By Peter Powell, AIA
I have designed over 60 passive solar homes over the last 35 years and have lived in seven of them. Based on that experience, I have come to a few conclusions which, although contrary to conventional passive wisdom, I have found to be valid. I must qualify these comments by saying that most of my experience has been in the Northeast, primarily in Maryland and Pennsylvania, in areas with 4,000 heating degree days and up. The following comments mostly apply to new construction in similar or colder climates.
Overhangs to shade south-facing windows are ugly, unnecessary, and counterproductive
You don’t need them. The theory is that overhangs should be designed to provide full shading of the south-facing glass in the summer and no shading in the winter. Part of the problem is that most of the standard summer designs for these overhangs are based on June 21 at noon. Unfortunately, this is neither the hottest time of day nor of the summer.
Likewise, winter designs are usually based on achieving no shading on December 21, which is certainly not the coldest day of the winter.
In my experience, these overhangs are not really needed. Because of the high solar altitude in the summer, over half of the solar radiation striking the glass is simply reflected away; another significant fraction of the solar radiation is rejected by the glass coatings.
Most overheating problems in the summer are due to east and west windows — window which can’t easily be shaded and which obviously should be minimized in the design.
The usual reason for overhangs is supposedly to reduce air conditioning costs. I contend this is a red herring, in part because AC is grossly overused in houses. We don’t need to maintain precisely 75°F year around, as some of my clients seem to want. (I even had a client once demand that his house include no operable windows.)
The easy solution to most summer overheating is to open windows. The major cause of summer thermal discomfort is caused not by excess solar gain but by high ambient temperatures and humidity. We shouldn’t throw out the baby with the bathwater by installing overhangs which are counterproductive most of the year and only marginally helpful in August (at least in my climate).
Solar gain in the spring and fall should generally be maximized and the heat stored to minimize any overheating. Overhangs, which by design will be shading as much as half of the glass in these seasons, should be avoided.
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.
Any new house site should be evaluated with one of the solar path devices to ensure that NO shading will occur within 45° or more east and west of the south elevation, any time of year.
A related problem with tall trees to the south is the likelihood of shading any rooftop thermal or PV collectors in the winter when the sun is low. Such shading can be extremely detrimental to their performance.
A house doesn’t need to be perfectly oriented
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.
In general, I favor orienting a passive house at least 10° west of south to improve late-day heat gain while slightly increasing east-facing window area to improve early-day warmup.
It’s possible to have more south-facing glazing than allowed by most rules of thumb
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.
I have designed a number of homes using this approach, which involves using a thermostatically controlled fan (usually the furnace blower) set to start when the temperature at the ceiling or ridge of the space rises above a set point of about 75°F.
This same system is also useful to store and distribute excess heat on days when a woodstove is being used. I have used a number of hollow floor systems for this storage including Airfloor, Flexicore precast planks, and most often a double 4 in. concrete slab with 2-in. EMT at 8 in. on center sandwiched between and connected to a supply plenum. (This forced air radiant slab can also be used for air conditioning if necessary.) Pulling stratified air through a hollow 6 in. or 8 in. concrete block wall is also very effective since all rooms exposed to either side of the wall benefit from the radiant heat. This hollow wall storage is also very effective when the site is north facing with no opportunity for significant south glazing. A design with south-facing clerestory windows can work as sort of a hybrid active thermal collector with the heat then mechanically stored in the mass wall.
Peter Powell is an architect in Rays Cove, Pennsylvania.