Half a Chestnut: attic

So, this is the rock wool insulation that was in my attic when I bought the house. Every so often a hunk of it would fall off into the attic space below. I used duct tape to hold it up but finally it was time to remove it all and reinsulate.

This space is tall enough to walk in and I wanted to keep it a warm space for storage so I chose to insulate under the roof as opposed to insulating the floor and knee walls. I explained how I came to that decision in an earlier post.
I started off with 4x8 sheets of foiled backed polyisocyanurate- or Thermax or Tuff-R.

I hauled them home on the roof of the Passat.

I pulled them upstairs and got to work.

As I explained in an earlier post, I wanted a 1 inch air space between the roof and the insulation. There has been some debate as to whether or not that is needed but I decided to go with it. To create the airspace, I first used some 1/2 inch x 1 inch long boards nailed the full length of the rafters but that involved using a lot of wood and was going to add a lot to the cost. I decided I could use pieces of the insulation. So I cut that into lengths about an inch wide and used them to provide the spacing for the air gap.

I used adhesive to secure it to the rafters. This would prevent the insulation panels from making contact with the roof boards.

In retrospect, I should have applied the pieces to the whole length of the rafter. This would have sealed the area better. I had plenty of scraps of this stuff so there was no need to break it apart and space it like I did. My goal was to have no air infiltration from the ridge and soffit vents in to the attic space and using one long strip would have helped.

I measured between the rafters and cut each piece of polyisocyanate to fit. The rafters are approximately 3 1/2" wide. So there was one inch of air space and 2 pieces of polyisocyanate between the rafters.

It was very easy to cut and work with, lightweight but it was time consuming measuring and cutting each individual piece. I put the shiny aluminum side facing the roof rafters on the first layer but I was unsure about the second layer.

I decided to face the second layer out. I fitted each layer between the rafters. I filled any gaps with expanding foam insulation and then taped all the seams with foil tape.

It certainly made the space much brighter. I then used full sheets to add another layer of insulation on top of the rafters. I used screws and 1.5" plastic washers to hold the third layer up on to the slanted surface.