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Box Builder
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Discussion Starter · #1 ·
Doing a fairly large addition on my house. Working on it when I'm not doing real work.:rolleyes: I've got a few areas with a hot roofs. I've got a few options on how to insulate them, but want to choose the best one. All the roofs have been air sealed at the roof sheathing. They will also be getting 1 1/2"-2" of rigid foam on the underside of the rafters as a thermal break. Rafters are minimum of 2x10 and 2x12. So, here is what I've come up for as options.

-Flash spray the underside of the roof sheathing with 2"-3" of closed cell foam. Finish the rest with dense pack cellulose. thinking behind this is the closed cell gives me a vapor barrier and air seals anything I missed.

-Flash spray underside with 2"-3" of open cell (regular) spray foam. Finish the rest with dense pack cellulose. thinking here is don't worry about the vapor barrier.

-Dense pack cellulose the whole thing. already air sealed don't worry about the vapor barrier.

I would really appreciate any advice or suggestions. I want to do this right, but I also don't want to over do it and spend money where I really don't need to be spending money. thanks, Nick.
 

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Box Builder
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6,320 Posts
Discussion Starter · #3 ·
The rigid foam on the underside of the rafters is most likely going to be expanded polystyrene. It is only for additional insulation and thermal break of the rafters. Why would it not be necessary in option number one? Anyway, I'm not sold on the idea that I need a vapor barrier anyway. I'm not doing vapor barrier on the walls which are 10" double walls with dense pack cellulose.
 

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Sean
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5,532 Posts
I just pulled a quick image from another site to help as a reference for the design only - Now why would you have 2 thermal breaks? The reason you do a hot roof is either necessity (cathedral) or the attic becomes part of the conditioned space for ducts, etc... The vapor barrier should not be required in either option, but I am not fully sure on #3

Sorry, I also should have specifically stated that the foam was not needed under the rafters in #3 also. #1 & #3 are the only code tested and approved methods #2 as you wrote it is not (no matter what the salesman states - if they say it is, have them provide the proof not a sales brochure)

Have you reviewed the buildingsciences web site - they have a lot of good info & will help make better informed decisions
 

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Box Builder
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6,320 Posts
Discussion Starter · #5 ·
there is only one thermal break. It is the interior rigid foam. I couldn't justify uthe cost of putting the foam on the outside of the framing. I don't consider flash coating or spray foam on the underside of the roof sheathing a thermal break. The rafters are what I am trying to get a thermal break on. Building science doesn't have anything that excatly answers my questions. I'm zone 5 (Boston) so with the amount of insulation I am talking about the interior paint is all I need for a vapor barrier. But it looks like the code would like spray foam and then the remainder of the cavity filled with other insulation. Doesn't seem to specify open or closed cell foam though.
 
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