Crestone Eagle, May 2006:
Designing thermal mass for passive solar heat storage
by Paul Shippee
solar heating of homes is usually thought to be simple enough:
just let the sun shine in through south-facing windows on
winter days. Everyone knows that this technique works. Many
people have enjoyed cost-free warmth and comfort from the
sun in their homes on cold days.
However, during winter nights, which are much colder and
also twice as long as winter days, the thermostat tells the
furnace to turn on, and we return to our dependence on fossil
fuels to keep ourselves warm until the sun returns.
In a conventional wood stick-frame house with sheetrock on
the interior walls the daytime sunheating scenario described
above might contribute 5-15 percent to the total 24 hour heating
load of the house for sunny days. The other 85-95 percent
of the required heat must be supplied by a fossil fuel source,
such as propane or electric, or by a wood burning source.
Given the above conventional scenario it might be interesting
to ask: What would it take to reverse those percentages in
our southwestern high altitude climate? In other words, how
can we get 85-95 percent of our total winter heating requirement
from the sun and still retain the simplicity of passive solar
You may remember that it was explained in the previous article
in April that passive solar heating is defined as using the
house itself as a solar collector. It is for this reason that
I am using the word “simple” to describe passive
solar home heating. It is not a mechanical system and does
not depend on electricity for its operation. You might say
it is a natural system, using natural means of heat flow to
achieve comfort round the clock when designed properly.
It is common to oversimplify the application of passive solar.
It seems that builders, designers and architects often tend
to do just that. This is usually because they might not have
taken the time and trouble to fully understand and appreciate
the dynamic and critical role that thermal mass plays in achieving
much higher passive solar heating percentages of up to 100
percent. The practical result of oversimplifying is that,
because of inadequate thermal mass placement, the house overheats,
then begins to get cold immediately at sundown.
What is this dynamic and critical role that thermal mass
can play in a simple passive solar system? What is thermal
mass? Thermal mass in the solar heating context refers to
the capacity of dense materials to store significant quantities
of heat, at a reasonable rate, as their temperature rises
(and then release it as the temperature around them falls).
The two main materials used for thermal mass in passive solar
buildings are water (in containers) and earth materials in
the form of adobe, concrete, earthen plaster, bricks, and
stone. To get technical for a moment, a cubic foot of water
can store nearly three times as much heat as a cubic foot
of any earth material for each degree of temperature rise.
You could say that water is more efficient, space-wise, than
adobe or concrete for storing heat.
If a regular sheetrock house has a fair amount of south windows,
it will overheat on sunny winter days and cool down rapidly
after sundown. Such large interior temperature swings are
unacceptable from a comfort point of view, and a portion of
the collected solar heat is unusable and therefore wasted
The remedy for this untenable situation is to add interior
thermal mass materials so that interior temperature swings
are kept to 10°F, say between 65°F and 75°F. The
main functional idea behind deploying thermal mass materials
is that they suck up excess solar heat during the day, thus
preventing overheating of the house air. Then during the night
this heat is slowly released into the rooms. The stored solar
heat, you could say, is invited out of storage as the house
air tries to cool down.
The volume, location, and surface area of heat storage materials
can be a creative architectural element inside the house.
The main design strategy is to balance the relationship between
these four elements: desired solar heating fraction (percentage
of heating by solar); amount of south glass area; quantity
and location of thermal mass; and daily allowable temperature
swings (comfort factor).
These quantities should all be strategically coordinated
from an engineering point of view by using accurate numbers.
Careful and deliberate sizing of these elements will produce
the best results in terms of long-term fossil fuel savings,
as well as having an attractive and comfortable home.
Lastly, the actual location of thermal mass materials in
the home requires attention to solar architecture, as well
as heat flow engineering details. The three most popular types
of passive solar systems are direct gain, Trombe wall, and
sunspace. A rule of thumb for designing a comfortable direct
gain system, which lets sun shine directly into rooms, is
to distribute the thermal mass materials over a large area
of walls and floor, about nine or ten times the area of south
glass and about 4-6 inches thick (for earth materials).
For a Trombe wall design, a 12-inch concrete wall is placed
directly behind the south glass. For a sunspace design this
same wall is moved back several feet from the glass to create
a greenhouse area that is allowed to swing in temperature
more than the house interiors. The solar rays heat one side
of the wall and this heat begins to show up on the interior
side at evening time.
From an economic point of view, the added building cost for
deploying quantities of thermal mass materials can be compared
to the future life cycle cost savings of the fossil fuels
Free convection air flows, facilitated by various venting
schemes, can augment the distribution of solar heat to the
cooler interiors. The performance of any passive solar system
can be greatly enhanced by employing moveable insulation to
cover south windows during long winter nights. This aspect
will be covered in detail in the next article to follow in
June. For more information go to www.crestonesolarschool.com
to the Eagle!