Passive Solar is an architectural design technique that uses direct sunlight for heating. By wisely orienting your home and proper use of window design and building materials you can drastically cut, or even eliminate, your heating bills. In this post I will re-introduce you to this ‘lost art’ of building design.

Passive solar design was used by the Greeks thousands of years ago, long before the invention of modern heating. It is somewhat of a lost art in the modern world, due to the “brute force” methods of gas and electric heat. Today’s planned-urban-developments (PUDs) and track homes leave no room to properly orient a home for optimum sun exposure; each track home’s placement is subservient to the street curb or cul-de-sac clocking. However, with today’s sustainability movement and rising energy prices, passive solar heating deserves more attention.

Track Homes Don't Care Where the Sun Is

Track Homes Don’t Care Where the Sun Is

In a nutshell, passive solar design exploits the fact that summer sun and winter sun have vastly different sun angles. Winter sun is low on the horizon; whereas summer sun is high overhead. By orienting the windows on the south side of the house, you can capture the winter sun’s rays for heat during the day. Absorptive surfaces collect this thermal energy during the day, and then release it during the night. The summer sun is shaded by awnings, verandas or roof overhangs, etc.

Passive Solar Direct Gain Method

Direct gain is the most common, simplest method and the least obtrusive. The sun’s rays directly heat the interior surfaces (walls and floors) of the home. These interiors surfaces need to have ‘thermal mass’ to collect the heat energy. This can be done simply with dark colored masonry construction (not lumber and sheetrock). Through the cold night, this thermal energy is released into the home. The next day, the cycle starts again.

DIRECT METHOD for Passive Solar

DIRECT METHOD for Passive Solar

Passive Indirect Gain Method

The indirect method uses a thermal collecting mass between the sun and the area to be heated. This method is architecturally more challenging, but when done right, is more effective. The most basic indirect method is to have a masonry wall right up against the window (aka ‘glazing surface’) to most effectively collect the thermal energy. This does mean the window is blocked and not a good light source. A refinement of this technique is to use a water wall instead of masonry, as water provides superior thermal storage.

INDIRECT METHOD for Passive Solar

INDIRECT METHOD for Passive Solar

Another indirect method is the attached greenhouse. In this concept, a greenhouse on the south side of the home collects thermal energy during the day and this energy is stored in masonry wall separating the greenhouse from the main house. This combines the architectural benefits of the greenhouse (growing plants, sitting room, extra living space, etc.) with the benefits of passive heating.

GREEN HOUSE METHOD for Passive Solar

GREEN HOUSE METHOD for Passive Solar

Passive Solar Isolated Gain

The third concept for passive solar heating is to have the system isolated from the living sections of the home. This allows the system to function independently from the building and thus not interfere with the architecture. Basically, these systems use a separate solar collector and the heat is piped to the house using a convective loop (with either fluid or air). These systems begin to look more like solar water heaters which we will talk about another time…

Summary

The design techniques for passive solar are hard to find. What are the rules-of-thump for the area and types windows; amount and type of thermal mass, the length of shade awnings based your climate and latitude? Most books on the subject are way too superficial. On the other hand, there is very good modeling software for the architect and building design professional, but this is way beyond the price point the layperson or do-it-yourselfer (DIYer). By far, the best book I have read on the subject is The Passive Solar Energy Book, by Edward Mazria, 1979. All the figures in this post are from that book. Alas, is out of print, but still available here on Amazon.

If there is any interest, I will go deeper on this subject in later posts. Please comment!