For those who are interested, check out this in-depth discussion on passive solar design and pros/cons of radiant floor heat in low-load and passive solar homes: http://homeperformanceforum.org/discussion/11469
@Martin, I agree that with today's beyond-code homes, there's less benefit with passive solar design. However, it doesn't necessarily add cost. You make the valid point that mass is usually not cost effective. However, a low-load home doesn't require much mass.
A south-facing walk-out basement will have more than enough mass in the slab. Ditto for slab-on-grade. Fully below-grade basements are more challenging, requiring window wells and some means for shading south glass from summer sun. I advise clients against crawl space foundations, to avoid the extra cost and complication of encapsulation, which adds no usable living space. I also advise against high-mass above-grade walls. They work great, but as you said, the same performance can be achieved for less with conventional construction.
Most of your arguments against passive solar fall apart when we acknowledge that homes like the one pictured do not represent 21st century passive solar design. In particular, low load homes typically don't require *any* additional south windows. Moreover, medium gain sputter-coat IG units (e.g., Cardinal 180) have SHGC's in low-to-mid 40's and u-factors similar to the best performing conventional IG units, so there's no longer a hit in terms of heat loss. But it sometimes takes extra legwork to source these coatings.
Favorable orientation, room layout and engineered overhangs remain key elements of passive solar design.
You wrote: "...because of the sun’s lower angle in the morning and the afternoon, overhangs don’t keep out all the summer sun."
In summer, the sun rises and sets north of E-W axis, so by the time the sun illuminates the south facade, it's too high to bypass a properly designed overhang. Conversely, the sun rises/sets south of E-W axis in winter so south facing windows are illuminated all day. That said, since warm weather lags solar geometry, window shades are an essential design element and are typically needed during late summer or early fall, depending on latitude.
As you know, I specialize in designing mechanicals and envelopes for low load homes. What you may not know is that more than half of my projects over the past 10 years have been passive solar by design -- including my own previous residence in Charlotte (http://veryuniquehome.com) and my next home in high desert of SE Arizona, which should break ground next month.
When properly done, passive solar design is a no-brainer. The only question is Why not?
David Butler, Building Systems Engineer
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