Passive Solar Design Basics

Collaborate with the sun to provide free heat in winter...and cooling in summer.

Passive solar design takes advantage of solar orientation and other climate conditions to minimize (and in some conditions, eliminate) heating and cooling requirements for a building.

The name says it all...

PASSIVE = no mechanical parts & minimal if any maintenance

SOLAR = uses (free) energy from the sun

DESIGN = integrated into the building design

The result is free heating (& cooling) that works because of how a building is designed, so it can never break. What a win!

South-facing glass allows low winter light to stream in, while shading high hot summer sun

The Basics are Simple!

1. Properly orient your building relative to the sun

The easiest direction to control when the sun enters your space is the side that faces the equator...where I live/work, that's the side facing South (see #2 below). This means, that you want to orient the longest side of your building along an East-West axis and a maximum amount of your building faces South. You can rotate 15-degrees either way without much effect, and rotate up to 30-degrees off that axis and still be over 90% efficient, so you have some play. But for maximum efficiency, set your longest façade to face due South (North, if you live in the Southern hemisphere...) And make sure the windows facing South will receive un-obstructed sunshine from at 10 AM to 2 PM (9 AM to 3PM is even better)...this means no buildings, no trees, or anything else that will shade the South-facing windows. Whatever percentage shade you have, that is the percentage of efficiency loss you have.

2. Use the sun's angle to your advantage

Have you ever noticed that the angle of the sun in the sky is lower on the horizon in Winter and almost overhead in Summer? Well, you can use that fact to your advantage...bringing low Winter sunshine into a house to heat it (for free) when it's cold outside. But also shading shading from that hot Summer sun is easy because the sun is nearly overhead...and no sun streaming in during hot Summers means free cooling too (because no heat gain). You can see a graphic of that below, with a building section through a 2-story passive solar house. On the lower floor, there is a thermal mass cob wall, 1/2 of which is in full sunlight in Winter. That means the cob wall heats up (again, for free) during the day, and releases that heat slowly overnight. That same thermal mass wall is completely shaded in the Summer, so it is a heat sink for excess warmth in the air (ie, it provides free cooling power all Summer).

solar angles on the South side, Winter vs. Summer

3. Size your South-facing window appropriately

The biggest mistake made with a passive solar home is to make the windows on the South too large. This means you will end up with too much heat gain (especially in temperate Spring & Fall weather). The size of your windows is calculated at a percentage of the total space that window will heat. The percentage used in the calculation depends on the climate where you are building. Generally speaking, the window area should be between 7% and 15% of the interior floor area.

south facing window sizing depends on the interior floor area and your particular climate

You also need to consider the size of windows facing North, East, & West. The windows on those sides help tremendously with the comfort of your interior space, because windows on at least 2 sides of every room means better quality light & means excellent natural ventilation. But, consider these tips as well:

windows on the WEST are your biggest source of heat gain, because the angle of the sun at the hottest part of the day in Summer is streaming in at a low angle. The best way to shade to this side is some well-placed trees. If you don't have landscape shading, then minimize the window openings or add shady porches on the West.

windows on the NORTH provide excellent light quality and cannot contribute to heat gain, however, large North-facing windows can mean lots of heat loss in the Winter (since windows are not as insulating as walls. I use smaller windows to the North side, and try to put closets & bathrooms on that side of the house.

shows the west side dug into the ground to avoid summer heat gain (right side) and north wall with very few window just to balance light inside

windows on the EAST can contribute a bit to heat gain in Summer (for the same reason), EXCEPT, that the morning sun is at the coolest time of day, so not quite as detrimental to your interior comfort. I use windows to the East for balanced morning light and to improve airflow.

4. Super insulate

Keeping free solar heat inside in winter requires an insulating bubble around your space. The insulation materials slow down the flow of heat through walls and roof. They keep warmth inside in winter and outside in summer. The more insulation you have, the slower the heat flows. Insulation materials tend to be the light fluffy ones, with lots of air pockets.

Insulation is measured as "thermal resistance" or R-value. The higher the R-value, the more insulating the material is. Heat rises, so the most important place to insulate is the roof above you (60% of heat loss is up). I use a minimum of R-49 insulation in roofs (or attics). For walls, I mostly use strawbales, which provide at least R-36.

thick strawbale walls provide superb insulation

5. Add thermal mass inside

Thermal mass describes materials that act like a rechargeable battery for heat energy. These materials absorb (conduct) and and release (radiate) heat constantly. Good thermal mass materials are the heavy materials, like stone, clay soils, and even water.

The beauty of adding thermal mass is twofold:

Thermal mass can be charged by adding a heat source, such as the sun or a fire. Once "charged", the mass acts as a radiant heat source (in your insulated bubble) and keeps temperatures consistent inside.
When the mass is not being "charged" (sun angles are high or you don't light a fire) it pulls excess heat from the surrounding air to act like a free air conditioner.

Example of a mass wall that has sun shining on it to warm it up in winter. The same wall is fully shaded in summer and acts as a cooling element.

6. Build a closed room at your entry

This simple strategy is often overlooked, but is quite effective. The principle here is again simple: when you open your outside door, there is a nearly full air exchange between inside air & outside air. That means whatever room your door opens into loses warm air in Winter (or cool air in Summer) and is replaced by whatever temperature air is outside. What is the simple way to minimize the effect of that? Open the exterior doors to a smaller room that isn't part of your living space. This space should be insulated, but does not need heating & cooling (it will stay within 10-degrees of the rest of your house). But absolutely, positively, it needs a door between that entry room and the rest of the house. That's what keeps the air exchange to a minimum. The entry room can also be where you store coats, hats, boots, shoes, etc.