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Community and Q&A

Exterior Foam Thickness – Atlantic Canada

ekPzmknSfY | Posted in Energy Efficiency and Durability on


It seems that the concept of an interior moisture venting is quite new and some experienced builders brush off the idea.

I am looking at building a house I will personally live in, and I intend to live in comfort, on reasonable energy efficiency, and of course durability of structure.

The design I am currently looking at would consist of; from outside to inside:

-Properly screened air gap
– 2 x Layers of 2″ Polyiso with joints sealed and staggered
– Tyvec Drainwrap (adhesive membrane flashing)
– 1/2″ Plywood
-5.5″ wet blown cellulose in 2 x 6 framing 24″ c/c
-Gypsum board
-Class III vapor retardant paint.

Details would be as per the Remote Manual, by the CCRHC. Except on a thicker lumber stucture, which I assume is possible due to my climate being warmer the Alaska testing grounds.

The building would be located in northern New Brunswick, Canada. From climate data, the would be 5900 HDD (In Celsius) Would that be around 10500 HDD from the Fahrenheit system?

Data (1970 – 2000) 3 KM from proposed building

The daily minimum average for:
December -13 C
January -17.9 C
February -16.4
March -10.1

Daily Average:

December -8.6 C
January -12.8 C
February – 11.1 C
March – 5.1 C

My dew point for interior condition of 21°C (70F) and 35% RH @ exterior sheeting would be 5°C.
Which would be reached @ an exterior temperature of -15.4°C assuming 44 % of insulation would be to the inside.

Would anyone experienced in this type of wall be comfortable in building this?
I know it would create a safer dew point to frame in 2 x 4, but since the buiding would be 2 stories, I am more comfortable with 2 x6 rigidity wise.
Is there enough safety factor to building such a wall?

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  1. GBA Editor
    Martin Holladay | | #1

    Your two layers of 2-inch polyiso will give you about R-25 for your foam layer; that's well in excess of the minimum amount of foam you need for an insulated 2x6 wall, even in climate zone 8. Your climate zone in New Brunswick is probably warmer than the DOE zone 8 -- I imagine you are in a zone equivalent to zone 7.

    So you're fine -- don't worry. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

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