Passive Solar Home

Passive solar homes are houses that seek to control their internal temperature by using the sun. Passive solar designs must account for both heating and cooling and take into account a variety of topics spanning from building material choices to optimal window locations. A passive heat purist would aim to heat their home entirely with just the sun and without any "active" methods of climate control such as fans, furnaces or heaters. A more practical goal be to get all needed heat energy passively but then to distribute it actively with fans or vents. An ever-more-reachable goal would be to take advantage of passive heat where and when you can, reducing your power bills and your ecological footprint.

Passive Heat Tips and Techniques

The absolute best time to think about passive heat is before a structure is even built. That said, don't be discouraged if you have no intention of building a new house. You can take advantage of many of the following techniques by making minor modifications to your current home, and I'll try to highlight these where possible.

Windows, Roofs and Seasonal Changes in the Sun's Path Through the Sky

As you've no doubt noticed the sun takes a different path across the sky in the winter than it does during the summer. The sun's change of path is a result of a yearly tilt in the earth's axis - the very same tilt that causes the seasons. It's possible to build a roof and its awnings (overhangs) in a manner that the sun can shine in your windows during the winter but misses your windows during the summer.

• In Winter: the sun's path makes an arc which is lower to the horizon than during the summer
• In Summer: the sun goes high in the sky as it travels east to west

If you're trying to picture how this might work imagine this: it's a hot summer day and you're sitting in a living room peering out a big sliding glass door. It's a very sunny day. The roof outside of the living room door hangs out a little bit, and you can see it casting a shadow just outside of the sliding glass door. This is great, it's summer and the sun's thermal energy isn't making its way inside. Now let's pretend you're REALLY patient. SO patient that you're going to sit in this living room until winter. As the seasons change you'll see the sun dip lower in the sky. The light should creep its way into the living room reaching further and further inside everyday. Around December 21st the sunlight will be reaching as far inside as it's going to get, and each day after that the sunlight will begin to slowly retreat back outside to where it belongs during the summer.

Positioning windows and measuring roof overhangs might be best done when a house is being built, but there are still numerous options if your house is already built. If the sun actually reaches inside south-facing windows during the summer and you find yourself having to crank up the air conditioning, consider installing awnings. Even if your roof is not designed to handle seasonal changes in sun exposure, you could simply create your own shade with awnings during the summer and then remove them before winter.

Materials and Colors that Trap or Reflect Thermal Energy

Certain materials are better suited for heating a space using solar energy whereas others are better at ignoring solar energy.

General Guidelines:

• dark colors absorb light (that's why they're dark) and then emit heat
• light colors reflect light and stay cool
• stone and masonry require lots of energy to heat up, retain heat longer, and release heat slowly
• carpet acts as an insulator and transfers very little heat

In an above example we imagined a passive solar home that was carefully thought out to let sun into its living room during the winter and keep the sun out during the summer. Can you guess what material would be best for the living room floor? Any sort of dark masonry work or tile would be able to capture a lot of the sun's energy and release it slowly, likely retaining residual warmth for several hours even after the sun went down. Likewise if you lived in a house who's poor design included a dark stone floor that got sunlight during the summer you would probably want a way to stop your house from turning into an over sized oven. A light colored area rug would reflect a large potion of the sun's light and make things more comfortable.

Another excellent way to make use of masonry's ability to absorb heat is a Trombe wall. Best of all it is 100% passive.

Trombe Wall

A Trombe wall is a sun-facing wall designed to absorb heat during the day and release heat at night to the interior of the home. The most common design consists of a sheet or two of glass placed on the exterior of the chosen wall. A small gap exists in between the glass and the wall therefore insulating the wall from the exterior but allowing light to hit the wall freely. The temperature of the wall can exceed the temperature of the air outside of the glass, and if the temperature of the wall exceeds the interior temperature then heat will flow into the house. Depending on the thickness of the wall energy may still be traveling through it as much as 6 hours after the sun goes down. A more recent modification of the Trombe wall design includes air vents that go through the wall to the gap of hot air on the outside, allowing hot air to circulate during the day.

Insulation

Perhaps the most important single topic in any passive solar design is insulation. Regardless of how much heat is pumped into a house it matters if you can keep it in the house.

Insulation Tips

• Insulate your attic! This is considered to be a major short sight in many existing houses.
• A lot of heat is lost through windows, close your curtains at night.
• Use rigid insulation on exterior walls
• Use reflective foil in walls behind interior heat sources

And don't forget even though the primary goal is to stop the transfer of heat between your house and the exterior there will be times that you will want to vent your home. Windows that open up high as well as attic vents can both help discharge heat from a stuffy interior./