Designing a Passive Solar Slab

Intermediate

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A passive solar slab
A passive solar slab
This house has an 8.25% glazing-to-floor-area ratio on the first floor
This house has an 8.25% glazing-to-floor-area ratio on the first floor (with thermal mass).
Step 1: Frost-protected shallow (FPS) foundation
Step 1: Frost-protected shallow (FPS) foundation with exterior and slab-edge insulation preinstalled.
Step 2: Adding the below-slab mechanicals.
Step 2: Adding the below-slab mechanicals.
Step 3: Leveling and compacting the subslab fill.
Step 3: Leveling and compacting the subslab fill.
Step 4:  A layer of rigid foam insulation over the vapor barrier
Step 4: With the vapor barrier installed, a layer of rigid foam insulation over it provides a thermal break from the earth.
Step 5: Remesh or rebar placed over the rigid foam provides attachment points
Step 5: Remesh or rebar placed over the rigid foam provides attachment points for PEX tubing.
Step 6: Pouring and distributing the concrete.
Step 6: Pouring and distributing the concrete.
Step 7: The medium-brown integral color used to tint the concrete
Step 7: The medium-brown integral color used to tint the concrete will help absorb solar radiation efficiently.
Step 8: Smoothing the slab’s surface to its final texture.
Step 8: Smoothing the slab’s surface to its final texture.
Step 9: Powertroweling the slab
Step 9: Powertroweling the slab provides a smooth finish.
Step 10: Applying a curing sealer
Step 10: Applying a curing sealer over the finished concrete will help protect the surface from stains.
A passive solar slab
This house has an 8.25% glazing-to-floor-area ratio on the first floor
Step 1: Frost-protected shallow (FPS) foundation
Step 2: Adding the below-slab mechanicals.
Step 3: Leveling and compacting the subslab fill.
Step 4:  A layer of rigid foam insulation over the vapor barrier
Step 5: Remesh or rebar placed over the rigid foam provides attachment points
Step 6: Pouring and distributing the concrete.
Step 7: The medium-brown integral color used to tint the concrete
Step 8: Smoothing the slab’s surface to its final texture.
Step 9: Powertroweling the slab
Step 10: Applying a curing sealer

A passive solar home requires five elements to take full advantage of the sun’s free heat: apertures to let in the sun’s warming rays; a means of preventing too much solar gain in the summer; an absorber surface that minimizes reflection; thermal mass to store the heat until it’s needed; and a distribution system to move the heat to where it’s required. 

For a truly passive house, each of these elements should operate without mechanical power or occupant intervention. As examples, summer solar gain is handled by properly designed overhangs. The distribution system would be natural convection within an open floor plan, with storage and release handled by concrete—a massive and dense material with high specific heat (heat storage capacity per unit volume) and moderate thermal diffusivity (the propensity of heat to dissipate to all areas of the mass).

Focus on the Slab

Perhaps the least-understood elements of passive solar design, and the ones that plagued the early passive solar pioneers in the 1970s, are the ratios of south glass area to floor area and of south glass area to thermal mass. Without proper balance—and an appropriate absorber and mass storage—an otherwise well-designed house can be unlivable. 

Too much glass can mean:

  • overheating even in the dead of winter
  • overchilling at night
  • too little privacy
  • too little usable wall space
  • too much glare and shadow
  • too little sense of enclosure and security

But without sufficient thermal mass, even the proper glass-to-floor ratio can lead to daily or even hourly temperature swings and heat stratification that can make a home uncomfortable. The south-facing glazing design standard for today’s passive solar homes is a window area between 7% and 12% of floor area. (For example, a 1,000-square-foot space would have between 70 and 120 square feet of south glazing.) That ratio can apply to the entire house if all stories are to be passive solar designed, or just to the primary living floor. It’s often more appropriate to design a bedroom floor to be sun-tempered, with south glazing of 5% to 7% of the floor area, which doesn’t require any additional thermal mass beyond normal building materials and has the benefit of providing more privacy. Beyond 12%, we enter the active solar range in which direct-gain thermal mass is not sufficient to maintain a uniform and comfortable indoor temperature without fans or pumps to move the heat to remote storage and retrieve it on demand.

Thermal Mass & Foundation Fundamentals

The goal in designing a passive solar home’s thermal mass is to be able to store midday solar heat until the early evening, when it will passively return to the living space. Thermal mass operates like a flywheel that dampens any sudden changes in acceleration or, in this case, changes in insolation—the amount of solar energy entering through the apertures—which would otherwise raise indoor air temperature.

Comments (2)

Fred Golden's picture

I found this book very helpful in deciding how thick to design a floor slab, and how many windows to use.

Passive Solar House: The Complete Guide to Heating and Cooling Your Home by James Kachadorian (Sep 15, 2006)

Mr Kachadorian's home is heated with the sun and used 12" concrete bricks laid on their sides, forming air channels running north to south collecting excess heat during the day and distributing it at night.

Robert Riversong's picture

I see you're putting this same comment on every article about passive solar, but Kachadorian's claims are largely fraudulent and his system excessively complicated and non-functional, as well as producing significant mold problems. The picture on the cover of his book shows a house that is almost entirely problematic in terms of passive solar design.

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