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What is the best way to insulate our new home?

Agronomer | Posted in Energy Efficiency and Durability on

After reading several of the articles on insulation of walls and vaulted ceilings lately, I am a bit confused. We live in SW Montana. Part of the home includes a vaulted ceiling in the great room, the rest of the ceilings are standard 8′ heights. The plan calls for 2×8 rafters in the great room. My plan was to 6″ bats in the ceiling, then cover with closed cell rigid board (up to 2 “) then gyp rock to achieve a decent R value. Likewise, the walls are 2×6 so planned on batts with closed cell on outside and then installing cedar board/batten.

Any suggestions on what is best (foil wrapped iso board ….) Spray foam for us is out of the question due to the cost. Our primary heat is an outdoor wood boiler with underfloor radiant heat and baseboard in upper floor including a wood stove in great room.


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  1. GBA Editor
    Martin Holladay | | #1

    You live in Climate Zone 6. In your climate zone, the building code requires a minimum ceiling insulation level of R-49.

    Your fiberglass batts are about R-20, and 2 inches of polyiso would give you another R-12, for a total of about R-32. That's not enough insulation for your climate zone.

    You'll have to come up with a better plan. This article should tell you everything you need to know: How to Build an Insulated Cathedral Ceiling.

  2. davidmeiland | | #2

    2X8 cathedral rafters shouldn't be on the plan unless you are going to put thick foam over the deck or perhaps under. 2x12 with r-38 batts in and 2" foam under will work.

  3. wjrobinson | | #3

    Also 2x8 if you have snow loads like we do here in the Adirondacks are too small unless the span is small. I have built many a home with 2x14 in the old days so as to up the insulation and carry the loads for large cathedrals in log homes.

    If there is no building code where you are, you may be doing fine since you are a wood burner.

    Air sealing up to a point is good, adding some kind of ventilation is good though woodstoves do that for you in a normal air leaky home.

    Adding rigid foam is good but comes with ugly. Has to be done right, done right you lower air exchange, then you need to be careful about fumes and quality air and ventilation, it gets complicated.

    GBA read read read... you learn so much that you know not... happens to me.

  4. Agronomer | | #4

    Thank you kindly for the information. After a careful look at my design plans, I see that, indeed, the plan calls for 2x10 (minimum) rafters above the vaulted ceiling, which still pushes the edge of obtaining adequate R values. Based on Mr. Holladay's article and all of your comments, I am thinking it best to go with a vented roof (roof is 12/12 pitch) assembly, 2x12 rafters, faced fiberglass insulation with 2" polyiso under the rafters, then gyp board. Additional information … the home is going up in an areas that only get 10-12 inches of annual precip with about 2-3 inches of snow on the ground at any one time. Sound right?

  5. wjrobinson | | #5

    Sounds like a windy location? If so rafters need to be tied well with proper high wind details. Other plan sets you can borrow should show this. Search rafter ties, and high wind construction details etc.

    No building codes or inspections ?

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