Using the sun for free heat: passive solar basics
Passive solar design heats a house with winter sunshine through well-placed south-facing windows, stored in heavy interior materials and kept in by good insulation. In Canada the trick is collecting enough winter sun without overheating the house in summer.
What is passive solar design?
The house itself acts as the collector. No pumps or fans are needed. Sunlight enters through windows, lands on floors and walls, turns into heat and stays in the building. The design works with the sun's seasonal path: low in the winter sky, high in summer.
That seasonal difference is what makes the approach workable at Canadian latitudes. A fixed overhang can block high summer sun and still let low winter sun reach the glass.
Which direction should the windows face?
South, within some tolerance, collects the most useful winter sun. East and west glass admits sun at low angles that is harder to shade and tends to cause summer overheating. North glass collects almost nothing in winter and loses heat all year, so it is usually kept modest. The direction guide for solar panels uses the same logic for roofs.
A long, narrow house on an east-west axis gives more wall length to the south. Lot orientation and street layout often decide whether this is possible, which is why the site matters at the earliest stage.
What does thermal mass do?
Heavy materials such as concrete slabs, masonry and tile absorb heat when the sun shines and release it later. Without mass, a sunny room overheats at noon and cools by evening. With mass placed where the sun reaches it, the swings are smaller.
Mass in the shade does little. A slab covered in thick carpet or a wall the sun never touches is part of the structure, not a heat store. Light timber-frame houses can still benefit from a modest amount of exposed mass in the right place.
How do you check a design before building?
An energy model will compare a few window layouts in an afternoon, and it is cheap compared with fixing a wrong layout in a finished house. Ask the designer to model at least two south glazing areas and show winter heating demand and summer peak temperatures for each. A site visit around the winter solstice, with a sun-path chart or a phone app, shows what the sun can actually reach, including shadows from trees and neighbouring roofs.
Check the overhang depth against your latitude too. A depth that works in Calgary will not suit Yellowknife, and summer shading that is too short will not do its job.
What are the limits and common mistakes?
- Too much glass. Windows lose far more heat than walls, so extra south glazing can lose on cold, cloudy days what it gains on sunny ones. Proportion matters, and an energy model is a reliable way to find it.
- Weak insulation. Passive gains help only if the shell keeps the heat in. Start with the insulation priorities and a tight envelope.
- No shading plan. Overhangs, deciduous trees or adjustable shades keep the same windows from cooking the house in August.
- Blocked sun. Neighbouring buildings and evergreens can cancel the benefit. A shading survey of the site in December shows what the winter sun will reach.
Window quality also matters here; see the windows guide for solar heat gain coefficients.
How does this relate to active solar heating?
Passive design collects heat where it lands. Active systems move it. At Drake Landing, flat-plate collectors on garage roofs gathered heat, a pipe loop carried it to a store in the ground, and the 52 R-2000 homes drew on it through heat exchangers. The mismatch between the sun's supply in summer and the demand in winter is why that project built seasonal storage instead of relying on window gains alone. The solar collection page shows the collector side, and the guide to solar space heating for homes compares the approaches.
Quick answers
Does passive solar work in cold Canadian climates?
Yes, as a way to reduce heating needs. In cold regions with sunny winters it can supply a meaningful share of heat, but it rarely replaces a heating system by itself.
Is passive solar design more expensive?
It can cost little when it is planned from the start, since orientation and window placement are design choices. Extra glazing quality, shading elements and thermal mass can add cost, so costs vary by project.
Can an existing house use passive solar ideas?
Partly. You can improve shading, use blinds on sunny winter days and clear obstructions to south windows. Changing orientation or adding large glazing is a major renovation that should be modelled first.