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Community and Q&A

Insulation Plan (Exterior/Interior)

bst_2024 | Posted in Energy Efficiency and Durability on

I am doing planning for insulation and air sealing details. I’ve been reviewing IRC requirements and looking over permeance information trying to understand what my best course of action would be to make sure moisture isn’t an issue (my brother-in-law works in water/fire restoration and tells me horror stories about when they open walls and the issues they find). I found this calculator that I have been using as a reference ( to see how my planned assembly would be. Here are my plans/thoughts so far:


– Cladding will be vinyl siding

– I want to use either zip system or forcefield as my sheathing / wrb layer

– I would like to do exterior insulation and believe doing an 1.5” layer would allow for easy installation (easy to do window bucks and vinyl manufacturer confirmed that as long as fasteners penetrate the recommended amount then fastening through foam vs. adding strapping or another sheathing layer is fine). At that thickness it would meet IRC requirements for Zone 5.

– I would like to do batt insulation as I do not want spray foam and loose fill in walls is not common in my area as I have learned from asking around. 


Curious on anyones feedback on those items so far. Below are some additional items/questions that I have.


– I was thinking of using R-19 batts (effectively r-18 once compressed in 2×6 wall) for interior and 1.5” ofexterior insulation which would be R7.5. This doesn’t meet the component r-values but appears to meet the assembly u-factor for 2021 IRC. Thoughts?

– Thinking that since wrb will be at the sheathing layer and will also be the air control layer I wouldn’t need to tape seams of exterior insulation. Am I thinking correctly?

– Using the calculator linked above it seems like using the Kraft facing of the batt insulation would be fine for the interior vapor retarder as class 2 would be fine. Understanding permeance and making sure my assembly is vapor safe is an item I’ve been struggling with. Is my understanding on this correct?


Am I overlooking anything else I should be thinking about?

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  1. Expert Member


    Everything you have laid out makes sense to me and sounds like good practice.

    The only thing I'm a bit cautious about is that the WRB behind the foam has no way of draining any water that gets though the cladding and foam. Maybe it would be safer with either a dimpled WRB over the Zip, or having the foam taped to exclude water. I'd be interested in what others say.

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