# Basement XPS approach



## cjmason (Jun 12, 2015)

I'm preparing to finish my basement, and I'm trying to figure out what combination of materials to use for insulating the walls.

I'm going to use XPS, based on the information I've gathered from this site. I've considered 2" XPS with no cavity fill, but I'm leaning towards 1" XPS, 2x4 framed walls, with R-15 Roxul. I like the added R-value aspect of using the cavity fill; I also like the fire-safety aspect as I will be finishing the basement very slowly, and will likely not have the drywall up to cover the XPS for several months at least. I figure I can fill the cavities with Roxul as I build the walls, and have some peace of mind. 

Looking through the various threads, I see that depending on your climate, using 1" may or may not be the right approach. The concern seems to be condensation on the XPS due to the additional insulation not letting enough the boards warm up enough. 

What I'm not sure about is how to determine my risk for this occurring. I see some threads where the average temperature in winter time is used as a basis, and other times where the average low is used. Not sure what method is more appropriate to use, or whether there's enough difference in the temps to really matter.

I might consider 1.5" XPS for the peace of mind, but I'd rather avoid it if I can to save the extra expense. 

Some notes on my situation: For my area (Baltimore, MD), the average wintertime temp is 36°F. The lows average 24°F. I've got a poured concrete foundation, 7.5 feet with about 2.5' above grade.

What does everyone think? Is 1" XPS with the R-15 Roxul a safe bet ? Or should I take another approach ?


Thanks for any help you can provide.


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## Windows on Wash (Aug 30, 2011)

1" XPS around here has worked quite well.

Once the drywall is up and done in a fashion that should make it airtight, the condensation potential is even less. 

Make sure that you get the ribbon/bandjoist areas sealed up well. 

Search some of Gary's posts with the references to the condensation. We have used 1" here with just the approach you are referencing with 100% success.


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## cjmason (Jun 12, 2015)

Finally getting around to starting the basement, and decided to start with the rim joists. Couple of questions for you guys:

- Most of what I read on insulating the rim joists with foam board seems to show the mudsill being left alone / not covered. Should the mudsill be insulated? Or is it not as important as the rim joist itself, for some reason.

- I had read awhile ago that 2" unfaced xps + air sealing the edges might be a good way to insulate the rim joists. But I saw some posts more recently that seemed to indicate a foil faced foamboard might be better. Any recommendations on which approach might be better for my climate (central Maryland) ? It would be easier in terms of materials to double-up 1" xps foam leftovers I'll have from the walls, but I if there's a good reason to use foil-facing I could probably go that route.

Thanks! Here's a pic to give you an idea of what I'm looking at.


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## cjmason (Jun 12, 2015)

I didn't do a great job searching existing threads before posting; I found lots of good info on foil-facing and why to use it on the rim joists. It seems like leaving the mudsill alone is OK too. 

For foil-faced foam board on the rim joists, the only kind I can get easily looks like Super Tuff R (http://www.homedepot.com/p/Super-TU...ft-R-13-Insulating-Sheathing-192653/100322373)

It looks like in the threads I've read here, that the foil-faced side should be in contact with the rim joist - correct ? I ask because I've seen lots of pictures online with the foil facing towards the room when installed at the rim joist.


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