Elements of a high-performance passive solar thermal mass slab:
Must support structural loads and be supported by the ground
- Perimeter frost wall, grade beam, or thickened edge for footings
- Interior linear or pad footings for load-bearing partitions or concentrated loads
- Well-compacted, mixed aggregate fill under slab
Must contain all first-floor fixture and floor drains
- Carefully planned and placed drain-waste-vent pipes
- Inside perimeter radon vent
- Other underground utilities, including wood heater combustion air supply
Proper concrete mix, reinforcement, admixtures, colorants & sealants
- Higher-strength concrete reduces the likelihood of surface or settling cracks
- Steel reinforcement (either rebar grid or welded wire mesh), placed 1 inch from bottom of pour
- Short-fiber polyester added to boost tensile strength and improve surface integrity
- Premixed colors for an easy finished floor
- Curing sealer applied after power-troweling to prevent dry-out and preserve color
Good solar access
- Sun sweeps across slab in fall, winter, and spring
- Windows are shaded with overhangs in summer
- Maximum slab area in direct sun during peak heating season
- Minimal insulating floor coverings or obstructions in areas of direct gain
Proper balance of glass to mass
- No additional mass required with 7% floor area in south glazing
- 6 feet2 of direct-gain slab or 9 feet2 of direct gain, thermal mass wall for every foot2 of glass beyond 7%
Good surface solar absorptivity
- Medium-to-dark earth tones
- Matte or textured surface—not glossy, unless thermal mass is in wall
Ample thermal storage material
- Dense: concrete, tamped earth, or masonry
- High specific heat (heat capacity per unit volume)
- Moderate diffusivity (distributes heat internally, but not too quickly)
- Good infrared emissivity (ability to radiate heat)
- About 4 inches thick for floor, or 8 inches thick for free-standing wall
- Good diurnal heat capacity (ability to store and release heat in 24-hour cycle)
Good environmental isolation
- Insulated from ground below (but not too much for good earth-coupling)
- Well-insulated at slab edges (where most heat loss occurs)
- Protected from water & water vapor with air/vapor barrier
- Subslab radon venting to evacuate carcinogenic soil gas
Natural convection distribution
- House designed with open floor plan
- If house has more than one story, central open stairway to move heat upstairs
- Perimeter floor grills for return flow (balanced with noise & privacy concerns)
Appropriate maintenance
- Re-seal (with siloxane) when floor begins to absorb water or stains
