HOW MUCH PASSIVE-SOLAR HEATING DO YOU WANT IN A WARMING WORLD?

Guest Post by Doug Hansmann

With daytime high temperatures staying in the 70s this week, we’re getting a much-appreciated reprieve from what’s been a roasting summer here in Wisconsin.  And as Denise and I finalize the orientation of the passive-solar house we intend to build next year, minimizing all possible summer solar heating  is definitely on the front burner.

So does that mean we’re waffling on a passive solar design?  Definitely not!   But we are going to do passive solar with a twist –  a twist, to the east.

All good passive solar designs call for facing a lot of windows, equaling 8 – 13% of the total square footage of your house, toward the south.  Check out Geoflo Energy Services  and Build it Solar for primers on the subject.

But what if your building site doesn’t quite face due South, or there are obstructions to the view of the sun during the day?  Our house site has a deciduous woods going up the hill to the east, which even encroaches on our exposure to the southeast sun on summer mornings.  Should we turn our long southern exposure away from this morning shade?  Should we turn toward it?  Does it matter?

Our building site, seen from the south west.

The books will tell you that turning 15 or 20 degrees away from due south doesn’t matter much.  I wanted to confirm this advice in more detail, so I pulled data from the US Naval Observatory tables for our building location.  You might also try SunPosition.info.

During the early and late hours of the day, the sun only enters through your windows at a glancing angle.  It’s as if your windows are only a fraction of their full size.   At our 43degN latitude, the winter sun rises a full 20deg south of due east, and it sets 20deg south of due west.  This means there will be little or no decrease in the amount of winter sun you’ll collect if your house is pitched as much as 20deg off of due south.  But, if you pitch to the east, your windows will be poised and ready to catch the first morning sun, because they will be presenting a bigger cross section to the sun, so you’ll warm up sooner in the day.

Full disclosure: Pitching to the east will also cause some loss of solar gain at the end of the day because your windows will be presenting a smaller cross section.  But in general, you’ll still get over 95% of the available solar gain relative to a due south orientation, with the added bonus of that early morning solar kick start.

And from Denise’s and my past experience with a passive solar sun porch on a house we retrofitted 30 years ago, it’s very possible to get too much solar heating during the course of a sunny January day, so I don’t think a small loss at the end of the day will bother us.

A hugely important aspect of passive solar design is to extend relatively long soffits or other shading devices above your windows  in order to deflect the hot summer sun.  But soffits have their limitations, and I’d be lying if I told you that a perfect passive solar design is possible for every day and every weather pattern.

To drive home this point, consider that the solar gains and solar shadings are equivalent in early May and early August.  At our site, early May temperatures generally don’t call for heating or cooling.  But even a little solar gain in a sweltering, early August like we just had would NOT be welcome.

So in early August, how’s that 20deg pitch to the east or west of south going to work for you?  If you pitch to the west, you’ll avoid some early morning solar heating because your window cross sections will be smaller for the low sun that slips below even the longest soffit.  But you’ll suffer mightily from low afternoon rays unless you’ve got some trees or other shading over there.

With natural shading to the east, our site calls out for the opposite – a pitch to the east.  Not only will those deciduous trees block the low early-morning summer sun, but the low late-afternoon sun will be comfortably past our windows by mid afternoon, when the outside temperature is peaking.

So while there won’t be much loss in solar heating for us in the winter with this twist to the east, and in fact we’ll warm up a bit sooner with this orientation as solar power reaches us through leafless branches, we’ll also stay cooler when the woods to our east leaf out in the summer.

Depending on your latitude and your specific site profile, your analysis may be different than ours.  But with a warming planet, summer heat waves are going to become more common, so avoiding solar gains on summer afternoons is definitely worth planning for.

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8 replies

  1. And then, of course, there’s also some sort of window shade that can be pulled down or covered over that section of late afternoon sunny window in summer. In my mind, it’s much more important to have the warmth in the winter. But then, I’m finding that as I get older, being warm is MUCH more important. (Which is probably why so many people go south during the winters around here.) :o)

  2. Hi Monique.
    Very valid comment about window shades. We plan to have insulated curtains for the windows for winter nights and hot summer afternoons.
    Being warm can be very comfortable, but you can get too much of a good thing in that department. I sure enjoyed the cooler temperature today better than our warm weekend.
    That slightly cooler temp today made all the difference as Doug and I worked with the tree prep crew cutting branches and peeling bark off selected trees today. I know I worked as hard as I did when they were here before, but did not have to wring the sweat out of my clothes at the end of the day, and somehow don’t feel quite as beat tonight.

    • Hi Dennis,
      I’m far from a pioneer woman. Would a pioneer woman blog? But I do admire our pioneer forbears for their willingness to get in there and sweat for what they wanted.

      My architect/daughter was just telling me this morning that no one in the history of their company has every leaped down into the trenches with them to work on all the phases of the building project as we have. But I find I always appreciate things more if I truly understand how they come about, and there are so many interesting steps in this process.

      I will say that I feel like we feel a bit like pioneers in choosing to build in new/(sometimes old) ways that could be part of the energy/environmental solutions that we need to find. Some people have got to step into new territory and be early adopters of building with unmilled timbers and straw bale and sod roof, etc. if these techniques are going to get a chance.

      Long answer to a short question. Nice to hear from you.

      • One of the biggest frustrations with being a pioneer in novel situations is not being able to find what you know will be useful. Big agricultural machinery fuelled by European Union grants means trying to source more appropriately scaled equipment is really hard. Might have to get some things made, is that a gap in the market or a market needs to be created for small things which don’t damage the ground so much?

      • Very good comment, Joanna.
        We are on the same path, but behind you there. Simple equipment that requires a minimum of fuel and compacts the earth as little as possible seems like the best plan in so many cases, and yet is hard to find.

  3. Oh, and you do follow the ArchDruid Report by John Michael Greer, don’t you? For the past year he’s been talking about many different lo-tech ways of reducing one’s energy consumption and improving one’s ability to deal with the coming disruptions of society. I wonder if we aren’t seeing the early results of increasing oil shortages in the Arab Spring, London riots, and political deadlock in the U.S. JMG is a very good writer and quite provocative, with many good comments by his readers. And they are not inflammatory or dug-in debates: JMG insists on that old-fashioned thing called civil courtesy. So his blog is a pleasure to read. I’m amazed by the things that people are doing to “adapt in place” (Sharon Astyk’s phrase).

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