Hi GBA,

We are in the process of designing an indoor pool building based off of the general concepts in Dr. Joe Lstiburek’s article “In the Deep End” (https://buildingscience.com/documents/insights/bsi-055-in-the-deep-end), however, mine is a residential building with wood frame construction and an asphalt shingle roof with wood trusses. I’m in Zone 4 (Maryland).

In the article, he recommends a liquid applied vapor barrier a liquid-applied vapor control layer yielding a Class III rating (something between 1 and 10 perms wet cup) for the ceiling vapor barrier (he says that if you are in Climate Zone 6 and higher, go to Class I and consider a foil-backed gypsum and either way, to stick with something that sticks to the gypsum board: either a coating on the inside, or a foil-facing stuck on the topside). His design, however, uses a drop ceiling, so it doesn’t matter if you use a foil-backed gypsum (which to my knowledge is only manufactured for exterior applications with the non-foil side not being a “finishable” surface) or if you apply a liquid membrane on the inside of the ceiling, because in either case they would be hidden by the drop ceiling.

We would like to avoid a drop ceiling in our design and are looking for some guidance on the best way to proceed. We’ve the following considered 3 options and would greatly appreciate your feedback on these or any other ideas or suggestions that you may have:

1) Option 1 – Use same Bituthene 3000 membrane that we’re using for the exterior walls. We could wrap a foot or so of extra Bituthene over the top plates on the walls before the trusses are set and then connect them to more Bituthene that we attach to the bottoms of the trusses before the ceiling drywall is set, creating a continuous vapor barrier. We see 2 main disadvantages with this approach: a) the Bituthene 3000 is very expensive so we’d prefer not to use more of it than we need to; and b) the install would probably be very difficult because we’re concerned about getting it to stick to the bottom of the trusses and not sag too much before the ceiling drywall is set.

2) Option 2 – Attach paperless gypsum board ceiling panels directly under the trusses (like standard construction) and use a liquid applied vapor barrier membrane on the inside (bottom) of the paperless gypsum board. Then in order to cover up the painted or sprayed on membrane, instead of using a drop ceiling we would just attach another layer of paperless gypsum board under the first layer (we would attach using glue and drywall screws). We see one main disadvantage with this approach, which is that the first layer of drywall isn’t the best surface to use for attaching the second layer of drywall, especially if they may see some humidity/water vapor from time to time.

3) Option 3 – Same as Option 2, except we would attach plywood panels under the trusses, apply the liquid applied vapor barrier membrane on the inside (bottom) of the plywood and then attach the layer of paperless gypsum board under the first piece (we would attach using glue and drywall screws). This would be a little more expensive than Option 2, but the plywood would be a better surface to use for attaching the bottom layer of gypsum board. One disadvantage is that the plywood might be a little less vapor permeable than the paperless gypsum board (although both are pretty permeable).

As mentioned above, we’d prefer to avoid Option 1 if possible. Do you think we’d have moisture issues with Option 2 or 3 caused by sandwiching the vapor barrier directly between two other layers (either 2 layers of drywall or 1 layer of plywood and 1 layer of drywall)? Since all of these materials are pretty vapor permeable (10 or more perms) they should be able to dry (either to the inside for the bottom layer or to the exterior (the vented attic) for the top layer if there are imperfections in the vapor barrier) and condensation shouldn’t be an issue since all of these layers will be inside of the thermal control layer (R38 blown-in cellulose insulation sitting right on top of these layers in the attic). Does all of that sound right, or if we have imperfections in the membrane (as stated in the article, its bound to happen) and water vapor penetrates and wets the top layer (whether its gypsum board or plywood) should we be concerned that the cellulose would get wet or would prevent the top layer from drying to the exterior (vented attic).

We’d love to hear any other ideas as well. Thank you all in advance for your help with this!!

Best,

Dan Theman