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Hawaii metal stud home — attic vent humidity concern

user-7027641 | Posted in Energy Efficiency and Durability on

I’m having a steel stud home built on Oahu, Hawaii climate zone 1. The house is ¼ mile from the ocean on the windward side of the island. Lots of wind will hit the house. I’m going with steel for termite, mold and hurricane related reasons. I’m thinking about using 4 inches of sealed polyiso rigid insulation on top of OSB with a ribbed metal roof.

My question relates to the attic. I’ve scoured this site and BSC and looked at lots of studies based out of Florida. I was originally thinking sealed attic to keep the marine air out of the roof assembly to decrease chances of rust, dust and especially and mold from getting into the attic. Then I read this article which suggests that unvented attics have higher humidity levels. I don’t plan on air conditioning my house all the time as most of the time I can leave my windows open and be very comfortable. (It is getting much warmer though…times are changing)

Most of the research that I’ve read is 10-15 years old in regards to vented and unvented attics. What do you think I should do? I’m confused as hell.

Also, with steel studs, I’m planning on staying away from insulation between the rafters. For hurricane reasons I don’t want to go too thick with the polyiso. Is there any way I can get an increased roof R value without going thicker on my polyiso and spraying foam between the studs?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name?

    I think that an unvented conditioned attic makes sense for your location. If you want to avoid the attic humidity problems mentioned in my article about open-cell spray foam, then don't install open-cell spray foam.

    If you need your house to meet wind-resistance targets, you'll need to have your details developed by an engineer. Among the options are exterior rigid foam, structural insulated panels (SIPs), or nailbase (panels like SIPs but with OSB on only one side). Talk to an engineer about developing a fastening schedule for one of these approaches to get you the wind resistance you need.

  2. user-7027641 | | #2

    @Martin -

    I knew you were going ask me about my name. For some reason I can't get my user name Ryan Price to show in the question. Spent 30 min trying to figure it out.

    Yes, in going back and looking at the article, you do talk about open cell foam. Thanks setting me straight and for confirming my initial hunch about going with an unvented conditioned attic. I'll continue with my original plan to use 3 or 4 inches of polyiso rigid insulation.

    Your daily contributions to this building community is very appreciated.

  3. Expert Member
    Dana Dorsett | | #3

    In US climate zone 1, the code-max U-factor for roofs is R28.6 "whole assembly", that includes the thermal performance of the roof decking, roofing, air-films, etc. At 4" of polyiso you're at R23-ish if using fiber faced 2lb goods, but R26 using some of the R6.5/inch foil faced material, eg. Dow Tuff-R:

    A 3/4" OSB structural roof deck adds about R0.9 to the stackup, so you're up to R26.9...

    ...the exterior air film is good for R0.17 pushing it up to R27.07...

    ...the interior air film adds R0.67, so now you're at R27.74...

    ...a 3/4" nailer deck over the foam onto which the roofing gets mounted adds another R0.9, bringing it to R28.64.

    That's a hair under U0.035, and that's even without counting the thermal value of any roofing underlayments or ceiling gypsum, etc.

    Putting a gypsum layer on the underside of the steel rafters would add another R0.45, and would give you couple of air films (albeit thermally bridged by the steel, but it looks like your totally up to code performance even without it.

    In this climate using foil-faced roof decking with the foil facing the attic or rafter-bays would also be worthwhile, and adding another bit of "effective R". Using a perforated aluminized fabric radiant barrier on the under side of the rafters facing the foil faced roof sheathing would be an improvement as well.

    Bottom line, you don't need more than 4" of polyiso to meet CODE, if it's a sufficiently high R/inch.

    Were you looking to beat code performance by some margin? If yes, by how much?

  4. user-7027641 | | #4

    @ Dana -

    Thank you for your thorough response. Sincerely appreciated.

    I was shooting to get to 30, so based on your calculations, I'm right there.

    Kind Regards - Ryan

  5. Expert Member
    Dana Dorsett | | #5

    R30 (insulation-only) is the usual path to code compliance if/when the insulation is thermally bridged by joists or rafters, which makes calculating the U-factors more complicated. When it's continuous insulation, no bridging, it's easier to just run the U-factor numbers to see where it lives. (In your case it's already pretty much there.)

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