Is Passive Solar Design Irrelevant? 6

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Thursday, January 28th, 2016
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Passive solar design principles have been recognized for decades and are on display in Frank Lloyd Wright’s Solar Hemicycle designs, to cite one high-profile example.

Once lauded for providing “free heat,” passive solar design principles are now recognized as being less effective at maintaining indoor comfort than superinsulated buildings.

As a new generation of do-it-yourself builders got busy experimenting in the 1970s, passive solar design ideas proliferated. Builders claimed that attached sunspaces, solar collectors, trombe walls, earth ships, and many more ideas could keep a house comfortable with minimal or zero use of fossil fuels.

Superinsulated designs, likewise, saw intense development in the 1970s, and have come to the forefront of green building thanks to the Passive House standard. This demanding approach focuses on improving air-tightness and insulation, decreasing thermal bridging, and setting rigorous levels for energy use.

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Prescott Passive House

For a time, fans of both approaches debated their merits, but it’s clear that the superinsulated approach has won the battle. Hefty amounts of insulation coupled with rigorous air sealing result in a house that uses less energy and remains more comfortable than passive solar houses.

Martin Holladay, writing for Green Building Advisor, examined this topic and admitted that, though he sided with the superinsulation crowd, he still hung on to some passive solar ideas. Deliberate solar orientation and proper roof overhangs, for example, could yield energy savings if properly designed, he reasoned.

Lately, however, I’ve begun to wonder whether there is any technical justification for these recommendations,” he wrote. “Do these design principles result in energy savings? Or am I just dragging around the stubborn legacy of my hippie past?”

Passive solar design must be optimized for both cold and hot climates. In hot climates, shading of all windows is important, and builders should skip the sub-slab insulation. Reflective roofing is preferred, and roofs should be insulated to R30. In cold climates, windows should be arranged for maximum solar gain, while roof overhangs should be designed to let in the sun during the cold months, and block the sun in the warm months. Orienting the house along an east-west axis allows for optimal window placement in both cases. Thermal mass is required to maintain a more even internal temperature in both hot and cold climates.

Many of these ideas have a certain instinctive appeal, but according to Holladay, their performance is lacking.

For one thing, passive solar buildings never worked all that well,” he wrote. “Even back in the 1970s, they were cold on winter mornings and hot on sunny afternoons. But most solar enthusiasts were so excited by the idea of ‘free heat’ that we accepted uncomfortable conditions as a necessary part of the brave new solar future we were all busy creating.”

Windows, in particular, are not very energy efficient. More technologically advanced windows help, but they’re expensive.

At night and on cloudy days, large expanses of south-facing glass lose significantly more heat than an insulated wall,” Holladay wrote.

A better approach would be to place windows where they’re most aesthetically pleasing, and invest in a photovoltaic array for on-site energy harvesting.

Holladay refers to cold-climate buildings, but hot-climate buildings are also better-served by the superinsulation approach. A superinsulated and airtight home uses less energy whether it’s in a hot or cold climate, and maintains a comfortable temperature with less variation than a passive solar home.

In addition, passive solar design requires a specific orientation to the sun, which is not always possible in an urban setting. Plus, in the context of attached dwellings, the superinsulated approach is much more feasible than the passive solar approach, especially in an urban neighbourhood.

That said, some passive solar design features may be desirable for other reasons. Windows are necessary in superinsulated homes just as in other homes, so designers have the freedom to place them for optimal views, daylighting, and breezes. Generous overhangs provide shading for windows and a feeling of enclosure, and an east/west orientation is friendly for photovoltaic installations.

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6
  1. Peter Tomkinson

    If people want to live in the equivalent of an air tight styrofoam box, then a super-insulated living environment will suit them. BUT the Indoor Air Quality will be seriously un-healthy. Low energy is way too narrow a focus for a healthy living space.
    And it is so easily defeated, that it is a joke.

    As soon as people indoors want some fresh air as they will do in a sealed environment, the open window(s) and/or doors to achieve some airflow cancells the air-tight design feature. Alternatively, turning on the air-conditioning system or HRV to achieve some benefit can put a huge dent in the energy efficiency gains. And again defeats the design intent.

    Air tight super insulated living spaces are definitely not a place people should spend much time in. Not if they consider their health important, especially long term health.

    • Ed Davies

      Heat-recovery ventilation does not use a huge amount of energy – significantly less than would be used to heat air exchanged with the outside without heat recovery.

      Better indoor air quality is a good reason for having good air tightness combined with mechanical ventilation. Instead of having insufficient ventilation on some days (e.g, still ones) and too much on others (windy) it's possible to arrange to have the right amount for the current circumstances.

    • Anna Evangelisti

      I agree with Peter – I have designed and now live in solar passive house and have managed to survive one of the hottest summers in Perth. Sure there have been one or two days that were a bit uncomfortable but while the temperature reached 41 degrees outside I managed with an internal temperature of only 31 degrees. For the rest of the time I have enjoyed living in an open light filled house with beautiful cross breezes and the occassional need for overhead fans during the night.
      Anyway why does it have to be either/or – I have insulated my roof to 4.2 and walls to 3.5 and have exposed concrete block walls internally for thermal mass – a small heater keeps the place cosy in winter. All of this depends on the context, size of the house and orientation and getting the right balance can be tricky but is achievable with a good design. Peter is right – who wants to spend their time living in an esky totally isolated from the natural environment!

  2. Klara Marosszeky

    While superinsulated, rigorously sealed buildings may give us the best energy outcome, energy efficiency is surely only one of the things to consider. In determining that airtightness is the ultimate way to achieve energy efficiency are we forgetting to assess whether airtight buildings are healthy for human beings with all of the off-gassing furniture, finishings and fittings that are enclosed in them? If our buildings contained only natural materials superinsulated buildings may be the ultimate answer, but the data suggests that indoor air quality in most buildings is very poor. Passive design often goes hand in hand with using natural materials and it also incorporates natural ventilation and inexpensive options for heating and cooling. I agree that some passive design features may be desirable for other reasons. I also believe that it would be good to think more broadly on the pros and cons of airtightness. It may be the holy grail if people are not just being enclosed in buildings with a myriad of chemicals.

  3. philip pollard

    The article assumes that either people in hot climates are prepared to lock out the breezes and remain in a sealed box- albeit one in which it's somewhat cooler than the outside air – or alternately, they have to switch on the chiller and mechanical ventilation – which ups the energy consumption considerably- and still leaves the occupants in a sealed in a box. Air-to-air heat exchangers can be useful, but are dependent upon running mech' ventilation (at the least) or air-conditioning, and the living space remains isolated from the outside environment.
    Most people would prefer not to live in a seled-up box- especially when the outside conditions are reasonably comfortable- which is quite a bit of the time in temperate climates.
    I subscribe to a mixed approach- which includes a very well insulated "core" space or snug spaces that can be sealed and that, in more extreme temperatures of cold or heat, can be efficiently heated or cooled using heat pump technologies. Outside this operable core zone, are other spaces that are more spacious and which provide a more seamless interface with the outside world. Good orientation, use of insulation, strategic thermal mass, external shading of glazing and use of landscaping for additional shade & microclimatic cooling all assist.
    As noted by others, the "passive house" standards bring with them a need to look very carefully indeed at indoor air quality- which of course extends to furniture and furnishings as well as the building fabric itself.
    My observations and client feedback suggest that a combination of the best elements of passive solar design with the insulated sealed box approach, gives occupants the best outcome – critically maintaining an interaction with the environment.

    • Thom Bohlen

      I can address this issue of living in a tight and insulated box directly, having lived in a residential tower for almost 7- years with only one operable sliding door to a balcony, with the remainder of the insulated windows fixed. Even though fresh air is supposedly pumped into the flat mechanically, the flat still is uncomfortable at times. A blend of passive design and passive house insulating and sealing techniques is the true answer. This argument reminds me of the 70's during which the design community was convinced to build windowless schools, only to discover that even though energy was saved, kids were getting sick and poor learning ensued. All of these schools were later remodeled to bring in natural light and ventilation, as is now the general rule in school design. Energy reduction must be balanced with human comfort.