Project specific factors usually determine what insulation method would be suitable. For our roof those factors were a target R-value of 60 or more, the existing low slope roof with its ten inch roof joists, the spatial constraints of the south attic, and the need for some roof deck repair down the road.
We reconciled these sometimes conflicting factors with an insulation assembly that started with three layers (total of 10 1/2 inches) of rock wool batts (R-45), followed by four inches of XPS foam boards (R-20). That should give us an R-value of 65 anywhere between the roof joists.
It looked like we had the perfect roof insulation for our project. Yet, it was also risky, which suddenly made it a lot less perfect.
What I neglected to consider in my schedule and material driven decision making, was the building science – the issue of moisture management.
From cold to wet to rotten
The current insulation assembly is so effective that I created a cold roof deck. During winter time, its temperature will be close to the outside air temperature. In other words, it will be around or below freezing.
Any water vapor in the assembly is likely to condense at the cold roof deck – or, to be more precise, the water vapor will be absorbed by the boards and the upper (cooler) sections of the roof joists.
That could drive up the material moisture content. Once the moisture content rises above 28%, rot or biodegredation may set in. This could compromise the structural integrity of the roof deck and roof joists.
We did fairly successfully airseal the 2nd floor, including the roof plane. That is, however no guarantee that the roof assembly is vapor tight. Plus the stack effect will constantly pound the insulation assembly with warm and moist air. Some of it will get into assembly. But I am not sure if it has an effective way to get out again.
The accumulation of moisture in the assembly would probably be a slow and gradual process. To get to and sustain a risky moisture content would probably take years. So should I lose sleep over it?
Yes. And I already lost a lot of sleep over it! We plan to be in this building for many years to come. The aspect of durability is very important to us. It makes economic sense and is sustainable. Plus we plan to put us a green roof. I don’t want to ignore the problem now only to risk costly structural roof issues 5, 10 or 15 years down the road.
Our current insulation assembly is upside down. If I move the XPS insulation on top of the roof deck, I should be able to keep it warm enough to reduce the sorption risk (often referred to as condensation).
The rock wool insulation can stay in place as is. It is vapor open and allows for seasonal drying of the now much warmer roof structure. With that, the moisture content of the roof deck and roof joists should remain in the safe zone, which would be below 20%. (Mold growth is likely to occur if the moisture content is in excess of 20%)
I don’t like doing things twice. But in this case it is the lesser evil. I will now spend a few days taking down our carefully installed XPS insulation. Reinstalling it atop the roof deck will happen sometimes down the road.
Trying to determine how to insulate or ventilate a roof isn’t easy. Here is a list of articles that guided me in the process:
- Insulating Low-Slope Residential Roofs
- How to Build an Insulated Cathedral Ceiling
- How to Install Rigid Foam On Top of Roof Sheathing
- Dense-Packed Cellulose and a Wrong-Side Vapor Barrier
- BA-1308: Moisture Control for Dense-Packed Roof Assemblies in Cold Climates: Final Measure Guideline
- Smart Vapor Retarders