ASHRAE Journal - August 2012 - 67

Photo 11: Roof-to-Wall Air Seal. Note the connection of the new roof deck membrane overlapping the trimmed back original gable overhang and catching the “white” housewrap water control layer and air control layer behind the wall exterior insulation (the “blue” layer of foam). The three layers of rigid insulation have yet to be installed over the top of the roof decking at this point. This air seal is pretty darn good. Note that there was no air seal at this location originally. Dumb. I have to come clean here as well. I resided 17 years over one layer of continuous rigid insulation. Not enough. Dumb. Should have installed much more. I will get around to fixing this eventually as well. Yes, I will share the experience. I can point out just how dumb I was on the walls as well 17 years ago. But the real mistake was not making the air seal at the top of the walls 17 years ago. No excuse, I knew better then. Got sloppy, careless. It happens, even to self-proclaimed gurus. later—maybe a lot later—the rafter extension approach was the one I went with. One of the keys to over-roofing is to tie the air control layer of the roof assembly to the air control layer of the wall assembly. On the gable ends this was pretty easy. The fully adhered membrane wrapped down over the cut back roof deck to tie into the wall water control layer and air control layer (Photo 11). On the front of the roof edges the original roof deck sheathing was cut back flush with the wall and the fully adhered membrane folded down around the rafter tails to catch the wall water control layer and air control layer. This was then “foamed” to complete the air seal. Go back and check out Photo 10. The finished roof hides the insulation over-roofing (Photos 12 and 13). Compare the trim details of Photo 13 with the original trim details of Photo 5. Not bad, eh? This stuff needs to be “invisible” or it will not happen. Find yourself an architect who gets it, and helps make it invisible. Notice the increase in overhang dimension on the gable ends. This is not just to match the overhang on the front. Apparently, I

Photo 12 (top): New Shingles Installed. Neighbors were pretty nervous until the shingles got installed. What is the crazy guy up to this year? I sure hope it does not end up looking dumb. I heard he had ants. Sheesh. Talk, talk, talk. Neighbors are such a pain. Photo 13 (above): Finished Trim Details. Hah! Can’t tell this roof is a super insulated over-roof, which was the whole point. This stuff needs to be “invisible” or it will not happen. Find yourself an architect who gets it, and helps make it invisible. did not add enough insulation to the walls 17 years ago, either. The increased overhang sets the stage for my next project, over cladding the walls.** Later. Maybe much later. Just remember, whatever you do, don’t do ugly things. Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Somerville, Mass. Visit www. buildingscience.com.

**Took the opportunity to get rid of an unnecessary chimney and upgrade the skylights. The skylight thing was more complicated than it should have been. Do you realize there is only one manufacturer of triple glazed gas-filled spectrally selective skylights in the United States? Amazing. One day folks will actually think that energy might be important. It wasn’t easy, but at the end of the day I got my R-5 skylights with the 0.2 SHGC—kind of filling my thermal holes in my R-70 plus roof. Don’t say it. Don’t go there. Remember the attic cave analogy.

August 2012

ASHRAE Journal

ASHRAE Journal - August 2012

Table of Contents for the Digital Edition of ASHRAE Journal - August 2012