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.
continued
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.
•••••••••••••• ••
•••••• •••••••
••••
• • • • •
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.
DIFFUSION
AIR LEAKAGE
4x8 sheet of
gypsum board
Interior at 70 F
and 40% RH
4x8 sheet of
gypsum board
with a 1 1/2 inch
hole
Interior at 70 F
and 40% RH
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