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Walls, zone 6, Montana

Lesley_Farm_Girl | Posted in General Questions on

My husband recently said that he would be okay with putting 2 inches of exterior insulation. We are putting up our ICF basement. I can’t get him to put down insulation in the slab. (I’ve given up on that argument.) 

Anyway, we are going to have 4×6 was prebuilt by the Amish. I believe they use plywood on the outside. So, what would be easiest most cost effective solution for the exterior walls? I would like to use Rockwool, but I think I might be able to convince him more on using Polyiso boards (2inches).

I’m feeling conflicted over costs and easy of insulation, and effectiveness.

We plan on using corregated metal siding. Would we have to use furring (sp?) Strips? 

We need to know these details to order windows and get thermal bucks. 

Also, would you use blown in cellulose rather than spray foam? I want good R value but to have safe air as well. 

I also get confused on air/water barriers. I sort of need an exact sandwich of insulation suggestion from exterior to interior.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Lesley,

    These articles will give you the details you need to design a wall with continuous exterior insulation and will help you sort through the water-barrier/air barrier details:
    Walls that Work
    The Four Control Layers of a Wall

    Some metal siding profiles offer enough drainage and airflow that you don't need furring strips, others don't. So, it depends. But if you use a significant amount of exterior insulation, you'll likely need them because the insulation will be too thick to fasten the siding through and the furring strips do the best job of holding the thick insulation to the wall.

    My personal recommendation is to use cellulose. Not only are their less risks when it comes to odors and off-gassing, it is more cost-and climate friendly for cavity insulation.

  2. user-2310254 | | #2


    Do you mean 2x6 for the walls? Two inches of foam might not be enough for Zone 6, but it would be fine with a 2x4 wall. Perhaps Dana will give your assembly a real-world assessment. (In the meantime, see and for more on this topic.)

  3. Lesley_Farm_Girl | | #3

    Yes, I meant 2x6's.

  4. Jon_R | | #4

    With 2x6s and 2" of foam (preferably unfaced EPS), use a Class II vapor retarder (preferably variable). Or any other wall that meets these guidelines:

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