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What’s the best thickness for polyiso roof insulation?

GBA Editor | Posted in Energy Efficiency and Durability on
Construction detail.jpg

I am working on a pool house design and have questions for roof insulation. Please see the attached drawing. Please see where Polyisocyanurate is indicated.
1. Are the locations of the rigid insulation correct?
2. Is the thickness 6” appropriate for this size of house?
3. Which product do you recommend? Please give me the names of the products.
Thank you very much for your assistance.

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Replies

  1. John Brooks | | #1

    KEEP THE RAIN OUT........
    the outside paving flush with the finish floor and lack of drainage is very disturbing

  2. Akino | | #2

    This drawing is not complete yet. My question was the only about roof insulation.

  3. Armando Cobo | | #3

    Sorry Akino, I agree with John, even an incomplete drawing should show the outside finished floor at a lower level of the pool house floor.
    Incomplete questions and drawings usually get incomplete answers:
    1. What is your location and/or climate zone? The size of the building has nothing to do with the insulation thickness. Are you on the performance or prescriptive code?
    2. You show rigid insulation around the envelope, then why the R38 above the collar ties? Are the knee walls in the conditioned space?
    3. You show vents on the Soffits, but where are the vents at the top? Ridge Vents? Are you venting each cavity?
    4. You show open headers. Are they insulated?
    5. You show a floating slab with no moisture barrier and crushed aggregate under the concrete slab. Am I reading it correctly? Wouldn’t you set the slab on a ledge on the concrete walls?
    6. A rain screen would be a good idea in a humid climate, especially if you are installing wood siding.

  4. GBA Editor
    Martin Holladay | | #4

    Akino,
    Armando and John nailed the important questions. I'll chime in to agree:

    1. We need to know your climate.

    2. The R-38 insulation on top of your collar ties is unnecessary.

    3. Your sub-slab details are bad. Crushed stone, please! And if you eliminate the useless insulation above your collar ties, you may be able to use the money to buy continuous horizontal insulation under your entire slab.

  5. Akino | | #5

    The location of the building is Newcanaan, CT.

  6. GBA Editor
    Martin Holladay | | #6

    Akino,
    Consult your local building code to verify whether your proposed R-39 insulation (6 inches of polyiso) meets minimum code requirements.

    In Connecticut, I would DEFINITELY recommend continuous horizontal rigid foam insulation under your entire slab.

  7. Armando Cobo | | #7

    Akino,
    In Climate Zone 5A you are required to have R38 in the ceiling/roof assembly. If you use 6” of polyiso (5.6-8 Rv/Inch) you may or not have enough depending on the product you buy. You need to make sure what the R-value of your polyiso is from your supplier. But, just bellow the 6” Polyiso you have 5 ½” R21 & 2 ½” R8 Insulation and you also have 12” R38 above the collars; what are those?
    A 2x8 Rafter has an R-value of 9, therefore you run the risk of having Thermal Bridging, which can facilitate condensation and ultimately mold, especially in a moist/coast climate.
    I believe a better solution for your job would be 2” of rigid insulation on top of the roof sheathing and then 5” of closed-cell foam insulation under the roof sheathing. This will solve your venting and thermal bridging problems plus a better seal package; and your cost should not be that much more in a small job like this.
    Also, see all previous notes.

  8. Akino | | #8

    5 ½” R21 & 2 ½” R8 Insulation - they were the original idea. I want to substitute with rigid insulation.
    12” R38 above the collars - I realized that it is not necessary according to the answers above.

    Armando, Thank you for your solution. For all, thank you for your comments. I am learning a lot. If you see any other mistakes and missing info. Please feel free to let me know.

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