February 2, 2006
Frost Patterns on Roofs
Last Friday, Jan. 27, 2006, a heavy frost appeared on the
roofs of all buildings
due to the surface temperature falling below the freezing point for
wate. For frost to form, instead of frozen dew, the dewpoint must
also be below freezing. Notice how the rafters show up on this
photo. The frost pattern is quite sensitive to the surface
temperature. The surface temperature of a roof for example is
very senstitive to the temperature of the space enclosed by the
building and the quality of insulation in the building. This
building, Spidel Hall on the Warren Wilson Campus, recently had extra
insulation installed on the undersides of the roof boards in the
attic. The insulation was a spray of polyurethane foam of about 2
inches thick. As the frost pattern indicates, the roof over the
rafters is warmer than the roof surface between the rafters. This
shows that the insulation is doing its job! The roof deck
consists of the shingles plus about 3/4 inch decking. The
rafters, however, are beams about 6 inches thick. The rafters
hold their heat from the previous day and thus keep the roof warm
preventing some frost from accumulating at the rafter position.
the above image of the recently insulated Spidel to another building on
campus. In the image on the right, the rafters appear to be
whiter and frostier than the roof decking between the rafters.
This building is clearly losing heat through the roof, and the rafters
are providing additional insulation that is absent on the main roof
Frost patterns are an inexpensive way to monitor your roof for thermal
efficiency. If you see less frost on the roof in areas between
rafters, or where the roof is not part of the overhang, you are losing
considerable heat through the roof. Improved insulation and lower
indoor temperature settings will minimize the thermal loss. A
much more expensive and quantitative method to monitor heat loss is to
employ a special infrared imager that senses the small variations in
surface temperature. Such a device will work in any weather.
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