Category Archives: thermal mass

Mas Mass

As discussed in my last post, thermal mass helps buffer temperature extremes in passive solar buildings. Including it in new construction is easy, and, may I suggest, that failing to design and build climatically-appropriate passive structures is not only ignorant but borders on criminal. Climate change and peak oil will define the rest of our lives. Enough said.

Our challenge is adding mass to a 100 year-old villa built at a time when folks had more of an excuse to ignore energy efficiency. Although you have to wonder why the English immigrants to New Zealand faced their houses toward the South Pole when clearly the sun was at the back door. I thought all the psychopaths and criminals were sent to Australia.


At this stage in the project we are focusing on increasing our solar gain (as mentioned in a previous post) but plans for adding mass are being carefully laid and materials collected. At this time they include:

Our second-hand, vintage Shacklock multi-fuel cookstove will occupy the northwest-facing wall of the kitchen and include a brick surround for fire protection. The heavy iron stove and brick will only receive direct sunlight in mid-winter when the sun is low in the sky and its rays penetrate deep into the house. In summer, when the sun is high in the sky, direct light is largely excluded from the interior. In true permaculture fashion, the stove and surround will serve multiple functions: cooking, space-heating, water heating, thermal mass and disposal of unpainted wood pulled out during the renovation. Plus, the wife thinks it is cute as hell.

While not everyone can add iron and brick to their home as we plan to do, our next strategy is one that can be installed cheaply and easily by nearly anyone. If you have ever picked up a sheet of Gib (aka ‘drywall,’ ‘Sheetrock’) you know how much mass it contains – a lot! Installing an extra layer of Gib board on top of existing Gib walls is practically invisible and fairly low-cost. Our plan is to add an extra layer of Gib to as many walls as practicable in the northern corner of the house.

Added bonus: Covering up hideous wallpaper with another sheet of Gib saves hours of steaming and scraping.


The next post will include two more strategies we plan to use and one experiment in the making.

– M.C. Estwing

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.