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Heating and cooling load calculations

canadianexpy | Posted in General Questions on


So the plans for my house are complete and now I need to get load calculations done, anyone know of any good company’s in Southern Ontario( Toronto area).

Would like to use Mini splits, the house is going to be tight and well insulated. So hopefully I can find someone that can understand my needs and not just try to sell over-sized unit.

With determining R-value of a wall that will have 6″ of Polyiso on the outside of a 2×4 stud wall filled with R14 Roxul, what rating would I use on the Polyiso knowing the outer will de-rate some.?
And will it matter much in the load calculations?

As the Polyiso seems to given a rating between R5-R6 per inch/ depending on temp and age.

Thanks Again…. In helping me get my dream green home built.

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  1. Expert Member
    Dana Dorsett | | #1

    When most of the R-value is polyiso, assume R5.5/inch at your 99% outside design condition, higher than that on average. The outer inch will perform less than R5.5, but the middle inches will outperform higher.

    A 2x4/R14 wall with gyprock interior and typical siding options will run about R10, so with 6" of polyiso (R33-ish) you're looking at about R43 or U0.023, give or take. If you worst-cased it and assumed R5/inch it would be about U0.020, but that's unrealistically conservative.

    Your window losses are likely to dominate the heat load numbers, unless you assume a lot of air leakage or high ventilation rates.

  2. canadianexpy | | #2

    Thanks Dana I was hoping you would chime in, as I feel you are one of the experts when it comes to this!
    Of course you could be a crazy guy in the basement making S*** up... but I'll take it either way.
    Now to try and find the expert up here, to do the calculations.
    I like math and problems but I assume these calculations. are over the average guys head.?

  3. GBA Editor
    Martin Holladay | | #3

    The person who performs your load calculations doesn't necessarily need to live close to your home. For more information on this topic, see Who Can Perform My Load Calculations?

  4. canadianexpy | | #4

    I guess that is true, they just need design/drawings and details. Thanks Martin.

    Alright well now I'm just looking for anyone that can do a good calculation.

    Any idea on what the going rates are? I assume the US is always cheaper than up here.


  5. GBA Editor
    Martin Holladay | | #5

    Once again, I urge you to read the article that I linked to. The article discusses rates.

  6. canadianexpy | | #6

    Will do, Thanks

  7. Expert Member
    Dana Dorsett | | #7

    "Of course you could be a crazy guy in the basement making S*** up... but I'll take it either way."

    I usually just make it up as I go along. So? :-)

    At +20C indoors, -17C outdoors (the 99% outside design temp for Toronto ), with R14 in the cavities and R30+ on the exterior, the warm side of the polyiso will be at about +5C, and the mean temp through the foam no colder than -6C, which is a tiny bit below the cold end of the curves in this graph:

    So perhaps R5/inch would be a more realistic estimate to be using for a heat load calculation (not that it'll move the whole-house loss numbers much), even though at the average winter outdoor temperatures in Toronto performance would be more like R5.5/inch.

    Even in January the mean outdoor temperature in Toronto is about -5C, which would move the temp at the warm side of the foam to about +10C, and the mean temp through the foam would be about +3C / +37F.

    So most of the polyiso samples tested would average fairly close to R5.5/inch in your stackup and location over the winter months, higher during the shoulder seasons. But only R5-ish at the 99% outside design temp.

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