The attic is stuffed with rock wool, but that is only two thirds of the story.
The other one third is the XPS foam board insulation that is mounted to the bottom of the roof joists.
Edit: The described insulation assembly does not comply with important building science principles. For more information read the following post: Do-over dilemma
We had narrowed the material choices down to foam board, either XPS or ISO. This wasn’t the challenge. But where to find the material for a reasonable price, and at what thickness, was.
Let’s break this down.
We needed a total of four inches of foam board insulation under the roof joists. Working with four inch thick foam board would be the most efficient and require the least handling. Two inch foam board is much more common, but would require twice the amount of handling.
And foam board doesn’t come cheap. The common sizes (such as two inch boards) appear more economical compared to less common sizes (such as the four inch boards).
Salvage with a capital S
You would think that the well of good fortunes on this project had dried up by now. Yet it still had a little surprise in store for us.
While scouring the salvage markets for insulation and reaching out to my fellow green building nerds, I landed a positive response from our friend John Edel. He is working on his own sustainable building and manufacturing project, called The Plant, and was trying to find a good home for a whole bunch of salvaged four inch XPS foam board insulation.
The insulation came from the walk-in coolers and smokers of the old meat processing plant, which he now is converting into a urban food production hub. The XPS insulation was faced on both sides and ideal for the attic. Not only that, but he actually had enough boards for us to finish the job.
The four-inch boards required the least amount of handling – but we still had the maneuvering issue to deal with. The only way to get the boards into the attic and under the roof joists led past the ceiling joists. We had to be strategic about what board sizes we could fit through the ceiling joists into the attic and how we staged the installation without boxing us in and leaving attic sections inaccessible.
The rear section of the attic was again easier to manage as we had no ceiling joists in the way. We still had to be strategic about the staging and mindful about the fitting in the corners and around the vertical blocking.
How did we attach the four inch boards to the ceiling joists?
We pulled the facing off one side of the boards. That side (the blue side) faced down.
I purchased a large box of two inch diameter plastic washers, specifically developed for foam board insulation installation. The washers have two small prongs that allow us to push them into the foam board where they stay put.
I also purchased a large box of 6 inch screws – a special order over the internet. The screws were long enough to reach through the four inch boards and the 1/2 inch XPS strips under the roof joists. That left me with 1 1/2 inch that I could sink into the joists.