The drawing above, or some version of it, has been part of every sustainable design class since about 1970: have lots of south facing windows carefully shaded by properly designed overhangs, with the winter sun heating up that thermal mass of the floor. Frank Lloyd Wright did it; I did it; everybody did it. But what if we were all wrong? Over at Green Building Advisor, Martin Holladay looks at what was almost a religious doctrine and questions its tenets, writing:
...certain aspects of the passive solar approach — an emphasis on careful solar orientation, a concern for proper roof overhangs on the south side of a house, and a preference for south-facing windows over north-facing windows — seem embedded in my DNA. Lately, however, I’ve begun to wonder whether there is any technical justification for these recommendations. Do these design principles result in energy savings? Or am I just dragging around the stubborn legacy of my hippie past?
And indeed, when Martin looks at what has been happening lately, he finds that high thermal mass floors are not particularly comfortable, that south facing windows as an energy source are counterproductive and “should be limited to that necessary to meet the functional and aesthetic needs of the building.” That careful orientation doesn't really matter any more because nobody needs that extra solar gain.
While large expanses of south-facing glass help heat up a home on a sunny day, the solar heat gain doesn’t come when heat is needed. Most of the time, a passive solar home has either too much or too little solar heat gain, so much of the solar heat gain is wasted. At night and on cloudy days, large expanses of south-facing glass lose significantly more heat than an insulated wall.
What's changed? Insulation and sealing. Holladay quotes building expert Joe Lstiburek:
We were here in the late 1970s when ‘mass and glass’ took on ‘superinsulated.’ Superinsulated won. And superinsulated won with lousy windows compared to what we have today. What are you folks thinking? Today’s ‘ultra-efficient’ crushes the old ‘superinsulated,’ and you want to collect solar energy? Leave that to the PV.”
Now this is not the first time we have discussed this on TreeHugger; Alex Wilson of BuildingGreen came to the same conclusion a few years ago, going from imagining in the 70s " in our youthful idealism, that within ten years all new houses would be oriented on East-West axes and rely on south-facing windows and thermal mass for heating."
It's a different world today, with triple glazing, low-e coatings, and gas fills pushing center-of-glass window R-values above R-8 and insulation levels commonly reaching R-40 for walls and R-60 for ceilings—at least within the green building community.
Over the last year I have certainly gone through a conversion myself, from Grandma's house to Passive House. I have even accepted that in a properly designed house, air conditioning isn't necessarily evil.
There are still good reasons to practice some of the things we used to preach; as Martin notes, an east-west orientation is great for solar panel installation on the roof. Windows can frame great views and sunny rooms are nice to be in. But in the end, we have to accept that the world has changed.
The new doctrine: high quality windows, tons of insulation, a tight seal and hey, while you're at it, Passivhaus certification.
Just to make us all feel even worse, Bronwyn Barry points to a 1978 study that compared the Saskatchewan Conservation House (superinsulated) to a Passive Solar design (mass and glass) of the period, and the conservation house won hands down, hiding in plain sight.