Tuesday, January 12, 2010

Passive Solar Roof Design

Planning for sustainable retirement

For the vast majority of people, retirement means living on a fixed income – one decidedly lower than the one we experience during our working lives. Since our 30 acres in the Texas hill country was purchased for that very reason – retirement property – we are researching many ways to manage the resources we have in the most efficient manner possible.

Conventional energy usage is not sustainable

Living in a suburb surrounding a major city, one gets used to several elements of everyday life that are both unsustainable and expensive to maintain. Energy usage is one of the best examples of this. Most typical suburban homes today are built to a certain standard of efficiency regarding insulation, and energy usage. This standard relies on cheap energy; since these homes all use electricity, natural gas, and/or heating oil to pump either warm or cool air through the structure, to heat water for bathing, and for washing dishes and clothes.

Passive solar design is a sustainable alternative

Passive solar design, known to the ancients, has been completely abandoned in the past few hundred years when it comes to modern home design and construction. Almost no one these days plans a new home’s construction around basic concepts like: site selection, solar orientation, roof pitch(angle) to utilize passive solar energy, window placement and design, cross ventilation, or integrated thermal mass.

These concepts are simple to understand, easy to implement, energy efficient, sustainable, and financially sound.

Let’s start with passive solar roof design. This concept is based on the principle that the sun’s light streaming into your home’s windows generates heat. Therefore, you want to allow the sun’s light to shine on windows (or even exterior walls) only when it is cold outside, and you want to block the sun’s rays when it is hot outside. The beauty of this idea is it’s simplicity. The sun’s angle shifts annually on a predictable schedule:

passive solar roof design. sun orientation. sun orientation through seasons.
Sun Orientation

In the northern hemisphere, the sun’s angle is high during the summer months and low on the horizon during the winter months. Passive solar design takes advantage of this by designing a home’s roof pitch and overhang to allow solar heat gain during the winter months, and block it during the summer.

allows winter sun, blocks summer sun, roof overhang, thermal mass, insulation, ventilation for cooling effect.
Natural convection and ventilation
The average American home is constructed with no consideration of this basic concept, therefore most houses fight against nature to stay cool in the summer and warm in the winter. The results are extremely high energy bills during the summer in the South due to air conditioning requirements, and extremely high energy bills in the North and Midwest due to heating oil requirements.

A home utilizing passive solar design works with nature to heat and cool itself. During a recent green home tour in the Texas Hill Country area, Tammy and I saw several homes with no HVAC system whatsoever. We toured one home in particular on a day when the outside temperature was 96 degrees F. Inside the home the temperature was in the mid-70's. This was a home with no air conditioning, no ceiling fans, no HVAC system whatsoever. The home used passive solar design, a radiant cooling system in the floors and walls, and thermal mass wall design. The roof of this home also employed a heat-shield system whereby the roof is actually a layered structure with an air gap between the roof surface and the home's decking system, allowing convection to pull cooler air up through this gap and insulate the home from the searing Texas heat.

A typical HVAC system installation cost around $10~$15K. Our typical electric bill during the summer is around $400. Additionally, we spend around $700/year maintaining our HVAC system. Imagine being able to simply do without these inefficeint, expensive, unsustainable systems and use the natural cycles of nature to heat and cool your home.

The key is in the design, site selection, solar orientation, roof pitch and overhang, and thermal mass wall implementation.