An alternative take on passive solar design

Architect Peter Powell has an interesting post on GreenBuildingAdvisor.com about passive solar design principles. Powell, who say he's designed over 60 "passive solar homes" over the last 35 years, challenges some of the most widely held principles of passive solar design. The full post is here, but I've selected a few snippets and copied them below.

Powell advises against using trees to shade south-facing facades, writing:

Using deciduous trees to shade the south elevation in summer is a major design error. This is a myth that won’t go away. Don’t do it! This theory holds that deciduous trees and vines will shade south-facing windows in the summer and reduce heat gain, while in the winter, when the leaves are down, sun will be able to enter and heat the house. It doesn’t work.

The problem is that the limbs of any tree tall enough to shade the windows in the summer will significantly block the lower winter sun, even with the leaves down. In my area (central Pennsylvania), most of the leaves don’t fall until November anyway, after there have been many cool days and nights when the solar gain would have been useful.

He also says that getting your building orientation 100% perfect isn't necessary...

I keep reading articles about how to locate true south, using everything from computer programs to measuring shadows through different seasons. Just get a simple compass and correct for declination if you must.

Orienting a house east or west of true south by up to 20° will have no significant effect on solar performance. For example, I currently live in two virtually identical passive solar buildings, a residence and a studio. The studio faces due south and the residence faces 25° west of south. By the end of a sunny day, the overall solar performance of the two buildings is almost identical, with the house performing slightly better in the spring and fall and the studio doing slightly better in the middle of winter.

...and that it's okay for designers to specify more south-facing glazing than most rules of thumb allow:

The area of south-facing glazing area can comfortably be 15-20%+ of total floor area as long as there is adequate storage mass and “active” movement of the heat into the storage.

To maximize solar gain while minimizing overheating, immediately move the stratified heat into storage. Don’t rely on natural conduction and convection to move the heat.

Most rules of thumb for south-facing glazed area assume that the storage mass is located only in the south-facing rooms, and they assume direct conduction of the solar heat into that storage. This method is relatively slow and inefficient, except for the limited surface areas which are directly in the sun, and often results in overheating. It fails to take full advantage of the available mass in rooms remote from the sun, as well as the advantages that occur when heated air is moved through hollow masonry walls or hollow floor slabs to remote mass storage.

A house with a larger-than-normal glazed area (which could be 30%+ of the floor area of the south-side rooms) can still be comfortable and productive if solar heat is moved mechanically out of these areas and circulated into other rooms and, most importantly, directly into supplemental storage mass.