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I'm building a 1400 sq. ft addition which will be mostly covered with a flat (very low sloped) roof, with a second storey bedroom over the rest.

I'd like to have 4" of continuous rigid exterior insulation (in addition to cavity insulation), so to keep my overall wall thickness down, I'd like to have the walls framed with 2x4s. My architect said that because of the 10' high walls they would need to be framed with 2x6's (according to Ontario building code), but I'm nearly certain I've seen recent builds that have 10' high walls that were framed with 2x4's.

I've not been able to specifically find anything to prove/disprove this yet.

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    A 2x4 wall won't give 4" of insulation. 2x4's are only 3.5" wide...
    – brhans
    May 1 '17 at 11:54
  • Just to be clear, my plan was to fill the 3.5" wall cavity with batts, and then add 4" of continuous rigid insulation on the exterior.
    – user19284
    May 2 '17 at 14:08
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    For me, it isn't a matter of code. Walls that tall tend to get wavy if framed with 4" lumber. Warpage and bounce are real concerns. I decided 20 years ago that I wouldn't even frame a garage, which tends to be 10-12' high, with 2x4. You don't lose much real-world insulation value by dropping your continuous to 2" and using R-19 in the stud cavities. I'd do that.
    – isherwood
    May 2 '17 at 15:39
  • Thanks for the insight. I think this is exactly what I'll end up doing.
    – user19284
    May 4 '17 at 17:21
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The national building code of Canada specify's exterior walls must be framed with 2x6 or 2x8. Interior non load bearing may be framed with 2x4. I hope this helps.

https://www.saskatoon.ca/sites/default/files/documents/community-services/building-standards/tall_walls.pdf

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  • Thanks for sharing, but this doesn't seem to be a complete source. Just because the exterior walls table doesn't show 2x4 doesn't necessarily mean that it's not allowed. For example it doesn't show that 2x6's can be framed 24" OC, but I know that is allowed.
    – user19284
    May 2 '17 at 14:48
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I think I found what I was looking for: Table 9.23.10.1 (Size and Spacing of Studs) in the Ontario Building Code at https://www.ontario.ca/laws/regulation/120332 indicates that an Exterior Wall supporting a Roof with or without attic storage plus one floor must have studs with a minimum size of 38mm × 89mm (2" x 4") with 406mm (16") spacing and a maximum height of 3.0m (9'-10")

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  • So is the answer 'no' in your case, seeing that your walls are 2 inches taller than code allows?
    – isherwood
    May 2 '17 at 15:40
  • Yes this seems to be correct.
    – user19284
    May 4 '17 at 17:21

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