U. Mass

Thermal mass is the unsung hero of passive solar design – the Rodney Dangerfield of eco-construction. Back in the 70’s idealistic but thermodynamically-impaired hippies built passive solar dwellings with plenty of glazing for solar gain and plenty of insulation, but little or no thermal mass. The result was dramatic indoor temperature swings from day to night. In some cases, on sunny midwinter days with outdoor temperatures around freezing, indoor temperatures in the high 80s F/ low 30s C required the opening of windows to let the excess heat escape. By the next morning, however, the house would be cold.

Let's face our house towards the sun, man. - Groovy idea dude, totally groovy.

Last year, we did some house sitting in just such a place. At 6 a.m. when I got up for coffee and PhD research, the temperature in the kitchen was in the low teens C / 50 F. On sunny days it rose to 30+ C / 86+ F by late afternoon only to return to sweater weather (‘jumper weather’?) the next morning.

The role of thermal mass is to moderate these extremes – to buffer the system. It is like a bank account where an excess ‘wealth’ of heat can be ‘deposited’ on sunny days and ‘withdrawn’ at night. Think of a concrete stoop that has been sitting in the sun all day. It holds heat after sunset. Please note, thermal mass in passive solar design must be within the building envelope – that is, inside the glazing and the insulation.

Many new homes are built on insulated concrete slabs that can serve as thermal mass. This is smart design, but our 100 year-old villa is up on 1 meter piles with air flowing beneath the floor. The challenge is incorporating thermal mass inside our envelope in effective, attractive, eco-thrifty ways. The next post will explain some ways in which we plan to do this.

-M.C. Estwing

This picture is posted to authenticate that two real hippies were used in the making of this post. We have done our best to ensure that no hippies will be injured during the completion of this project.