3" CC or 8" OC for attic | Insulation/Energy/Building Envelope Q&A | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

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Author	Comments
Thomas Dasch Posted: Apr 21, 2011 06:01 AM	3" CC or 8" OC for attic House on the MD/PA line. 2"x6" roof 18" OC, unvented. Attic is used solely for storage/junk. Going to have the underside of the roof sheathing SPF including the gable ends. My question deals with which of the 2 options would be most energy efficient: 1. 3" CC under roof and on gable ends. 2. 8" OC under roof and on gable ends. The difference in price between the two options is only $300. The reason im debating with the 8" of OC is because 8" will completely cover the 2x6's thus reducing heat loss/gain through the wood. Thoughts? 3" CC or 8" OC. Thanks in advance.
jimcoler I have over 10 years of experience specifying and installing open and closed cell spray foam. I've sold my business but I'm still selling for the new owners and consulting on large and custom specific jobs. I've expanded my knowledge into t Posted: Apr 21, 2011 07:18 AM	I would go with the 8" OC because of the thermal bridging which occurs through the wood. The 3" of CC would only allow more of the heat to pass through the wood which is only an R-3 at the edges of the foam. So, not a very bright idea.
Thomas Dasch Posted: Apr 21, 2011 07:45 AM	Thermal bridging, thats the term I was looking for. Thanks Jim! Searching the web I can't find information on how much thermal bridging really affects things but it seems there is a general consensus that there is some affect (duh right). Any problems with using OC in terms of moisture getting caught between the roof sheathing and the foam?
Bryan Kwater Posted: Apr 21, 2011 08:39 AM	You would also need to check into vapor barriers with open cell and thermal barriers for open and closed cell. Just make sure you understand everything required before you begin.
Thomas Dasch Posted: Apr 21, 2011 09:27 AM	Thermal barrier is taken care of by a spray on product that goes onto the SPF (darn expensive to boot). OC needs a vapor barrier? None of the installers I had in said anything about that, nor priced OC with a vapor barrier.....hum. Doing a quick google showed that you do need a VB, it seems on the underside of the SPF. Without a VB I assume there would be moisture buildup along the roof sheathing and the SPF? I wonder if with the added cost of a VB it wouldn't make more sense to go with the CC. I still wonder about the impact of thermal bridging.
Brian Currie Posted: Apr 21, 2011 09:46 AM	If you can cover the 2x6 with open cell you can do the same with closed we do it all the time if nothing is being attached to it after
Thomas Dasch Posted: Apr 21, 2011 09:58 AM	You mean fill the entire cavity with CC? That would be great except that doubles the cost and would be overkill. The two options are either 3" or CC or 8" of OC. Learning more about VBs and the tighter sealing properties of CC is now swaying me towards CC. I worry about the heat loss/gain from thermal transfer through the 2x6's though.
quentin Posted: Apr 21, 2011 10:02 AM	At that depth the open cell should be a vapor barrier anyways. Most are by the time you hit about 5 inches. Either way will work but if you go closed cell have them do a flash over the beams and you will be fine. It won't take more than about .5 inches to make a good thermal break in that situation. Done it many times both ways and both systems have worked very well with no complaints.
John Shockney Posted: Apr 21, 2011 10:30 AM	Doesn’t wrapping one inch of closed cell over the rafters require a lot of additional foam? There is a lot of surface to cover when you adding the sides of the rafters. But for me it’s a no brainer 3inches of closed cell is only R-21 at the most and 8inches of open cell is at least R-30 plus you remove the thermo-bridging through the 2x6 rafters (R-7to10). With 8inches of open cell there will be no need for a vapor barrier as Jim said on another thread 3-4 inches of OC will act as an air barrier and there will be no way for warm moist air from the house to penetrate the foam to a layer that is cold enough to cause that moisture to condensate into water and cause a problem. This has always been the argument that the guys only spraying closed cell have used to push their product, but I will say that closed cell is a better product and if you can afford it I would recommend that you spray 4.5-5 inches of closed cell and wrap the rafters to get a high enough R-value 3inches isn’t enough CC foam for me in that application. Airpro
Bryan Kwater Posted: Apr 21, 2011 04:42 PM	Just out of curiosity, where do you find listed the thickness of open cell foam so that a vapor barrier is not needed? I looked at 3 brands of foam and couldn't find it anywhere in their documents. Isn't a vapor barrier needed on anything greater than 1 perm? Demilac has listed 3.3 perms @ 7" thick and 2.3 perms @ 10" thick. I could only find 4.4 perms @ 3.5" thick for NCFI and 13 perms @ 1" thick for Gaco Western. I know the info has to be there somewhere, I must be blind.
Posted: Apr 22, 2011 05:44 AM	(since this is gonna be "just attic storage of junk and stuff",,,dont forget the thermal barrier coating eh jim??lol,,i think,,) not sure,,,mi,,but hillbilly reasoning would make me think that you could extrapolate the numbers and get there,,much like you do with cc,,,let see,,13 perms @ 1",,,1 perm @ 13",,,and if 3.3 perm is to 7" then solving for 1 perm @ x inches would be 23.1 inches (wow now thats air permiable),,but hiilbilly reasoning also says this could be flawed as these thicker apps probably come in at higher densities,,,thus lower perm/inch,,,gawd im so confused,,,

Author

Comments

Thomas Dasch

Posted: Apr 21, 2011 06:01 AM

3" CC or 8" OC for attic

House on the MD/PA line. 2"x6" roof 18" OC, unvented. Attic is used solely for storage/junk. Going to have the underside of the roof sheathing SPF including the gable ends.

My question deals with which of the 2 options would be most energy efficient:

1. 3" CC under roof and on gable ends.
2. 8" OC under roof and on gable ends.

The difference in price between the two options is only $300. The reason im debating with the 8" of OC is because 8" will completely cover the 2x6's thus reducing heat loss/gain through the wood.

Thoughts? 3" CC or 8" OC.

Thanks in advance.

jimcoler

I have over 10 years of experience specifying and installing open and closed cell spray foam. I've sold my business but I'm still selling for the new owners and consulting on large and custom specific jobs.

I've expanded my knowledge into t

Posted: Apr 21, 2011 07:18 AM

I would go with the 8" OC because of the thermal bridging which occurs through the wood. The 3" of CC would only allow more of the heat to pass through the wood which is only an R-3 at the edges of the foam. So, not a very bright idea.

Thomas Dasch

Posted: Apr 21, 2011 07:45 AM

Thermal bridging, thats the term I was looking for. Thanks Jim!

Searching the web I can't find information on how much thermal bridging really affects things but it seems there is a general consensus that there is some affect (duh right).

Any problems with using OC in terms of moisture getting caught between the roof sheathing and the foam?

Bryan Kwater

Posted: Apr 21, 2011 08:39 AM

You would also need to check into vapor barriers with open cell and thermal barriers for open and closed cell. Just make sure you understand everything required before you begin.

Thomas Dasch

Posted: Apr 21, 2011 09:27 AM

Thermal barrier is taken care of by a spray on product that goes onto the SPF (darn expensive to boot).

OC needs a vapor barrier? None of the installers I had in said anything about that, nor priced OC with a vapor barrier.....hum. Doing a quick google showed that you do need a VB, it seems on the underside of the SPF. Without a VB I assume there would be moisture buildup along the roof sheathing and the SPF?

I wonder if with the added cost of a VB it wouldn't make more sense to go with the CC. I still wonder about the impact of thermal bridging.

Brian Currie

Posted: Apr 21, 2011 09:46 AM

If you can cover the 2x6 with open cell you can do the same with closed we do it all the time if nothing is being attached to it after

Thomas Dasch

Posted: Apr 21, 2011 09:58 AM

You mean fill the entire cavity with CC? That would be great except that doubles the cost and would be overkill.

The two options are either 3" or CC or 8" of OC. Learning more about VBs and the tighter sealing properties of CC is now swaying me towards CC. I worry about the heat loss/gain from thermal transfer through the 2x6's though.

quentin

Posted: Apr 21, 2011 10:02 AM

At that depth the open cell should be a vapor barrier anyways. Most are by the time you hit about 5 inches. Either way will work but if you go closed cell have them do a flash over the beams and you will be fine. It won't take more than about .5 inches to make a good thermal break in that situation. Done it many times both ways and both systems have worked very well with no complaints.

John Shockney

Posted: Apr 21, 2011 10:30 AM

Doesn’t wrapping one inch of closed cell over the rafters require a lot of additional foam? There is a lot of surface to cover when you adding the sides of the rafters.

But for me it’s a no brainer 3inches of closed cell is only R-21 at the most and 8inches of open cell is at least R-30 plus you remove the thermo-bridging through the 2x6 rafters (R-7to10).

With 8inches of open cell there will be no need for a vapor barrier as Jim said on another thread 3-4 inches of OC will act as an air barrier and there will be no way for warm moist air from the house to penetrate the foam to a layer that is cold enough to cause that moisture to condensate into water and cause a problem.

This has always been the argument that the guys only spraying closed cell have used to push their product, but I will say that closed cell is a better product and if you can afford it I would recommend that you spray 4.5-5 inches of closed cell and wrap the rafters to get a high enough R-value 3inches isn’t enough CC foam for me in that application.

Airpro

Bryan Kwater

Posted: Apr 21, 2011 04:42 PM

Just out of curiosity, where do you find listed the thickness of open cell foam so that a vapor barrier is not needed? I looked at 3 brands of foam and couldn't find it anywhere in their documents. Isn't a vapor barrier needed on anything greater than 1 perm?

Demilac has listed 3.3 perms @ 7" thick and 2.3 perms @ 10" thick. I could only find 4.4 perms @ 3.5" thick for NCFI and 13 perms @ 1" thick for Gaco Western.

I know the info has to be there somewhere, I must be blind.

Posted: Apr 22, 2011 05:44 AM

(since this is gonna be "just attic storage of junk and stuff",,,dont forget the thermal barrier coating eh jim??lol,,i think,,)

not sure,,,mi,,but hillbilly reasoning would make me think that you could extrapolate the numbers and get there,,much like you do with cc,,,let see,,13 perms @ 1",,,1 perm @ 13",,,and if 3.3 perm is to 7" then solving for 1 perm @ x inches would be 23.1 inches (wow now thats air permiable),,but hiilbilly reasoning also says this could be flawed as these thicker apps probably come in at higher densities,,,thus lower perm/inch,,,gawd im so confused,,,

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