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help on installing xps on cinder block

Posted by phoenix_nj (My Page) on
Wed, Sep 17, 08 at 22:23

OK, the 3rd code official I spoke to (he's an assistant code official) has said we can put XPS on the foundation wall and frame and sheetrock over it. Initially, they said the drywall had to be in contact with the XPS.

We're going to mechanically fasten the XPS onto the wall, using probably 2'x8' tongue and groove 2" XPS board. No one seems to sell the 4'x8' which I prefer in order to have fewer seams to tape. I have the following questions:

1. If the goal is to just fasten to the wall, then will 2 screws (one at center top and center bottom) be enough? Or should we use 4 screws per sheet (at the corners)? Or is there a benefit to screwing one in the center of the board in addition to the 4?

2. I know we need to tape the seams, but what do we do about at the top and the bottom of the board? Do we tape there or use foam spray?

3. We've got an interior perimeter drain that channels liquid to the sump pumps. Should we not tape the bottom of the boards then to allow any condensation or water to drip to the drain?

4. I've got cinder block columns as shown in the picture at the link below - do we just score XPS pieces to cover as much of the columns as possible (ex: along the sides of the column)? We will be framing 1/2" along the wall and then around the columns to maximize space

5. We've got what looks like an old window (see picture at the link) that was covered with cement, but it isn't flushed with the rest of the wall - do we need to cut XPS pieces to fasten directly on the window or do we just ignore it b/c it's fine as long as this space lies underneath the 2'x8' board?

6. It seems tapcons and fender washers are used - what about standard masonry nails?

7. What should be done at the rim? Use unfaced fiberglass insulation? It seems like a lot of work to cut XPS and then cover it with drywall (mandatory with XPS) at the rim in between all the joists.

thanks in advance for your help.

Here is a link that might be useful: foundation wall pictures

Follow-Up Postings:

RE: help on installing xps on cinder block

AS you say, the Wallmate would really be a lot faster. Check directly with Dow or at lumber yards, as opposed to the Big Boxes.

Use as many or as few fasteners as needed to keep the XPS tight to the wall, including the bottom. I've not tried concrete nails. If you can get them to hold the washers tight, why not? Please use goggles! (I once warned a neighbour about that. He brushed me off. The next day he was sporting a massive patch on his eye after a visit to the ER.)

Adhere the XPS directly to the blocked window. The idea is to keep the XPS directly on the wall; this prevents condensation and keeps the R Value as high as possible.

Use XPS on the columns (actually, they're pilasters) too.

If you use T&G, it's not really necessary to tape the joints as long as they're tight.

The rim area is actually the most important area to insulate tightly as it's above grade.

Pull out all that fg; it was just filtering the drafts anyway. And if you put plastic over it, you'll get mould and mildew.

Caulk any spaces between the blocks and wood. Then fill the spaces between the joists tightly with either XPS or EPS (extruded polystyrene. Fill any gaps with handheld spray. Tape too, if necessary. It won't look pretty! But it should be tight.

AS an alternative, at the rim, you could have professionals spray in cc foam. This is what I do on new homes when I'm not spraying the entire basement wall. (Some at this board say they've used the DIY sprays successfully. I have no experience with them.)

Glad you persisted and got through to a Code Official who understood what you were doing!

RE: help on installing xps on cinder block

Worthy, Others:

Re: Adhere the XPS directly to the blocked window. The idea is to keep the XPS directly on the wall; this prevents condensation and keeps the R Value as high as possible

I have NEVER seen any mositure on my walls. If I have never seen any, why do I need the XPS. Does the fact that I construct an insulated wall and inch away from the concrete increase the possibility of there being condensation? I am just at a loss as to why I need the XPS.

RE: help on installing xps on cinder block

Water vapour is moving in and out of your basement all the time. You just can't see it. But impede its flow with water sensitive materials and the trouble will begin.

RE: help on installing xps on cinder block

Worthy, thanks so much for your advice.

Regarding the rim, if I put XPS on it, won't I need a fire-resistant covering, such as drywall? This would be a lot of work - is there another fire-resistant covering I could use that would be easier to install over XPS after caulking?

I read somewhere that if a high enough R-value (like R-19) for unfaced batts at the rim is used, I could prevent condensation from taking place there and still allow warm vapor to breathe in and be dehumidified in the room (through dehumidifiers). I understand that if there are air gaps, that the batts would deteriorate with time. Any thoughts?

Also, do I have to insulate the rim joists before putting up the walls? It's probably easier before, but could it be done after the walls are framed?


RE: help on installing xps on cinder block

The rim is the most important part because that's where there's the most exposure to the outside air.

Lstiburek in Understanding Foundations says:

"Of particular concern are rim joist areas
which are cold not only during the summer but
also during the winter (Goldberg & Huelman,

Basement walls should be insulated with non-
water sensitive insulation that prevents interior
air from contacting cold basement surfaces
the concrete structural elements and the rim
joist framing. The best insulations to use are
foam based and should allow the foundation
wall assembly to dry inwards."

The simplest way to provide the required thermal barrier for the rim joist area when it is insulated with any type of foam is to drywall the ceiling. After all, you're drywalling the rest of the area. Why leave off the ceiling?

When one client insisted on just that, I used a gypsum-based sprayed plaster that's used for fireproofing industrial buildings. There is also now a paint-like product that can be applied. Sprayable Portland cement is another possibility.

Here is a link that might be useful: Understanding Foundations

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