We are trying a new attic insulation and air sealing detail on this house. We have been a big believer in conditioning the attic space in all of our houses and have been spraying open cell foam on the underside of the roof sheathing down to the top plate. Typically, we have been running the duct work for the 2nd floor in this attic space, so it makes sense that it is terribly inefficient to run duct work that is delivering cool air in a hot attic space. Also, because we mostly use low slope shed roofs, the volume of the attic is not generally that much of a space to condition.
For the Passive House standard, we need to achieve a ceiling R-value of 60. To do this in foam is just too expensive for us (more surface to cover with a gable vs shed roof), so we find ourselves returning to dry blown cellulose, which you see in most houses. Traditionally, the big downside for cellulose is that it won’t prevent air infiltration. The system of just blowing the cellulose over the ceiling drywall leaves potential areas of air leakage around lights, through drywall cracks and around the inside edge of the top plate. This was not going to work for us.
To get a great air barrier, we actually nailed up 1/2″ plywood to the underside of the trusses. Before even setting the trusses, we made sure to put down a small 6″ ripper of plywood on top of the cap plate of the wall and taped that outside corner of the plywood to the exterior sheathing really well. Once the trusses were set, we continued this plane of plywood all the way across the ceiling and taped all the seams. We made sure this ripper of plywood projected past the interior side of the wall to have enough room to tape the seam.
We did have to notch into the plywood to attach the hurricane clips. We did our best to tape those notches, but I think I’m going to have to use a little foam there to seal the deal.
We are going with all sconces for lighting and will also put the smoke detectors on the wall, so we don’t have to worry about any penetrations of the plywood. If we were to have ceiling lighting, we would have to fir down a chase on the underside of the plywood. John Semmelhack had an interesting idea if we had to go this route of actually building a mini floor system on top of the wall plates of 2x4s or 2x6s and plywood. This would be installed before setting trusses too and the tape would have to be installed on the sky side of the plywood instead of the underside. Besides giving a chase in the conditioned space to run recessed lighting or duct work, this would create a platform to set trusses on, which ups the safety and ease of installation for truss setting.
As far as costs, I really don’t think there is much of a cost increase from this method. Because cellulose is so much cheaper than foam, the insulation cost savings makes up for the cost of the material and labor of installing the plywood. It’s a slightly different story if we were to build a false floor system though, as the labor and material costs from that would be moderately significant.