Replies

1. GBA Editor
   Martin Holladay | May 19, 2016 06:42am | #1

   Eric,
   If sheathing gets damp, it makes sense to provide some ventilation air to ensure fast drying. Most builders find that the easiest way to do this is with the rainscreen approach -- creating a ventilated air gap between the WRB (which is vapor-permeable) and the siding.

   In theory, you could instead provide a ventilated gap between the inner surface of your sheathing and the outer surface of the insulation installed between your studs. The main reason builders don't do this is the difficulty of establishing an exterior air barrier. It's fairly easy to tape the seams of the OSB or plywood sheathing to create an air barrier; with your suggested technique -- inserting dozens of ventilation baffles between the studs -- the creation of an exterior air barrier becomes far more challenging.

   If I were you, I would stick with the usual location for this ventilated air gap -- between the WRB and the siding.

   For more information on walls, see How to Design a Wall.

2. 
   ecolbeck | May 19, 2016 07:10am | #2

   Thank you Martin for your patience in responding. I apologize if my question made you "sigh." :) I am curious, however, as to why the exterior air barrier is paramount with wall construction but not with ceiling construction. What am I missing here?

3. GBA Editor
   Martin Holladay | May 19, 2016 07:16am | #3

   Eric,
   It's true that many builders are sloppy about ceiling air barriers. That's why so many cathedral ceilings have moisture problems ("raining" ceilings due to condensation, or sheathing rot).

   In fact, ventilation baffles should always be installed with attention to airtightness. That's what GBA has recommended for years. The fact that most builders ignore our advice is not encouraging.

   For more information on this topic, see Site-Built Ventilation Baffles for Roofs.

4. 
   charlie_sullivan | May 19, 2016 09:53am | #4

   As are as the question of why the exterior air barrier is considered more important for walls than for ceilings, there is the difference in recommendations: attic insulation is considered OK without an air barrier on top of it, even though detailing the sides with an air barrier is recommended (though not always practiced--the supposedly premier insulation company in my area recommended using fiberglass batts for the insulation dam at the soffit!). There are a number of reasons for accepting attic insulation without an air barrier on top. One is that the direction of potential convection loops and wind washing makes open-top attic insulation less problematic. The other is that even if the insulation effectiveness is reduced by those effects, it's easy and cheap to increase the attic insulation thickness to compensate, just by blowing more in, whereas in a wall, making it thicker can be expensive.

   That said, the open top is part of the reason that cellulose is better attic insulation than fiberglass--it retards the convection loops better and blocks radiation from penetrating into the insulation, an effect that degrades the fiberglass performance a bit. You could probably get fiberglass attic insulation to perform more like cellulose by putting an air barrier over the top, but I've never heard of anyone doing that, since it's cheaper to just use cellulose.

   Back to your idea of having double-sided sheathing ventilation, the way I would answer the question of "what do people do this on a roof but not on a wall" is that a roof needs to be waterproof, and the waterproofing prevents it from breathing well, so you need an alternative. But I do think it's worth thinking about now that we have walls that are so well insulated that we aren't drying the sheathing with heat from a 120 kBTU/h furnace.

   Essentially, you are suggesting separating the functions of the sheathing into two layers--a baffle that holds the insulation in and provides the air sealing, and an OSB layer that provides the structural bracing. I assume the advantage of that would be that it would allow using a baffle material that is more vapor permeable than OSB. I'm not sure what you'd want to use for that--a plastic baffle, as is sometimes used in rafter vents, would be a vapor barrier, so that's no good. Cardboard can be used in attics, but I don't think it would be strong enough to support dense packing. So the options I know of would be fiberboard or a robust housewrap material like Mento.

   But then, if you are only using the sheathing to provide bracing, why not do the bracing with wood diagonal members? In a traditional stud wall, that requires let-in bracing which is labor intensive. But in a double stud wall, the bracing can go in the space between the two walls. That's quick and easy, and then whatever material you decide is adequate to hold in the insulation and act as an air barrier can go on the outside of the wall with no sheathing. Still, you might be nervous about the robustness of the outside layer, to stay intact as an air barrier, and to keep the insulation in place. But I think that design would be easier to construct than the double-vented sheathing design.

   If you want one material that can do all three functions--air barrier, hold the insulation in, and provide bracing--and you want something with more vapor permeability and moisture resistance than plywood (even though plywood is better than OSB on both counts), consider DensGlas fiberglas/gypsum sheathing.