time for a new XPS question

15 years ago

I used 1.5" XPS around the perimeter of the first area of my basement. I did this mostly for fear of water somehow getting in and as a mold protectant. In some areas I framed against the XPS with R11 insulation.

In the fall I will move to the 'easy' to finish portion of my basement. I say 'easy' because it entails framing out 3 exterior walls. The area is 14 X 20. The 14' wall is exposed to the side of the home, potentially water could get in here though it never has. I'll use the 1.5" XPS again. The first of the 20' walls abuts the inside wall of the garage. I can not imagine water getting in here...it has no exposure to water from above. Likewise the second 20' wall abuts our sun room so it also has no exposure to water from above either.

I'd like to put some XPS on the walls to help permiate any incidental mositure that may come thru the wall. I am doing this prevent the possibily of mold. I'll frame against this XPS and add R11 fg. Can I get away with 1/2" XPS here in Eastern PA? It has the advantage of coming in 4' wide sheets and is substantially cheaper and $$$ are running low!

Comments (9)

worthy
15 years ago
Building Science Corp. says 1" minimum, as long as you use mechanical dehumidification. They also say EPS is okay, but obviously thicker as 1.0p pcf typically has an R factor of 3.85 per inch. Neither are waterproofing, but they are not affected by water.
andrelaplume2
Original Author
15 years ago
...wonder why they even sell 1/2" then. I have dehumidification via a dehumdifier and now a few AC ducts in the basement. It may be a choice between 1/2" or nothing...not sure what the lesser of 2 evils is....maybe it even comes in 3/4".....really like to use 1/2? rather than just leaving a 1/2" air space...
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worthy
15 years ago
Virtually all of Pennsylvania is in the cold zone as per the DOE's hygrothermal map.
In a cold zone, BSC recommends 1" XPS as a minimum.

What if you are in the handful of Pennsylvania areas that are classified as "mixed-humid"?

Building Science Corp says this:

"The rule of thumb that BSC uses is 1/4-inch of rigid insulation per 1,000 heating degree days (HDD); one inch of rigid insulation works well for this climate."

wonder why they even sell 1/2" then

For moisture management and insulation value in restricted spaces--under exterior sheathing, for instance.

Again, consider EPS, which is cheaper and also approved by BSC for interior foundations.
andrelaplume2
Original Author
15 years ago
how thick must the EPS be...
worthy
15 years ago
EPS has an R Value (at 75 degrees F.) of 3.7-3.85 per inch vs. typical XPS value of R5. So 1.5"-2" thickness should do it. Installation is the same. Use only adhesives specific to foam and mechanical attachment.
andrelaplume2
Original Author
15 years ago
the problem is the thickness in this area of the basement I need as much space as possible. I need to frame for outlets and such...that costs me 4" on each side of the room right there. I can use fg in there for R value. 1.5" XPS plus the framing is to big...I can put 1/2" to 1" of something in back of the framing and 1" is even tight. Cost is a secondary but important issue. I am just looking for some mold protection..I'l like to use whatever is least expensive and least thick...not worried about R value...sound like 1/2" XPS or EPS won't provide that protection so I either squeeze in the 1" or skip it all together....wonder what the mold chances are along those two walls...one abutting the garage and the other the sun room...?
andrelaplume2
Original Author
15 years ago
I will be doing the joist area not matter what w/1.5" XPS...that I learned makes a noticable difference.
worthy
15 years ago
To save space you can easily use 2x3s. That gives you room for the 1" XPS. The R value of the foam boards determines their effectiveness in keeping the wall below the dew point. All I can convey to you are BSC recommendations.
andrelaplume2
Original Author
15 years ago
good idea, thanks.