Regional Adjustments for Direct Solar Gain

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One of the most infuriating aspects of passive solar design is that one size never fits all. A design that works perfectly on your site may be ineffective two miles away on the other side of a hill, on higher ground, or in a rural area.

With that in mind, regional adjustments are not too difficult. The basic design can stay much the same in areas with similar climate, but you will have to adjust four factors:

  1. The amount of insulation installed.
  2. The amount of glazing on the side facing the sun.
  3. The amount of thermal mass.
  4. The size of the overhang to shield windows from the high summer sun.

Each of these factors must be weighed and compared against the others, to find the best balance. Let's look at some of the basic principles you need to know. Remember that these apply to direct gain passive solar homes.


Basic insulation requirements depending on your local climateMost passive solar home designers use extra insulation wherever they can. Exceeding local building codes is normal; exceeding the International Energy Conservation Code is better; aiming for the EPA Energy Star level is common. When choosing insulation, remember the following important points:

  • Cold climates need more insulation than warm climates.
  • Window insulation is extremely important in direct gain designs.
  • Don't forget that windows can be insulated with shutters and shades as well.
  • Floor insulation is critical, especially if your floor is also your thermal mass.
  • Efficient, effective foundation insulation is absolutely essential for thermal mass floors.
  • Floors over crawl spaces need more insulation than slabs laid directly on the foundation.
  • Air infiltration must be controlled.


Solar glazing

The toughest part of the balance is, without a doubt, the solar glazing. The basic rule of thumb – more heat requires more glazing – can get you into awful difficulties with heat gain and loss. Too much glazing will bake you in summer, let too much heat out and freeze you at night (or on cloudy days) and can even make the home uncomfortably hot in winter.

The 7% rule states that the heat generated by glazing equal to 7% of the total floor space in the home is covered by intrinsic thermal mass. In other words, if the surface area of all your sun-facing windows comes out at 7% of the floor space area, you'll be fine.

All passive solar homes should aim for glazing in the 7% to 12% range. In warmer climates, the lower end applies; the higher end is for cold areas. You can go above the 12% mark but you will have to make some serious design choices – such as a thermal wall placed directly in sunlight – but you should never go beyond the 20% mark.

Thermal mass

As important as the solar glazing, thermal mass stores the sun's energy as heat and releases it once the house cools, maintaining a comfortable temperature for longer.

  • Thermal mass should be spread throughout the direct solar gain space (as far as is practical).
  • The more mass you can site in direct sunlight, the better.
  • Use lighter colors closer