When the Problem Is Heat
How to keep the heat outside when you live in the desert Southwest and daytime temperatures can top 100°F
If you have problems dealing with the heat, you probably wouldn't like the desert Southwest, especially when conventional air conditioning is simply too expensive to use on a regular basis.
That seems to be the case for a GBAGreenBuildingAdvisor.com reader who's trying to learn more about building in a climate where the challenge of staying cool far outweighs the minor and occasional inconvenience of staying warm.
"I live in the low desert southwest near Phoenix," Anneal G writes in a post at GBA's Q&A Forum. "Living is very different here than in cold weather areas. And very different than Florida or humid areas of Texas."
Outside temperatures are more than 100°F in the shade for five or six months at a stretch, and may hit 115°F. At night, low temperatures may be only 95°F for a couple of months. Humidity is very low, and there are few drought-resistant trees to help.
A lot of people work at home, and few of them set their air conditioners to anything less than 79°F.
"We need to keep heat out rather than keep in or keep cool in and heat out, and this is apparently handled different than in cold weather," Anneal asks. "But how? That is what I would like to know.
"I am not looking for perfection like the Passivhaus. Just what should an average person with a low-to-medium budget really ask for to build a basic energy-efficient home in my low desert environment?"
Anneal's query is the focus of this Q&A Spotlight.
Start by orienting the house correctly
GBA senior editor Martin Holladay suggests that Anneal follow the recommendations in one of his articles, Hot-Climate Design. In that article, Holladay recommended that homes in hot climates be oriented so that the long axis of the house has an east-west alignment; have a hipped roof with wide roof overhangs; have as few east-facing and west-facing windows as possible; include details that keep the windows shaded; have all ducts inside of the home's thermal envelope; include adequate roof insulation or ceiling insulation; have a thermal envelope that is carefully air sealed; have energy-efficient appliances and lighting; and have ceramic tile flooring rather than carpeting.
Orienting the house so its long axis runs in an east-west direction means that in the middle of the day, the angle of the sun on south-facing windows is high, writes Dana Dorsett, and that allows much of the heat to be reflected. By shading windows with overhangs, most of the direct solar gain is blocked.
"East- and west-facing windows get sun on them when the sun is lower in the sky, and the incident angle on the windows reflects very little of that solar energy," Dorsett adds. "By having the longer axis running east-west means less window area on the east and west sides to soak up the heat. If possible, design for no west-facing glass and minimal east-facing glass."
North-facing windows are the best for daylightingUse of sunlight for daytime lighting needs. Daylighting strategies include solar orientation of windows as well as the use of skylights, clerestory windows, solar tubes, reflective surfaces, and interior glazing to allow light to move through a structure., Dorsett says, since it's shadow-free and never as harsh as direct sunlight.
A simple gable roof with the ridge running east-west also would give Anneal the most productive rooftop solar (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) installation.
"Even if you don't install it on day one," he says, "within 5 to 10 years the costs are expected to fall by more than half (again!), which will make it one of the cheapest energy sources available. The average installed cost of PV right now in much of the U.S. is about $4/watt, or about $40/square foot. In some places it's getting down closer to $2/watt or $20/square foot."
In Arizona, an array with a capacity of between 10,000 and 15,000 watts should cover most if not all of an air conditioning bill, providing the attic is well insulated.
What about adding thermal mass?
Anneal wonders about the differences in building techniques between Florida, another hot spot, and Arizona. "I am a little confused about that, too, since rammed earth, adobe, and straw-bale construction are mentioned in this area for thermal mass (?) but the (Florida) Hot-Climate Design article mentions only needing 2 inches of rigid foam (R-10) for the walls," Anneal writes. "I would have thought more than R-10 would be needed for the walls? Am I missing something else?"
Adding thermal mass to exterior walls is expensive, says GBA senior editor Martin Holladay, but it is an effective strategy in hot climates.
Except, adds Dan Kolbert, in areas where it doesn't cool off very much at night. "Sooner or later," he writes, "the mass will heat up, too, and then what?"
This is certainly the case where Anneal lives. Temperature data over the past three or four years shows that high temperatures in July and August can range as high as 116°F, while low temperatures are in the high 80s.
"Dan is right," Holladay adds. "Once summer weather sets in, and nighttime low temperatures are higher than your indoor temperature, thermal mass does no good. Under those circumstances, you still want good old-fashioned insulation.
"The reason that less insulation is required in Phoenix than in Fairbanks is that your delta-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures. [the difference between inside and outside temperatures] is smaller," he adds. "Even when it's 110 degrees outside, your delta-T is only 30 or 40 degrees. In Fairbanks, the delta-T can be 120 degrees."
Design a form appropriate to climate and site
Malcolm Taylor suggest Anneal look at the work of architects who have developed successful designs for similar climates. He cites the work of Australian Glenn Murcutt, plus a house designed for the California desert by Lloyd Russell.
The Russell house features a steel canopy that shades the house, plus garage-style doors that can be rolled up to let in light and air.
But the California house ignores the Passivhaus-like strategies that work well for almost all climates, says John Brooks: extremely airtight building envelopes with ample insulation, not too many windows and careful window orientation and shading.
"Overhead doors and sliding glass pocket doors are almost impossible to make airtight," Brooks says. "The canopy is a cool idea but [it] seems to be doing a very poor job of shading the glass ... not to mention the concrete thermal bridge."
Not so fast, replies Taylor. It was only recently that we stopped designing buildings to reflect their location and climate. The availability of cheap energy allowed designers to make a one-size-fits-all kind of house that could be built anywhere from Alaska to Arizona. "So we end up with an inappropriate basic shape we then try and make work by upping the efficiency of the building envelope," Taylor says. "At that point the only way to do this is to create the hermetically sealed enclosure characteristic of Passive Houses. Why not step back to look at the solutions that were always used before the advent of the ubiquitous modern tract house?"
Not every detail of the California house necessarily works, he adds, but the problem with Passivhaus design is that it seems arbitrary or inappropriate, he says. "They sometimes take on the superficial form of the region — say, barn-like in Vermont — but none of the essential elements that reflect the local climate," he says.
Our expert's opinion
Here's what GBA technical director Peter Yost had to add to the mix:
My first thought on key elements of design and construction for hot-dry climates is: Use the GBA Strategy Generator! It’s a great place to start to gather up ideas for your climate, by stage of construction.
And then for some thought-provoking examples go to these GBA Green Home projects:
- LEED Platinum House in a Warm Climate in Weatherford, Texas.
- Production Platinum Homes in Las Vegas, Nevada.
- A Modest New House Proves Green Doesn’t Mean Expensive in Prescott, Arizona.
- Luxury Home Earns Gold NAHB Energy Value Housing Award in Chino Valley, Aarizona.
I know that none of these homes are in exactly your climate, but each has some key design elements relevant to your environment.
My second thought is: check Building America resources from Building Science Corporation on design and construction for hot-dry climates:
No surprise that takeaways from all of these resources reflect insights already shared by Martin Holladay and company, but here several to round out the issue:
- Airtightness is key in any climate, no less so in hot-dry climates. And airtightness is as much about comfort and air quality as it is energy efficiency.
- Keep all HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. ducting and equipment in conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. . And this is as much about thermal comfort and air quality as it is energy efficiency.
- Decide how “active” you want to be in operating a home to reject heat. Window coverings can play a key role in managing solar gain (most effectively from the outside but inside as well), but their effectiveness is often tied to occupancy engagement.
- Real energy efficiency is always an integration of excellent design, materials, and construction. Look for architects or builders who are willing to guarantee the performance of their work.
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