New vented reroof over existing
Have gotten involved in a commercial building where the contractor value engineered out the air barrier (zone 5A). Shingle roofing on pitched wood truss construction with batt insulation stapled to the bottom chord and a ly-in ceiling suspended below. The plans called for a layer of drywall on the bottom chord that they apparently gave a credit back to the owner for not installing. The design also included most of the ductwork as well as hot water lines in the attic, yes above the insulation. Since construction (~5 years ago) there have been severe ice damming with roof leaks and condensation on the underside of the roof sheathing. The contractor has doubled the ventilation and added electric icemelt systems but the problems still occur not to mention high heat bills. The problems have been pointed out and the laywers are drooling but now we’re trying to come up with the best solution and we’ve presented the idea of a new vented roof system with rigid insulation above the existing roof sheathing and closing off the ventilation to the existing attic. This would be much less disruptive than trying to install a ceiling or work in the attic but wondering if anyone has done this and if there are details out there for this retrofit.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Chris,
I can't believe that 5 years ago, there were designers, builders, and owners who would build a new building without a ceiling air barrier. I'm glad that the lawyers are drooling -- whoever builds this type of building needs a strong lesson.
Your solution is a good one; commercial roofs are built this way all the time. Just be sure that the rigid foam you specify meets minimum code requirements for ceiling R-value.
There are three ways to do this: using layers of rigid foam with staggered seams; using nailbase (or nailbase over foam); and using SIPs.
Installing ventilation channels (2x4s parallel to the rakes) above the top of the foam, and then another layer of roof sheathing, is optional; but, if the owner can afford the venting, I think it's a good idea.
Chris,
Ditto with Martin, plus, I would not count on the Rvalue for the batt insulation; it’s probably worthless as a $3 bill (Grade 3 installation). I would consider taken it down and spraying open cell foam under the roof decking, so you do not have to install so much rigid foam on top and it'll help you do a good job on the air seal.