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Old 02-21-2011, 08:12 AM   #1
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Adding batt insulation to foam for increased R value


Just want to verify something and get some opinions.

I am going to have my basement walls sprayed with 2 inches of closed cell foam. The R value is 12. I would like to increase this if I can by adding some batt insulation (Roxul) in between the studs. There will be no polyethelene vapor barrier since it is not needed for closed cell foam.

My concern is with potential condensation on the surface of the foam and my understanding is that I must use the two-thirds rule in order to prevent this. ie, two thirds of the total insulating value must be in the foam portion of the entire system.

So, it seems that I can add a maximum of R7 batt insulation.
R7 + R12 = R19
R19 * 2/3 = R12 )

Conveniently Roxul comes in R14 batts, so I can simply divide them in half and insert.

Does anyone have any experience, or know of any disaster stories related to this sort of thing ?



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Old 02-21-2011, 08:22 AM   #2
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Instead of 2" foam, why not have the foam sprayed to the full thickness of the stud wall? More money, but a lot of R value and no condensation between different types of insulation.


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Old 02-21-2011, 08:46 AM   #3
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Well, the reason is because closed cell foam is very expensive. The contractor doing the work advises that another inch of foam will not return value; that 2 inches of foam will do 90 % of what 3 inches will.
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Old 02-22-2011, 12:55 AM   #4
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If you air seal real well, I don't think you are going to have any problem, but I'd refer you to People flash and batt all the time. The vapor barrier being one third of the way through the insulation rule will not, I believe, nuke you when using foam. Check on BS, though. Keep your sheet rock air tight but vapor open (use latex paint, and the airtight drywall approach).
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