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Passive solar design is a key element of creating resilient homes.

A passive solar home in Halifax, Vermont. High-SHGC, triple-glazed, south-facing windows were used to improve the direct-gain passive solar performance. Click on image to enlarge.

As I discussed in last week's blog, a resilient home is extremely well-insulated, so that it can be kept warm with very little supplemental heat--and if power or heating fuel is lost, for some reason, there won't be risk of homeowners getting dangerously cold or their pipes freezing. If we design and orient the house in such a way that natural heating from the sun can occur, we add to that resilience and further reduce the risk of the house getting too cold in the winter.

Passive solar heating

I had the good fortune of working in Santa Fe, New Mexico for a solar energy organization in the late-1970s, when the passive solar energy movement was just emerging. Northern New Mexico was the epicenter of research into passive solar--the effort, ironically, being led by Los Alamos National Laboratory, which, a generation earlier, had brought us the nuclear age.

It was an exciting time. The relationship between solar gain and thermal storage was becoming understood. It was discovered that very simple south-facing windows and high-mass walls and floors were not only far simpler than the very complex active solar heating systems that emerged (briefly) in the early 70s, but they also worked better. Direct-gain passive solar

The most common passive solar heating system is known as direct-gain. South-facing windows transmit sunlight that is absorbed by dark surfaces of high-mass materials in the house. In a sense, the house itself becomes the solar collector and heat storage system, with different components serving multiple functions. Those windows also provide views to the outdoors and bring in natural daylighting, while the thermal mass consists of the walls or floors that serve structural functions. We need those elements anyway, but by optimizing their area, placement, and configuration, they can become the primary heating system.

The challenge with direct-gain passive solar heating is to provide the right amount of glass in the proper orientations and incorporate the proper amount of thermal mass to minimize temperature cycling and prevent overheating. (Back in New Mexico in the late-1970s, there were a lot of poorly designed passive solar homes that overheated horribly.)

As window glazings have improved in the three decades since my days in New Mexico and as we have recognized the primary importance of highly insulated buildings (see last week's blog), the opportunities for passive solar heating have improved--but so has the complexity. With better glazings and reduced heat flow out of homes, one has to be more careful to prevent overheating or unacceptable temperature cycling. And we have to choose glazings more carefully, because the most insulating low-e glazings block too much of the solar gain. For passive solar, we want glazings with high solar heat gain coefficient (SHGC) ratings--values over 0.6 are great, but 0.5 should be considered a minimum when passive solar heating is important.

Fortunately, as the complexity has increased, the computer software tools for modeling energy performance of homes with significant solar gain have also improved. Such programs as Energy 10, EnergyPlus, and REM Design all do a good job at modeling energy performance and passive solar contributions to heating. With any such software, the designer inputs a location close to where the house is located to load the relevant solar gain and other climate data. Note that even with state-of-the-art software, hiring a designer with experience in passive solar design is key to achieving good performance.

Trombe walls

Direct-gain is the most common passive solar energy system, but it isn't the only one. With indirect-gain passive solar, the collection is only indirectly connected to the living space. The most common such system is a Trombe wall--a south-facing high-mass masonry wall with glass or plastic glazing held away from the wall in a frame. Sunlight shines through the glazing and heats the dark surface of the masonry wall. Heat moves into the wall where it is stored and gradually conducts through to the interior, where it radiates heat to the living space.

Some experts question whether it's better to simply add more insulation to that south wall and skip the indirect solar gain, while others argue that the solar is very important--especially relative to resilience. If other energy inputs to the house become unavailable for some reason, delivering heat with a Trombe wall could be very beneficial.


Finally, there are isolated-gain passive solar systems in which solar heat is collected in one place and brought into the house only when desired. A south-facing attached sunspace is the most common isolated-gain system. The sunspace heats up during the day and windows or vents connecting the house and sunspace can be opened to deliver heat into the house, or kept closed to keep that heat out. An insulated wall between the house and sunspace ensures that as the sunspace cools off at night (due to heat loss through the large amount of glass), it won't cool the house down. The sunspace serves as a heating system for the house, even as it also serves as a supplemental daytime living area and a place to grow plants (especially plants that can accept significant temperature cycling).

Passive solar and resilience

No matter which type of passive solar heating system is employed, it plays a key role in making a house resilient to power interruptions and loss of heating fuel. If there is no solar gain, even a highly insulated house will gradually cool off. The more insulation, the slower the temperature in the house will drop, but drop it will. With a reasonable amount of passive solar gain and a really well-insulated building envelope, enough heat will enter the house to compensate for most of that heat loss in all but the cloudiest weather.

In this resilient design series, I'm covering how to achieve resilient homes and communities, including strategies that help our homes survive natural disasters and function well in the aftermath of any event that results in an extended power outage, interruption in heating fuel, or shortage of water. We'll see that resilient design is a life-safety issue that is critical for the security and wellbeing of families in a future of climate uncertainty and the ever-present risk of terrorism.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.

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1 That extra coil in your air h posted by Robert Riversong on 01/11/2012 at 11:41 am

That extra coil in your air handler is for anything that heats water. Look into a solar thermal (solar hot water) panel system, which could be effective if you are in a region with lots of available sunshine in the heating season. When the sun is shining and the house is not calling for heat, you will need a place to "dump" the extra heat in the panels, so heating or preheating your domestic hot water would be the sensible option. That would likely require an additional hot water storage tank but the total cost should be less than the cost of an outdoor boiler (which tends to cause near-ground smoke hazards).

2 You present direct-gain, indi posted by Robert Riversong on 01/10/2012 at 08:38 am

You present direct-gain, indirect-gain, and isolated gain strategies as equally efficacious, when the net efficiency of the three approaches declines with increasing complexity. As with so many technologies, the simplest is often the best. Direct-gain solar heat is somewhat more efficient than indirect-gain, and markedly more efficient than isolated-gain. In both the case of a Trombe wall or a sunspace, optimized efficiency often requires a powered fan to move air to the interior of the conditioned space before the additional heat loss to the outside reduces the benefit. The use of a fan, of course, diminishes the resilience of the system. You also fail to note that an important element of power-out resilience in a cold climate that often offers little sun in the mid-winter months when it is most needed is earth-coupling. As you correctly suggest, even the most well-insulated house will eventually lose heat until it reaches equilibrium with the freezing outdoors. But a house on a slab that is not overly thermally isolated from the ground will equilibrate to a higher (often above freezing) temperature, since ground temperature in winter is always higher than average winter air temperature. Just as we learned the importance of balancing solar glazing with thermal mass, we still need to learn the proper balance between thermal insulation and thermal coupling to the earth.

3 I should add that a fundament posted by Robert Riversong on 01/10/2012 at 08:43 am

I should add that a fundamental flaw in the solar design of the Halifax house pictured above is the lack of summer solar shading over the large first floor windows. It is easy to remedy that with an attached pent roof or trellis, but far too often aesthetics trumps function in modern design. An overhang at each storey also increases cladding and finish durability as well as decreasing probability of leakage by reducing exposure to wind, sun and rain.

4 We have a house built to maxi posted by Ronald Gist on 01/11/2012 at 05:53 am

We have a house built to maximize the cooling. It has 10' porches on all four sides and well insulated. As a result our cooling cost are very low and easy to keep the house cool. Looking for an effective way to add a solar factor to our heating. We have a extra coil in our air handler for an out door boiler, but the $6000 plus has that pushed back. Any ideas? thanks.

5 This aricle puts the finger o posted by Traonvouez on 01/12/2012 at 09:13 pm

This aricle puts the finger on the risk of overheating in passive houses, while at the same time solar gain would be welcome, and I agree fully. Comments by Mr Fergusson are no-nonsense: being thermally insulated from the soil, passive houses don't benefit from ground freshness in summer, nor from warmer-than-air soil in heating season if heating breaks down; a loop+pump in the soil would allow some action if connected manually with a floor heating network, but it is not resilient unless conceived to work as thermosiphon. Regarding solar direct gains in winter, the issue is on two points: energy is welcome but it's transformation in ambiant heat at high speed is a problem, so some form of short-term storage would be helpful. I have recently been in a low energy office for a meeting; after one hour I felt hot and said it to my host, who had conceived the building; to this remark I got the following answer: "I know, above two persons in my office it happens all the time". So, there are some serious issues in these buildings.

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