Physics Photo of the Week

April 2, 2010

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 Spring!

Physics Photo of the Week is 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 

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 individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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