Pay attention to thermal mass

Insulation has metrics such as thermal conductivity, thermal mass (heat capacity), vapor resistance, and moisture content.

A quick question:

• Is a house where indoor temperature swings wildly a good house?
• Is it reasonable to install a large air conditioner just for the brief peak heat waves of midsummer?

The answer is obviously no — so what should we do?

The key is thermal mass.

Point 1

A house with high thermal mass dampens indoor temperature swings. No more being cold at night and hot during the day (reduced amplitude).

Point 2

It blunts the summer peak heat load. The higher the thermal mass, the more the timing of the indoor temperature peak shifts away from the outdoor peak (peak shift).

In Japan, where summer peak cooling load is high, building up thermal mass is important for keeping equipment costs down.