Venting a radiant barrier airgap in roof?
GBA Editor
| Posted in Green Building Techniques on
I’m designing an R30 roof. The top layer is 7/16 radiant barrier OSB, with the foil faced down. These will rest on 1x sleepers, which are attached to the rafters through a 2″ layer of foil faced polyurethane foam board. Cellulose fill between the 2×6 rafters. Roof is 4:12 pitch in Southern California – tops out at 90 degrees F.
I’m wondering if it makes sense to vent the airgap space? Normally, this area would be vented from the soffit through to the ridge vent. However, venting this way would allow dust to enter and settle on the foil face of the foam board. Does an airgap always need to be vented?
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Replies
Roof venting was originally prescribed by code to eliminate moisture, then justified further by the reduction in summer heat gain, increase in composite roofing life, and decrease in cold-climate ice dams.
With today's highly insulated and air-tight buildings, it turns out that the most important function of roof venting, particularly in cathedral ceilings, is to allow drying of hygroscopic materials in the event of a roof leak. In hot, sunny climates it also reduces solar radiant flux into the conditioned space, though not as effectively as light-colored and reflective roofs.
If you have a light-colored roof , particularly metal or tile, then the radiant air gap is not going to significantly reduce radiant gain since it's already interrupted at the exterior roof surface.
Since, with foil-faced foam board, you shouldn't have an interior-sourced moisture problem at the roof deck and since the foam will act as a secondary drainage plane, there will likely be little or no advantage to venting that space.