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Steel frame home insulation

JBFlorida | Posted in General Questions on

Climate zone 2
Ventilated Attic
We plan to use 3″ polyiso on exterior walls, taped, with an air gap and steel lap siding.
Plan is to use 6-8″ of polyiso on attic floor, and also putting HVAC system in conditioned space.
Radiant barrier on purlins with metal roof.

I’m looking for comments, concerns and advice on whether this amount of insulation is adequate?

or any other thoughts on this approach or things to consider with steel construction.


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is this amount of insulation adequate?"

    A. You are planning to install about R-18 of wall insulation and about R-36 to R-48 on your attic floor. This exceeds the minimum code requirement for wall insulation (R-13) in your climate zone. If you decide to only install 6 inches of polyiso on the attic floor, you will barely meet the code requirement for attic insulation (R-38) in your climate zone, depending on the R-value per inch of the product that you choose to install.

    Q. "I'm looking for comments."

    A. The use of polyisocyanurate to insulate an attic floor is unusual. Polyisocyanurate is expensive. If you choose to install cellulose instead of polyiso, you can get far more R-value for the same price.

    One more piece of advice: make sure that the insulation you install on your attic floor is continuous with the wall insulation. You don't want any discontinuities where these insulations meet. If you leave an insulation gap above the steel top plates of your exterior walls, you'll have a massive thermal bridge.

  2. JBFlorida | | #2

    Thank you for your comments. I am able to switch things up and make the attic unventilated or mostly conditioned. Example: metal roof to purlins, then rigid foam under purlins. With this I could still have a ventilated ridge and soffit system ventilating just under roof deck.
    In this scenario, would the additional insulation be more appropriate attached on roofline (under rigid foam or on ceiling floor)?
    Hot-Humid climate

  3. GBA Editor
    Martin Holladay | | #3

    Since you describe this as a "steel frame" home, I assume that your rafters are steel. So you can't install rigid foam insulation under the purlins, because the steel rafters represent massive thermal bridges through the insulation.

    If I understand your construction correctly, then the only way for your insulation to follow the roof slope is to install your proposed rigid foam insulation on top of the roof sheathing, followed by a second layer of roof sheathing. Alternatively, you could install a SIP roof.

  4. JBFlorida | | #4

    I may be able to get a 0.5 in rigid foam under the steel (on top of purlin). The design is to screw the metal panels directly to the purlins. No sheathing.
    would a small layer of rigid as described above create a sufficient thermal break?

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Would a small layer of rigid as described above create a sufficient thermal break?"

    A. No. With steel framing, the rule is simple (break it at your peril): All insulation needs to go on the exterior side of your framing. It's time to consider the installation of plywood roof sheathing.

  6. JBFlorida | | #6

    So, hat channel frame with sheathing (suggestion on material other than wood, Densglass?) then foam, then furred metal roof? A true unventilated system.

  7. GBA Editor
    Martin Holladay | | #7

    Or SIPs.

  8. JBFlorida | | #8

    I did some reading on SIPS and Metal Insulated Panels. MIPS are available 5" thick polysio with a high R.
    Question...since these are a one piece foam, would they be prone to the failures as in BSI-036?
    These do have the advantage of attaching directly to the purlin.
    Or, do we construct something similar with multiple layers of foam on top of a hat channel frame? How would one detail this?
    Thanks for your help

  9. GBA Editor
    Martin Holladay | | #9

    I don't know what you mean by "the failures as in BSI-036."

    Q. "Do we construct something similar with multiple layers of foam on top of a hat channel frame? How would one detail this?"

    A. If one doesn't know the answer to the question, one would hire an architect.

  10. JBFlorida | | #10

    thanks for your assistance.

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