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Resolving insulation issues for an illegal addition in Toronto

lowelllodesign | Posted in General Questions on

Hi Mr. Martin Holladay,

This is Lowell Lo sole proprietor Architect in Toronto. I am dealing with an illegal built 2nd storey addition. According to the contractor he used 2×6 studs and filled with fibreglass batts for the exterior walls. One of the walls is built on the property line. The current code in Toronto requires R-5 continuous outboard insulation. We cannot encroach onto the neighbour’s property. Can I proposed to add EPS insulation to the interior of the space with drywall already in place. If we tape and caulk the joints of the EPS would that be ok or would we trap moisture in the wall assembly? If so can would we need to make the exterior sheathing more vapour open? Another problem is that wall assembly will need to be non-combustible construction.

We also have the same issue for the flat roof which maybe 2×10 rafters filled with batts insulation I am assuming for now.

Thanks much again.

Lowell Lo
BArch, LEED AP, OAA  Architect


  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to a relevant article: "Walls With Interior Rigid Foam."

    Of course, just because interior rigid foam is fine from a building science perspective, doesn't mean it meets code. If you have code compliance concerns, talk to your local code official.

    If your exterior sheathing consists of board sheathing, OSB, or plywood covered by a vapor-permeable housewrap, your sheathing should be permeable enough to avoid problems.

    The roof situation is entirely different. Are you saying that you cannot add any exterior rigid foam to the roof?

  2. lowelllodesign | | #2

    Thanks for your reply again Martin!

    Thanks for the info for the walls. I was worried that I have to use cement board sheathing because of the non-combustibiltiy issue but I just found out from another Architect I can use regular plywood with non-comb. cladding and interior type x 5/8" drywall!

    The roof is already done and ceiling drywall is up too. So you don't recommend adding EPS rigid insul under the drywall ceiling? Would adding purlins to deepen the rafters and fill more with batts be an option? I am not sure yet if there is any vent space above either which be still be required in Toronto. If that doesnt work may be they have to resort to opening up the ceiling and use closed cell spray foam to get around the required venting issue in Toronto I am thinking now.


    1. Expert Member
      Dana Dorsett | | #4

      Is the ceiling already insulated with batts or something? (BAD IDEA, especially for a flat roof in snow country, with or without poly sheeting or venting.)

      The ceiling has to come down. See my response in #3 for the recommended stackups, none of which require interior side polyethylene, but if the inspector needs to see something, install 2-mil nylon (Certaineed MemBrain), which behaves as a vapor barrier when it needs to, but becomes vapor open if the assembly needs to dry.

    2. Expert Member
      MALCOLM TAYLOR | | #9


      Are you sure about the sheathing? It's been a couple of decades since I did any zero-clearance walls in Ontario, but I remember having to use exterior gypsum as sheathing. It might be worth confirming what's required.

  3. Expert Member
    Dana Dorsett | | #3

    It will be a lot easier to reliably air seal 1" foil faced polyiso (R6-ish) than EPS. The foil facers are a true vapor barrier, which means you could skip the interior polyethylene.

    In Toronto (the cool edge of zone 5A) with 9.25" of cavity space you could safely install 3" of HFO blown closed foam (R20-R21) on the underside of the roof deck and compress rock wool batts designed for 2x8 framing into the remaining 6.25" where it would perform at about R26- R27, for something pretty close to the IRC's prescriptive R49.

    If the inspectors won't buy it, install 4" of HFO blown foam (R27-R28) and compress R23 rock wool designed for 2x6 framing into the remaining 5.25", where it will perform at about R22. Any way you cut it that would be over R49, and with the majority of the R as low permeance exterior side closed cell foam, no condensation risk, even in climate zone 6.

    1. lowelllodesign | | #13

      Thank Dana,

      Sorry, jst gotten around to reading your advice. Good advice, I am guessing the flat roof rafters are 2x10" I hope.


  4. Expert Member
    Akos | | #5


    I think there are some confusions with the insulation requirements. OBC requires external insulation if using studs less than R5 value, 2x6 wood studs meet this so you don't need exterior insulation. All you need is R24 batt insulation in the walls.

    Cathedral ceilings, your 2x10 with R30 batt plus 0.5" regid insulation vent baffles will work. You will need venting for this to pass code, if you can't vent it then either sprayfoam underneath the roof deck or rigid above the roof.

    1. lowelllodesign | | #6


      No confusion here Akos. Now latest OBC in Toronto R5 continuous insulation is required outboard of sheathing for Res. Additions.

      Cheers for now.


      1. Expert Member
        Akos | | #12


        I haven't had to apply for a permit with the new SB-12 changes, so I could be missing something, but from my read package A1 from table is fine without exterior insulation.

        If you need a non combustible assembly (which would be the case if there are more than one residential units above each other), then wood studs won't work. With metal studs you would need a different compliance package which would definitely need exterior insulation to pass code.

        If you are required a fire rated assembly(mentioned 5/8 drywall) then you need type X sheating on the outside as well, plywood won't be enough.

        1. lowelllodesign | | #14


          I got your message, metal is better even with its high embodied energy is your advice sounds like.


    2. Expert Member
      Dana Dorsett | | #7

      R30 batts+ half-inch rigid baffles may meet the letter of code, but even when vented flat roofs are risky in that climate, since there isn't sufficient convective drive in a flat roof to effectively purge the vent channel air.

      1. lowelllodesign | | #15

        Hi Dana,

        Yes. I remember this point about venting from one of Martin's older articles I think. I also read an article from the FHB magazine about how they were saying the the next generation of spray foam with have a more ecofriendly binder instead of HGH? Has that new magic binder arrive yet may I ask?


        1. lowelllodesign | | #18

          it is the HFC blowing agent that gives off greenhouse gases Dana!!!

  5. GBA Editor
    Martin Holladay | | #8

    "Flat" (low-slope) roofs are difficult to vent. Here's a link to an article describing how to do it right: "Insulating Low-Slope Residential Roofs."

    I doubt if you have enough of an attic (that is, enough vertical height) to do it the right way -- so I'm guessing that you will end up with an unvented roof assembly insulated with closed-cell spray foam.

  6. jberks | | #10

    Hi Lowell,

    I am also in Toronto. Just for my own curiosity, what exactly do you mean by latest OBC in Toronto? The city recognizes the same OBC as the rest of the province.

    I also agree that you should double check the requirement for continuous outboard insulation on an R24 2x6 wall. I've done exterior insulation on my current build for solely for the purpose to use 2x4 construction and still have an R24 wall. If you're going to add extra insulation on the inside of this fire rated wall, I'd suggest rockwool sheets like comfortboard, as it's non combustible and still air permeable.

    In terms of the roof assembly I'm no architect so tell me of I'm wrong, but I don't believe there is a vented roof requirement in Toronto. That might be a plans examiner/inspector thing regarding code for vapour barrier. But spray foam or exterior rigid foam is quite common here. At least I can say I've designed and built an unvented roof without the mayor knocking on my door.

    My experience for what it's worth.


    1. lowelllodesign | | #17

      Thanks for your info Jamie.

      There is a table in SB-12 for addition to existing buildings requiring continuous insulation of R-5. I think it came into effect in Jan 2017.


  7. Jon_R | | #11

    > The current code in Toronto requires R-5 continuous outboard insulation.

    I see:

    " Continuous insulation (ci) is intended to minimize the thermal bridges in an assembly. It is generally uninterrupted across all structural members. ... Insulation may generally be installed on the interior or the exterior, ..."

    1. lowelllodesign | | #16

      Thanks Jon R. I will check on the definition myself now.


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