new framing cavities, then drywall.

On the framing that exists around the windows, i was inquiring if I could sandwitch some XPS between cotton batts, but it sounds like that is questionable, so I'll avoid it. I'll follow up with an inspector tomorrow to be sure before I go and buy a bunch of batts, but I like the idea of just batts all the way on the currently framed part. Seems safer...

Do you have a strong preference towards cellulose batts over cotton batts, or are they pretty much comparable? There is a place just down the road that carries UltraTouch cotton but not cellulose. They are on sale till the end of the year, so I figured it was a good option...

Thanks again,

jklingel 12-19-2010 10:29 PM

I have not heard of any big preference for cott or cell. Either should work, and no big itchy mess. Be sure to air seal, and have a good one. j

pasichnyk 12-20-2010 04:03 PM

Here is something i pulled from the code, which is the only thing I can find regarding use of a vapor retarder in walls in teh Seattle Residential building code so far...

502.1.6 Moisture Control
502.1.6.1 Vapor Retarders: Vapor retarders shall be installed on the warm side (in winter) of insulation as specified in the following cases.
EXCEPTION: Vapor retarder installed with not more than 1/3 of the nominal R-value between it and the conditioned space.

I'm not exactly sure what that EXCEPTION is saying. Is it that you can only install if not more than 1/3 R value is on the inside of the retarder, or the opposite? Regardless, this seems backwards for a concrete (facing dirt) wall. I can't find anything about special exceptions for concrete, but seems that its very different from the dynamics of a above ground framed wall... :(

Even worse, it seems that Seattle has cut out the possibility to ask questions over the phone and require either email or in person, which now may (will probably) require additional fees.

jklingel 12-20-2010 05:32 PM


Originally Posted by pasichnyk (Post 554393)

I'm not exactly sure what that EXCEPTION is saying. Is it that you can only install if not more than 1/3 R value is on the inside of the retarder...

Yes. Sometimes people will build w/ a double stud wall and put the vapor retarder on the outside of the inner wall. That leaves 3.5" (if using 2x4s) of space for pipes and wires, etc, which may then be filled in with insulation. This minimizes penetrations into the VR. Visit the code folks; all they can do is charge you, and it may be worthwhile. j

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