Expert: Richard Taylor, AIA - 11/28/2004

Question
Richard,
I'm thinking about moving to southern Arizona, desert country.  I want an energy efficeint home.  The choices are frame and stucco AND cement block or slump construction.  It seems to me that the cement walls would be better but I just don't know.  What do you think?  And one more ?  What do they mean by "built up" ceilings?  I notice that term when they talk about cement buildings.
Thanks
Lee

Answer
Lee:

Thanks for your question!

I don't know where you're moving from, but I know you'll like Arizona!  We're designing a house there now and I can't get enough of that sunny climate and unique landscape!

In a hot-arid climate, creating an energy efficient home goes way beyond the choice of wall construction.  Historically, “thick-wall” construction was a common method of slowing the heat gain into a building (it's still used in less-developed parts of the world), and made sense in desert areas where wood for framing was scarce.  Back before Carrier invented air-conditioning, thick masonry walls and deep surrounding porches were among the very few methods of making the desert heat tolerable inside the house.

But the reality of construction in the Arizona desert today is quite different, since just about any conceivable construction material is readily available.  And since it's not likely you won't be installing air conditioning, thick wall masonry construction isn't necessary.

The root of the uniqueness of the Arizona climate is the lack of humidity and the southerly latitude.  Humidity is what holds the heat in the air; that's why it cools down so quickly at night in the desert (no humidity to hold the heat).  Your house will go through rapid and dramatic temperature swings all through the year, but especially in the summer months.  What you're after in create an energy efficient home is mitigating those swings – keeping the heat out during the day and keeping the heat in during the night.  If you design properly, you'll achieve a balance that requires very little air conditioning in the summer and little heat in the winter.

In temperate and hot-humid climates natural ventilation isn't much use since the outside air is hot, humid, and sticky day and night.  But in AZ, the low humidity makes the air temperature relatively unimportant.  The air isn't hot since it has little capacity to hold in the heat; it's the house itself that gets hot from the sun's radiation.  So bringing in breezes during the day will cool the house no matter what the outside temperature.  At night, close the windows and doors to keep the heat in.

Your choices of building materials and techniques should be more influenced by aesthetics, cost, and the local environment; in this climate, energy efficiency is more affected by the overall design strategy of the house.  There are many pros and cons of wood-framed and solid-masonry wall construction for a house – too many to list here, but one big drawback of masonry construction is the lack of design flexibility in everything from locating windows and doors to placing electrical outlets and hanging pictures.  You should also note that the least expensive construction methods are the ones most common in the area; anytime you do something other than the usual, it's going to cost more.  What's common where you're building?  You need to consider seriously where you want to spend your construction dollars and what their potential return is.

Here are a few design strategies to think about in the house design that will help take advantage of the climate and control energy costs:

Use a “courtyard” design to maximize the perimeter of the house, creating more exterior walls for more rooms in the house – more surface area for windows and doors.  The courtyard, facing north, also creates a large shady area accessible from many rooms.  A pool or fountain in the courtyard will also help to cool the courtyard through evaporative cooling.

No unshaded windows facing the sun – use deep overhangs above south-facing glass to prevent the sun from getting in at all.  Windows on the north don't need them.  Tinted glass can also help tremendously.

Water at the edges – anywhere you can place water near the house it will help create cooling breezes.

Reflective roofing – use light-colored roofing to reflect the sun's heat

Porches, verandahs, loggias – outdoor covered areas, the more the better, will create pockets of cool that will buffer the house from the sun.

Vegetation – block the sun with lots of trees and shrubs; cover the ground with plant material – the sand and rocks in the AZ desert absorb heat and can radiate it towards the house.

Orienting the house on the site – maybe the most important strategy of all.  A careful examination of the sun angles and topography can influence the overall shape of the house, the location of spaces, and go a long way towards reducing the house's energy dependency.

Regarding "built-up" ceilings, do you mean built-up roofing?  BUR is a commercial construction system for "flat" roofs made of layers of felt paper held together with bitumen, a coal-tar extract.  Not often used in residential construction - there are other better systems for residential "flat" roofs.

Hope this helps!

Richard Taylor, AIA
President
Richard Taylor Architects, LLC
www.rtastudio.com