Refining the Difference Between Icicle Formation and Ice Damming – GreenBuildingAdvisor

This has been years in the making…

I am on a personal mission to get to the bottom of the difference(s) between icicles forming on a house and the presence of ice dams.

I have lived and worked in a handful of cold climate regions in the US northeast (from Boston to Vermont to upstate New York). The climate zones ranged from 5 to 6.

I have seen enough houses with icicles to cause enough anxiety to last a few lifetimes. I would bet that most of these houses either had active ice dams or were sliding by on the edge of “disaster.”

Only recently I have had to re-consider my position on ice dams versus icicles. My own house in upstate NY (CZ6) has been the catalyst for this.

I live in a region where old houses dominate the landscape (houses built throughout the 1800s). Icicles are everywhere you look. Roofs are often re-shingled with two courses of ice & water barrier, and retrofitted ridge vents.

My house is a true oldie (original structure dates to the late 1700s, with an addition added in the mid-1800s). I have icicles galore, but the signs all point toward zero ice damming. I was convinced that this was due to the exhaustive efforts I took when I gutted the house back in 2021. I was only able to achieve an R-value of R-25 for the roof assembly (all of which is cathedral style), but I was able to achieve an effective air barrier beneath the roof sheathing (using 1/2″ polyiso cut and cobbled between the rafters–and sealed with canned spray foam). The roof details include valleys, a couple of chimneys, and mortise & tenon connections for the original structure.

I’m looking for thoughts andor insights from the GBA brain trust. I’ve read a good deal of articles on the subject, and I’m still left wanting more.