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Rigid Foam –> 1 – 2″-layer or 2 – 1″-layers

idahobuild | Posted in General Questions on

House build in zone 5 (south Idaho). Is it better to use 1, 2″ layer of rigid foam or 2, 1″ layers?

Wall layers:
5.5″ studs (R23 batts)
7/16″ OSB
Mento 3000 Connect/Delta Vent SA
Rigid Foam (R10)
1×3 Battens

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  1. Expert Member
    ARMANDO COBO | | #1

    If you tape and stagger the seams, you can get greater envelope tightness. If you use an approved tape by the foam manufacturer, you cam make the outsulation a moisture barrier.
    You can research lots of free information about wall assemblies, rigid foam and WRBs in this website.

  2. idahobuild | | #2

    Thanks Armando,
    My original plan was to use 1 layer or 2", until a watch a Joe Lstiburek talk about place that he had built and 'kicked himself' (my words, not Joe's) for not thinking that he'd have been better off with the staggered seams on the exterior insulation from 2 layers. If I use 2 layers, stagger seams and seal with approved tape; would that make the Delta Vent layer obsolete/redundant?

  3. Expert Member
    ARMANDO COBO | | #3

    Joe's barn its a thing of beauty, remodeled into their office at first, and then it became a party hut nowadays and the epicenter of Summer Camp, and yes, by experience, he has taught many of us in the BS world that even he is mortal, and he's used his own lesson to teach us that its better to install two layers than one (or four or five as his barn got in the fix). Make sure you install bug screens too.
    You could delete the Delta WRB, but if you can afford it, leave it there as "belt and suspenders", I usually spec a less expensive WRB (taped).

  4. Expert Member
    BILL WICHERS | | #4

    Two layers is better for the reasons already mentioned, but note that they don't have to be the same thickness. Sometimes it is advantageous to use a sheet of 1/2" and another, thicker sheet to make up the rest. Use the thinner sheet on the inside side, so that the thicker, stiffer sheet will help to keep the finished surface flat. This type of stackup still blocks airflow through the insulating layer, but is sometimes easier or cheaper to build.


  5. Expert Member


    It's a trade off. There is no doubt two layers are better, but whether that advantage outweighs that they represent twice the labour to install, is probably very dependant on how much that labour costs.

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