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Community and Q&A

Upgrading with interior rigid foam in zone 6

Sofiane | Posted in Energy Efficiency and Durability on


I have the option to upgrade insulation from inside in a part of the house where we’ll be doing soundproofing.

House is currently brick veneer, air gap, asphalt felt, plywood, 2×6 with r19 fiberglass, polyethylene, drywall built in late 80’s

I was thinking going to :

R 22 rockwool I already own for non insulation benefits plus

Add one 1nch of polyiso on the south facing wall – no windows and

3/4 inch on the east and west facing wall with big windows that make up about 1/2 of the wall (my wife was against the idea of adding any insulation to those walls initially, we compromised).

Omit the polyethylene and pay really close attention to air sealing.

The 2 brands I can find in my area claim aged R6 for 1 inch and are foilfaced or R 5.5 unfaced.  

Does it make sense to add the 3/4 inch of polyiso? do the downsides of reduced drying to the outside outweigh the small bump in R value? Will the new assembly allow for slightly higher indoor moisture in the wintertime? I get pretty annoying symptoms whenever RH drops below 30%.

On another note, I’m currently upgrading my rim joist insulation from fiberglass batts to 6 inches of eps (mostly done) + rockwool for fire protection. I thought I could add a few inches of rockwool to go beyond the rim joist and increase R value, like I was using 2×8 to frame a wall rather than 2×6. Does that work? Or is it better to just add a small amount the fire protection?



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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Sofiane.

    I'll let someone with energy-modeling experience answer the question of whether this is worth doing from a thermal performance perspective. But it is certainly okay to do because the exterior side of your wall assembly allows for outward drying. You may find this article helpful: Walls with Interior Rigid Foam

    Make sure to air seal the foam well, which will prevent conditioned air from getting into the wall assembly where condensation can occur, though sub 30% is pretty low indoor relative humidity. Even in cold climates it is generally safe to keep it a bit higher than that. For this question, you might find this helpful: What’s the Ideal Relative Humidity in Winter?

  2. Sofiane | | #2

    Thanks Brian. Your advice is useful. I did read the article on walls with interior rigid foam. There was also a newer case study published recently. I am hesitant to pursue without further input as the issue of the colder sheathing was brought up. How much of an actual worry should this be? My understanding is it is much more significant in a double wall construction than it would be with my project that only adds a maximum of R6, but I would like to avoid creating a problem as I am trying to solve one.

    As for the RH, it has improved significantly since I recently sealed the leakiest part of our rim joist and it should improve more as I continue sealing the house although I am also being helped with milder weather. My understanding, is the condensation risk would be greatly reduced if I both air seal and replace my polyethylene vapor retardant with polyiso.

    I'm trying to see how high can I safely go without having to do a WUFI analysis? I would be satisfied with 35% RH although I completely get rid of my irritation from dry air at closer to 40%.

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