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Doug Crenshaw Posted: Jun 10, 2008 03:29 PM	Proper depth of open cell Hope you guys view my question objectively. I promise you I am not a knit picker. I was the general contractor on my own house and didn't have any ;major problems during the whole process. A foam contractor just completed my attic with open cell foam. The attic has 2"X6" roofrafters with some 2"X4" knee walls. I chose 3.5 inch because the future use of the attic will probably not be living space and we wanted the things we had stored in the attic protected. Also we could not justify the added cost to go to 5.5 inches. The problem arises when you check the depth of the foam between the rafters. It varies from 1.5 inches to 5.5 inches and more. Like I said, I'm not picking. I guess more than 30 percent is less than 3.5 inches. It was an extremely hot day when this work was done. My wife and I kept supplying ice water to the workers all day. The workers had great perservance in completing the job. I admire them for that. Question! Do we have problem. Is it acceptable to have such variances in depths?
Edward Brassington Posted: Jun 11, 2008 10:46 PM	A lot depends on where you are located. Open cell foam has an R value the same as fiberglass approximately R3.2 per inch. What are the code requirements for your area. 5.5 inches would be approximately R20. If vapour barrier is normally required in your area for fiberglass, then you will need it for the open cell foam. 3.5 inches is probably not enough.
Jim Coler Posted: Jun 25, 2008 09:04 AM	As an open cell installer and one who understands the building science behind it, I would have recommended you fir down the rafters with foam board to create a thermal break and spray 5" minimum in the cavities. Open cell is harder to spray smooth which makes it less desireable by some foam sprayers. The job doesn't always look pretty and smooth when you're finished like the closed cell can. It takes a sprayer who is an artist to get it smooth and consistent. So I say 5" min, but that would be 5-6.5" with the firred down rafters by 1". At thicknesses of only 3" in an attic underside of roof, you may get some condensation on the surface because the open cell is typically an R-3.5-3.9/inch depending on the product and testing that was done. Don't believe all you hear about R-value being the only measurement. If they measured MPG of a car in downhill conditions only, that kind of what we have in this industry with the R-value test. It's not a real life situations that the test is conducted under which is how fibergalss gets their higher R-value numbers from. In all actuallity, fiberglass is derated from 30-60% of the stated R-value right out of the bag. This is the training from the Energy Star Training organization so don't take my word for it.

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Comments

Doug Crenshaw

Posted: Jun 10, 2008 03:29 PM

Proper depth of open cell

Hope you guys view my question objectively. I promise you I am not a knit picker. I was the general contractor on my own house and didn't have any ;major problems during the whole process.

A foam contractor just completed my attic with open cell foam. The attic has 2"X6" roofrafters with some 2"X4" knee walls. I chose 3.5 inch because the future use of the attic will probably not be living space and we wanted the things we had stored in the attic protected. Also we could not justify the added cost to go to 5.5 inches.

The problem arises when you check the depth of the foam between the rafters. It varies from 1.5 inches to 5.5 inches and more. Like I said, I'm not picking. I guess more than 30 percent is less than 3.5 inches.

It was an extremely hot day when this work was done. My wife and I kept supplying ice water to the workers all day. The workers had great perservance in completing the job. I admire them for that.

Question! Do we have problem. Is it acceptable to have such variances in depths?

Edward Brassington

Posted: Jun 11, 2008 10:46 PM

A lot depends on where you are located. Open cell foam has an R value the same as fiberglass approximately R3.2 per inch. What are the code requirements for your area. 5.5 inches would be approximately R20.

If vapour barrier is normally required in your area for fiberglass, then you will need it for the open cell foam. 3.5 inches is probably not enough.

Jim Coler

Posted: Jun 25, 2008 09:04 AM

As an open cell installer and one who understands the building science behind it, I would have recommended you fir down the rafters with foam board to create a thermal break and spray 5" minimum in the cavities.

Open cell is harder to spray smooth which makes it less desireable by some foam sprayers. The job doesn't always look pretty and smooth when you're finished like the closed cell can. It takes a sprayer who is an artist to get it smooth and consistent. So I say 5" min, but that would be 5-6.5" with the firred down rafters by 1".

At thicknesses of only 3" in an attic underside of roof, you may get some condensation on the surface because the open cell is typically an R-3.5-3.9/inch depending on the product and testing that was done.

Don't believe all you hear about R-value being the only measurement. If they measured MPG of a car in downhill conditions only, that kind of what we have in this industry with the R-value test. It's not a real life situations that the test is conducted under which is how fibergalss gets their higher R-value numbers from. In all actuallity, fiberglass is derated from 30-60% of the stated R-value right out of the bag. This is the training from the Energy Star Training organization so don't take my word for it.

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