Physics Photo of the Week
Positive and Negative Rafter Images
When frost forms on roofs during cold nights, one can see the outline
of the roof rafters. These two images were made on the same day
(Jan. 24, 2008) of different buildings on Warren Wilson College.
Why do the rafters show up white on the left image whereas the rafters
show up dark on the right image - like a photographic negative?
The answer is due to the differences in insulation of the two
buildings. The building on the right has recently had insulation
applied to the undersides of the roof deck whereas the building on the
left has no under-roof insulation, but has some insulation in the attic
floor. With the insulated roof on the right (where the rafters appear
dark), the roof is so well insulated that heat from the attic is
prevented from reaching the roof allowing the roof to cool more quickly
to the ambient air temperature. Thus frost collects on the
exterior of the roof. The rafters, however, have a much larger
thermal mass than the thinner roof boards. Because of their
larger thermal mass, the rafters retain their heat and prevent the
exterior from cooling to the frost point.
With the roof on the left where the rafters show the white of the
frost, the relative lack of insulation allows heat from inside the
building to keep the roof warm. The rafters on the left building
act as the only insulation and allow the exterior of the roof to
cool. Thus the frost images indicate a substantial thermal leak
and energy waste.
This will be the last wintry image to be shown on Physics Photo of the
Week for awhile, unless we get an April snowstorm. Time for
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to firstname.lastname@example.org.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
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