Basement Insulation

tavertyJuly 5, 2013

Sorry to start another basement insulation thread but I'm trying to determine the most cost effective way to properly insulate my basement.

I've read the Building Science's site. It seems 2" of xps against the concrete would be one of the best solutions. But 2" xps is really expensive and I have a plumbing drain pipe that hangs down in the top foot of my concrete wall and it only give me at most 1" between the concrete and the pipe.

Since I'm limited to 1" xps and Roxul. Would it be OK to drop to 3/4" xps and Roxul? I didn't want to get condensation on the front of the xps because it wasn't thick enough. My local Lowes sell 3/4" xps for almost $10 a board less than 1". The 3/4" has a R-value of 3.8 vs 5 for the 1"

I also have I-Joists and lots of obstructions in my rim joist bay (bolts for deck ledger, electric, etc). Is it OK to only use Roxul in the rim joist or should I cut xps and then roxul? I was also thinking with the money I save doing 3/4 and roxul I could do spray foam in the rim joist. The spray seems like it would be the easiest way to effectively seal my rim joist.

I have about 3-4.5 feet of exposed concrete, the rest is underground. 9 feet floor to ceiling in basement.

I live in Montgomery County Maryland and supposedly it's in the mixed-humid zone 4. R10 continuous or R13 framed.

Thanks in advance for the help!

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The 3/4" will work as long as you provide for mechanical dehumidification, according to Yost and Lstiburek.

See here. Scroll to post 19 Sept. 2008 at 17:30.

This post was edited by worthy on Fri, Jul 5, 13 at 13:51

    Bookmark   July 5, 2013 at 12:31PM
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Worthy, thanks for the reply. I've read a quite a few posts by you and they're all great.

I have a dehumidifier running now. Since I've been in the process of taking down all the builder installed foil backed fiberglass insulation it seems the dehumidifier runs pretty much 24/7 to keep the humidity between 50-55%. I think I'm getting more drying to the inside due to about 50% of the insulation missing from the walls. There's no moisture on the walls and I haven't had any liquid water. It's a little pricy to run the dehumidifier all summer.

If I move to 1" xps can I run the dehumidifier less and keep the humidity level around 55%? I guess I'm trying to understand if more xps will lower the humidity level or will I have to run a dehumidifier anyway to keep the humidity level to the recommended 50%?


    Bookmark   July 5, 2013 at 5:01PM
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The thicker the XPS, the less permeable it is, thus restricting vapour flow into the living space and requiring less dehumidification.

I'm now in a 1964 home with no foam on the basement walls. It takes double the dehumidification to reach the same RH level as in our larger home with 1" XPS insulated walls.

Foil-covered foam insulation (foil to the interior) would work even better. Lstiburek has changed his view on the need to dry to the interior.

âÂÂI made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesnâÂÂt matter. ItâÂÂs OK for the concrete to be wet. The concrete doesnâÂÂt have to dry to the inside.âÂÂ

Glad to be of help!

Foil-covered Thermax insulation in a basement wall.

This post was edited by worthy on Fri, Jul 5, 13 at 19:51

    Bookmark   July 5, 2013 at 7:41PM
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