Proper Rafter Vent Construction
In building rafter vents in 2×8 rafters with a 2 inch vent at the top of the rafter bays allowing air flow from the soffit vents to ridge vent, is it better to use plywood vs foam board to create the vents because plywood may have a higher perm rating which may increase the chances of water vapor drying to the outside? Or would there be a greater risk of moisture condensing on that plywood causing it to rot over time. This plywood would be air-sealed with a quality air-sealing tape, resulting in moisture having nowhere to go except back into the attic space which is heated passively by the heated living space below. Also, the rafters below the constructrd vent will be insulated with Rockwool and drywall.
Would using foam board that is adhered to the rafters with spray foam adhesive to create the rafter vents solve the potential rotting plywood problem, but increase the potential problem of rotting rafters because the foam board can’t dry to the outside whereas maybe plywood can?
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Replies
dajn,
Plywood is definitely safer. Impermeable foam relies on moisture moving through the framing members to dry. The plywood baffles are not at risk as they have good drying potential into the vent cavity above. Another alternative is to use house-wrap for the baffles.
https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs
Thank you, it sounds like plywood is the optimal way. The house wrap is a good idea, I just prefer something more solid.
dajn,
Plywood is a lot easier than house-wrap in each rafter bay. If this is new construction though this might be simpler than both options, and yields a really well performing roof assembly:
https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
These sealed vent baffles are generally not needed and if done improperly can sometimes create moisture issues. It only really makes sense if you are dens packing the rafter bays.
The better option is to detail the drywall or membrane above it as the main air barrier and only put baffles by the soffit intake area. If you use high density batts, you can skip the rest of the baffles as these don't suffer from wind washing.
If you really must have vent baffles, the order I would go:
-house wrap
-fiberboard
-CDX
Housewrap is pretty easy. Get a small roll and cut a bit less than the width needed on a miter saw. Unroll and staple to your blocking/underside of I joist flange as you go.
I like this idea, it would save time and materials. But, if there are no vent baffles, and thus no air seal at the the top of your insulation, would cold outside air infiltrate the insulated rafter bays, thereby reducing efficiency of the insulation? Owens Corning Eco Batts say they're high density, are these acceptable, or is there a certain type of high density batt that is needed?