Slab thickness?
Claire Anderson
| Posted in Green Building Techniques on
I’m trying to calculate the “ideal” slab thickness for our passive-solar home in Medford, Oregon. Does anyone have any calculators they’d recommend? I used James Kachadorian’s CSOL program, but it doesn’t factor in underslab insulation (we’re insulating to R-15) and putting in hydronic tubing for radiant heating.
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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.