Using polystyrene in a basement

millerjoe49December 12, 2007

I'm starting to plan my basement remodel project and one thing that I've been researching is hanging 1/2 inch polystyrene (XPS) insulation on the walls before I frame. The house is only 3 years old and the builders insulated the walls from the outside. I'm also not planning on insulating the floor. Is it necessary that I hang the XPS or am I just wasting time and money?

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homebound

That's what I did and I have no regrets. It's quite nice down there.

There was a long discussion on this. Do a search on "insulating basement walls" or similar.

    Bookmark   December 12, 2007 at 5:41PM
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bungeeii

If it was insulated adequately outside, you wouldn't need it inside. Same with the floor. Some folks think far enough ahead to insulate the ground *before* the slab is poured. (I'm not one of those folks, so I have a cold basement floor.)

Worthy has linked building sciences website in this forum several times. Take a look.

    Bookmark   December 13, 2007 at 12:14PM
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millerjoe49

I know the walls were insulated with XPS outside...not sure about the floors. I live in southern WI and its been in the teens and twenties over the last couple weeks and i checked the floor and walls last night and it wasn't too cold. I guess, if I'm not going to notice a difference enough to justify the cost why waste the time/money. I'm looking at about $175 to cover the projected finished space with XPS. I was planning on putting kraft faced FG insulation between the studs anyway.

    Bookmark   December 13, 2007 at 12:28PM
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formula1

Hold on! Insulating the interior basement walls with fiberglass and no foam may be setting yourself up for problems in the long run. Like mold and mildew in the walls.

Check out the www.buildingscience.com document linked below. You probably need the foam board on the walls, and if you use fiberglass in the stud walls next to the foam, no kraft paper.

Here is a link that might be useful: Understanding Basements

    Bookmark   December 13, 2007 at 4:44PM
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worthy

If it's an either/or decision, skip the interior fiberglass and just adhere the XPS to the inside. As formula1's link explains, it's especially important at the rim (top) which is likely not to be adequately insulated from the exterior.

    Bookmark   December 14, 2007 at 4:16PM
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greif

hi

living north of you in wisconsin I would out 2" of foam against your wall then frame in leaving a 1/2 air space between them. the 1/2 foam will not do much for you

    Bookmark   December 15, 2007 at 6:05PM
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millerjoe49

I read the articles in Building Science and it recommended either damp cellulose or unkraft fiberglass in front of XPS. Since my exterior walls are already insulated, would 1/2 (or 1 inch) XPS, frame, then use unkraft fiberglass be sufficient? I also talked to a guy at Home Depot last night (not an employee) who said I need to glue a sheet of plastic directly to the concrete, insulate with fiberglass, then use another sheet of plastic stapled to the framing before I drywall. Now, unless I read a totally different article then most of you, is that about the worst thing you can do?

    Bookmark   December 15, 2007 at 9:15PM
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worthy

Double vapour barrier. Yes! But, sadly, that was the standard required in new construction here and elsewhere for years thanks to Code writers.

With exterior XPS in place, you've got a number of safe alternatives: for instance, XPS or EPS on the interior with fiberglass beween the studs, or fiberglass or other batts by themselves, or XPS or EPS by themselves.

    Bookmark   December 15, 2007 at 9:29PM
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shortlid

Would a house with a poured basment in Central New Hampshire in the early 70's have this type of exterior insulation? I am guessing not but just thought I would ask.

    Bookmark   December 16, 2007 at 4:51PM
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worthy

When you shovel and thaw out next summer, you can always dig down. But, unless it was a custom, extremely unlikely.

    Bookmark   December 16, 2007 at 6:31PM
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patsfan

I have read the buildingscience article but was wondering if anyone could answer a related question.

While I have never had any water in my basement, the group around my house saturates quickly, so I'm sure there is moisture behind the wall.

My contractor's advice is to simply build a standard framed wall a couple inches inside my poured concrete wall for airflow, keeping any unfaced fiberglass away from the concrete. Would it be better to put XPS on the concrete and then frame a wall an inch inside of that, leaving a gap between the XPS and the FG? The buildingscience article seems to put the FG directly against the XPS.

Anyone have any experience with framed walls inside of XPS on poured concrete?

Also, while buildingscience recommends a 2-inch XPS panel, I am finding it impossible to find. Lowes carries Dow WallMate, but only 1.5 inches thick. HD carries InsulPink, but I don't think they even make it 2 inches thick. If I combine 1.5 inch XPS (R7) plus R13 FG, it seems like that would be sufficient. Opinions?

    Bookmark   January 13, 2008 at 3:33AM
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Brewbeer

The XPS is a vapor retarder. Put it right up against the foundation. Then build your framed wall up against it. It us better to use XPS in the insulation system than not to, as advised by your contractor.

The HDs near me carry Insulpink foamboard in 1, 1.5 and 2 inch thicknesses. If you can't get the 2 inch, the thinner board is acceptable. Use unfaced insulation in your stud bays.

    Bookmark   January 14, 2008 at 7:57AM
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patsfan

I think you may have misunderstood my post.

My contractor didn't say anything about using XPS. He recommended to only build the framed wall away from the foundation.

I have read many people recommending using XPS (directly attached to the foundation), but I don't understand how it is acceptable to put fiberglass up against the XPS. As you say, the XPS is a vapor retarded, meaning that *some* moisture will pass through it, right? If so, wouldn't moisture eventually reach the fiberglass? Would an airgap between the XPS and the fiberglass help to prevent the fiberglass from moisture? Or would the airgap somehow make things worse?

Thanks for your help!

    Bookmark   January 14, 2008 at 9:40PM
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worthy

, but I don't understand how it is acceptable to put fiberglass up against the XPS.

XPS is a semi-permeable material. So while it will allow water vapour in a gaseous state to pass through, it will remain dry and it is completely acceptable to place fiber glass next to it on the dry (interior) side.

(See Renovating Your Basement in the link below.)

Your contractor is correct in not placing the wood framing in contact with the foundation. He is out of date in recommending fiberglass alone for a below-grade installation.

For further info., see Building America., a joint private-public programme under the aegis of the US Department of Energy.

Here is a link that might be useful: Renovating Your Basement

    Bookmark   January 15, 2008 at 12:19PM
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Brewbeer

The permeability of the XPS is lower than that of the unfaced fiberglass insulation and drywall. The fiberglass and drywall will shed moisture to the inside of the basement faster than it can come through the XPS.

Worthy is right: your contractor is recommending a method that is out of date.

    Bookmark   January 15, 2008 at 9:19PM
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patsfan

Thanks for your responses.

Would there be any advantage/disadvantage to having an airgap between the XPS and the fiberglass?

    Bookmark   January 15, 2008 at 10:26PM
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worthy

More like no point.

Besides, if the fiberglass isn't resting against something firm at the back, it'll fall out of place. Building Science says that even if you use a vapour barrier in this setup (as I have been forced to do in new construction) there's no problem as long as you keep the humidity less than 50%.

    Bookmark   January 15, 2008 at 10:43PM
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millerjoe49

Well, I went with 1 inch XPS and started gluing to the walls. I figure with the XPS and unkrafted FB, I'll end up with an r-18 value in the basement. Is that sufficient? Also, should I duct tape, caulk, or expandable foam the seams for the XPS?

    Bookmark   January 28, 2008 at 4:02PM
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worthy

Is that sufficient?
Depends where you are. But it should be sufficient in any "cold" region in the Building
Science hygro-thermal region map:

should I duct tape, caulk, or expandable foam the seams for the XPS?

Use a building tape, not duct tape. Tuck or Tyvek brands are popular with builders. Foam is good anywhere there's a gap.

    Bookmark   January 28, 2008 at 7:40PM
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