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Slab dimensions for passive solar

Christa Campbell | Posted in Green Building Techniques on

I found the following answer and need clarification on the last paragraph. When Robert Riversong says ” floor area”, does he mean the whole floor area of the house or just the floor that could be exposed to sunlight?

For both radiant thermal mass and passive solar thermal mass, the ideal thickness of a poured concrete slab happens to be exactly the same as the industry standard for residential applications: 4″.
For reasonable response time, it’s best to have radiant tubing at about 2″ from the slab surface. Heat conducts through concrete at approximately 1 hour per inch. So a 4″ slab will absorb and release solar heat during an approximate 8-hour cycle. This corresponds well with absorption during the heat of the day (noon) and release by early evening when the sun is down and the house is cooling.
For passive solar thermal mass, the area of slab in direct sun relative to the area of south glazing is the more important factor. Since a sun-tempered house (5%-7% of floor area in south glazing) requires no additional mass, a passive solar house (7%-12% of floor area in south glazing) requires approximately 6 sf of 4″ thick thermal mass for each square foot of south glazing beyond 7% of floor area. Mass not in direct sun is 1/3 to 1/2 as efficient as a storage medium.
ANSWERED BY ROBERT RIVERSONG
Posted Mon, 10/12/2009 – 21:06

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  1. GBA Editor
    Martin Holladay | | #1

    Christa,
    Q. "Does he mean the whole floor area of the house or just the floor that could be exposed to sunlight?"

    A. The whole floor area of the house -- although the formula works better for homes with a relatively open floor plan (and a shape assumed to be rectangular, with the long dimension being east-west).

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