As with all forms of solar power – solar thermal, PV panels, solar chimneys and others – the amount and quality of sunlight to which a home is exposed is extremely important in holistic home design. Thankfully, again in common with many other forms of solar power, even areas that might appear at first sight to be far too gloomy can, in fact, provide enough energy to reduce heating bills by 50% or more.
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When you read about solar systems, especially solar thermal and passive systems, you may come across the "10-Degree Rule". The rule states that, wherever possible, a building should be constructed with the long axis facing within 10 degrees of true south (not magnetic south).
The reason for this rule is very simple: south-facing walls get the most sun (in the northern hemisphere). You want as much of a building as possible facing that way to gather as much heat and energy as possible.
Direct gain passive solar homes have many advantages and disadvantages compared to other approaches. Historically, the first direct gain homes were often a complete mess, because the designers had little understanding of the need to balance things like solar glazing and thermal mass. Modern designs are much better, especially since analysis software is now available to help make the tough decisions.
Thankfully, the days of cookie-cutter houses built quickly to exactly the same design are on their way out. Even in places where the homes are all based on the same floor plan, individual home owners add and change things to suit themselves. One of the easiest things to change in any home is the color of the exterior walls.
That title might fill you with apprehension and, in many ways, you'd be right: analyzing such a complex thing as a home's energy performance without using software is a daunting task. But it is possible. There are even several books which include worksheets and detailed descriptions of how to build a more accurate assessment than the one we'll outline here.
Analyzing your passive solar home's energy performance by hand is long-winded, cumbersome, and prone to error. Between ten and twenty years ago, a variety of software tools appeared on the market, ranging from DOS-based utilities to basic Windows apps.
Over time, most of these programs have disappeared. Some were discontinued because they weren't popular enough, some vanished because they were based on obsolete operating systems, and others were simply discontinued through lack of support.
Designing a passive solar home is not the end of the road for sustainability. There are many more choices you can make to improve your home's sustainability and, at the same time, reduce the impact of traditional living on the environment.
At first glance, you might think that it's impossible to have too much or too little thermal mass. After all, if there's loads of it, your home will store more heat; if there's very little, you'll still get the benefits of what you do have.
But it's not as simple as that.
In an ideal passive solar world, homes would be built as a long string of single rooms, so that every room would be heated by the sun and there would be no need to transport warmth from warmer to cooler rooms. In practical terms, long houses are rarely appropriate: traffic flow inside such buildings is often difficult and housing lots in towns and cities tend to be square or rectangular, rather than extended oblongs.
Most, if not all, proponents of solar energy will exhort the long-term value of installing passive (and active) solar systems. The return on investment over the life of the installation is always impressive on paper and professional installers will happily spend hours explaining how the cost works out as a tiny percentage of your income. Then they'll just as happily bill you five figures!