spraying foam to prevent ice dams?
Background: My parents have lived in their 2-story English Tudor home located in Iowa for 40 years. Roof is a steep pyramid with two ‘hip’ dormers. I’d estimate the attic floor area to be about 900 sq ft. Ten years ago they had the roof re-shingled. During the reshingle, they had some moulding & soffet work done (don’t know exactly what was done) and replaced a small electric attic vent fan replaced with a single 12″ ‘wind-powered ‘ vent'(rotation driven by the wind to pull out more air). Insulation below the plank attic floor is blow-in or poured-in and cotton-like consistency (as opposed to a cellulose or a paper-based material). Given the age, I doubt if there is a vapor barrier.
Problem: A couple of years ago, they started getting ice dams at the upper edge of the dormer (where the roof of the gable meets the main roof) on the south side of the home. They thought they cured the problem with ice melting cables, but had evidence of the water leaks last year. I went up and checked the attic soffet vents. There’s only four soffet vents (about 20″ x 3″ each) that were clogged. No soffet vents on the dormers (new trim may have covered them., but there’s only 2″ of soffet behind a 2.5″ fascia board…so there may not have been any vents before). The attic vent doesn’t turn, as it is in the wind shadow of the roof peak.
Questions: Since they lived in the home for years without a problem until the re-roofing, I figure I need to undo I’ve need to go back to what worked. I’m doubtful that just clearing the four vents will solve the problem, and plan to I re-install a powered fan. But then I’m not sure the fan would run in the winter (doesn’t the thermal switch need a high temp to switch on?) And though the dormers have 1-2 sq feet of opening to the main attic, I’m guessing I need to have a soffet vent for the dormers? I don’t know if the framing will even facilitate anything more than drilled holes and those circular vent inserts.
I’ve read the archives on cold roofs and ice dams, and the FHB articles on cold roofs. Here’s the final question. Would it be simpler and have a higher probability of success (talbeit expensive) to spray the main roof and dormer roofs? (I think I can gain enough access for a spay gun.)
Thanks
Jeff
Replies
How about more insulation
and air-sealing of the attic floor? Right now you probably have a fair amount of warm, moist air from the house finding its way up into the attic. During the winter it warms the underside of the roof deck enough to melt a little bit of snow and cause the ice dams. You might also be getting condensation on the underside of the roof deck, if the wood is cold enough and the air is warm and moist enough.
I would remove the existing attic floor insulation, air-seal all of the penetrations from below, and replace the insulation to at least R-40 if not 60 or more. Use cellulose. Build a weatherstripped and insulated cover over the stairs/ladder.
It may be that the roof replacement changed the ventilation pattern of the attic. Did they add plywood or OSB over skip sheathing? Did they remove a wood shingle roof and install asphalt shingles?
Spray foaming the underside of the roof might help, but it might also cause other, larger problems. If you do that, you are going to want to bring the attic inside the thermal envelope, which will mean sealing the vents, etc. Definitely not the first thing I'd do...
It should be noted that most folks who've studied them regard those turbine vents to be useless.
Strange to replace the powered vent w/ a turbine vent. I agree w/ you ... the vent is not the issue/cause of the ice dams. In reroofing and other work, did they change the way the eaves are vented? You kind of imply something about the vents 'getting plugged' w/ insulation.
Maybe they had some other work done. Ice dams are caused by 'heat leaks' through the ceiling that affect the temp of the roof/roofing. Can partially be caused by plugged vents so that natural heat loss through the ceiling has a chance to warm up the roof a little. Maybe they installed some recessed can lights or otherwise compromised the ceiling somehow. Classic casuses of ice dams is things like 'cool' recessed lights that people like to install ... but it could be any type of penetration of the ceiling that would allow excess heat to get into a small area of the attic.
If no other changes were done, I'd be suspicious of the reroofing (and associated soffit) work altered their attic air flow enough to affect the thermal properties of the roof/ceiling enough to cause this.
The vent is not related to this problem.
Another cause of ice dams is sunlight hitting the roof on the other side, or just higher up (where trees or structure shade the lower part) and warming up the attic enough to melt snow.
The eaves are generally the coolest part of the roof, so if the temp's just a bit below 32 then you're set up for ice dams. Any heat in the attic that is enough to cause snow melt then leads to the actual phenomenon.
The solutions are: 1) keep heat out of the attic (insulation), 2) get heat out of the attic (ventilation), 3) warm the eaves (heater cables).
air sealing, i agree is the the best thing to do. and bump the floor insulation up R40 at least. using celluse helps with the airsealing.remember it does settle some so over fill. make sure to foam or caulk all penitration threw plates and if voids(bulk heads use xp w/foam to close off. to suggest spray foam to fix this problem would be very costley. twice the expense of everybodies suggestions. I did a job recently and cut in ridge vent into a house. use science. hot air always rises. dont rely on the fans... not worh it. unplug all sophet vents and install the round puck style where its needed. can use a whole saw. for the small dormers, with out vents. hope this helps
Good points, although for ice dams to occur from solar freeze/thaw cycles .. you'd have to have a lot of them and generally, for winter conditions, there wouldn't be a lot of solar freeze/thaw opportunities, I'd think. Seems like conditions would have to be right. Too much snow and you may not get enough melting to really run down the roof.
We have one part of our roof that sees these fairly often -- none last winter -- wrong conditions -- but some winters it gets moderately bad once or twice (but nowhere near as bad as some houses in the area where ice is cascading down the siding).
You have to remember that snow on a roof melts from the top down, since the ridge is exposed to the sunlight first. It's not a "freeze-thaw cycle", but rather warm temps at the top and cooler near the ridge, just like the other scenarios.
I stand corrected ... I meant the melt freeze cycle ... melts at the top, freezes at/near the eave.