Builder's Tip: Using Wall-Sheathing as Insulation Stops I switched to blown-in cellulose insulation in the attics of new homes a few years ago. But while watching the installers prepping one job, I saw all the trouble that they had to go to — stapling cardboard insulation stops between the rafters or trusses above the exterior-wall plates ― just to keep the insulation from falling down into the soffit. So I came up with the idea to make the job easier and just as effective. As you can see in the accompanying drawing, I let the exterior-wall sheathing extend above the top plate and become an integral insulation stop. Here’s what I do: - Instead of trimming one foot off the last row of the sheathing so it is flush with the top plates, I let it run above the plate.
- I calculate the height between the bottom and top truss chords at the point above the outside face of the exterior wall.
- Then I deduct 1-1⁄2 inches from the distance — to leave a ventilation slot between the top of the wall sheathing and the underside of the roof sheathing ― and cut off the rest.
- After the wall sheathing is nailed on, I mark the roof-truss layout along the top edge of the sheathing and squared down the lines to the top-plate level. Then, before lifting the walls, I cut 1-3⁄4-inch slots for each truss to drop into.
The slots have an added benefit for truss installation: They quickly position each truss and allow me to float the walls beneath until they can be tweaked straight. Then I nail down the trusses.
I use expanding spray foam to seal any gaps between the truss and the wall sheathing so that no insulation can slip by. All in all, the extra work saves me more than it costs me in time because the insulators don’t have to charge for crawling on their bellies to staple up cardboard stops. Plus, I think it’s a better detail. — Mike Guertin, East Greenwich, R.I. | 
