Are plastic ventilation baffles a condensation risk?
ranson
| Posted in Green Products and Materials on
I was looking at the design of smart baffles, and I don’t understand why they’re a good idea. They seem like a cold side vapor barrier. Wouldn’t any moisture that migrates through the adjacent insulation in mid-winter condense and be trapped at the surface of the baffle?
–John
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
John,
I addressed your question in my article, Site-Built Ventilation Baffles for Roofs.
In that article, I wrote:
One of the reasons that builders install ventilation channels is to help damp roof sheathing dry out. Researchers now realize that ventilation channels can help a little bit at this task, but not as much as some people think. (In fact, during the summer, ventilation channels can actually add moisture to roof sheathing rather than remove it...)
The most important way to prevent the migration of moisture from the interior of a house to the roof sheathing is have a good air barrier at the ceiling. The reason is simple: the usual transport method for this moisture is air leakage, not vapor diffusion. Because of this fact, installing ventilation baffles that are airtight makes this type of roof assembly more, not less, robust.
What if interior moisture is able to reach the underside of a ventilation baffle — isn’t it possible that the moisture might condense against the baffle (especially if the baffle is cold)? If so, isn’t this a good argument in favor of using vapor-permeable materials (for example, fiberboard, cardboard, or thin EPS) for ventilation baffles?
The answers to both questions is a qualified yes. Anyone worried about this possibility should probably make their ventilation baffles out of a vapor-permeable material.
That said, there really aren’t any reports of failures or problems resulting from the use of vapor-impermeable materials — for example, polypropylene, vinyl, or foil-faced polyiso — to make ventilation baffles. The main reasons:
Not much moisture manages to make its way to the ventilation baffles (especially in homes that pay attention to airtightness);
The air in the ventilation channels is often warmer than outdoor air, a fact which limits condensation; and
Any moisture that does make its way there seems to be incorporated into the rafters via sorption. The ventilation channels are able to remove a limited amount of moisture from the rafters, and it appears that the rate of drying exceeds the rate of wetting.