Idaho Earth Builders doesn't sell a product or service called Passive Solar.    So why are we bothering to dedicate a portion of our web site to it?   Because we believe strongly in it and our products and services help make it a more effective solution for our clients.

Effective use of Rammed Earth and Adobe thermal mass walls working in harmony with the warmth of the sun can make a dramatic impact on the comfort of any living space, in both cold and hot weather.

Passive Solar is not a product - it's a design approach that uses the natural cycles and energy of the sun to heat and cool a structure.   Factors such as building orientation (which way the building faces), glazing (windows), roof overhangs, thermal mass and ventilation are all leveraged to heat and cool a space passively.   'Passively' means that, ideally, we accomplish all this without the use of mechanical systems - but more on that in a bit.

Passive solar heating:

In an effective passive solar design the roof overhangs and the position and size of windows allow the low winter sun to penetrate and warm a building when the weather is cold.   Thermal mass (a Rammed Earth or Adobe wall) also absorbs that warming sunshine, storing it up through the day.   Through the night that stored warmth is released into the space as the air cools, allowing you to heat the room with sunshine long after the sun has set.

Passive solar cooling:

Those same factors - roof overhangs, position and size of windows keep the high summer sun out, shading the interior spaces when the weather is hot.   Thermal mass again helps cool the spaces.   As the air in an interior space tries to rise in temperature, it also comes into contact with those massive earth walls , which have been cooling through the evening and night.   Those walls help pull the heat from the air, slowing and moderating the rise in temperature.

Blowin' in the wind

Now, it may be easy to see how the sun can be used to provide heat, but to provide cooling?   How can that be?   The answer lies in the last factor I mentioned at the beginning of this section: ventilation.   Passive solar heating can be used to cool a space. That's right, I said we can cool a space by heat.   Here's how.

Warm air tends to rise.   When that air rises, it creates a vacuum that must be filled.   That vacuum will be filled by relatively cooler air - which tends to sink.   These tendencies of warm and cool air set up a natural flow of air - or ventilation.   No fans necessary.

We've got it made in the shade

An effective passive solar cooling design provides a means for rising warm air to escape and for relatively cooler air to be drawn in to replace it.   A shaded space acts as a reservoir of cooled air that can be drawn into a room that is trying to heat up.   As the cooled air is drawn from the shaded reservoir, fresh air enters.   But on a hot day, that fresh air is also warm.   If there is no means of cooling this new air before it is drawn into the room we'll quickly deplete the reservoir.   It will be filled with warm air just like the air in the room and the ventilation will naturally shut down.

By placing thermal mass in the shaded space we provide stored 'coolness'.   The air drawn into the shaded space may be warm when it enters, but the thermal mass will work to cool it and thus keep a supply of relatively cooler air flowing into the interior space.

It should also be pointed out that the strategic addition of water features and vegetation are also very effective passive cooling mechanisms.   Water, like earth, also provides thermal mass.   The leaves of shrubs or trees and blades of grass provide a great deal of surface area.   This surface area, along with the moisture content of the plants, acts as a heat sink.   So, air drawn across water or vegetation drops a few degrees in temperature, giving up some of its heat.   Place that water and vegetation on the shaded side of the house and you crank up its heat absorbing efficiency.

It's Cool to be Hot

Not only can a cooled thermal mass wall be used to cool a space, but a warmed thermal mass wall can also contribute to the cooling effect.   By placing a dark Rammed Earth or Adobe wall (called a trombe wall) so that it will absorb the sun's warmth on hot days we create a super-heated surface.   The air in contact with the surface will rise dramatically in temperature and rise along that thermal mass wall just as dramatically.   The quickly rising air creates a more powerful vacuum effect.   This draws the cooler replacement air into the room more quickly.   Since air at any temperature when in motion feels cooler than the same air when at rest (think 'wind-chill factor') the accelerated movement of the air provides a more dramatic sense of cooling.

Once the sun has gone down and the ambient air begins to cool, that thermal mass wall is still working.   The Rammed Earth or Adobe soaked up all that warmth during the day and is still very warm after the sun has gone down.   The wall continues to heat the air along its surface, providing the power source for the natural ventilation cycle to continue.

What about those mechanical systems?

All the heating and cooling effects described above are achieved without one watt of electricity, one cubic foot of natural gas, or one pellet of woodstove fuel.   You pay for it once when you build your home and it works for you day after day, season after season at no additional cost.

But the problem with the sun is that you can't rely on it.   It's a pretty good bet that it will rise and set, but you just never know if it will actually shine on you - or if it will shine on you in spades.   If the sun hides behind the clouds day after winter day, it can be hard and even impossible for those thermal mass walls to gather much warmth from it.   If it bakes you day after summer day, it can be hard for those walls to shed their heat during the night so they can help keep you as cool as you'd like the next day.

So, yes, even the best passive design can be challenged by nature.   When this happens, it's nice to have insurance.   But that is the key difference between a passive solar design and a 'contemporary' design.   A contemporary design focuses on overpowering nature and imposing an interior climate.   The problems arise when those mechanical systems break down or become costly to operate.   When this happens the design and construction of the structure usually offers no 'natural' relief.   So you're locked into either using those systems constantly or being miserable.

The passive approach takes the attitude that good design, construction and taking advantage of what nature provides can keep you quite comfortable the vast majority of the time.   But for those times when Mother Nature doesn't cooperate, it's nice to have some modern conveniences to fall back on.   One additional advantage of good passive solar design is that the size of those mechanical systems can often be smaller and less costly.   They only need to be used occasionally and then only to give a boost to the effective, designed-in features of the house, instead of compensating for the lack of them.