Could truss uplift create breaks in the ceiling vapor barrier?
David Adams| Posted inEnergy Efficiency and Durabilityon
在明尼阿波利斯。我们的工程建筑计划calls for using LVL ridge beams and I Joist rafters. Our general contractor has proposed using roof trusses in part to save money that we could back into the project elsewhere. We had planned the ceiling vapor barrier to consist of gaskets around any openings and a vapor retarding primer. I am concerned that truss uplift could break the vapor barrier by causing cracks in the ceiling drywall. Is this a valid concern? Are there any other problems or concerns I should have switching from a ridge beam with I joist rafters to roof trusses?
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Replies
David,
I think you are confusing air barriers and vapor retarders.
Ceiling cracks can interrupt an air barrier -- assuming you are using the Airtight Drywall Approach.
However, ceiling cracks will have no discernable effect on a vapor retarder.
Your sentence -- "We had planned the ceiling vapor barrier to consist of gaskets around any openings and a vapor retarding primer" -- makes no sense. Vapor retarders and vapor barriers don't require gaskets; however, air barriers do.
I will defer to Martin's response for the air barrier and comment only on truss uplift. If it is severe enough that it causes drywall cracking, it is not built correctly. The supports have to be tied down by code. Interior walls can theoretically be a problem but thiis should also be accounted for in the design. If not, the savings you may achieve is not really a savings.
Martin,
You are right I was using the wrong terminology. I was concerned about the potential that uplift could cause cracks between interior walls and the ceiling. The air barrier would be the gaskets and the drywall ceiling. The vapor retarder would just be the primer which it seems would also be compromised if any uplift caused cracking in the drywall and thus also the primer layer.
Corian,
As you posted if built correctly it should not cause cracking. Therefore if it did I assume the contractor would be obligated to correct the problem. I guess I would need to confirm this with the local building inspector. Thanks for your input.
David,
If a crack interrupts 1% of an area covered by vapor-retarder paint -- and that would be a huge crack -- then the paint remains 99% effective at reducing diffusion through the area under discussion. In other words, when it comes to vapor retarders, cracks don't matter.
Here's some more information on truss uplift solutions:
http://www.finehomebuilding.com/how-to/qa/dealing-with-truss-uplift.aspx
If you do a Google search on "truss uplift," you'll get lots of information on the issue, and advice on how it can be handled to avoid ceiling cracks.