Passive, Concrete Slab Heat Storage
on the First Floor
The main reasons for this passive mass are to prevent the Active Area from overheating during sunny days by collecting heat and later release the stored heat during the evening – to stretch the solar day without supplemental heat.
Sunlight striking an object (directly or indirectly) will warm the object (unless it is highly reflective). The object gives off heat to the extent that its temperature is greater than: line-of-sight objects (radiation); the ambient air (convection); and, adjacent objects (conduction). Lightweight objects quickly become hot and most of the energy is quickly dissipated into the surrounding air. If the object is dark and massive enough, it will absorb most of the energy and warm only moderately.
Concrete is an excellent thermal mass material. It holds about 23 BTUs per cubic foot per degree. It can be used structurally and it can be a finished surface.
Daniel Chiras discusses solar glazing and the mass needed to keep from over heating in his book The Solar House. One point he makes is that “incidental” mass, the mass of typical building material, is sufficient for spaces with up to 7% glazing to floor area. He then estimates that about 5.5 square feet of direct lit floor mass (stone, concrete, etc.) and/or far greater amounts of floor not directly lit or wall space are needed for the glazing above 7%.
The Active Area is our priority space. We want to maintain a consistent, comfortable temperature (68 to 74 degrees). The Active Area (about 710 S.F.) is thermally isolated from the rest of the house (insulation in the ceiling and back interior walls). We have placed most of windows (investment) in this area, with 101 SF of south facing glazing. This is about 14% (glazing to floor area) or about twice the glazing typical building material mass can handle. We need thermal mass to prevent overheating and to provide heat after the sun sets. We also want to provide radiant heating or cooling, under certain conditions, to specific areas on the first floor.
Because of the above requirements and Chiras’ guidelines we decided to cover the entire first floor with 3.5” of concrete. This will provide both the thermal mass and an efficient distribution medium for the PEX radiant tubing. This concrete is not structural – think of it as a big piece of concrete tile.
Because we may want to heat the Active Area in the morning, the floor is zoned to allow heating the kitchen area (east end, indirectly lit floor) without heating the larger west end where the main windows are. This strategy minimizes heating the prime thermal mass area, which would limit the amount of solar energy the mass could then capture during the day (warmer objects absorb less heat than cooler ones). We may also put PEX in the ceiling of the kitchen area to provide quick heat with little warming of the mass!
The mass also assists with other goals. Because we will want to cool the floor (no, not to the dew point) we installed ice & water barrier under the concrete to insure against any moisture migrating to the deck and framing.
Our bedroom, closets and bathroom are in the rear of the house (north
side). We may occasionally want to heat the bedroom; the bathroom we
expect to heat every morning. One of the keys to my design is to control
where and when heat is used as much as possible. So while the whole
first floor is concrete mass, there are thermal breaks to minimize heat
from moving in an unregulated fashion to the north. Pouring the whole
floor allowed zoned PEX to be installed throughout and eliminated the
need to build two floor surfaces to the same height. Finally, in the
summer, the entire thermal mass will moderate the first floor’s room
temperature. The only disadvantage of extra mass is that it will slow
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