16. Passive Solar Radiant Slab


A passive solar home requires five elements in order to take full advantage of the free and plentiful heat of the sun: an aperture to let in the sun’s warming rays, a control device to keep them out in the summer, an absorber surface to receive the rays without too much 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 house to be truly passive, each of these elements should operate without either mechanical power or occupant intervention. The control system, for instance, is ideally properly-designed overhangs. The distribution system would be natural convection within an open floor plan. And the storage and release device would be 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).

Solar builders are offered a wide array of “apertures” or window options, and with sufficient demand perhaps manufacturers will begin to offer the kind of highly insulating windows that also offer high solar heat gain coefficients (SHGC, as listed on the NFRC label on new units). At this point, only some Canadian manufacturers and a few high-end custom window makers in the US are selling windows that are ideal for passive solar applications. The current federal energy conservation rebate program excludes high solar heat gain windows.

Good residential designers are beginning to understand the importance of overhangs, both for rain protection and for preventing summertime overheating in passive solar homes. Many also appreciate the value of a south-side open floor plan, with private rooms, entries and utility/storage spaces in the north, as well as an elongated east-west axis to enhance a southern exposure that’s oriented within 15° of true south.

But perhaps the least understood element of passive solar design, and the one that plagued the early pioneers in the 70’s, is the ratio of south glass to floor area and the ratio of south glass to thermal mass. Without the proper balance and the 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 as well as over-chilling at night, too little privacy, too little usable wall space, too much glare and shadow, and too little sense of enclosure and security.

Even the proper glass-to-floor ratio, without sufficient thermal mass, can lead to daily or even hourly temperature swings and heat stratification that makes a home uncomfortable and annoying. The design standard for today’s passive solar homes is between 7% and 12% of floor area in south glazing (a 1000 square foot space would have between 70 and 120 square feet of solar glazing – that means the transparent glass dimensions, not the entire window unit which includes sash and frame). That ratio can apply to the entire house if all storeys 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%-7% of floor area, which doesn’t require any additional thermal mass beyond the normal building materials and provides 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.

The goal in designing a thermal mass element is to be able to store mid-day 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. It also operates like a sponge, soaking in large quantities of solar heat, measured in BTUs (British Thermal Units – about the heat of a wooden kitchen match), and allowing it to be “squeezed” out later.

The optimum thermal mass is direct-gain, meaning in the direct path of the sun, and uniformly distributed throughout the living space. A thermal mass floor fits this need quite well, and the simplest and most cost-effective thermal mass floor is a concrete slab-on-grade.

It’s possible to pour a light-weight concrete slab on top of a wood-framed floor, if the structure has been sized properly for the extra weight, or use a tile or masonry floor finish. But to combine the structural floor with the thermal mass, design it to be earth-coupled to make the house frost-proof, and perhaps integrate a radiant floor central heating system, is the height of design elegance: using one element to serve multiple essential functions.

To do this well, however, requires designing the home literally from the ground up, integrating multiple systems, and understanding the engineering requirements of each step in the process. Every material choice and methodology decision must build toward an integrated system.

If the slab is also part of the foundation, then gravity loads and soil-bearing capacity as well as foundation insulation must be considered. While a monolithic slab – a floor slab with thickened edges to act as footings – is popular in some areas, I prefer to thermally decouple the floor slab from the foundation. A frost-protected shallow foundation (FPSF) often works well for this, at least in well-drained soils, and the reinforced concrete grade beam becomes the building’s foundation. In wetter areas, I will use a rubble-trench foundation – a perimeter trench dug to below the frost line, drained to daylight, and filled with clean mixed stone to grade – and then pour a grade beam at the surface on top of the stone. A more conventional alternative would be a frost wall, though that requires three pours: footings, walls, and slab.

I build my exterior wood-framed walls 12″ thick with a Riversong Truss system, so I can use a 12″ wide grade beam which includes 2″ of exterior XPS foam and a 2″ x 6″ piece of slab-edge insulation within that width, and still cover the interior edge insulation with the wall. For a thinner wall section, the grade beam can be stepped to allow the slab-edge insulation to hide under the bottom plate, with the slab supported by the foundation.


The building foundation, whether grade beam or frost wall on footings, needs to be sized to carry the design or code-specified live and dead loads, including snow loads, into the ground, and the ground has to be of sufficient load-bearing quality to receive them. Typical soils, with the exception of loose sand, soft clays and sandy loams, will carry at least two tons per square foot. Though codes may mandate wide footings as a general practice, most two-storey homes don’t need more than an 8″-10″ wide perimeter footing reinforced with ½” steel rebar. Don’t forget to place additional steel-reinforced interior linear footings or pads under center bearing walls and point loads, like chimneys and posts.

Clean, granular gravel or sand fill is often required to bring the interior up to the level of the sub-slab insulation, and that fill must be mechanically compacted in 6″ to 8″ lifts. But first, all sub-slab mechanicals must be carefully placed, since there is no way to move things once they are cast in concrete. This would include any first-floor drains, a water or well line, underground electrical and telecom utilities, and underfloor DWV pipes. It’s also wise practice to install a sub-slab radon vent in any new construction, since radon soil gas is found in all geographic areas and is carcinogenic. Then a radon/vapor barrier must be placed on the compacted fill (I use tear-resistant 4 mil cross-laminated Tu-Tuf®), and sub-slab insulation, leaving 4″ for steel reinforcement (either 6″ welded wire mesh or steel rebar) and concrete.

How much sub-slab insulation to use depends on the climate zone (2009 International Energy Code standards for foundations are R-10 for zones 4 and 5, R-15 for zones 6 and 7), the amount of exterior foundation insulation, and whether the slab will be part of a radiant heating system.

Extruded polystyrene (XPS), known by the trade name Styrofoam®, is the industry standard for sub-grade insulation because of its durability, compressive strength, low moisture absorption, dimensional stability and high R-value of 5 per inch. It is also highly resistant to acids, alkalis, mineral oils, glycols and beer (no kidding). For a heated slab, I would recommend an additional R-5 beyond code minimums, except with a FPS foundation which relies on heat loss downward to maintain the earth temperature above freezing, and which has an additional R-5 to R-10 exterior foundation and wing insulation. As important as sub-slab insulation is for a radiant or solar heated slab, the greatest heat loss from a slab-on-grade occurs at the vertical edges and this is why I prefer to pour a slab separate from the foundation and isolated with R-10 edge insulation.

Any foundation type, particularly a slab-on-grade, must also be hydraulically isolated from the ground with capillary breaks – material that prevents the migration of moisture – between the earth and foundation, between foundation and slab, and between foundation and wood framing. The sub-slab vapor barrier and edge insulation serve this purpose, as does the sill seal and metal termite flashing installed on the grade beam before wooden sills. But few builders bother to include a capillary break between footings and foundation wall to prevent the wicking of water up the concrete, which has a theoretical capillary height of 6 miles! I prefer a brushed-on latex masonry waterproofing such as UGL DryLock®.

Codes require that all concrete in residential construction have a minimum 28-day strength of 2500 psi, but since it tends to get extra water during pouring and because stronger concrete is also more waterproof, I order 3500 psi mix for foundations and 4000 psi mix for slabs. In addition to 6″ welded wire mesh reinforcement (which because of its grid pattern makes laying out and securing radiant tubing simple), I also specify short polyester fiber reinforcement in the slab mix. Steel reinforcement, whether a grid of rebar or welded wire mesh, offers tensile strength to resist cracking from settling or ground movement. The short fibers, invisible once the slab is power troweled, help prevent the small shrinkage cracks that can occur if mix water is allowed to evaporate too quickly. All concrete work should be covered by plastic or kept wet for three days to allow initial curing to occur.

If the slab surface is to be the finished floor, then any of several dozen standard colors can be premixed at the batch plant for a small extra fee, and this saves the cost of additional finish materials and labor. Any solar absorber surface should have low specularity (be somewhat matte textured, not glossy) and a medium hue such as brick red or terracotta brown, which have solar absorptivity factors of 70% to 80%. Even untinted concrete has an absorptivity of 65%, which is within the acceptable range. A finished concrete floor also needs to be sealed to prevent water absorption and staining. There are both surface coatings, which tend to create a sheen until worn in, and penetrating sealers made of siloxanes, silanes, or silicates. Siloxanes are the least volatile, penetrate well, can last for 10 years, and can be applied on slightly damp materials. Silicates, unlike the other two, have poor water repellency, poor water vapor permeability and a shorter working life. A surface film or paint can be applied, but that will have to wait until complete cure, which can take three months.

If a solar slab is to be fully or partially covered by other floor finishes, those coverings must have good solar absorptivity and very little thermal resistance – no more than R-0.5. Tile or masonry works well, and a 3/8″ laminated prefinished hardwood strip flooring can be installed with mastic without too much loss of thermal mass function. The wood alternative makes a concrete floor “softer” on the feet and offers a “warmer” feel to the house, but use material with a matte finish for low specular reflection and enough color for good solar absorption. The thin, laminated hardwood will be more stable than solid sawn lumber, which is important on a floor that will be changing temperature. Throw rugs are OK if the total area is limited to perhaps 20% of floor area.

Dense materials, like concrete, which have a specific heat of 28 BTU per cubic foot per degree F (about half that of water), tend to allow heat diffusion at a rate of about one inch per hour. So the heat of the noontime sun will penetrate to the bottom of a 4″ thick slab by about 4 PM and all that heat will have returned to the interior by about 8 PM. For this reason, a 4″ slab is ideal for solar thermal mass. If the heat is moving in one direction only, such as in a Trombe wall, then 8″ is ideal.

Because a house does not require additional thermal mass until the south glazing area exceeds 7% of floor area, a rule of thumb is to allow for 6 square feet of direct-gain 4″ thick mass for each square foot of south glass beyond 7%. Thus, a 1,000 square foot house with 120 square feet of south glazing (12% – the maximum for passive solar) would require 300 square feet of slab floor available to the sun. That would be 60% of the south half of the house, or no more than 40% of the floor blocked by furniture and coverings.

If that small house was super-insulated as well, even in the cloudy northeast it could get close to 50% of its annual heat requirement for free from the sun. For the small incremental cost of additional south windows and careful design, coupled with a highly-efficient thermal envelope, the heating costs can be more than halved. Upgrading from energy-code standards to super-insulated and passive solar might add 5% to the construction costs of the home, but the energy savings more than offset the additional mortgage payment so the payback is the first month that the bills arrive.

If the passive solar design is complemented by a radiant heating system (made more cost-effective by supplying domestic hot water as well), then sunshine or clouds, the floor will be warmer than room temperature. Human thermal comfort requires warm feet and cool heads. Most heating systems – particularly forced hot air – cause air temperature stratification, with the ceiling warmer than the floor. This detracts from occupant comfort. A radiant floor, however, whether solar or mechanically heated, increases the mean radiant temperature of the living space, which has a greater impact on human comfort than air temperature. What makes radiant floor heat more efficient is that the thermostat can be lowered a few degrees without any sacrifice in comfort. One thing to avoid with a radiant floor, though, is a setback thermostat. There is too much thermal inertia, or lag time, to make changing the floor temperature a strategy for saving energy.

Some building experts discourage the mixing of passive solar with radiant floor technology because this lag time can make it more challenging to maintain uniform temperatures when the floor is warming from below and then the sun comes out from behind the clouds. Even if the thermostat shuts off the radiant circulation, the heat already in the floor will continue to emerge while the sun is also heating the space. But this can be an asset because, since the sun is raising the slab temperature, there will be less heat exchange from the radiant tubing in the south half of the slab and more heat available to the north half which doesn’t have the benefit of the sun. This selective heat redistribution function would be most efficient if the radiant tubing was looped in a north-south orientation.

Throw in an efficient, outside-air-coupled woodstove, and the balancing act becomes more delicate. But the learning curve for occupants who have chosen this mix of heat inputs would be short, and the benefits should outweigh the liabilities. With or without supplemental radiant floor heat, a passive solar slab can be a cost-effective multiple-function element of a well-designed and energy-efficient home.


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by Robert Riversong: may be reproduced only with author attribution for non-commercial purposes and a link to this page

4 Responses to 16. Passive Solar Radiant Slab

  1. Gerald says:

    A sincere thank you. This is a well-written and very informative article. I stumbled across it in my Google search results. I’m looking forward to reading ‘the other 23’. Have a great day!

  2. Kris Koppy says:

    Thank you very much. Why aren’t all houses being built this way?

  3. marshallbart says:

    Really excellent blog, Robert. Thank you very much. This piece brings together everything I have been researching about passive solar in one article and is the most well-thought out, comprehensive approach I have seen. Perfect for my needs. I am building in southern Colorado at 8000 feet–cold but plenty of sun.

  4. Mick J says:

    Hi, Robert, excellent article. FWIW I have translated it into French.
    Best regards, Mick J/France

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