I only had two major snafus on this project, both self-inflicted. One had to do with the roof insulation – or – to be more precise, the attic insulation.
I did an excellent job insulating the attic, first with rock wool followed by foam board. And in that process I created a cold roof deck. A cold roof deck during winter runs the risk of getting wet over the years (for more information go to: Do-over dilemma).
With the old roofing torn off, I found to my delight a bone-dry roof deck. Maybe this was an indicator that the risk of a cold and wet roof deck was marginal, but nevertheless I’m glad I made the decision to sandwich the roof deck between layers of insulation (rock wool below and foam board on top) to keep it warm.
Roof insulation with cold roof deck 
Roof insulation with warm roof deck
What type of insulation to use?
I used a fair amount of salvaged extruded polystyrene insulation (XPS) on the project and thought of using it on the roof too. But my roofer, Pablo, balked at that idea. I quickly realized that all the roof insulation I ever saw was polyisocyanurate insulation boards (or polyiso in short). And there is a good reason.
Although I didn’t find any independent publications, articles from the roofing industry and manufacturer associations indicate that polyiso is fairly fire resistant and does not melt and drip like polystyrene. That is a rather important factor, considering that we planned to install a modified bitumen roofing system, also known as torch-down roof. And as the name suggests, it involves a torch and heat. So having fire resistant insulation boards is – let’s say – imperative.
The downside of polyiso insulation is its cold weather performance. To quote Martin Holladay from Green Building Advisor:
“At temperatures below 50°F, polyiso performs worse than it does at a mean temperature of 75°F”
And what does that mean? In warm conditions, polyiso outperforms XPS insulation. Under cold conditions, polyiso is about on par with XPS (R-value of 5 per inch). And under very cold conditions, it may drop below an R-value of 5 per inch.
When I was sitting at the roofing material supplier to order the materials for our project, the price for the polyiso boards was less than I expected, which made me suspicious. After combing through the material specifications, I realized that the boards in questions had a cardboard based facer.
Unlike XPS or expanded polystyrene (EPS) insulation, polyiso always comes with a facer on both sides. The facers contain the foam core during the production.
Having a cardboard facer, which runs the risk of disintegrating or deforming when it comes into contact with moisture, was not acceptable in a roofing situation. And the potential flammability of a cardboard backing may negate the fire resistance the polyisocyanurate provides. A fiberglass facer would be the material of choice. Slightly more expensive and a special order item (because most roofers don’t want to spend the extra money), but moisture resistant, dimensionally stable and safer.
Installation
We installed two layers of 1 ½ inch polyiso boards across the roof. I did pick 1 ½ inch boards because they fit with the solar blocking, but more about that in a later post.
To maximize the thermal performance, we staggered the joints of each layer. The polyiso boards, like most materials, expand and contract with rising or falling temperatures. When contracting, the tight butt joints may morph into a slight gap, which would allow thermal energy to escape. By staggering the joints, I have at least another layer of foam board over that gap that would slow that escape.
We fastened the boards mechanically to the roof deck so that they don’t blow away and are a solid foundation to which we can adhere the roofing system. To do so, we used long insulation screws with insulation washers.
We have, what you would call, a typical low slope roof. What is not so typical is that the bottom of the slope is blocked for about 12 feet by our staircase extension. To prevent water from ponding up against the extension, we added some tapered insulation to add a slope that would allow for positive drainage.