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Passive Solar Part II – Reduce your home heating costs

If you are looking to build a new passive solar, energy-efficient home or would like to see your existing home benefit from some much-needed energy improvements, it is important to work with an home designer who is experienced in passive solar design strategies. I have designed and built many homes and remodels that see great savings in heating costs due to simple, yet skillfully implemented, passive solar strategies. The photo to the left is the living room of Liberty House, a small (why small see this link), sustainable, energy-efficient home that I both designed and built. The photo illustrates passive solar concepts in a nutshell: sunlight pours through a large South-facing window array, landing on a dark concrete floor where the solar heat is stored.

Although, passive solar design is simple in methodology, a designer who is experienced in passive solar strategies will ensure that your home’s solar potentials are utilized most effectively. The key principles of passive solar design are as follows:
1. Building site properties must be evaluated and exploited.
2. The home must be designed so that it captures the solar radiation.
3. The home has a floor plan that is fairly open in order to promote convective air cycles.
4. Building materials must increase in mass for heat storage. i.e. masonry, concrete, stone, thick tiles.
5. Mass is in correct location in regards to solar aspect.
6. Windows or apertures are at right heights and face due South.
7. Building overhangs to the south are deep enough to shield the sun from the interior during the summer months to prevent overheating.
8. An aggressive strategy of cross ventilation through windows is designed into the building.
9. Windows are glazed with the correct type of glass. This type will change depending on its aspect i.e. facing North, East, South, West.

Every building site has its potentials and weaknesses in regards to utilizing the sun’s energy. Sites that are sunny and fairly free of tall coniferous trees, especially to the southern aspect, have great passive solar potential. During the Summer, deciduous trees on the south side of the home help screen the sun in the summer to reduce excessive solar gain. During the colder months, deciduous trees drop their leaves, allowing the lower angled suns rays into the home.

Hills and mountains can either be an attribute or a hindrance to passive solar energy gains. An ideal situation is one where there is a hill to the west or east, but definitely not a mountain to the south. As this diagram shows, the sun angle in North America changes in its trajectory and angle through out the year. In the summer, the sun rises more to the Northeast and sets more to the Northwest. During mid day, the sun is directly overhead. In the winter, the sun moves to the South. It is due East as it rises in the morning and due West as it sets. Also, the sun angle is lower in the sky at noon, which allows maximum sun penetration into the home. A hill to the West is not necessarily bad, because the focus is to harvest heat mid-day during the winter. A hill will help keep the lower angle afternoon suns rays from penetrating into the home and reduces overheating and solar glare. This is also true for coniferous trees to the West. The same situation applies to hills and trees to the East, although it is not quite as important. In the morning, the temperature in and out of the home is a bit lower, so more sunlight not only helps to wake the inhabitant, but also helps to heat the home.

When designing a passive solar home, the main goal is to maximize solar gain to the south. The floor plan or layout is very important in achieving this. Ideally, your main open space is on the south side of the building. Having the living room, dinning room, and kitchen as a combination open space not only gives a spacious feeling, but also creates an ideal situation for direct solar gain. In these spaces it is common and sensible to have hard floor surfaces for easy cleaning. More importantly, these hard surfaces become the heat mass that capture and store sunlight from the south. Usually, more open living spaces allow for enough solar gain to occur and with a good layout, furniture can be kept clear of these surfaces. Note: Darker floors will increase gain, lighter colors reflect the radiation, which is not desirable for solar gain.

Making direct radiant gain work is dependent on the flooring mass. It is important that window heights are close the floor, so that the sunlight falls directly on it, without being blocked by soft-scape obstructions like sofas and chairs. Tables are okay because the sun usually will filter around the table legs. It is important to avoid floor rugs in this area. A minimum of 1.5″ of concrete slab thickness as the flooring works very well because it holds sufficient heat. The slab can also have radiant hydronic heating coils, or “Pex,” laid into it. It can be applied over typical 3/4″ plywood subfloor as long as the joists span meet the increased load rating. On some South-facing floors, it is possible to pour slab on grade if the elevation of the building works out correctly. This can be poured up to 8″ thick, although this is considered almost wasting material. The slab must be insulated to the ground with hard insulation or the floor will wick cold in and heat out. Proper insulation below such radiant gain surfaces is a must. Again, heating hydronics in a slab makes sense because relying solely on the sun’s energy can be difficult when the sun doesn’t shine for extended periods of time. Through combining passive solar heating and hydronic slab heating, there is much to be gained. Hydronic coils bring heat back into the system, which flows throughout the floors of the home, helping to reduce the need to frequently run the boiler. If relying on mass alone, it must be thicker. I know folks who live in Northern climates that rely on an insulated 6″ concrete slab with southern glazing and just a wood stove. They live very comfortably through the coldest months. The concrete slab stores solar heat so well, they often don’t have to light their wood stove for days.

How the mass works to your advantage has to do with its ability to store the heat and then emit it throughout the night-time hours. If concrete is not a desired interior floor, it can have masonry tiles or stone laid over it. Even a typical plywood sub-floor with 1/2″ of durarock then overlaid with tile is better than no mass at all. This is common in remodels of homes for achieving more radiant gain.

As said before, it is preferable to have the windows close to the mass floor as this gets the sun’s radiation to land directly on the slab closest to the glazing. It is advantageous to use specially-designed glazing on the South side of the home. Double layer thermal pane glazing with a low emissivity coating are the norm for passive solar gain (L-e 173). Note: The higher the Low E number, the less solar heat gain you will get, but it will have better U-value, or insulative properties. Higher series such a 300 and up Low E is preferable on the aspects of the building where one wants to reduce heat gain, increase insulative properties, and reduce ultraviolet radiation destruction of furniture and soft materials.

Cross-ventilation in a passive solar home is very important. Height differences of walls across the room promote cooling and air convection. I found that by having awning windows low to the floor on the South-side and a high awning windows on the highest walls, helps to promote a air draft that encourages air flow and convective cycles. The advantages here are very important because in the summer it encourages passive cooling or evaporative cooling. This is free air conditioning. Air pressure differences due to hot and cold differences in the building allow heat to escape through the awning windows. Air inlets create an upflow that encourages evaporation of moisture on our skin and gives the sense of cooling. Rather than spend a bunch of money on air conditioning, spend the money operable windows and a home designer with experience in convective design. You will save you money in energy costs!

In a nut shell, a home does not need to employ expensive and energy-consuming mechanical systems in order to heat and cool. The heat gained from a passive solar strategy will reduce yearly heating costs. By hiring a skilled designer to properly configure glazing, ventilation strategies, window layouts, and appropriate flooring mass, it IS possible to achieve a net zero home heating system. At Greenovision, we are passionate about passive solar design and other energy-efficiency systems and have a lot of experience implementing these strategies into new and remodeled homes. Please come to Greenovision for see how how passive solar design can save you money. See reasons to get off the corporate energy grid on my blog Part 1 of Why Passive solar?.

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3 Comments

  • Mechteld Abelli on Mar 07, 2013 Reply

    Maximizing the use of natural light and ventilation is an excellent way to reduce HVAC expenses. Incorporating this concept in a home’s architecture is brilliant. I once visited a house that was constructed with this concept in mind, and it was beautiful.

  • RobH on Jan 29, 2013 Reply

    The hydronics and the passive gain would seem to work against each other because the floor will produce heat regardless of the gain from the sun. How can we avoid overheating on a day with lots of passive gain?

  • Mark@greenovision.com on Dec 30, 2012 Reply

    Hi Rob, The boiler heats the fluid to keep up with the thermostat setting in the main part of the room (away from the heat gain or out of the sun). There are sensors in the slab that establish the slab temp. The boiler is computer controlled and with a bit of monitoring and tweaking you find a ‘sweet spot’. The fluid being sent back to the boiler is essentially preheated. The swing in temperature is very slow due to the mass of the slab having what is called ‘thermal lag’. But nevertheless the fluid is being heated slightly by the warming slab and is being sent through the tubing to rooms where the slab my not be in the sun and colder. Its a mechanical form of convection is kind of how I envision it.

    Hope this makes it a bit clearer.

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