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Flash and batt vs XPS and batt in basement


jaholliday's Avatar
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VA

02-15-14, 12:36 PM   #1  
Flash and batt vs XPS and batt in basement

Hi All,

I'm working on finishing my basement in southwest VA and am confused about my options. We've already got the 2x4 walls up (about 1" setback from poured concrete walls) and I realize now I should have put up XPS behind those. So that's not an option now. After reading a lot, it seems the recommended thing is 1-2" XPS behind stud walls, followed by unfaced fiberglass in stud bays. My question is, there seems to be some controversy about the flash and batt method (spray foam then fiberglass), but this would seem to be identical to XPS + fiberglass. What am I missing? Perhaps it's that people try to do this with <1" of foam and end up with not a sufficient thermal break and hence condensation on the warm (interior) side? I don't want to pay for full spray foam so was thinking of about 1.5" of foam then fiberglass.

A second question - much of my basement is above grade. All poured concrete walls (why it wasn't built with 2x6's for the above grade portions I don't know). Does this substantially alter how I should be insulating in the above grade portions? Can I just use fiberglass there, understanding that I don't want it to touch the concrete? Or will this just provide a cold space for condensation?

Thanks for any advice you can give!

 
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calvert's Avatar
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02-16-14, 02:11 PM   #2  
Jaholiday, the flash and batt approach would be fine as long as you follow a guideline of keeping approx., (at least) 1/3 of the total r-value of the assembly provided by the foam. If you use 1-1/2" of closed cell foam that should equal approx. r-12 r-14. If you install another 3-1/2" high density fiberglass batt you would be adding another r-13 or 15.

I would spray foam all the wall assembly equally then add the fiberglass batt. I would think that the area with a greater portion of the wall above grade is even more critical because it will present itself as a condensing surface under the right circumstances. Remember that closed cell foam is a relatively good vapor retarder. This means that you have to pay attention to a few details.

How old is your house...with cast concrete you have a lot of water involved and to dry the concrete can take 2+ years. If the wall has sprayed foam on it then the drying will take place to the exterior. The concept also applies to moisture that wicks into the concrete from the footer as well as the wall area that is in contact with the soil UNLESS you have a good exterior water/vapor retarder applied to the buried part of the foundation. If vapor cannot dry to the interior then under certain conditions ....as it dries to the exterior you can create spalling of the concrete or stucco finish that is on the exterior. I see that you are in VA but I don't know how cold your winters get. The above concern is related to the intensity and duration of cold. BTW, a bituminous coating (tar) applied to the exterior of a foundation does NOT constitute an acceptable vapor retarder as typically done in most areas of the country, (thinned out and applied by the most inexperienced laborer on the crew).

 
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02-17-14, 10:01 AM   #3  
Hi Calvert,

Thanks for the detailed response. It sounds like this is a reasonable approach, though perhaps I should wait a little longer. The concrete was poured about 14 months ago, we've been in the house for about 10 months. The exterior of the above grade portion is finished with a mix of vinyl siding and fake stone. As to the below ground, I don't know what exactly what they did but I recall seeing both tar and a layer of what looked to me like sheets of XPS (though I didn't think to look closely at the time). I asked the builder thinking it may be insulation and he said it was just waterproofing. Guess maybe I should inquire further.

So if we have a pretty good waterproofing on the exterior below grade I guess it's even more important to wait for the walls to release the moisture from when they were poured? If I went ahead this summer (about 20 months post-pour) do you think it would be ok?

 
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02-17-14, 10:16 AM   #4  
By the way - I am in Blacksburg VA. This winter has been unusual with a few sub zero nights, but in general we get down into the teens at night from time to time, usually not for very long or very often.

 
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02-17-14, 04:45 PM   #5  
jaholliday, I wish I could tell you the 20 months is enough. I simply cannot make that call. I will try to get you some information regarding a test you can perform to help you make that determination.

If the insulation that was placed against the foundation was indeed xps, that would of course be to your benefit as a reasonable water/vapor retarder.

If I had to make a wild call based on your climate and details you have described then I would follow the instructions I presented earlier and not be too worried about the issues of spalling, but I would get a few more months of drying in.

Are you also planning to foam the rim/box joists? This can be an area of considerable air leakage. Just be sure you understand any stipulations your exterminator may have with regard to that as it can be frowned upon in termite territory.

 
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02-18-14, 05:05 AM   #6  
I asked him specifically if it was XPS but he said no, it was 'waterproofing'. I didn't follow up but I assume some kind of membrane.

I thought about the rim joists, though termites are an issue here. We do have fiberglass insulation in the floor joists (i.e., in the basement ceiling) but I understand this isn't going to do all that much good at keeping air out in the rim joist area (though the insulation does continue into those areas).

Thanks again for your help.

 
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02-18-14, 09:33 AM   #7  
The contractor may have interpreted the foam as "protection board" for the waterproofing they applied.

Check around the perimeter of the house for an exposed edge of the material.

This is why it is always good to take pictures when construction is going on.

 
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02-18-14, 11:50 AM   #8  
Good point. I imagine I can just dig down a little to see what's there. There are a lot of things I will do differently next time I build, if that day ever comes! Live and learn I guess...

 
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