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Community and Q&A

Insulating an existing roof

Lolalo12 | Posted in Energy Efficiency and Durability on

So I’ve been reading the blogs about the best way to insulate a roof, but it always seems that they are dealing with new roofs or gables. We purchased a house a few years back and have been steadily renovating it. The roof was “new” asphalt at the time of purchase – seller added additional layer of asphalt shingles so not a complete tear off. In either event, I dont wish to redo the roof at this time, but i need to insulate it as we intend to begin converting it to bedrooms. The roof is a hip roof with three dormers (one at each cardinal direction except west). From the various posts it seems like i will need to treat it as an unvented cathedral ceiling. What i’m debating is the combination of insulation to employ. Spray foam concerns me because of cost and that it conceals leaks. I am in Zone 5 (connecticut) so all seasons are of concern.

Existing roof construction is as follows: Asphalt shingles, plywood, purlins, and 2×4 rafters (built 1920, lumber is really 4″)

I have two thoughts.

OPTION 1: seal Eave and all joints at the perimeter with spray foam then fill cavity with 3 1/2″ of roxul, vapor permeable air barrier on the inside face of rafters, 2x furring and 1/2″ gypsum. Later, reroof-complete tearoff and put down 3-4inches of polyiso followed by 3/4″ plywood, A/V barrier and standing seam metal roof.

OPTION 2:seal Eave and all joints at the perimeter with spray foam then fill cavity with 3 1/2″ of roxul, 2 layers of 2″ rigid over rafters – staggering joints, Vapor barrier, 2x furring and 1/2″ gypsum.

Both options would be around R41, but head height is a concern so ideally with option 2 i would prefer not to do more than 2 inches of Rigid. I definately do not wish to trap water and by code i should be around an r49. Advice would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    I want a job that pays $500 an hour and includes two months of vacation per year. But you can't always get what you want.

    Hipped roof, low ceiling, three dormers -- and you want to turn it into bedrooms. Not many options there. Just because you can't get what you want, doesn't mean you're getting bad advice.


    1. Hipped roof and the presence of dormers precludes vented options.

    2. The fact that you don't want to install rigid foam above the existing roof sheathing means that you have to use closed-cell spray foam on the underside of the roof sheathing.

    3. Other options (including the options your propose) can lead to moisture accumulation and rot.

    More information here: How to Build an Insulated Cathedral Ceiling.

  2. Lolalo12 | | #2

    Thank you.

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