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Energy Solutions

Resilient Design: Passive Solar Heat

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.
Image Credit: Alex Wilson

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 to 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.


  1. RobFisher | | #1

    Alex, do you have any
    Alex, do you have any suggestions for good books on passive design?

  2. user-1017420 | | #2

    Passive Solar
    Yes Sir! I agree. I invite you to look at our latest blog ( regarding a house we are building on the Italian Alps. I would have no fear of leaving this house without power for months if necessary. I do believe that most aspects of passive (or near passive) are simple common-sense and the firm belief that you cannot and SHOULD not cheap out on building a house.
    You may contact me at [email protected]

  3. user-651028 | | #3

    Great topic Alex.
    I've been thinking a lot about passive survivability since you brought it up a few years ago, Alex. Passive solar and high levels of insulation / air sealing are a perfect combination to accomplish this.

    I'm an architect living in an underground home in NH with a wall of south-facing glass. It was done in the '70's by the previous owners (with help from Malcolm Wells) using by-guess and by-golly engineering. Check it out at

    When it gets too hot in the winter we open the doors a crack and get fresh air; nice problem to have. In the summer there's a fabric awning which keeps us cool. We don't have a thermal mass to keep temps from cycling, we do it by opening and closing doors at appropriate times. When there's a power outage we can easily keep from going below 60’s with just our little wood stove insert at night and on non-sunny days. Even if we didn’t fire it up nothing would come close to freezing. Keep it simple. Just don’t use low SHGC glazing.

    Thanks for writing this Alex

  4. metamerman | | #4

    Another passive solar scam (i.e., greenwashing).
    Argh. Once again GBA publishes an article promoting passive solar design without an mention of movable insulation or the fact that without it wintertime heat loss through those large windows overnight and on cloudy days will exceed heat gain on sunny days. While passive solar design may keep pipes from freezing in optimal climates like New Mexico, and so be an example of "Resilient Design", large windows without movable insulation is just a recipe for wasting energy and uncomfortable occupants (or possibly even frozen pipe disasters) in more demanding climates. Better to install a woodstove and only the amount of windows necessary for lighting and/or ventilation in those areas, IMHO.

  5. ecdunn | | #5

    Reply to Scott Raney
    Scott, I think you missed the point Alex made about the need for mass inside that both receives direct sun and even "mass the sees the mass that sees the glass". Thermal mass is critical for comfort of the occupants and moderating the temperature swings throughout the day. As to movable insulation, that introduces the possibility of air currents that will actually carry heat away as it has been pointed out in GBA articles about energy myths. Insulation, insulated curtains with radiant barriers, etc need to be tightly seal to keep those drafts from happening. I don't know where you are but it looks like David Ely is up in NH, far from NM, and it still works for him. As he mentioned, tightness and optimum insulation are where you start in a passive solar design. Just doing those two things right will give you a head start on bringing in the sun's energy.
    Comfort is the big plus from a properly designed and built passive solar house. I sell the idea based on the aesthetic value of cold climate comfort. Comfort is something the client is willing to pay for more than saving money on heat or saving the planet.
    As to insulation values. follow the guidelines from Oak Ridge National Labs, not the Model Energy Code. When the MEC was first published it had the same values as ORNL but was later down graded. I wish I knew why.

  6. Alex Wilson | | #6

    A new book on passive solar
    Rob Fisher asked about good books on passive solar. There's a new one out that I highly recommend: "Passive Solar Architecture: Heating, Cooling, Ventilation, Daylighting, and more Using Natural Flows," by David Bainbridge and Ken Haggard. I've known David and Ken for probably 25 years, and I think they've both been designing passive solar homes in California for over 30 years. The book is published by Chelsea Green Press (2011).

  7. debbie_at_sunplans | | #7

    Passive Solar Books and Software
    It is so refreshing to read an article about simple, inexpensive, common sense passive solar. Solar hot water and PV panels can of course be added to passive solar designs on a case by case basis determined by the building owner's lifestyle, budget, and priorities along with the tax incentives for the particular location.

    Regarding books, thanks for the reminder about the new book on passive solar. In 2007, I last updated my book on passive solar: The Sun-Inspired House: house designs warmed and brightened by the sun. Martin Holladay gave it a favorable review. The basic principals of passive solar remain the same and I was pleased to see this mentioned in the article.

    Regarding software, a few very basic comments.
    1) Energy-10 is not very friendly and has not been updated in a while.
    2) Energy Plus is very complex.
    3) REMrate only models passive solar with sun spaces and not direct solar gain. Actual results of passive solar homes can be much lower than what the program shows. Some Home Energy Raters raters have told us they know how to tweak the data to make it moreaccurate, but I don't believe that is common knowledge. In speaking with the developers of REM Design about this, they seem to be concerned which means hopefully in the near future the software will more accurately reflect passive solar which would be great since it is my understanding that approximately 95% of HERS raters use their similar REM Rate software.
    4) I really like BGW2004 even though the name reflects the date last updated. It evolved from the software developed by Doug Balcomb in the 1980's in conjunction with the Passive Solar Industries Council (now the Sustainable Industries Industries Council). Originally it was a manual method for builders. I developed a spread sheet to speed the process, then they developed a DOS version, and finally Fred Roberts put it into Windows. I assisted him some with the user interface with the goal of making it user friendly for the average architect, designer or builder. To me it has the same simplicity that passive solar ought to have. I especially like how the program show the percentage of annual heat obtained by the sun. (It also shows cooling savings, but does not address hot water, lighting or auxiliary heating.) The heating and cooling results seem to be very close to a home's actual performance. (I have no financial interest in this software)

  8. Alex Wilson | | #8

    Software for modeling passive solar
    Thanks Debra for this detail on software. I'd love to hear other thoughts and experiences with software packages that do a good job with passive solar. What have other readers found most useful?

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