Grace Triflex: Not on my roof!

It's time to replace our roof. I've talked to three different roofing contractors after finding them on Angie's List. Two of them understand the issues that my 1930s New England bungalow, with its unvented roof, presents. The other one was trying to push a bunch of bad product onto my roof.

If I were building a new roof that was properly vented and I needed to dry it in for a month in Wisconsin in January, I'd probably want to use something like Grace Triflex 30 as an underlayment. It's water repellent, strong, doesn't tear, and won't degrade from UV light (for a while at least). But with a permeability of about 0.5, it also doesn't breathe worth a damn when compared to traditional roofing felt/tar paper. In new construction roofing with proper ventilation, this doesn't matter. In an 80 year old house with an unvented roof and an improved attic with knee walls and all kinds of loose, leaky construction, this is a Very Bad Thing To Do, especially when you slather Grace Ice and Water Shield all over other parts of the roof.

The problem is that unvented roofs need to breathe water vapor out the roof. Ice and water shield doesn't pass water vapor. Triflex sort of does, but not very well. My suspicion, after doing a bit of research, is that putting this stuff on an old roof with poor roof ventilation will greatly accelerate dry rot. It's a great product applied in the wrong situation, like using peanut butter when the situation calls for Astroglide.

Check out some research on this topic from RCI, Incorporated:

Overview of Underlayments

Overview of Underlayments Part 2

Roofing Upgrades Gone Wrong

Interestingly, when I made this claim to the roof salesman, he said "This stuff hasn't been around long enough to demonstrate that it causes dry rot." The guy was doing a hard sell. It was weird to have a roof salesman do a hard sell. It's a roof--not a car. Of all the contractors I've talked to over the years, this was the first one with a real hardnose sales pitch, which made me suspicious. But his comment made me even less likely to use the product. It hasn't been around long enough to prove itself. Roofing felt and tar paper, while imperfect, are well understood technologies whose upsides and downsides can be worked around. I'm sure Triflex is a great product for the right house. But I doubt it's the right thing for older, unvented roofs.