Getting the Roof Right a Key to Good Design

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Monday, July 21st, 2014
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Designing and building high-performance structures offers many chances for under-performing details, particularly when it comes to roofing.

Some design elements are problematic in and of themselves, and some components can be problematic unless they’re installed perfectly. Aesthetics are one thing, but practicality is another. For practical reasons, it is imperative to get roofing details right.

CAD systems make designing a complex roof fairly easy, but even if a roof performs fine in the modeling software, it still has to be built, insulated, and air-sealed to specs. A simple gable or hip roof, or as close a design as you can come up with, will reap rewards for the designer, builder, homeowner, and remodeler.

“Chopped-up roofs with a variety of intersecting planes are hard to frame, hard to keep watertight, and hard to vent,” said former builder Martin Halladay, who writes for Green Building Advisor. “Every nook and cranny creates somewhere for pine needles and ice to accumulate. You don’t want any nooks and crannies on your roof.”

Roof design doesn’t have to be complex to look good, and simple designs are more functional, cheaper, and easier to re-roof. Plus, simple roof designs offer more space for installation of PV panels.

Another key to ensuring good practice is to ditch the idea of skylights, or “roof windows,” as they’re sometimes called. Since they are really windows, they’ll perform as such, with an R-value of about two to four. This means you can have a well-insulated, high-performance roof that’s built to last, combined with a few holes that waste a lot of energy.

“A skylight in the roof of a house will typically lose 35 percent to 45 percent more heat during cold weather than the exact same window installed on the side of the house,” say the writers at Energy Realist. “That’s because heated air rises.”

Skylights can let massive amounts of heat into a home, as well. Energy Realist states that “a 2-foot by 4-foot skylight made with a single pane of clear glass will allow enough heat into a home to make a typical air conditioner use an additional 240 kilowatt hours of electricity per year. Based on an average kWh rate of about 8 cents, that means that one clear skylight could boost your electric bills about $19 per year for extra cooling.”

They are also prone to leaking. Although manufacturers have certainly made great strides in preventing skylight leaks, a lot depends on the quality of installation, so the potential for leaks will always be present.

“In one place we could poke a pencil through gaps in the flashing, on all four corners, to the outside air,” said one Energy Realist writers, who works as a home energy auditor, of a particular installation.

Another roof hole that’s easy to avoid is downlights or recessed can lights in insulated ceilings. They’re notoriously hard to seal, and therefore allow air to flow back and forth into and out of the conditioned space. They also permit moisture to flow, which can lead to moisture issues in attic insulation. And finally, a lot of heat can move into the cooler space, whether that’s the attic or interior space, through leaky fixtures.

This test of light bulbs demonstrates how much heat even LED bulbs generate:

● An LED lamp at 28°C room temperature showed a heat sink temperature of 60°C to 100°C depending on the make and model of the LED bulb, room temperature and airflow.
● A CFL lamp in the same test was running a glass temperature of 120°C and electronics temperature of 85°C.
● Incandescent and halogen bulbs were as hot as 181°C, and sections of the glass on a CFL bulb were as hot as 131°C

While there are plenty of ways to ensure good design, a look at oft-ignored roof issues shows how simple common sense can save money, hassle and headaches down the line.

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