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?
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.