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jphedrick 10-30-2012 09:45 PM

Finishing Basement - Insulation Question
Helping a friend finish a room in a mostly below grade poured basement. My research indicates 2' foam insulation before the stud wall is installed and then adding fiberglass batts to the stud wall. Is this necessary in Virginia or is this a procedure typical of more Northern climate?
If I don't need that much xps insulation, what is the alternative for a dry basement?

joecaption 10-31-2012 01:13 AM

Look on the top right of the page, see where it says search?
There's thousands of post on this one subject, check it out and come back if your still in trouble.

jphedrick 10-31-2012 07:27 AM

Thanks, but I have already spent several hours using that search box for the answer, and I still have the same question.
Do I need to use the xps before the wall is installed or just a vapor barrier in southwest Virginia?
I thought someone may have a quick answer that would save me a few more hours reading through "thousands of posts" hunting for just the right one.

Beepster 10-31-2012 12:13 PM

To find out what is "necessary", give a call to your local building inspector.

Look through the posts in the "Insulation" section. There are literally hundreds of replies from GBR with attachments to figure out what is needed in your zone.

No vapor barrier.

None of this will prevent your basement from getting wet. It will only direct the moisture downward. To keep your basement dry, make sure every precaution is taken outside to keep/move the moisture as far away from the house as possible.


scottktmrider 11-01-2012 01:54 PM

I would use dry lock on the concrete walls than use batt insulation in the stud wall with vapor barrier.

Setzer1994 11-01-2012 03:18 PM

Whatever you do, DON'T use a vapor barrier. All you'll do is trap the moisture and that'll lead to mold.

Here's the proper basement "sandwich" as I like to call it for Northern climates:

Concrete Foundation Wall -> Foil-Backed Foam board (1" is plenty) -> Normal Framing w/ Pressure Treated Base Plate -> Roxul Batt Insulation (rock-wool, doesn't mold, sold at Lowes) -> Moisture-resistant Drywall (green board)

As stated above, the foam board is more for letting moisture condense on the foil-backing then travel downwards instead of through your insulation and wall. Use Liquid Nails to adhere it to the concrete.

Roxul is seriously the best insulation ever, especially for basements. It's a little more expensive, but its moisture- and vermin-resistant. Plus it's super easy to work with (cuts with a bread knife!) and makes insulating the sill plate a breeze.

This is how I did my basement much further north than you (CT). Dry as a bone and plenty warm in the winter.

Gary in WA 11-04-2012 05:09 PM

Welcome to the forums, JP! The foamboard warms the cavity and is the first condensing surface in the insulation make-up. Using only 1" (R-5) rigid, may not be enough to limit condensation for the three lowest temperatures in your area. Eg.- using Lebanon as your city- average lows of Jan., Feb., and Dec. = 24*

At 7o* inside temp, your inside face of the f.b. would be 37* with 1" (R-5), giving you condensation there at anything above 29% RH in the room. This is with cavity insulation of R-13.

At 70* w. 2"fb (R-10), the RH will be 40% and above for condensation. If you do use water-resistant drywall, the framing requires 12" o.c. for the ceiling. I would not use it in a basement environment. Dryloc is optional though it will direct the wall moisture to the wall/slab joint (from gravity) to show as pooling rather than letting it through the f.b. in a controlled manner, same with the foil-faced f.b. Roxul is good. Use a foam sill-sealer under the p.t. (code required) bottom plate for an air/thermal/capillary break from the cold/wet slab;

Use f.b./canned foam, not air-permeable insulation as f.g. or Roxul on the rim joists to stop air infiltration/exfiltration and condensation problems there;

See; Calculating shrinkage" in sidebar, pp. 51:

Thick enough foamboard will let moisture through without condensing in the cavity, you can use a vapor barrier if;


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