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Attaching and Taping XPS for basement walls and floor

Posted by fish7577 (My Page) on
Sat, Oct 10, 09 at 10:26

What is the easiest way to attach and seal XPS board to the foundation walls (interior)? We're building a new home with block walls. I plan on finishing the basement (eventually). I bought 1" 4'x8' XPS for the walls and three layers of 1" 4x8 XPS for under the floor. We'll be putting radiant heat in the concrete. The three layers was MUCH cheaper than buying 2" + 1".

Can I glue the XPS to the walls? Should I still tapcon? Should I tape the seams on the walls and floor? I"ll have the seams staggered for the floor. I planned to have 1" of XPS on the walls meeting the XPS on the floor, so the concrete would come right up against XPS on the wall.

I'll build a stud wall against the XPS with fiberglass. The exterior of the foundation has Delta MS without any insulation board.

Follow-Up Postings:

RE: Attaching and Taping XPS for basement walls and floor

Sounds good! Especially having the XPS on the wall meet that under the floor. Since you're into detail, slop a layer of asphalt onto the top of the footings as a capillary break to kill rising damp. (BSC recommendation. See Link below, esp. RR-0202)

Mastic designed to adhere XPS to the block walls should work. However, you will have to use something to hold the XPS to the walls until the mastic sets. Concrete screws and fender washers detract from the continuity of the insulation; if you need them, use them in conjunction with long 2x2s. Tongue and groove (t&g) XPS works best. If you're not using t&g, tape the joints with building tape such as Tuck or Typar brands. Fill any gaps with handheld urethane foam (Great Stuff etc)

Here is a link that might be useful: Understanding Basements

RE: Attaching and Taping XPS for basement walls and floor

Thanks for the response- I was hoping you'd still be on this forum.
Is mastic something like liquid nails, or a specific type of adhesive?
Since my stud wall will be 2x4's, I was hoping to not have to use anything else to attach the XPS. From an insulation standpoint, I could build the wall with 2x2's, but figured a 2x4 wall would be easist for wiring. Perhaps some temporary furring strips run horizontally , then take them off and foam the holes.
What about a vapor barrier under the XPS for the floor? I can put a 6mil sheet under and lap it over the footer, but wasn't sure if it is needed with 3 layers of xps.
There is asphalt/tar sprayed onto the exterior of the foundation, then the Delta MS. I'm not sure if the spray was a good idea or not with the XPS inside.
The XPS is not tongue and groove, so I'll plan on taping. For the rim/joist area, I was considering damp spray cellulose (that's what I'm considering for the whole house). It's tough to get an experienced honest opinion on the cellulose. The only people with any experience with it seem to be the ones selling it.

RE: Attaching and Taping XPS for basement walls and floor


The easiest to use comes in tubes and is applied like caulking. It should be available at any building supply company that carries XPS.

You could leave the furring strips in place too if you want. It gives you more space for the fiberglass batts.

I'd still put the plates on top of 6 mil or better yet, strips of XPS. This protects the plate from liquid water that might come in the basement from flooding--burst water tank etc.

For the rim joist area, I use oc and cc foam and wouldn't use anything else. (Ok, in small retrofits, you can laboriously cut, insert and seal XPS boards and use small spray cans.)

Spray dampproofing and Delta MS on the outside is what I use and is the almost universal combination for builders here. The Delta type membranes were mandated; but when they were made elective, most builders continued to use them. For the last 20 years, I've used sprayed cellulose for the attics with satisfactory results on the four new homes I've built for myself and others for clients.

Typical Fire Stop Basement Detail
Source: Fairfax County, Virginia

When you use any foam it has to be covered by acceptable fire resistant barriers and separated by firestops, whose composition and placement varies greatly from one jurisdiction to another.

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