Wont extruded rigid foam poly trap moisture?

jimracJune 13, 2007

HI all,

I contacted ownes corning about insulating my below grade basement walls. No water problems whatsoever.

I mentioned, I was going to use their fanfold ext rigid polyestrene and attach it to the concrete walls, then add framing, then fill will batt insulation, then sheetrock.

Rep told me, whomeever gave you that advice is not serving you well,, and we are asking for a mold problem, since the foam board will trap the moisture.

Any thoughts?

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According to the Building Science Corporation, extruded polystyrene is a preferred method of basement insulation. (See here, document 1, on Building America, a project of the US Department of Energy.)

I'm a builder and I did my own home with the method you're proposing., except that at the rims I used open cell foam.

XPS (extruded polystyrene) is not impermeable. It is a semi-permeable material that permits water vapour to pass through without being affected, so it works fine in basements, being affected by water in neither liquid or vapour state. (The only vapour that would pass through, by the way, is on the portion of the wall that is abovegrade.)

They key to a good job is to affix the XPS as tightly to the wall as possible. I used Tapcons and fender washers, and sealed all joints with building tape (the red sticky tape, such as Tuck brand). You might be able to use foam adhesive to better effect if your basement is unit masonry (blocks).

If you use two-inch XPS, you don't even have to bother with the batts. And you can use thinner wood framing or even 2x2 steel. Be sure to rest the plate on strips of EXPS to provide a thermal break with the floor and to prevent water absorption should there ever be a flood.

On a later project, I've upgraded to sprayed closed-cell insulation, (BASF Walltite), at R-6 per inch; it adheres perfectly and is applied quickly.

Whichever method, you're way ahead of the "standard" mould-encouraging batts and polyvinyl vapour barrier.

Good luck! (And don't ever take a salesman's advice as gospel.)

    Bookmark   June 15, 2007 at 7:46PM
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What is EXPS? or did you mean XEPS (extruded polystyrene) in the following statement?

If you use two-inch XPS, you don't even have to bother with the batts. And you can use thinner wood framing or even 2x2 steel. Be sure to rest the plate on strips of EXPS to provide a thermal break with the floor and to prevent water absorption should there ever be a flood.


    Bookmark   June 16, 2007 at 10:01PM
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Sorry! (Unfortunately, this site doesn't have any editing facility.) I meant to write XPS for extruded polystyrene such as Dow Styrofoam.

    Bookmark   June 16, 2007 at 10:52PM
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Hi Worthy,

I appreciate you taking time to answer my question. Now, what weight doe the building science corp carry in construction knowledge? It looks like their info was presented back in 2002.

Now, I used the 3/8" extruded poly and I attached to the below grade foundation walls ( raised rach in the northeast) with that popular adhesive, "nails.. somehting"It has bonded great.

I am noiw going to frame with 2x4, then add unface insulation. But maybe it makes sense to use more extruded poly, cut to fit in each stud cavity?

Thanks once again,


    Bookmark   June 18, 2007 at 9:09AM
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Here's another one that's more recent, 10/2006.

Here is a link that might be useful: Updated Doc

    Bookmark   June 18, 2007 at 9:45AM
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In their own words, Building Science Corp. "is a building science consulting firm with offices in Boston, Massachusetts and Waterloo, Ontario and clients throughout North America. Building Science Consulting specializes in building technology consulting. Our focus is preventing and resolving problems related to building design, construction and operation. We are internationally recognized for our expertise in moisture dynamics, indoor air quality, and forensic (building failure) investigations. We are also on the leading edge of the design of sustainable communities and buildings. We believe in promoting energy efficiency and environmental responsibility within the constraints of marketable and affordable building technology."

Best of all, their publications put into understandable language, the conclusions and findings of technical experts published in professional building journals.

Liquid Nails makes a variety of products. Several are suitable for adhering the foam to concrete.

3/8" of XPS will only yield maybe R-3. So you'll have to add more insulation. (I would have used 1".) You can either use batts or cut to size extruded or expanded polystyrene. If you use batts, Building Science Corp. advises you to use mechanical dehumidfication in the summer to keep humidity below 50%.

    Bookmark   June 18, 2007 at 12:23PM
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No basement wall made of poured concrete is ever true, so you'll have gaps between the wall and the XPS. These gaps can condense water vapour and thus water can get trapped and cause mold.

Having said that, it's still way, way, way better than the traditional "gap" that's used between the studs and the concrete wall.

    Bookmark   June 26, 2007 at 12:31PM
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I'm doing something very similar, but want some confirmation.

I just had a french drain installed in my finished basement because I've always had a water problem there. I had a flood last year, probably because my window well is all concrete and had no drain. I would always see some water along the front of the house when it would rain.
So, I cut out the front wall (left the metal framing, which sits about 3" off the foundation wall) and had the french drain installed along with a drain installed in the well that connects to the french drain through the foundation wall directly below the well. As part of the french drain system, there is a molding installed along the bottom of the foundation wall that connects to the french drain to allow any water that drips down the wall to drain be evacuated by the drain. from the Seems to have worked. There are some cracks in the wall and areas where the thoroseal on the wall has failed, and the wall appears to be a very large block wall. It is completely below grade. So, now I want to install insulation and refinish the wall. My house is in the northeast US (NYC).

I bought 2" XPS and am trying to squeeze it behind the metal framing of the wall I cut out - a difficult task that requires cutting the XPS into smaller pieces. I intend to insulate as well as can be around the plumbing and electrical in the utility closet (location of meters, main panel, waste pit, etc.) that is also located along that wall. The finished wall will angle inward to the house to create the closet, so the interior of the closet will be open to the gap between the XPS and the wall finished wall. I'm using construction adhesive only (no furring strips). Then I was going to install R-13 faced batt insulation face-side inward between the XPS and sheetrock (excluding the utility closet, that will not have sheetrock on that wall).

So, does this sound like a good plan? Also, what should I use to fill the gaps between the edge of the XPS and the window frame, sill plate, etc? What about between the pieces of xps that may not (okay, do not) fit perfectly together? Also, can I get away without adding a dehumidifier (I'd hate to waste electricity)?


    Bookmark   April 14, 2011 at 6:59AM
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Since there seems to be liquid water still coming to the interior and running to your inside drains, I would not use the fiberglass insulation. However, the 2" of more or less continuous XPS meets the NYS Energy Code Building Envelope Requirements for new homes. As well, even liquid water will not affect its insulative value. You can seal the seams and gaps with polyurethane spray foam (Great Stuff, eg.) and building tape such as Tuck or Tyvek. Not duct tape!

Dehumidification is almost always necessary, at least in the warmer months of the year. The electrical cost is just the price of keeping the space comfortable and healthy.

    Bookmark   April 14, 2011 at 11:51AM
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"The colors depend on the manufacturer."

That would best be described as a small swimming pool.

    Bookmark   April 14, 2011 at 2:19PM
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