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Insulating and envelope in East Ontario

DanDug | Posted in General Questions on

Hi, Thanks for all the writing and information. I’ve been digesting as much as I can. I am not a builder, so much of the technical details on implementation vs theory are likely missed by me. As in what works and is deployed in my geography.

I am planning a single family, slab on grade, 4 bedroom, 2900sq ft home near Ottawa, ON (DOE zone 6A, almost into 7 I think.) There will be a loft above the garage. South facing house for the most part. 

I have a few ideas from builders. One ICF builder with radiant floors and forced air AC, One Stick frame with exterior foam and Batts (looking into BIBs) and forced air furnace (96% efficiency) and AC.

My current thought is 3″ exterior foam (R 12), BIBs (R-23) for an R 35 wall. The builder does 2×6 16″o.c. 

As for the Slab. I want lots of insulation. We can source reclaimed EPS, 4″ I think for underslab (R-16). I looked at insulation rafts (legalett), FPSF, and frost wall. Not sure I can get these built.

Attic floor: Thinking 2″ Closed cell (R12) to air seal and blown in on top to R 70. Would foam board work instead of spray foam. Thinking 1.5″ foam board taped with drywall over it. Thoughts?

The ICF builder uses Nudura block and underslab insulation. I think I can get more bang for my buck doing Stick and forced air with the first builder.

I am trying to grasp windows right now. I think the idea is high solar gain on the South  facing and cheaper on the North. 

Any thoughts on this approach? I hope to use beOpt software to model and get a Manual J too. But this stuff is still a little beyond my knowledge still.

Also, just getting into the details about where HVAC ducts go. Like Plenum trusses or soffit. The ICF builder says he does a drop ceiling (9′ ceiling interior but a 1’cavity above that for ducts. I think the stick frame builder could do the same, but it’s more sq ft to build so more money.

Lots to go through. Struggling with many aspects and just trying to get some thoughts and ideas. Appreciate our time and insights.


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  1. this_page_left_blank | | #1

    The wall you describe will be more like R30, due to thermal bridging. Check out this calculator for determining total wall R values:

    If you're planning on using solar gain as part of your design, I'd recommend using the PHPP software package to determine the proper values. You might want to just pay a consultant to do it.

    I think the foam on the attic floor is not money well spent. You should be employing an air sealing strategy that encompasses the entire house, and if you do that you don't need to try to air seal with the attic insulation. Blown cellulose is just about the best option for the attic, both in cost effectiveness and carbon footprint.

    Dropped ceiling is a good idea for ducts.

    One other thing about windows; you may not want high solar gain windows on the north side, but that doesn't necessarily mean cheaper. You want high quality, low U-factor windows all around.

  2. Expert Member
    Dana Dorsett | | #2

    Based on the ~4500 HDD(C) climate, Ottawa is pretty much in the middle of zone 6A, not the cold edge:

    The prescriptive minimum exterior R in the IRC for 2x6 walls is R11.25, but the presumptive cavity fill is ~R20. To keep the same R-ratio (which is what determines the temperature of the sheathing midway between the studs would require R13. At temperatures that matter EPS would deliver that performance, but it it's close- not a lot of margin. If using 3" of polyisocycanurate instead of EPS it would have a bit of margin. Dow Thermax (tm) polyiso allegedly does not lose performance at lower temp the way roofing polyiso does, and would have quite a bit of margin at 3", but it's expensive.\

    The 2" of closed cell foam on the attic floor is about the most expensive way to air seal the floor. At 3" open cell foam would be about 1/5th the cost, air seals at least as well, and even that is a fairly expensive way to air seal an attic floor. If local codes require a vapor barrier at the ceiling under a vented attic, 6-mil polyethylene is dirt cheap compared to 2" of closed cell foam, so is vapor barrier latex primer paint. If building with a service plenum space for ducts, etc. air sealing the layer that supports the insulation with the appropriate tape / caulk /mastic can be as tight as 2" of closed cell foam at a tiny fraction of the cost.

    As a rule ICF is always more expensive than stick-built at the same thermal performance, but it's a lot more durable and easier to air-seal too. In hurricane/tornado zones the higher structural capacity can be deemed worth the upcharge too, but it's tough to make a financially rational case for ICF on thermal performance alone.

  3. Yupster | | #3

    I see questions like this sometimes and wonder what other house designers in Ontario (and the rest of North America) are doing if their clients have to come to a web forum to get the most basic design questions answered. It's somewhat absurd.

    I'm an hvac and architectural designer working for an engineering firm in Ontario. You should be able to source 2"+ reclaimed polyiso for your walls from Kijiji for a decent price. You will get some pushback for using thick foam but that can usually be overcome. Getting rigid foam installed is much easier in Ontario since it became pretty wells standard with the latest building code basically requiring it. ICF is a good system but if you take the extra money you spend on the icf and spend it instead on more insulation and air sealing you will get a better performing system for the same money.

    If you are aiming for a high performance house, the radiant is typically money spent that would be better spent on insulation or better windows. The costs add up fast for a secondary hydronic system! If you put lots of sub-slab foam, you won't have any problems with cold floors. R16 is pretty good but a little more is good when you are so close to grade. Not sure what your plan for your slab on grade foundation is but frost walls will definitely be the easiest to get built around here.

    For your ceiling, assuming it's a simple flat ceiling plane with a vented attic, poly is good air barrier for ceilings that local contractors have lots of experience installing and generally do a very good job with. The spots usually missed are penetrations like wiring and plumbing vents in partition walls to the attic. Cover that with all the cellulose you can afford. Make sure the heels of your trusses are at least 12" high and you installed nicely sealed vent baffles. If you want to use something more robust than poly, osb with the seams sealed with 3m 3015 works well.

    Northstar makes a good quality & relatively cheap triple paned window. Go for the low solar heat gain coating, somewhere around SHGC 0.2. Without Passive focused construction, the solar gain in the winter doesn't really offset the cooling gains in our climate and the comfort issues caused by big temperature swings when the sun is blasting in the summer are a problem. Find a window guy who likes triple paned windows, many of the window installers don't like the extra weight and will overprice the triple paned and then try to stick you with a double paned window and tell you it's basically the same performance.

    If you are building a high performance house, consider using ducted minisplits for your house. Cheaper than oil and propane. Natural gas is definitely the cheapest option by far if you can get it but not the greenest. Ontario has a reasonably green electricity infrastructure, always good to take advantage of that. Depending on the layout of your house, one or two ducted minisplits might work well for you and can often allow you to keep the ducts to a minimum. dropping the whole ceiling is probably un-necessary unless you have a very complicated footprint or hate the look of bulkheads.

    That's my perspective anyway. Hope that helps!

  4. DanDug | | #4

    @Trevor Thanks very much. I appreciate the info and thoughts on windows. As you suggest, low U-value on the north windows, not cheaper. I was being imprecise in my language in the OP. I also now, between you and Dana, understand the air sealing of the attic floor.

    @Dana Thanks for clarifying my zone. I also appreciate the clarity on the attic floor. I can skip the spray foam and seal with primer or 6mil poly as suggested, then blow in Cellulose.

    As these things add up (insulation upgrades, soffits and chases) the cost difference and technical work between the stick bulid and ICF build are becoming closer and closer. I feel like I'll be asking for a very custom job from the stick builder vs a common job from the ICF builder. As in, I worry the technical detail of sealing and properly building the well built stick frame may be beyond the local contractors comfort zone, but the ICF builder knows his product well. It's a frustrating problem.

    Can I get away with standard ICF R-23 wall in this climate, or should I ask to add R value inserts or equivalent methods to increase R-value.

  5. DanDug | | #5

    @Yupster I responded prior to seeing your post, and THANKS! Your local advice is appreciated and accurate in relation to the foundation. A frost wall is almost a certain outcome at this point. Also, the SHGC recommendation and window manufacturer name is really appreciated. I'll take your advise about dealing with window contractors, since I am sure I will hear the exact argument you presented about double being as good as triple.

    We have a simple layout I think. I'll add a picture. I considered bulkhead, my wife hates the idea. We have a well built house with basement 8' ceiling and bulkhead now so we have some experience with them. Although, the main floor slab on grade hose would have 9' ceilings so a bit of a different effect.

  6. amach | | #6

    Are you working with an Architect or Architectural Designer? Seems you are approaching this with builders. If you need a designer, I can help you out.

  7. DanDug | | #7

    @Anthony I have talked with a few builders. I did intend on engaging an architect for more design input. I am over my depth for sure. Also talked with a few guys who built a great deal of their own home. Both used ICF and swear it is the best. Thanks for your post and I think you are wise to suggest a design expert. I have an architect in mind who is local and worked with the builders many times.

    @Trevor, I tried the calculator in the link you provided. That's pretty neat. Thanks. My wall came out to R value 33.8 @ $6.90 /sqft. The builder will likely suggest 1.5" EPS exterior foam (Isoclad if anyone is interested) bringing the wall to R value 26.8 @ 5.60/sqft. Thoughts?

  8. Expert Member
    Dana Dorsett | | #8

    "Can I get away with standard ICF R-23 wall in this climate, or should I ask to add R value inserts or equivalent methods to increase R-value."

    Even in zone 7 an R23 ICF would meet the IRC 2018 code minimum (=R21 in zone 7 for mass walls. R20 min for zone 6), as long as at least half the R value is on the exterior side.

    A code-min stick built in zone 6 a 2x6/R20 + R5 continuous insulation meets the thermal performance, but doesn't have sufficient dew point control at the sheathing to get away without interior side vapor retarders. The 2x6/R13 + R12 c.i. approach would measurably beat IRC code minimum and would not need interior vapor retarders, but code officials still seem to trust 4-6 mil polyethylene better than the dew point control math- you might have to sell them on it, or use a "smart" variable permeance vapor retarder to preserve the drying capacity.

    At 1.5" EPS doesn't have sufficient R value for dew point control on R23 fiber in the cavities, and an interior vapor retarder would be needed.

    1. DanDug | | #10

      Would you mind if i could continue this line of questions to you? We are nearing final decisions.

      I would like your thoughts on exterior polyiso in Ontario winters vs EPS vs XPS. I am sure I can get reclaimed for the Slab, Foundation, and walls. So, at 3" or 4" thick (overlapping layers) would you choose any one of them over another.

      Second, would you put the below grade foam on the interior or exterior of the wall. I like the idea of putting the continuous layer of foam all the way from the footing up, but that would mean the interior below grade studs are up against the concrete (likely 10" block) foundation wall. But more interior sq. ft available then putting foam interior.

      Third, would you modify the idea of BIBs in 2x6 cavity (R 23) and 4" foam (EPS,XPS, PIC) for above and below grade, and spray foam header area to R 16 approx (open of closed cell?). So the header has Rigid foam exterior and spray foam interior (R 36ish total foam dependent) and the above and below grade are R 35 to 40 depending on foam choice and de-rating...i don't even know what to rate the foam as anymore! Aged, wet, temp dependent, thick layers...I am unsure. All i know is in Ontario we have warm days and cold days. And I would like the foam to work well in both and last a long time.

      Thanks so so so much to everyone contributing to GBA and other similar forums! I have learned just enough to make myself confused but also so glad!

  9. DanDug | | #9

    Thanks for the Clarity to all the respondents. I appreciate your time. Really helped.

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