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Community and Q&A

Insulating a roof with Icynene and sheetrocking to the rafters

7JrJEnfJbr | Posted in Energy Efficiency and Durability on

We have a cape style house in southern Mass. close to the ocean. The second floor ceiling was low so we took it down and hoped we could have cathedral ceilings to gain some height. I am a little nervous about having this sprayed directly to the underside of the roff. Any thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    1. You are located in Climate Zone 5. According to the 2009 IRC, your ceiling should be insulated to at least R-38.

    2. If you install closed-cell spray polyurethane foam, you need at least 6 inches of spray foam. If you install open-cell spray polyurethane foam -- the most commonly installed type of Icynene is an open-cell foam -- you need at least 10 inches of spray foam. If your rafters aren't deep enough, you may have to fur down your rafters or sister deeper rafters to your existing rafters. Since older homes often have undersized rafters, sistering new rafters might be a good idea anyway.

    3. Assuming you have enough rafter depth, you don't have to install the spray foam directly against the roof sheathing if you don't want to. Many people prefer to leave a ventilation air space between the top of the insulation and the underside of the roof sheathing. If you decide to have ventilation channels, it will be easier to repair future roof leaks or sheathing rot. The best ventilation baffles are site-built baffles made from thin plywood and 1x1 sticks.

  2. 7JrJEnfJbr | | #2

    Thank you. Do you think it is a good idea or would I be better to deal with the lower ceiling height? My contractor thinks the closed cell insulation would be the way to go.

  3. karlkorp | | #3

    Though Martin is right about the R-values, consider the existing R-value of the sloped ceiling. I would recommend spray foam in the existing cavity, no matter the depth, but also include the existing sloped ceiling portion, which will no doubt substantially increase the existing performance of the assembly even by increasing the effective area of heat loss. Use Martin's method to further increase the performance
    I further concur with Martin's suggestion on sistering the rafters but this must be assessed further if you intend to remove the ceiling joists. These exisitng ceiling joists are intermittently supporting the span of the existing rafters/roof joists. If you remove these you must compensate to span the greater distance, easily figured out using span tables.

  4. GBA Editor
    Martin Holladay | | #4

    Karl is right about the effect of removing the ceiling joists. Consult an engineer before removing them.

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