Passive Solar Homes Glass-To-Thermal-Mass Ratios

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If you're designing a direct gain passive solar home, you will need to maintain a careful balance between two vital parts: the amount of glazing on each side of the house and the amount of thermal mass available inside to store and later release heat. Getting the glass-to-mass ratio wrong will result in too much or too little heating.

The first rule is the most basic: "the 7% rule" states that the first 7% of the glazing in a direct gain passive solar home is covered by the incidental thermal mass in the building. In other words, if your solar windows' surface area (those on the hot side of the house) equals 7% of the total floor surface area of the building, you do not need to include any extra thermal mass. Confused? Let's look at an example.

In a house with 2,000 square feet of floor space (which was the US average for "ideal home size" in 2010, according to a variety of surveys), the 7% rule comes out as follows:

7% of 2,000 sq. ft. = 140 sq. ft.

This house could have 140 square feet of solar glazing on the south side without any need for additional thermal mass to contain the heat. If you add more glazing, you'll need extra mass or your home will bake you like a cake!

Ratios to calculate how much thermal mass you need compared to window sizeSo how much extra mass do you need to allow for extra glazing? The rule is a little complicated because thermal mass comes in three types: floors in direct sunlight, floors out of direct sunlight but in the same room, and walls (in direct sunlight or not). It is important to note that all thermal mass on the floors and walls should be at least 4-6 inches thick to be effective.

  • For floors in direct sunlight, you need 5.5 square feet of surface for every extra square foot of glazing (over the 7% mark).
  • For floors not in direct sunlight but in the same room, you need 40 square feet for every extra square foot of glazing.
  • For walls, you need 8.3 square feet of surface for every extra square foot of glazing.

You're probably wondering why there aren't two figures for walls. It's because there is some disagreement on whether walls retain a given amount of heat (as stated by the Sustainable Buildings Industry Council ) or retain more when they're in direct sunlight because they heat up more, as some experts say.

Using the above ratios and figures, let's take another look at our example house. The 7% rule says we can have 140 square feet of solar glazing. What if we want to put 220 square feet (an extra 80) on the south side? How much extra thermal mass do we need?

Floor space in direct sunlight = 80 x 5.5 = 440 square feet
Floor space out of direct sunlight = 80 x 40 = 3,200 square feet (!)
Wall area = 80 x 8.3 = 664 square feet

Of course, in the real world, your thermal mass will be made up of a