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Community and Q&A

Comfort and cost concerns – 1 1/2 inch vs 3 inch exterior Comfortboard

Eric M | Posted in Energy Efficiency and Durability on

Hello,

I am planning the wall system for a future home build in Calgary (climate zone 7A). My planned wall system is as follows (inside to out):
5/8″ gypsum drywall
Intello plus smart vapour barrier
2×6 studs @ 16″o/c (note: cant go to 24″ due to tall wall height)
Either R-22 or R-14 comfortbatt mineral wool between studs
5/8″ plywood
Solitex Mento 1000 WRB
Either 1.5″ orĀ  3″ comfortboard mineral wool panels
2×4 battens
Either hardiplank siding, or cement board with thin brick veneer.

I would like to use the thicker exterior 3″ mineral wool comfortboard, but that would add a fair bit of cost (extra $1.35/sq. ft for comfortboard and extra ~$1.00/sq. ft for longer fasteners).

My questions are:
1. Would I notice any big difference in comfort worth the price of going from 1.5″ to 3″ exterior wool? (I would be going from an effective total wall R value of about 26 to 32)
2. Do I run the risk of condensation on inside face of ply with only 1.5″ of exterior insulation? My thought is that with the wool being permeable, its not the same risk as with using thin exterior rigid foam. Is this correct, or no? I also plan to use the smart vapour barrier to help let out any tapped moisture, but is this enough to keep the wall dry?
3. If I did upgrade to the 3″ exterior wool, would it be advisable to downsize to an R-14 batt between studs to warm up the inside face of the wall sheathing to avoid condensation issues (as asked about in Q2)?

The home will use an HRV, and will have AC for the short but sometimes hot summers we get. I want to get the best bang for my buck for a comfortable home, and would like to avoid foam products where possible. I am also working with limited space for wall thickness, as the house will be on a skinny lot where every inch of wall thickness counts.

Thanks,

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Replies

  1. Jon R | | #1

    Using Table 2B, R6.3 of exterior insulation is enough for R18 of cavity insulation. Factor in the favorable exterior to interior perm ratio (enough for no exterior insulation with Intello/plywood if you interpolate) and it will be fine with a bit more cavity R - don't deliberately reduce it, with 1.5" or 3".

    I wouldn't expect any noticeable difference in comfort with an extra R6.3 - spend the money on better windows. Always air seal well and blower door test. Interior (Intello) and exterior (taped plywood) is better than either alone.

    No idea about your code requirements.

  2. Jason S. | | #2

    1. No. The effect on comfort would be negligible and would have more to do with your HVAC systems, floor assembly and window selection.
    2. Some risk, yes, but only if the Intello isn't installed airtight enough. See Jon's note on blower door testing.
    3. Yes. If you upgrade to 3" of the exterior mineral wool and hold the cavity insulation to R15 or less you could omit the Intello and make the airtight plywood sheathing be your air and vapor control. This should get you better condensation control, more continuity to your insulation and better air barrier location at the plywood.

    You'll have to crunch numbers to determine 'bang for buck'. Window details will certainly be fussier with 3" comfortboard.

    Best,
    j

  3. Jon R | | #3

    If you use 3" and no Intello, you can meet the 2B guideline with a Class III and R15 or exceed it significantly with a Class II and R23. I'd pick the latter - with the variable perms of Class II vapor barrier paint, I don't think the difference in inward drying is worth the increased risk and R value loss. I'd also consider using cellulose (better performance in most ways) or faced fiberglass (lower cost).

  4. Eric M | | #4

    Thanks you two.

    Looks like I'm on the right track thinking the 1.5" mineral wool panels are alright given my selection of plywood and intello. Definitely the more cost friendly choice.

    And yes I actually plan to seal all plywood joints using prosoco joint and seam filler before I stalling the WRB, and of course will do a blower door test.

    Code is my area is exceeded with this setup. Inspectors in my area always want a vapour barrier (even when it's a bad idea lol), so I figure it makes my life alot easier if they just see the intello, regardless of if I have thick enough exterior wool insulation to eliminate it or not.

    Thanks!

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