May 7, 2025UPD May 7, 2025
On a small in-progress addition site, the builder forgot to put in the insulation.
I initially thought that with reinforcement insulation we could secure nearly equivalent performance — but to be safe, I ran a thermal-bridge analysis. The result: without doing anything, about a 20% drop in performance; even with reinforcement insulation, about a 7% drop.
You can simply call this "insulation performance dropped" — but in fact, for slab-edge / under-slab insulation, thinning it can lead to reduced cooling load. Especially in Climate Region 6, the annual heating+cooling load can actually go down.
Of course, this trend doesn't appear in every house — it's a story for high-performance Passive-House-level housing.
When I design Passive Houses, the summer cooling and dehumidification load is often the bottleneck. So I sometimes "tune" by deliberately raising the heating load a little in exchange for lowering the cooling/dehumidification load. Insulation under the slab is one of the easiest places to make that adjustment.
The point: "thicker insulation is always better" isn't necessarily true.