CLT walls, insulation, and dew points

This is a technical discussion between builders and designers, not something that touches most homeowners directly, but it's worth understanding if you're planning a build with CLT (cross-laminated timber, a solid wood panel product used for walls and floors) or any thick, dense wall material in a cold climate.

The question centers on a common cold-climate building problem: figuring out where moisture will condense inside a wall as warm indoor air meets cold outdoor temperatures. Builders call this the "dew point" — the spot in the wall where water vapor turns to liquid. If that spot lands inside a solid wood panel rather than in the exterior foam or mineral wool insulation, the wood can stay damp and eventually mold or delaminate. One builder designing a cabin found that meeting code-minimum insulation left the risky condensation point inside the wood itself, and that pushing it safely out into the exterior insulation required much thicker insulation than expected — thicker, in fact, than local code technically requires.

Responses in the thread suggest two practical fixes contractors already use: putting rigid mineral-wool insulation on the outside of the wood panels (so any moisture buildup dries outward rather than getting trapped), and sealing the wood panel's surface well so indoor humid air can't leak into the wall in the first place. There's also a reminder that dew-point calculations should use the average temperature over the coldest few months, not the single coldest recorded temperature, which changes how much insulation is actually needed.