Insulating roof over framing
mikevashon
| Posted in Building Code Questions on
Hi Martin, building a new home with a butterfly roof on one end. Roof is framed and sheeted with plywood. Plans callout for over framing above roof to get enough slope to drain roof to butterfly side. Because roof pitches up to the perimeter from the center except on butterfly side, traditional venting will not work. My understanding is that closed cell foam must be used on under side of over framing, is that correct? If so do you know of a way to get closed cell foam into the space between the under roof and the over framed roof?
Any help would be greatly appreciated.
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Replies
Mike,
First of all -- and this advice is for GBA readers who come across this Q&A thread, not you -- the time to figure out your insulation details is before construction begins, not after framing is complete. It sounds like you have already framed your roof, so this advice comes too late for you. For more information on this issue, see "Plan Ahead for Insulation."
If you want to create an unvented insulated sloped roof assembly, you have two choices. Either:
1. Install an adequately thick amount of closed-cell spray foam on the underside of the roof sheathing, with or without additional fluffy insulation in direct contact with the cured spray foam; or
2. Install an adequately thick layer of rigid foam on the exterior side of the roof sheathing, with or without additional fluffy insulation between the rafters and in direct contact with the underside of the roof sheathing.
If you choose option 2, you'll need a second layer of roof sheathing above the rigid foam.
Both of these options are explained in the following article: "How to Build an Insulated Cathedral Ceiling."
Mike -
We don't know the slopes of the roofs involved in your project but most butterfly roofs are too low in pitch to vent. Just an FYI -
Peter
Hi Martin, thank you for the very quick reply.
For clarification, are you saying we would need to spray foam between the lower(main roof)and the over framed area?
Mike,
I don't know what you mean by "the over framed area." I assume that you are talking about furring strips or shims installed above the roof sheathing in order to create the butterfly roof slope.
If you choose to insulate your roof assembly with closed-cell spray foam (or with the flash-and-batt approach), then the answer to your question is: Yes, the spray foam needs to be installed on the underside of the lowest level of roof sheathing.