Attic foam depth? | Ask Mason Knowles - SPF Consultant | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

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John Kish Posted: Sep 09, 2008 10:23 AM	Attic foam depth? Mason, I'm considering sealing my attic via spray foam and my head is spinning from receiving conflicting information - help! I have R30 blown fiberglass in my Atlanta home and I'm trying to determine the best coarse of action to reduce infiltration, lower heat around top level air handler/ductwork, keep squirrels and such out of my attic, and reduct pollen and dust in the house. So far, I've had a suggestions to put 5" of open cell foam vs 2" of closed cell foam with leaving the blown insulation in place. I worry about the 2" CC foam depth compared to the R19 code and the possibilty of condensation. What is the right answer in both the OC and CC foam installations for required depth? Thanks for any guidance.
quentin Posted: Sep 09, 2008 01:13 PM	Either way you HAVE to meet code which is 3 in of CC and it is a better barrier against the things like squirrels. Either one will do a good job though. Just with CC you get the perm barrier a lot easier than with open cell and you gain some structural value. Look at your budget and decide what are the most important things for the end results for you first off. List them in order and see which one does more of what you want and if that is worth more money or not if the CC does it better. If not then cross that item off your list and keep moving down it. Either way, dump the fiberglass and donate it to the local habitat for humanity or etc instead of paying to dump it, gain a few dollars for tax credits with it and seal the entire attic as part of your building envelope. You may also want to have the duct work sprayed separate on top of the attic so it is sealed.
John Kish Posted: Sep 09, 2008 01:24 PM	Thanks for the reply. A question on meeting code: If the fiberglass is left in place, would the 2" CC meet code? I realize that it is probably best to remove or have the option to remove, but is that combination a valid one from a code perspective?
mason Posted: Sep 09, 2008 06:54 PM	This is a common question on my forum and you can go back to some of the topics in the last month and obtain information. go to the search feature and type in unvented attics and sprayfoam thickness. Also check out my article "learning the differences between 1/2 lb and 2 lb SPF on my website masonknowles.com But to answer your question, 5 inches of open cell foam to the underside of the attic will provide a rated R value of approximately 19 - 20. Two inches of closed cell foam will provide a rated R value of approximately 12 - 13. Both the open cell and closed cell sprayfoam have been tested to perform 20 to 40% more effective than equal rated R values of fiberglass installed to the floor of the attic. (testing conducted at Oakridge National Labs, did not include effects of air infiltration, so the performance differences were based on the sprayfoam reducing convection in the attic space.) In wall assemblies, open cell and closed cell sprayfoam has tested again 20 to 40% better than fiberglass insulated walls. This testing did add air infiltration. In both tests the closed cell foam performed slightly better than the open cell foam in percentages of rated R value vs actual measured performance. I attribute this difference to the fact that in both tests, some voids were discovered in the open cell foamed walls when the foam was removed. If the voids were not there I would expect the open cell foam to perform equal to the closed cell foam (in terms of energy performance) Also, get rid of the fiberglass, it is doing hou no good and may actually keep the attic warmer in summer and colder in winter, the opposite of what you are trying to achieve by installing sprayfoam to the underside of the roof. Hope this helps
Gerry Wagoner Posted: Sep 10, 2008 07:32 PM	"Also, get rid of the fiberglass, it is doing hou no good and may actually keep the attic warmer in summer and colder in winter, the opposite of what you are trying to achieve by installing sprayfoam to the underside of the roof." My thoughts too Mase. og

Author

Comments

John Kish

Posted: Sep 09, 2008 10:23 AM

Attic foam depth?

Mason,

I'm considering sealing my attic via spray foam and my head is spinning from receiving conflicting information - help!

I have R30 blown fiberglass in my Atlanta home and I'm trying to determine the best coarse of action to reduce infiltration, lower heat around top level air handler/ductwork, keep squirrels and such out of my attic, and reduct pollen and dust in the house.

So far, I've had a suggestions to put 5" of open cell foam vs 2" of closed cell foam with leaving the blown insulation in place. I worry about the 2" CC foam depth compared to the R19 code and the possibilty of condensation.

What is the right answer in both the OC and CC foam installations for required depth?

Thanks for any guidance.

quentin

Posted: Sep 09, 2008 01:13 PM

Either way you HAVE to meet code which is 3 in of CC and it is a better barrier against the things like squirrels. Either one will do a good job though. Just with CC you get the perm barrier a lot easier than with open cell and you gain some structural value.

Look at your budget and decide what are the most important things for the end results for you first off. List them in order and see which one does more of what you want and if that is worth more money or not if the CC does it better. If not then cross that item off your list and keep moving down it.

Either way, dump the fiberglass and donate it to the local habitat for humanity or etc instead of paying to dump it, gain a few dollars for tax credits with it and seal the entire attic as part of your building envelope. You may also want to have the duct work sprayed separate on top of the attic so it is sealed.

John Kish

Posted: Sep 09, 2008 01:24 PM

Thanks for the reply.

A question on meeting code: If the fiberglass is left in place, would the 2" CC meet code?

I realize that it is probably best to remove or have the option to remove, but is that combination a valid one from a code perspective?

mason

Posted: Sep 09, 2008 06:54 PM

This is a common question on my forum and you can go back to some of the topics in the last month and obtain information. go to the search feature and type in unvented attics and sprayfoam thickness. Also check out my article "learning the differences between 1/2 lb and 2 lb SPF on my website masonknowles.com

But to answer your question, 5 inches of open cell foam to the underside of the attic will provide a rated R value of approximately 19 - 20.

Two inches of closed cell foam will provide a rated R value of approximately 12 - 13.

Both the open cell and closed cell sprayfoam have been tested to perform 20 to 40% more effective than equal rated R values of fiberglass installed to the floor of the attic.
(testing conducted at Oakridge National Labs, did not include effects of air infiltration, so the performance differences were based on the sprayfoam reducing convection in the attic space.)

In wall assemblies, open cell and closed cell sprayfoam has tested again 20 to 40% better than fiberglass insulated walls. This testing did add air infiltration.

In both tests the closed cell foam performed slightly better than the open cell foam in percentages of rated R value vs actual measured performance. I attribute this difference to the fact that in both tests, some voids were discovered in the open cell foamed walls when the foam was removed. If the voids were not there I would expect the open cell foam to perform equal to the closed cell foam (in terms of energy performance)

Also, get rid of the fiberglass, it is doing hou no good and may actually keep the attic warmer in summer and colder in winter, the opposite of what you are trying to achieve by installing sprayfoam to the underside of the roof.

Hope this helps

Gerry Wagoner

Posted: Sep 10, 2008 07:32 PM

"Also, get rid of the fiberglass, it is doing hou no good and may actually keep the attic warmer in summer and colder in winter, the opposite of what you are trying to achieve by installing sprayfoam to the underside of the roof."

My thoughts too Mase.

og

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