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3/4" exterior rigid foam retrofit over insulated 2x4 wall?

timstokes81's picture

Hi there, I have a circa 1905 house in Toronto (Zone 6 I believe). We are hoping to get 2 walls of aluminum siding replaced due to a wonky installation and years of dents and holes for cable boxes etc... The 2 contractors that have made it to have a look so far have recommended the 3/4" rigid foam insulation over the sheathing. This seems like a bad idea to me, given Martin Holladay's most recent article about exterior foam, as well as the following factors:

- The house has had a major renovation in 2014 (not by me unfortunately), so the exterior walls have fiberglass batts and interior polyethelene. The attention to detail behind the renovation seems highly suspect to me, so I'm sure the poly has lots of air leaks, and the insulating job isn't what you would call top notch. 

- Though the rigid foam is supposedly vapour permeable, it seems like it would drastically reduce the ability of the wall to dry to the exterior, and has essentially zero ability to dry to the interior (due to the polyethelene).


Since 2x4 walls with fiberglass insulation is supposed to be adequate in this climate, adding some R-value with 3/4" foam hardly seems worth the tradeoff of reduced drying ability. Am I missing anything?



R14 in a 2x4 wall do not meet (post #215461, reply #1 of 6)

R14 in a 2x4 wall do not meet current building code requirements, not did it meet the requirements in 2014.

Ontario  Supplementary Standard SB-12 defines those requirements.  For Toronto, above grade walls should be R24 minimum if you have a high efficiendy heating system  AFUE 90% or greater.  For lower efficiency heating systems, with wall should be R29.


Did you buy this house knowing that the renovation was not to Code? 

You can use a mineral wool board (Roxul ComfortBoard) as exterior insulation, which will dry to the outside.  You will need 2.5" thick to get to R24,  4" to R29

Exterior insulation should be installed as a Rain Screen, to provide the ventilation for drying.

Don't use foam with poly on the inside.

Thanks for the post! I (post #215461, reply #2 of 6)

Thanks for the post! I actually did not know that R24 is code - I'm surprised. So I guess the answer would be no, we did not know the myriad of things that were not done to code during the 2014 renovation (or the 112 years of renovations before that). 

I really like the idea of mineral wool on the outside, but I'm fearful of the 2.5 - 3" addition to the wall thickness, and how that will affect the roofline (which is a low-slope roof with no overhang on the side in question, and thus would have to be extended).

I think in the end I will reluctantly just end up replacing any bad sheathing, fresh tyvek, new siding, and calling it a day. It seems to me that (while certainly not the most efficient) this route will keep the sheathing warmer (hopefully about the dew point), and allow it to dry.

It should be noted that, for (post #215461, reply #3 of 6)

It should be noted that, for a wall that was originally built before about 1980, installing Tyvek (and being obsessive about taping and sealing it) will do much more good than another 2" of insulation.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

This makes a lot of sense to (post #215461, reply #4 of 6)

This makes a lot of sense to me.

OTOH, (post #215461, reply #5 of 6)

FWIW: If you are already going to pull all the siding, you do have an opportunity to pull the sheathing and the FG and replace with spray cc foam.

It won't be quick, and it won't be cheap; but it will be very good. I had several knee wall attics in Chicago foamed two years ago, the results are very impressive. 

Good luck. 

Exterior foam. (post #215461, reply #6 of 6)

 Just read Martin Holladay's article and it made me do some thinking. Come next spring I'm gonna change my vinyl siding and was gonna install 1" polystyrene but now I'm wondering if it's not a good idea for the 2 x 4 walls which do have polyethelene vapour barriers. Here in my area of eastern Canada a lot of homes are using the 1" polystyrene nowadays more than the Tyvek or the Greenguard, mostly because of our cold winters and the rising energy rates. People are simply installing it to make their homes warmer. I wonder what these homes will be like over time after reading Martin's article. Maybe nothing will happen at all.