Physics Photo of the Week

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.

Compare 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 deck. 

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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