Buildings Magazine - Buildings 2/2015

At 70 degrees F. and 40% relative humidity, a 4-foot by 8-foot sheet of gypsum board passes about ⅓ of a quart of water over one heating season in a cold climate. Given the same timeframe and conditions, if the board has a 11/2-inch square hole, the resulting air leak passes 30 quarts – 100 times more – and plenty of heat along with the moisture.

Fortunately, most roof systems include components
that function like air barriers, including gypsum board,
may be R- 25 or less. That degradation could have a signifi-
cant impact on heating and air conditioning systems that
have been designed to work with a higher R-value.

While skylights may reduce R-value, they can also provide daylighting that reduces energy for artificial lighting. The tradeoffs should be weighed.

3) Roof Surface

The roof surface can be a critical energy component, especially in warmer climate zones where insulation is minimal. High solar reflectance and high thermal emittance are very important properties in these zones.

White roofs reflect more heat energy than black roofs, resulting in a lower temperatures at the roof surface. For example, a white roof might reach a temperature of 110 degrees F., while a black roof in the same location might reach 175 degrees F.

Thermal emittance is the amount of energy that is given up by the roof. Metal roofs, for example, heat up quickly but also give off their heat quickly, much like a piece of aluminum foil in a toaster oven. Depending on the building and the location, high emittance may or may not be a desirable property.

As roofs get dirty over time, their solar reflectance may decrease, particularly if they have a low slope. After four years, a roof’s reflectance value might go from 0.7 to 0.55. In that case, the roof may require cleaning in order to regain its original reflectance.

White roofs are only one alternative for warm climate zones. Other ways to manage heat include installing vegetation or ballast on the roof surface.

4) Air Barriers

Air leakage is as important as insulation to thermal resistance. In fact, the more insulation that is required, the more important air leakage – and the heat that the air’s moisture carries – become.

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WIND UPLIFT resistance can be increased with metal bars near edge flashing or coping. If the flashing or coping is blown off, the bar may prevent a catastrophic progressive failure.

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METAL EDGE FLASHING
COPING
Continuous bar over membrane
Additional bar over membrane if
parapet height exceeds 12 inches
⅓ quart of water

30 quarts of water Continuous bar over membrane

AIR LEAKAGE IS AS IMPORTANT AS INSULATION to thermal resistance. The leak through a 11/2-inch hole passes 100 times more moisture, along with the heat contained in the moisture.