Dow Protected Membrane Roof EN - Manual (Page 16)

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1 5
Conditions, Issues and Ratings
S P E C I A L C O N D I T I O N S A N D I S S U E S
Cold Rain
Phenomenon
THE ISSUE
“Cold rain phenomenon” (or
“cold water wash”) occurs dur-
ing periods of cold rain and/or
melting snow or when the ambi-
ent condition is 33°F to 50°F.
In these conditions, the deck
temperature may be temporarily
reduced. The issue is that there
may be additional heat loss, and
in buildings with high humidity,
such as pulp and paper mills,
the likelihood of condensation
increases.
DISCUSSION
Increased heat loss: Heat loss
studies have shown that extra
heat loss in PMR systems during
periods of “cold rain” is a tem-
porary phenomenon, occurring
only during the short time of
cold rain in a heating season. In
fact, cold rain in the cooling
season creates a cooling advan-
tage for a PMR system. Studies
comparing a conventional versus
PMR assembly show only a 3
percent overall heat loss disad-
vantage for the PMR assembly.
High temperature/high humidity
buildings: According to NRCA, a
building with 45 percent RH is
considered high moisture occu-
pancy. Other buildings, such as
pulp and paper mills, textile
mills and natatoriums, can have
an even higher internal humidity.
Combining high humidity with
a higher than normal operating
temperature results in a “high
temperature/high humidity”
building environment that
requires special design
consideration.
The severe operating conditions
of high temperature/high humidity
buildings are particularly prob-
lematic for conventional roof
systems. The high temperatures
drive the high humidity up
into the roof system, resulting
in severe condensation and
premature deterioration of the
insulation and roof deck.
A PMR system offers an inher-
ent design solution for this
moisture problem. The water-
proof roof membrane is an
excellent vapor retarder. With
the membrane directly on the
roof deck and the insulation
above the membrane, the mem-
brane effectively blocks water
vapor from reaching the insula-
tion. Also, the membrane is
maintained at a temperature
near that of the interior, dramat-
ically reducing the probability of
condensation on the membrane
and minimizing the possibility
of premature roof failure.
The “cold rain phenomenon”
can change this situation. Cold
rain, filtering past the insulation
to the membrane, can cool the
membrane and the deck below,
resulting in a temporary con-
densation condition on the
underside of the deck. For some
businesses, like pulp and paper
mills, this dripping condensation
can create problems with the
manufacturing processes and
products, causing decreased
productivity and increased
production costs.
To solve this problem, a thin
layer of insulation can be placed
below the membrane. This layer
keeps the roof deck warm during
brief cold rain periods – main-
taining the inherent advantages
of a PMR system while mitigat-
ing the problem of the “cold
rain phenomenon.”
See TechNote 507: “STYROFOAM ™
Insulation in the Optimum
Design System for Pulp & Paper
Mill Roofs” for additional details.
CONCLUSION
The effect of “cold rain phe-
nomenon” is temporary and
does not have a significant over-
all effect on the performance of
a PMR assembly. Generally,
thicker amounts of insulation
are not required to counteract
the negative effects of cold rain.
In high humidity and high
temperature applications, sand-
wiching the membrane between
two layers of insulation, coupled
with a vapor retarder on the
roof deck will address condensa-
tion problems in high humidity
roofing systems. Remember that
the thicker insulation layer
should be above the membrane
to ensure the dew point is above
the membrane.