XPS in walkout basement

Matt5416November 26, 2012

Hi, I am planning to finish my walkout basement. As you can see in the first photo below, the basement wall is 1/2 poured concrete with some type of sprayed on coating (I am the second owner so I am not sure what that is) and then upper half is a normal framed wall with bat insulation. The second photo is showing a close up of where the bottom of the framed wall meets the concrete. As you can see there is a bit of a lip there (it is not uniform around the house, in some areas it is set back 1/2" rather than overhanging). My original plan was to put up 2" XPS and then a 1x3 grid and then drywall. However I haven't seen a lot of examples of covering a framed wall with bat with XPS here, coupled with the fact that there would be gaps between the XPS and either the framed portion of the wall or the concrete due to the overhang in the second photo, I thought I would ask for some opinions on how best to insulate.

Thank you in advance for any replies.


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I'd pull out the existing batts and put XPS or EPS between the studs and add new fiberglass, or reuse the batts if they're not full of mould. Leave no space between insulating batas as convective air currents will degrade their R value. No vapour barrier. I'm assuming mixed or cold climate.

Here is a link that might be useful: Building Science Corp. on basement insulation systems

    Bookmark   November 26, 2012 at 4:51PM
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It is a cold climate (Michigan). So after putting XPS between the studs and putting the batts back (no mold), would you then put a floor to ceiling sheet of XPS up? I would think I still need something over the poured concrete.

Also, not sure why i have a third picture turned on its side. I guess I posted something incorrectly.

    Bookmark   November 26, 2012 at 5:23PM
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I would run the XPS from the floor to the top of the concrete wall, then fill the rest--bottom and top--with fg. I wouldn't sandwich the fg with XPS because the lower permeability of the XPS might impede the drying process.

    Bookmark   November 26, 2012 at 5:59PM
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Thank you for the reply. I drew up what I think you are saying to make sure I understand. Please take look and tell me if option 2 is what you are saying.


    Bookmark   November 26, 2012 at 8:16PM
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Yes, option #2.

A couple of further suggestions:

* put strips of 1" XPS under the wood baseplate to create a thermal break with the concrete floor. This also protects the wood from moisture drive and raises it above any minor flooding that might occur.

* use caulk or single-component polyurethane foam at the existing baseplate.

Air sealing your home is at least as important as insulation in improving the building envelope.

    Bookmark   November 27, 2012 at 12:47AM
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Worthy, Thank you for your help! I have redrawn option 2 with your suggestions. I have also added a few other things that I was planning to do based on the other reading I have read on the site. Please take a look and tell me what you think. I have a few concerns about mounting the XPS to the existing poured concrete wall as it certainly isn't flat (see the textured coating in the pic) and isn't exactly straight. The other thing that I am bit worried about after reading some other pages is a fire break on the XPS. Do I need to do something at the top of the XPS sheet? I can't really visualize it because of the way my walls are (half and half). Usually I see the firebreak at the rim.

    Bookmark   November 27, 2012 at 10:24AM
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XPS definitely goes up easier on a smooth block wall than on an irregular poured wall. The concrete that comes through the form edges is the biggest headache. And the older the forms, the more irregularities you have to deal with. One-inch thick XPS conforms better to irregularities than the two-inch thick boards you're planning to use. (If you're in the Very Cold climate zone of the Upper Pennisula, I'd lean towards the thicker board.)

The most important thing is to keep the boards as tight to the wall as you can. Use an extra concrete screw where necessary and/or fill in any cavities with Great Gap etc. handheld single component foam. When you frame against the boards, use shims--cedar shakes work well--to keep the board tight to the wall.

Sometimes this can be a bit finicky and time consuming. That's why I have used spray ccspf on some jobs.

Yes, you have to provide a firebreak. That's necessary no matter insulation you use. Strips of 1/2"drywall are the cheapest material to use.

See the diagram below.

    Bookmark   November 27, 2012 at 3:13PM
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I think I will switch and go down to 1" XPS then. My walls are not perfect, with the corners being the worst (due to the forms). I may need to fill some of the corners with spray foam. I am just outside of Detroit so not near the next climate zone (I am in zone 5). I updated the diagram to reflect that and include the firebreak as I understand it (on top of the XPS vs on top of the wall). I don't really understand how it will help at that point but put it in there anyway.

I apologize for all of the back and forth. Your assistance has been very valuable. And hopefully the diagram will help others when it is finalized.

    Bookmark   November 27, 2012 at 4:16PM
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The firebreak should be at the top of the wall. The purpose is to separate the basement wall space from the ceiling space. In a typical stud wall the top plate would serve as the firebreak; an offset wall needs an extra piece to cover the gap.

BTW, the R value of the basement wall is optimally R-19-20, according to the International Energy Conservation Code.

I apologize for all of the back and forth.

No need! That's what these forums are for.

Speaking of which, what illustration software are you using?

    Bookmark   November 27, 2012 at 5:20PM
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Thanks Worthy, I misunderstood the purpose of the firebreak. I thought it was to contain the XPS, not the entire wall. I have attempted another version that shows where the current top plate is and where the floor joists connect.

I use the website http://www.diagrams.ly to create the drawings. When you create something there, you can save it to your pc (or google drive) as a .xml. You can export it to a .png (which is what I have been doing) or embed the diagram in a web page (that usualy works for me, but didn't on this site). It isn't nearly as robust as something like visio, however it has many stencils and is pretty good once you get used to it.

    Bookmark   November 27, 2012 at 9:00PM
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This is a great thread (pics are extremely helpful). I have another question: With the setup above is a vapor barrier still required? If so, would it go beneath the drywall? I am from Saskatchewan, so would this basement wall be suitable for very cold winter temps? Thanks.

    Bookmark   August 21, 2013 at 1:03PM
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R 23.8 is the minimum basement wall insulation value for the Energy Star 2012 Standard in either of Saskatchewan's very cold climate zones.

The cheapest retrofit interior way to reach that minimum while avoiding condensation in the basement wall would be two-inches of XPS against the interior of the foundation wall, (R-12) followed by fibrous insulation between wood or light steel framing with no vapour barrier. Building scientists Joe Lstiburek and John Straube, among others, recommend this system.

(Variations that either eat up more interior living space or cost more in most cases, use ccspf, expanded polystyrene boards (EPS) or polyisocyanurate boards.)

The key detail area is the space around the rim, which, ideally, should be sprayed with closed cell spray polyurethane or fitted tightly with foam board, sealed and caulked.

This post was edited by worthy on Wed, Aug 21, 13 at 14:14

    Bookmark   August 21, 2013 at 2:08PM
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Thanks for the info, really appreciate it. Good point about the rim.

When the house was built the existing stud wall portion of the basement was "finished" with batt insulation and vapor barrier. So is it recommended to remove the vapor barrier from this portion of the wall and proceed like you suggest above?

Another part to the problem/question: Our basement is a walkout, so the walkout wall is simply 2x6 construction with again batt insulation and vapor barrier (like any of the other outside walls above grade). Is this sufficient, or is there a reasonably affordable way to improve the R-value of that wall. As far as I know, all new homes (minimum code) are built with 2x6 wall with batt and vapor barrier in this area of the country. Thanks again.

    Bookmark   August 21, 2013 at 2:51PM
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Yes, get rid of the vapour barrier. (See link below.) If you're redoing the wall, redo the insulation.

For about 25 years, vapour barriers beneath the drywall were the enforced standard in Ontario and elsewhere, too. Thanks to building science research, it's realized they're not always appropriate. Most moisture enters the wall not through permeable surface materials, but through actual air leaks around electrical and service boxes, drywall to wood connections and wood-to-wood joints. Vapour retarder paint is sufficient in most climates.

Here is a link that might be useful: Building Science Corp.--Basement Insulation Systems

    Bookmark   August 21, 2013 at 6:33PM
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This post was edited by worthy on Wed, Aug 21, 13 at 20:16

    Bookmark   August 21, 2013 at 6:37PM
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This post was edited by worthy on Wed, Aug 21, 13 at 20:15

    Bookmark   August 21, 2013 at 6:45PM
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Windows on Washington Ltd

Good Post Worthy.

One thing to add would be to seal the seams in the foam board so as to avoid any air contact with the foundation wall as well.

ADA drywall.

    Bookmark   August 21, 2013 at 8:47PM
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seal the seams in the foam board

Or if using two layers of 1", stagger the seams. I use shiplap, but I understand it's not available everywhere.

    Bookmark   August 21, 2013 at 10:44PM
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I can get the shiplap foam here. So what is best to use in the seams, the adhesive or just an acoustic sealant, or do you just Tuck tape the seams?

    Bookmark   August 27, 2013 at 11:01AM
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Tuck is fine indoors; for foam sheathing used as a primary drainage barrier on the exterior there may be more effective choices.

This post was edited by worthy on Tue, Aug 27, 13 at 20:16

    Bookmark   August 27, 2013 at 5:14PM
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