Appropriate details help keep a house cool
People who live in Florida or Texas often accuse energy-efficiency experts of having a cold-climate bias. They’re right: most energy-saving tips are written with cold-climate buildings in mind — perhaps understandably, since Americans spend about twice as much for residential heating as they do for cooling.
Whatever the origins of this pervasive cold-climate bias, it’s time to rectify the situation with a few hot-climate design tips.
We’re not in Kansas anymore
Most builders know that house designs need to be climate-specific. In areas of the country where air conditioning bills are higher than heating bills — as they are along the Gulf Coast and in much of Florida — homes should be designed to reject exterior heat.
So what are the most important factors governing hot-climate design?
- In Florida, a home’s building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. faces a much smaller delta-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures. than in Maine. (Delta-T is the difference between indoor and outdoor temperatures.) While the maximum delta-T in Maine might be 90 F°, it’s probably only 30 F° in Florida. One consequence of this fact: insulation (especially floor and wall insulation) can be thinner.
- It’s just as important to minimize air leaks south of the Mason-Dixon line as it is up north. Air leaks not only increase a home’s sensible cooling load; by introducing humid exterior air, they also increase a home’s latent cooling load, making the air conditioner work harder to wring moisture out of the air. Tight houses are much easier than leaky houses to keep cool and dry.
- While cold-climate designers have to worry about heat escaping from the envelope in all directions, the heat-gain concerns of hot-climate designers are overwhelmingly dominated by just three major factors: windows, ceilings, and internal gains. On summer afternoons, one large west-facing window with the wrong type of glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. can make a huge difference in a home’s heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss..
Over the past decade, our understanding of the best building practices for hot climates has been significantly advanced by researchers working at the Florida Solar Energy Center. Useful hot-climate research has also been conducted by engineers from the Building Science Corporation; much of this research has been funded by the Department of Energy’s Building America program. The following advice represent a distillation of researchers’ findings.
- Orient the house with the long axis east-west.
- A slab foundation should have perimeter insulation (unless termite concerns preclude it) but no sub-slab insulation. Uninsulated slabs can actually reduce a home’s cooling load.
- Shade is good. Roofs should have wide overhangs, ideally 3 feet wide or wider. Hurricanes like to grab onto roof overhangs, though, so be sure to secure roof trusses or rafters to top plates with adequate hurricane clips. Since a hipped roof can shade all four sides of a house, hipped roofs are preferable to gable roofs.
- Most windows should face north or south. Because they are harder to shade, east- and west-facing windows contribute much more to overheating than north- or south-facing windows; so east- and west-facing windows should be minimized.
- Every effort should be made to shade every window. Windows can be recessed into thick walls or protected by projecting architectural elements. On the east and west elevations, it’s often best to protect any windows with a wide porch.
- It is critical for a home’s air handler and all ductwork to be within the home’s thermal envelope. One way to do this is to “cathedralize” the attic by spraying closed-cell foam insulation against the underside of the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . Of course, duct seams should always be carefully sealed; slightly oversized ducts are better than undersized ducts.
- Ceilings or roofs should be insulated to at least R-30.
- If the house has an unconditioned attic, specify radiant-barrier roof sheathing.
- Use highly reflective roofing — ideally, white metal roofing or white concrete tile roofing.
- Wall insulation is much less crucial down south than it is up north; 2 inches of rigid foam (R-10) is probably plenty. If the house has concrete-block walls, install the insulation on the exterior, not the interior.
- Specify windows with a solar heat-gain coefficient (SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.) below 0.32; if possible, aim for 0.28 or 0.29.
- The home’s thermal envelope should be carefully air sealed.
- While high internal loads — that is, waste heat from lights and appliances — benefit cold-climate houses in winter, such loads hurt the performance of hot-climate houses in summer. So in Florida and Texas, it’s particularly important to install CFLs or LEDs rather than incandescent bulbs and to specify the most efficient available appliances, including refrigerators and televisions.
- Ceramic tile floors are best. Avoid carpeting.
Although many of these suggested measures may seem obvious, they are widely ignored. When I last visited Nevada, I noticed that large residential developers like Pulte Homes don’t bother to orient their buildings to the points of the compass; rather, new homes are plopped down at random. Driving around Las Vegas, it’s easy to see hundreds of new homes with stingy roof overhangs and unshaded windows exposed to the full glare of the Nevada sun.
Mixed climates may require compromises
Remember, if you live in a mixed climate — that is, one with significant heating as well as air conditioning bills — you’ll need to include a selection of measures that help a home retain heat in the winter as well as reject heat in the summer.
Last week’s blog: “Passivhaus Windows.”
- Florida Solar Energy Center
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