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Ed Fischer Posted: Jun 02, 2009 09:36 AM	Rafter Roof - Open vs Closed Cell My roof system is 2x10 rafter construction 14/12 pitch, 16" oc. One side facing north and one south. The north side has been replaced several times due to condensation and needs replaced again. I am planing on replacing both sides from the outside. Removing all the sheeting and present insulation, down to the tuff-r nailed to the underside of the rafters. I have had several spray foam insulators make proposals for fixing my roof problems and I would like your oppinion as to which one would be best. #1, Spray the cavity full of open cell foam, shaving off the excess to insure an airtight fit when the sheeting is installed over the foam, then installing the roofing, either shingles or metal. ( non-vented roof) #2 Spray 3 to 5" of closed cell foam onto the tuff-r surface leaving a 3-4" air space above the foam in a vented roof system. One roofing contractor even recomended stripping the rafters with 2x4's in the closed cell foam system to allow a cross ventilation between the 2x10 spaces. This would also put ventilation in the spaces above the sky lights and chimney. Then sheet and install the roofing. You advice would be greately appreciated, for I would like this to be the last time I have to roof this house.
mason Posted: Jun 02, 2009 02:06 PM	Either open or closed cell foam can be used installed to the underside of the roof deck. But, if in colder climates you require a vapor retarder element over the open cell foam. I am guessing this is a cathedral ceiling? Then install the foam without venting to the underside of the roof deck. You mention the decking has deteriorated and been replaced twice. What is the humidity inside the structure? It sounds like they need to address excess humidity inside the house with vents to the outside. I wonder if they have vents going into the attic but not to outside, that could cause this type of problem. Or they need to reduce the amount of water vapor generated inside the house. Do they have a lot of pets, house plants or aquariums. Do they take tons of showers or dry their clothes on clothes lines in the basement?
Circle-D Posted: Jun 06, 2009 10:06 PM	Mason, Sounds like this guy wants to do this from the outside, so the underside of the roof deck is not accessible. I did this to my own home last summer.I sprayed 5" closed cell and left a 4" vent open between the foam and sheathing, with a vented soffit and ridge. So far, so good, heating bill was down about 30% this winter.
mason Posted: Jun 08, 2009 10:16 AM	Thanks for the clarification circle-D I missed that he wanted to spray from the outside. So, I presume that there is drywall on the interior surface so that they cannot spray the foam on the underside of the roof deck. (Still my preference if you can do it.) But, like circle d I have sprayed that configuration, but this was a wine room that required a vapor barrier coating over the foam. So I sprayed the foam down then the coating then the roof deck was installed over the foam and coating. The space was vented over the foam to the outside. It worked well. Only problem I see is if there is a roof leak that goes through the decking that cannot get out.

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Ed Fischer

Posted: Jun 02, 2009 09:36 AM

Rafter Roof - Open vs Closed Cell

My roof system is 2x10 rafter construction 14/12 pitch, 16" oc. One side facing north and one south. The north side has been replaced several times due to condensation and needs replaced again. I am planing on replacing both sides from the outside. Removing all the sheeting and present insulation, down to the tuff-r nailed to the underside of the rafters. I have had several spray foam insulators make proposals for fixing my roof problems and I would like your oppinion as to which one would be best.
#1, Spray the cavity full of open cell foam, shaving off the excess to insure an airtight fit when the sheeting is installed over the foam, then installing the roofing, either shingles or metal. ( non-vented roof)
#2 Spray 3 to 5" of closed cell foam onto the tuff-r surface leaving a 3-4" air space above the foam in a vented roof system.
One roofing contractor even recomended stripping the rafters with 2x4's in the closed cell foam system to allow a cross ventilation between the 2x10 spaces. This would also put ventilation in the spaces above the sky lights and chimney. Then sheet and install the roofing.
You advice would be greately appreciated, for I would like this to be the last time I have to roof this house.

mason

Posted: Jun 02, 2009 02:06 PM

Either open or closed cell foam can be used installed to the underside of the roof deck. But, if in colder climates you require a vapor retarder element over the open cell foam.

I am guessing this is a cathedral ceiling? Then install the foam without venting to the underside of the roof deck.

You mention the decking has deteriorated and been replaced twice. What is the humidity inside the structure? It sounds like they need to address excess humidity inside the house with vents to the outside. I wonder if they have vents going into the attic but not to outside, that could cause this type of problem. Or they need to reduce the amount of water vapor generated inside the house. Do they have a lot of pets, house plants or aquariums. Do they take tons of showers or dry their clothes on clothes lines in the basement?

Circle-D

Posted: Jun 06, 2009 10:06 PM

Mason,
Sounds like this guy wants to do this from the outside, so the underside of the roof deck is not accessible.
I did this to my own home last summer.I sprayed 5" closed cell and left a 4" vent open between the foam and sheathing, with a vented soffit and ridge. So far, so good, heating bill was down about 30% this winter.

mason

Posted: Jun 08, 2009 10:16 AM

Thanks for the clarification circle-D I missed that he wanted to spray from the outside.

So, I presume that there is drywall on the interior surface so that they cannot spray the foam on the underside of the roof deck. (Still my preference if you can do it.) But, like circle d I have sprayed that configuration, but this was a wine room that required a vapor barrier coating over the foam. So I sprayed the foam down then the coating then the roof deck was installed over the foam and coating. The space was vented over the foam to the outside. It worked well.

Only problem I see is if there is a roof leak that goes through the decking that cannot get out.

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