Passive solar design rules for a cold-climate house

What are the basic rules of passive solar design?

Five ideas carry most of the weight.

1. Orient the house. Put the long side and most of the glass within a modest angle of true south, which differs from magnetic south by an amount that depends on location. Our guide to the best solar panel direction in Canada explains the difference.
2. Size the south glass sensibly. Enough to bring in winter heat, not so much that the house overheats or loses heat at night.
3. Shade for summer. Fixed overhangs, deciduous trees or adjustable shades.
4. Add thermal mass. Dense materials inside the sun's reach soak up daytime heat and release it later, as discussed in thermal mass in homes.
5. Insulate and seal. Without a tight envelope the heat leaks out as fast as it comes in.

The overview in our passive solar design guide covers the concepts, and this page works through the rules in more detail.

How do you work out the overhang?

The answer comes from sun angles. The sun's height at noon equals 90 degrees minus the latitude, plus or minus up to 23.4 degrees for the season. Take 51 degrees north, roughly the latitude of the Calgary area, as an assumed example. At the winter solstice the noon sun sits about 15.6 degrees above the horizon. At the summer solstice it is about 62.4 degrees up.

Now take a 1.5 metre tall south window with an overhang at its top edge that projects 0.8 metres. In June the shadow drops about 0.8 times the tangent of 62.4 degrees, close to 1.5 metres, so the window is shaded at midday. In December the shadow drops only about 0.2 metres, so most of the glass sees the sun. The numbers change with latitude, window height and the time of day, so use them to understand the principle and have a designer check the actual building.

How much south glass is too much?

Designers often cite a rule of thumb that ties south glazing to a share of floor area, but the right share depends on insulation, mass, window quality and climate, so treat any single number with caution. Too little glass wastes the free heat. Too much causes summer overheating, winter glare and heat loss on cloudy nights and after sunset.

Window quality changes the balance. Modern glass with good insulating value and a suitable solar heat gain coefficient lets sun in while holding heat, which is why window specifications matter, as covered in energy-efficient windows in Canada. Energy modelling software or an energy advisor can test options before the walls are built.

What mistakes do passive solar houses make?

• Overheating in shoulder seasons. Spring and autumn sun can be strong while the heating is off, so shading and ventilation must work in those months too.
• Mass in the wrong place. A concrete slab covered by thick carpet or furniture never sees the sun.
• Leaky construction. A house that fails a blower door test, described in air tightness and the blower door test, wastes the gains. Tight houses also need controlled ventilation, covered in the heat recovery ventilator guide.
• Ignoring the site. Trees, hills and neighbouring buildings can shade the south side at the low winter sun angle.
• Relying on it alone. In a Canadian winter a passive solar house still needs a heating system for the dark, cold weeks.

How does this relate to Drake Landing?

Drake Landing Solar Community in Okotoks, Alberta, used active solar rather than passive: 800 collectors on garage roofs, fed through pipes to a ground store. Its 52 homes were R-2000 and Built Green Gold, with envelopes tight enough to make low-temperature heating practical, as the homes page explains. Passive and active solar share the same foundation, which is a very good envelope, and the difference between the two approaches is set out in solar collector orientation for heating and R-2000 homes explained.

Quick answers