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Does adding 2″ polyiso to the interior of an unvented cathedral roof assembly cause any problems?

gDGd63H2ch | Posted in Green Building Techniques on

I am building a new home with cathedral ceilings in a cold climate (zone 6) and have a question about the roof assembly. The plans from the architect call for a 2×12 rafter furred with ripped 2×6, giving a rafter depth of about 14″. My builder is uncomfortable with the connection of the furred 2×3 to the rafter and has suggested an alternative. He has proposed skipping the furred strip and just go with the 2×12 rafter and then use 2″ foam and apply that to the inside of the rafter. The foam could be installed before the cellulose and then the cellulose could be pumped in behind it. Might make for an easier job installing the cellulose. I have seen lots of unvented cathedral roof assemblies with foam on the exterior but none with it on the interior. Does this sound like a good plan?

Replies

  1. GBA Editor
    Martin Holladay | | #1

    Geoff,
    The system your builder is proposing is fairly common in New England, but may not meet code. To learn more about code requirements for cathedral ceilings, see How to Build an Insulated Cathedral Ceiling.

    The system will meet code if you include ventilation baffles between the top of the cellulose and the roof sheathing. To be sure that the rigid foam is well secured, most builders install 1x4 strapping under the foam to hold the foam in place. If you use foil-faced polyiso, this air space provides a bit of extra R-value to the assembly..

  2. user-659915 | | #2

    Or you could just get your builder to furr out the ceiling as specified. The usual method is to secure the furring with scraps of plywood or OSB plated across the sides of the assembly.

  3. gDGd63H2ch | | #3

    Thanks Martin and James for the responses. The more I think about the idea of adding the foam to the inside of the rafters, the more I like it. By not furring the rafters, I lose about 3 inches of dense pack cellulose or about an r value of 11.4 (r3.8 x 3"). By adding the polyiso foam to the inside of the rafters, I gain an r value of 14 (2" x r7) which more than makes up for the furring. In addition, the initial plan of furring the rafters still had the problem of thermal bridging which is eliminated with the foam.

    Martin, I had read your mentioned article on "How to Build an Insulated Cathedral Ceiling" and it was very helpful. I still had questions about the best way to approach the roof assembly. My roof contains several dormers and windows which seems to point me to an unvented assembly. If I use dense pack cellulose and the foam on the interior, what isn't to code exactly? Is it that I need to add a vent channel between the sheathing and the cellulose? Am i better off using something other than cellulose for the rafter bays?

    Thanks for the advice

  4. GBA Editor
    Martin Holladay | | #4

    Geoff,
    Q. "If I use dense pack cellulose and the foam on the interior, what isn't to code exactly?"

    A. If you want to build an unvented cathedral ceiling with cellulose between the rafter bays, the code requires you to install rigid foam insulation ON TOP of the roof sheathing. The reason for this requirement is to prevent moisture accumulation in your roof sheathing during the winter.

    Q. " Is it that I need to add a vent channel between the sheathing and the cellulose?"

    A. That's one possible solution. However, your dormers will complicate everything, and it sounds as if your roof is not a good candidate for a vented cathedral ceiling.

    Q. "Am i better off using something other than cellulose for the rafter bays?"

    A. If you choose spray polyurethane foam instead of cellulose, it will simplify everything.

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