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We compared an ASTEC-coated metal roof without insulation with
another metal roof having 10 cm of polyurethane foam. This allowed
us to calculate the surface temperature under identical simulated
environmental conditions. In addition, the roofs were given a
"typical" 40 m length to determine the thermal shock reduction
provided by ASTEC.
The following steps were followed in the calculations:
1. The thermal properties (for conduction) of the materials (metal
roof, ASTEC, polyurethane foam) specified were based on ASHRAE
Fundamental Handbook data.
2. Environmental conditions were given as follows:
a. Outside Air Temperature: 35˚ C
b. Inside Air Temperature with Air-Conditioning: 24˚ C
c. "Clear day" solar radiation factor: 905 W/m2
3. Constant Values:
a. The Stefan-Boltzmann constant (for radiation).
b. Air film coefficient (for convection): we used identical air film
coefficients for both, the outside air and the inside air.
CONCLUSION:
The ASTEC coated roof surface temperature was calculated at 37.55
C˚.
The roof sprayed with PU foam has a surface temperature of 86.83 C˚.
If the PU were to be sprayed on the underside of the roof as it is
sometimes the case to protect the PU from ultra-violet degradation
and from water induced delamination, we measured the thermal shock
on the bare metal roof as opposed to an ASTEC covered roof. Using
the coefficient of expansion for galvanized metal and the delta in
roof surface temperatures (T1 - T2) for a given length (40 m), the
calculations further conclude that the ASTEC coated roof would
reduce thermal shock from 28.82 mm on the metal roof to 1.42mm on an
ASTEC covered metal roof. This represents a 95% reduction based on a
40m in length metal roof.
It should be noted that ASTEC is practically impervious to UV
attacks due to its hard ceramic component. Moreover, the ASTEC
"system" provided added benefits in the form of corrosion control
and waterproofing. The life cycle cost of ASTEC proves to be more
cost effective than conventional mass insulation.
In terms of heat transfer only, 4 inches (10 cm) of PU foam would
perform better than ASTEC alone. However, heat transfer alone is
only part of the solution. Cost effectiveness considers heat gain
for thermal shock measurement, heat transfer for energy savings,
waterproofing for roof preservation, corrosion control for
structural integrity, and many other value-added benefits only
offered by ASTEC. If a project owner insists on PU purely for heat
transfer purpose without consideration of cost effectiveness and
other value-added ASTEC benefits, the thickness of PU (therefore,
the cost) can be minimized and the energy savings increased by
adding the ASTEC finish coat to the system. The finish coat of
ASTEC
#900 would reflect some 85% of the solar radiation — thus enhancing
the efficiency of the PU in managing 15% of the absorbed solar
radiation instead of the 90% absorption rate of a metal roof.
Considering the "Total Solution" there is no doubt that ASTEC
outperforms 4 inches (10 cm) of polyurethane foam.
