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Community and Q&A

Retrofit exterior insulation over fiberglass and polyethylene interior – high performance

Foxwedge_DB | Posted in Green Building Techniques on

Hi! New member here.

I’m looking for suggestions to retrofit insulation from the outside to a 30-40 year-old home in what I think is climate zone 5b, north of Kingston, Ontario, Canada, County of South Frontenac.

The home is built with 2×6 walls and (TBC) 2×10 rafters providing a cathedral ceiling, seemingly all insulated with fiberglass and polyethylene “vapor barrier” to the interior.

We would like to re-do the exterior siding, and would like to take the opportunity to add further insulation before replacing cladding with Hardie shingle. I would like to have a long-term plan for the roof, but will be a later event.

I would prefer to go foam free, if possible, but cost is also a major consideration.
What would be an appropriate assembly for walls only? Walls and Roof? How would I tie in a later roof insulation?
Would the recommended assembly allow drying to the exterior?
It seems that slapping on polyiso could trap the moisture? Or is it permeable enough?
My first inclincation is rockwool rigid panels for the walls, for general ease, cost, availability, and vapor permeability.
I’ve never seen a rigid rockwool panel used on the topside of pitched roof sheathing, locally… Final roof is and always will be steel.
Shooting for R-40 walls, R60 roof. Seem reasonable?
But maybe that will require TJI outriggers or some such…

Any thoughts appreciated!!!

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  1. Expert Member
    Akos | | #1

    Since you have a class I interior vapor barrier, you can go with any amount of exterior rigid insulation, there is really no minimum.

    Going for a vapor permeable product is always better, there is unfaced EPS and GPS or fiber faced polyiso. Mineral wool is the most permeable but tends to be expensive and fussy to install (mineral wool is still a bit squishy so you have to adjust all your strapping screws to have the siding flat).

    With any painted siding, rain screen over the foam helps with drying and durability, generally a good detail. If you want to skip it, if you limit the exterior rigid to 1.5", most siding can still be nailed up direct through the foam.

    Important detail is to tape the seams on your sheathing when you strip the existing siding. You want to air seal from the outside as much as possible. This will make a bigger difference in energy use than the extra insulation, so definitely worth the effort. Make sure to also seal the sheathing to your foundation, this tends to be a large source of air leaks in most houses.

    There are lots details here on handling your WRB and flashing details. Check out the detail library on the site.

  2. Foxwedge_DB | | #2

    Thank you,
    But to clarify, it is ok to disregard the clear advice to avoid plastic on the interior of walls sheathed in foam?:

    Let's go ahead and assume the plan is to add 4" eps or polyiso on the walls. Same for roof at later date?

  3. Expert Member
    Akos | | #3

    With enough exterior rigid insulation you can skip interior poly, but if you already have it, it won't cause issues. My own home is built this way, I opened up a wall not too long ago and the interior was as pristine as the day built. Walls with double vapour barriers are not some evil assembly that should be avoided, you can read more about here:

    With interior poly, you do want some way for the wall to dry to the exterior, thus use permeable rigid (mineral wool, unfaced EPS/GPS/XPS or craft faced polyiso).

    Roofs are very different assemblies from walls.

    How best to tackle it depends on how it is vented right now, what the roof shape is and weather you are working from the inside or the outside.

    If you have a vented roof, usually the best is to insulate from the inside. You can install rigid against the underside of the rafters and drywall over that. You can use any amount of rigid on the interior, with R24 batts in the roof, you'll get most of your energy savings with about 2" of rigid.

    If you don't want to insulate from the inside, you need a way to plug the vent channels (usually best by dense packing for the existing FG batts with cellulose) then installing enough rigid above the roof deck for condensation control.

    In zone 5, this works out to 40% of your overall R value. Assuming 2x10 dense packed over FG, so around R30, you want at least R20 above the roof deck. 4" polyiso or 5" EPS would work.

    The R40/R60 targets are for new build. There is no ROI in going there in case of a retrofit. Keep it simple and the cost reasonable with a focus on the air sealing.

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