Oppose NASH as AS for E2 as it doesn’t cover external insulation

And putting batts between light steel framing is dumb.
Only R0.25 thermal break that is vapour closed will mould and does not meet the ASHRAE 160 criteria.
The Oak Ridge National Laboratory has found that thermal bridging in a wood-framed wall lowers the effectiveness of cavity insulation by 10%, but performance drops a whopping 55% in a wall framed with steel.
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“But from a hygrothermal standpoint, I can only add — to the excellent points already made regarding energy performance — these two:
  1. Condensation. Even small thermal bridging sets up for significant risk of interstitial condensation, exacerbated by the next point.
  2. Buffering capacity. This is the amount of water that a material or assembly can “hold” or tolerate without deterioration (for more, see this research paper). Joe Lstiburek is famous for many things, but his “Joe math” attracts a lot of us builder types. Here is one of my favorites: For a 2,000-sq.-ft. home framed with steel, the hygric buffer capacity is about 5 gallons of water; wood-framed, it’s about 50 gallons; and for masonry walls it’s approximately 5,000 gallons.
So, when condensation happens in a steel-framed assembly, it’s the really low hygric buffer capacity that ramps up the significance of that condensation. The wall may have very little tolerance for that moisture.
My conclusion: if you are considering light-gauge steel framing, use the BSC “perfect wall” approach. Keep all of your R-value to the exterior, and let that steel be part of the interior conditions (or nearly so) of the building. And the more severe the climate, the more stringent my recommendation is.”

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