Hello, what is the ratio I need to keep in order to prevent condensation when insulating a basement from the interior, with spray foam (2") and then Roxul without vapor barrior. thanks Alexis

Posted By Alexis on 12 Apr 2013 11:31 AM
Hello, what is the ratio I need to keep in order to prevent condensation when insulating a basement from the interior, with spray foam (2") and then Roxul without vapor barrior. thanks Alexis

That all depends on your anticipated average basement temp and your local climate. Where are you? (Nearest city/town with comparable climate, that has weatherspark.com datasets.)

In most of the lower 48 of the US and much of Canada 2" of closed cell foam (R12-R13) would be more than sufficient for R15 Roxul, and in many locations that can be bumped up to the R23 goods. In colder parts of Canada and AK we'd have to do the math.

Thanks a lot,

the building will be in st esprit, qc J0K 2L0

could i go with only 1.5" of spray foam? I want to keep a security margin...

With 1.5" of closed cell foam you have about R10 in foam, and R15 of Roxul (assuming 2x4 construction.) The January binned hourly average temperature in St. Esprit is about -11C. (http://weatherspark.com/#!dashboard;a=QC/St-esprit ) Assuming the interior is kept at 20C & 35% RH, the dew point of the interior air is about +4C.

As long as the average temperature of the foam in January is above +4C (or about 15C above average January temperature) you will have very low risk of moisture & mold problems from wintertime condensation.

Does it make it?

With -11C outside and 20C inside you have an average difference of 31C, divided equally across (R10+ R15=) R25, or about (31/25=) ~1.25C per R.

With R10 in foam and an exterior temperature of -11C, the interior side of the foam is then -11C + (1.25 x R10)= 1.5C.

That's a more than a couple degree too cool, and there is still some risk, but not a large risk.

With 2" of foam you would have R13 on the exterior R15 in the cavity for a total of R28 and (31C/R28=) 1.115 degrees per R. That would put the interior side of the foam at -11C + (1.11 x R13)= 3.43C still not quite there, but close enough to not matter. Using the January average rather than the winter average outdoor temp is usually quite conservative.

If you keep your house at 30% RH (the very low end of what health professionals recommend) the dew point of 20C/30% RH air is about 2C, and since January is the coldest month with the driest air, that would work.

The other way to get there is to reduce the R value of fiber by going with R11 or R13 batts rather than R15 Roxul:

R10 + R13 is R23, or 31C/R23= 1.34C per R. Then the temp on the inner side of the R10 foam is -11C + (1.34 x R10)= 2.4C, again just fine if you keep the conditioned space at 30% RH during the coldest weather.