Solar vapor pressure
kurtz
| Posted in Energy Efficiency and Durability on
Could someone please direct me to some literature discussing the concept of solar vapor pressure? (Sun hits asphalt shingle roof wet from recent rain shower and drives moisture through into the attic where it condenses on the insulation that is cold from the AC.)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
David,
Here is an excerpt from an article ("Sunlight Drives Moisture Through Asphalt Shingles") that I wrote for the January 2003 issue of Energy Design Update:
"Because of night-sky radiational cooling, roofs are typically wet with dew at dawn, and some of the dew is drawn between the laps of the shingles. When the sun heats the roof — often to 150°F or higher — the moisture evaporates and is driven into the asphalt felt and roof sheathing.
“The phenomenon of solar vapor drive through asphalt shingles exists,” says Lstiburek. “It’s there. We should have known about it all along, and we would have if any of us had bothered to look at a psychrometric chart — duh.”
Whether or not the moisture driven into a roof assembly by the sun will cause any problems depends upon the roof construction details. According to Rudd, the unvented roofs in Houston were designed to exclude exterior moisture, so the solar-driven water vapor amounted to a “flaw.” Lstiburek does not anticipate any performance problems from solar-driven moisture into cathedralized attics, where the moisture can dissipate into the attic space, but he feels it could be a problem in cathedral ceilings with painted gypsum board as the interior finish.
“The phenomenon matters a great deal with unvented cathedral-ceiling roofs,” says Lstiburek. “In a cathedral ceiling, the moisture buildup could be cumulative. And we’re finding that this will happen as far north as Chicago.”
To prevent solar vapor drive into roof assemblies, Lstiburek and Rudd advocate installing a vapor barrier between the asphalt shingles and the roof sheathing. Ordinary polyethylene, which is degraded by sunlight and dangerously slippery, should never be installed on a roof. Lstiburek recommends the use of a vapor-barrier roof underlayment called Tri-Flex 30 (manufactured by Flexia Corp., Brantford, Ontario, Canada).
Mystery Ridges
Lstiburek says that switching from asphalt felt to Tri-Flex 30 not only solved the Houston inward vapor-drive problem, it also solved the problem of asphalt shingle buckling. Although Danny Parker, a senior researcher at the Florida Solar Energy Center, concluded that the mysterious ridges in asphalt shingles installed over insulated roofs were caused by plywood sheathing exposed to high temperatures (see EDU, May 2001), Lstiburek and Rudd are convinced that solar-driven moisture, not high temperature, causes the ridges. Although the exact mechanism is still uncertain, the ridging appears to be caused by either swollen asphalt felt or the swollen edges of the roof sheathing panels. Whatever it is that pushes up the shingles, the installation of a vapor barrier over the sheathing appears to solve the problem." "