| It has recently become
popular in cold regions to require more and more heat resistance in roofs.
Some designers advocate super-insulated roofs, where the roof insulation
might exceed the wall insulation by three or even four times. This raises
the first cost of the building and often cannot be justified by energy
savings. In cold country, the problem with this approach is that the thickness
or thermal capacity of the heat resistant material is not the only factor
in effective thermal roof function. Often cracks between insulating batts
allow warm, moist air to reach the outer roof surface, effectively circumventing
the insulation regardless of its thickness.
The retardation of moisture is as important as thermal resistance.
Warm air retains more moisture than cold air. It is desirable to keep
residential building at approximately 50% RH during winter months. If
the buildings are constructed without a proper vapor retarder on the
warm side of the insulation, condensation may occur either inside the
insulation (reducing its effectiveness) or on the under side of the
roof surface. This condensation may appear as roof leaks, even though
the roof surface itself is perfectly dry. The migration of moisture
through the roof may actually reduce the thermal properties of the roof
as the insulation increases. The higher the internal moisture (vapor
pressure) and the colder the outside temperature, the better the vapor
retarder must be. Vapor pressure is a powerful force and moist air can
find its way through the smallest crack.
Even in cold locations where there is very little precipitation, effective
control of the moisture inside a building is vital to the success of
roof design.
Great internal damage can occur to buildings whose interior air is warm
and moist if that air reaches the inside face of the exterior skin of
the building. For these reasons the cold must be understood and be considered
in the design of buildings.
Snow adds load to roofs where it falls on them. These loads are often
more like dead loads than live loads. On low-sloped (flat) roofs these
loads are often thought to be some fraction of ground snow load, but
this can be a dangerous assumption as will be discussed below. Most
designers assume that steeply sloping (4 in 12 or greater) roofs will
shed the snow and many codes allow a reduction in load for such roof.
But, the author's experiences have shown (Figure 2) that even slippery-sloped
roofs do not reliably shed snow. The snow can be held on the roof by
projections such as vent pipes, skylights or even standing metal ribs.
(Figure 1) Perhaps the most serious deterrent to sloping roofs sliding
their snow blanket is the ice dams that form at roof edges and behind
roof projections. Building heat or solar radiation can cause the snow
on the roof to melt. The melt water runs down the roof slope under the
insulating blanket of snow and refreezes when it comes in contact with
cold outside air.
The ice formed adheres to the roof surface and prevents the snow from
sliding as is discussed below. These ice dams can greatly increase roof
loads and can cause very serious leaks.
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