At first glance, you might think that it's impossible to have too much or too little thermal mass. After all, if there's loads of it, your home will store more heat; if there's very little, you'll still get the benefits of what you do have.

But it's not as simple as that.

Too little thermal mass

If you install too little mass in your home, you will suffer. The low winter sun will penetrate deeply into your home (as it is supposed to) and warm the available mass very quickly, then continue to warm the house. This can result in uncomfortable temperatures reaching up into the 80°F range (over 26°C).

When the sun goes down, the thermal mass quickly radiates all its stored heat back out into the home, but since there's so little mass available, the heat is quickly dissipated and your home gets very cold, very quickly.

The practical result is that your home suffers large temperature swings and you spend lots of money on backup heating systems to compensate.

Too much thermal mass

Most thermal mass offers proportionate gains the more you install. However, heat does not transfer at a steady rate through deeper materials, so there is definitely a point where installing extra thermal mass is just throwing money away for little gain.

Once you get beyond a depth of about four inches, the efficiency of thermal mass drops off quite sharply. Heat transfer becomes much less effective, so a six-inch thick wall is nowhere near half as efficient again as a four-inch thick wall. In fact, the extra two inches of thickness net you less than 10% extra efficiency!

Secondly, surface area is more important than depth (assuming the whole surface is hit by sunlight). It is far better to have one hundred square feet of two-inch thick wall exposed to direct sunlight than fifty square feet of four-inch thick wall. This is because the amount of energy per square foot from the sun cannot be affected (at least not without using lenses). The more surface area there is, the more energy hits and the more heat can be gathered.

Disagreeing with the theory

It is important to note that some builders disagree with the idea of too much thermal mass being possible. There are some construction companies who install as much mass as they can in every building – especially in the exterior walls – as they believe the enormous amount of storage offers a better temperature balance, year-round.

Unfortunately, there isn't much research to back up either point of view.

Alternative thermal mass

In certain specific circumstances, faster heat absorption and emission are desirable. In these cases, standard masonry and other dense thermal mass installations may not be appropriate, and water can be used instead.

Water retains at least twice as much heat as any solid equivalent, far outstretching basic earthen options. But it also releases that heat quicker. If this suits your needs, it is possible to include sealed glass tubes filled with water (usually placed in direct sunlight) in the walls or even