Abstract
A white roof, cool roof, is constructed to decrease thermal loads from solar radiation,
therefore saving energy by decreasing the cooling demands. Unfortunately, cool roofs
with mechanically attached membrane, have shown to have a higher risk of intermediate
condensation in the materials below the membrane in certain climates (Ennis & Kehrer,
2011) and in comparisons with similar construction with a darker exterior surface
(Bludau, Zirkelbach, & Kuenzel, 2009). As a consequence, questions have been raised
regarding the sustainability and reliability of using cool roof membranes in Northern U.S.
climate zones.
A white roof surface reflects more of the incident solar radiation in comparisons with a
dark surface, which makes a distinguished difference on the surface temperature of the
roof. However, flat roofs with either a light or dark surface and if facing a clear sky, are
constantly losing energy to the sky due to the exchange of infrared radiation. This
phenomenon exists both during the night and the day. During the day, if the sun shines on
the roof surface, the exchange of infrared radiation typically becomes insignificant.
During nights and in cold climates, the temperature difference between the roof surface
and the sky can deviate up to 20°C (Hagentoft, 2001) which could result in a very cold
surface temperature compared to the ambient temperature. Further, a colder surface
temperature of the roof increases the energy loss and the risk of condensation in the
building materials below the membrane. In conclusion, both light and dark coated roof
membranes are cooled by the infrared radiation exchange during the night, though a
darker membrane is more heated by the solar radiation during the day, thus decreasing
the risk of condensation.
The phenomenon of night time cooling from the sky and the lack of solar gains during the
day is not likely the exclusive problem concerning the risk of condensation in cool roofs
with mechanically attached membranes. Roof systems with thermoplastic membranes are
prone to be more effected by interior air intrusion into the roof construction; both due to
the wind induced pressure differences and due to the flexibility and elasticity of the
membrane (Molleti, Baskaran, Kalinger, & Beaulieu, 2011). Depending on the air
permeability of the material underneath the membrane, wind forces increase the risk of
fluttering (also referred as billowing) of the thermoplastic membrane. Expectably, the
wind induced pressure differences creates a convective air flow into the construction i.e.
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air intrusion. If the conditions are right, moisture from the exchanging air may
condensate on surfaces with a temperature below dew-point.
The definite path of convective airflows through the building envelope is usually very
difficult to determine and therefore simplified models (Künzel, Zirkelbach, & Scfafaczek,
2011) help to estimate an additional moisture loads as a result of the air intrusion. The
wind uplifting pressure in combination with wind gusts are important factors for a
fluttering roof. Unfortunately, the effect from a fluctuating wind is difficult to estimate as
this is a highly dynamic phenomenon and existing standards (ASTM, 2011a) only take
into account a steady state approach i.e. there is no guidance or regulations on how to
estimate the air intrusion rate. Obviously, a more detailed knowledge on the hygrothermal
performance of mechanically attached cool roof system is requested; in consideration to
varying surface colors, roof air tightness, climate zones and indoor moisture supply.