Part V: Passive Solar Homes and SunspacesChapter 8. Designing a Passive Solar House

Thermal Mass: How Much, Where, and What Kind

Concrete, water, and masonry as heat storage, and why mass in the wrong place does little.

Difficulty: Intermediate
Cost: Planning only

published

Thermal mass stores heat by getting warmer and gives it back by getting cooler. It does not make heat. If the sun never reaches it and the room air barely reaches it, it is furniture weight. The mass that matters for direct gain is in the sun, or is a large area coupled to the room the sun just heated.

A planning sketch used for direct gain is about 5 gallons of water, or about 1 cubic foot of masonry sitting in the sun, per square foot of south glass. If the mass only feels the room air and never the beam of sun, plan on several times more, because the coupling is weaker. Carpet over a slab removes the slab from the account. The calculator turns the sunlit rule into gallons, cubic feet, and the floor area of a slab of a given thickness.

These are rules of thumb for a temperature swing you can live with, not an energy-code compliance path. A room can be comfortable with less mass if you accept a wider swing or if you cover the glass at night.

Difficulty

Level: Intermediate
Time: An hour to measure glass and floors
Skills: Distinguishing sunlit mass from heavy objects in a back room

Cost estimate

Planning only

Uncovering a slab you already have can be cheap. A new masonry wall or a large water tank is structure and money. The sketch tells you the quantity before you buy either.

Materials

Item	Qty	Notes
South glass area	Square feet of actual glass	From the overhang page's fraction, not from a wish for more windows.
Sunlit floor or masonry	The area the winter sun hits	A rug, a wall-to-wall carpet, or a covered sofa does not qualify.
A structural yes or no	1	Water is about 8.3 pounds per gallon. A few hundred gallons means redesigning the floor to carry it.

What counts

A bare concrete slab in the sun counts. Four inches of slab is one third of a cubic foot per square foot of floor, so the "one cubic foot of masonry" rule wants about three square feet of that slab for each square foot of glass. A 100 square foot south window points at about 300 square feet of sunlit four-inch slab. That is a lot of room. If you do not have it, you have too much glass, you add water, or you accept a bigger afternoon swing and a stronger night cover.

Masonry walls count when the sun hits them. A dark surface absorbs more of the beam. A light surface bounces light deeper into the room, which helps daylight and still stores less of the direct hit. Do not paint a floor so dark that it glares and overheats the surface people walk on. A middle color on a floor and a darker color on a wall that is not at eye level is a practical compromise.

Water stores more heat per cubic foot than concrete, which is why the gallon rule is compact. It also leaks, it weighs a concentrated load, and it wants a drain under it. A water wall is a designed tank with a footing, not a row of bottles on a shelf. Bottles tip. They also sit at whatever temperature the room already is if the sun misses them.

Gypsum board and wood store a little and do not satisfy the rule. Phase-change products exist as manufactured panels with a published melt temperature near room temperature. Buy them as a rated product if you use them at all. Do not mix a homemade salt recipe and call it mass. The melt temperature will be wrong, and some of those mixtures are corrosive.

Sun on a bare floor counts. A rug on that floor, and a heavy wall in a room the sun never enters, do not.

How much, as a sketch

The calculator uses 5 gallons of water or 1 cubic foot of sunlit masonry per square foot of south glass, then spreads the masonry into a slab of the thickness you type. It also shows the heat stored over a 10 degree F swing, using about 8.34 BTU per gallon per degree and about 28 BTU per cubic foot of masonry per degree. Real concrete varies with density. Treat the BTU figure as a comparison, not a bid.

If your slab is covered, the usual first step in a retrofit is to uncover it, or to lay tile on the sunlit strip, before you pour anything new. The retrofit page puts that in order. Adding mass in a north room because the floor was easy to pour will store the furnace's heat and give some of it back at night. That is a comfort effect, not solar storage for the south glass.

Measure only the south glass
Use the area that faces the winter sun. East and west glass is a cooling problem, not a reason to add more mass in the middle of the house.
Mark what the sun actually touches
Floor, wall, or neither. Anything under carpet or in another room goes in a different column.
Run the sketch
Compare the slab area you have with the slab area the rule wants. The gap is extra glass, missing mass, or a wider swing you must choose on purpose.
If you add water, design the load and the leak
Pounds, a floor or footing that can take them, and a drain. No pallet of barrels on a bedroom floor.
Cover the glass at night if the mass is thin
Insulated shades make a modest amount of mass behave better. They do not make a rug into a slab.

Thermal Mass: How Much, Where, and What Kind

Difficulty

Cost estimate

Materials

What counts

How much, as a sketch

Mass for south glass

Measure only the south glass

Mark what the sun actually touches

Run the sketch

If you add water, design the load and the leak

Cover the glass at night if the mass is thin

Further reading