Ice dam potential prior to re-roof
Over the last year my house has gone from almost 100% shaded to 100% full sun with some town trees that have come down. Home is oriented E-W, with the front of the home a couple degrees south of true W. In August, almost 8 hours of sun hit the front side until the Sun eventually hit the treeline around 8PM.
I’m looking to re-roof, and curious how this might impact my ice daming risk and what precautions or answers I should be looking to get from contractors ie amount of ice+water shield, if needed? Rafters are R-31 OCF. Roof pitch is pretty steep, if that helps. Once or twice over the last 6 years, prior to trees coming down, the front gutter would ice up pretty good but it wouldn’t creep up much (and porch is 3-4′ out before conditioned space). I had also noticed a little ice build up where the shadow is where the two pitches meet for the bump out. Again, never water in the home, but occurred under more shady conditions, with a lot of snow as well. I probably should have roof raked that area in hindsight. Maybe this is moot as the trees don’t have their leaves anyways? Curious if GBA had any recommendations. House is outside of Boston.
Kevin
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Replies
Hi Kevin,
A bulletproof roof install may prevent damage from ice dams once they have formed, but roofing doesn't prevent ice dams from forming. Here's an article that will help you understand the insulating, air sealing, etc. that can help prevent ice dams: Ice Dam Basics
Thanks Brian- so solar gain doesn't have much affect on ice daming?
Hi Kevin.
Solar gain could definitely cause some ice dams as it happens when snow melts to water and then the water freezes. But I have never heard of solar gain as a significant cause of ice dams. The most common causes are heat loss through a leaky and/or under-insulated attic or roof. Some roof designs exacerbate the situation, but are not the cause.
>"But I have never heard of solar gain as a significant cause of ice dams. "
A rare (and mostly insignificant) exception sometimes seen in the Boston area is with deep roof overhangs on an unsoffitted south facing wall, where there's a column of rising hot air on exterior side of the wall creating a pocket of warm air a few inches further out from the wall when sun hits the wall.
The house in the picture isn't likely to have that issue.
Is that gable window in the picture to an open attic, or is the attic built-out space with kneewalls?
The street side gable window is an insulated, doored off storage space. The attic space running parellel to the street is conditioned living space with 1 mini split head. It's only spot heated in the winter as needed.
Ice dam prevention is more about air sealing and insulating that it is about the roof itself.
If you’ve had an issue with ice dams in the past, it’s worth putting in some extra protection. When the ice and water shield goes up, you want AT LEAST two roll width’s worth at the edge of the roof. One roll width isn’t really enough. Make sure the ice and water shield is installed ON TOP of the drip edge too. I’ve seen that done wrong lots of times.
Water needs to run off of everything, so anything that starts higher up has to end on top of whatever is lower down. You don’t want the water to find any edge that it can get under and work it’s way into the house.
Bill
Thanks Bill, that is helpful. From what I've read it wouldn't hurt to put some shield in the valley of the roof as well.
Even without ice damming problems or soffit-to ridge venting the valleys need something more waterproof than #30 felt. Valleys (even steep valleys like that) become major snow-traps during big nor'easters with snow depths often reaching 2-3x the average snowpack depth on the ground. (Ask me how I know! :-) )
The air tightness and insulation levels of the sidewalls on that shed dormer need to be up to snuff too.
Knowing the locations where the ice dams tend to form can provide important clues as to the proximate cause (and possible cures.) If there's a triangle cross section dead-space/micro-attic in that porch overhang with insufficient wall R or air leakage to the main house it could easily become an ice dam starter.
I saw a picture from a roofing ad that looked to have ice+water shield completely closing the existing ridge vent. Is that something I should do or not do with open cell foam in the rafters?
Above the porch with the dormer is a master closet- no insulation in the floor and I believe just batts with plastic in the knee wall- that room is cold in winter and hot in summer. I had explored injecting CCF into the floor, or adding PolyISO but never went forward as it was pretty expensive.
Between the dormer and bumpout is a small laundry room- leaky as well as dryer exhaust runs to the porch ceiling with a crappy access panel door. Not sure on insulation in the knee wall but the room is so small it's hard to move the appliances around to get back there. The washer drain has thickened up once in sub zero temps as it is unheated- we throw a space heater in there on occasion.
>"I saw a picture from a roofing ad that looked to have ice+water shield completely closing the existing ridge vent. Is that something I should do or not do with open cell foam in the rafters?"
If there is no ventilation channel between the open cell foam and the ridge vent the vent is providing no benefit, and it's fine to seal over it with Ice & Water Shield.
Open cell foam is a bit too vapor open to be completely moisture safe when unvented. A smart vapor retarder or vapor barrier latex applied to gypsum board on the conditioned space side of the ocSPF can substantially mitigate against rot risk on the roof deck. The riskiest section of roof is anything north-facing or otherwise substantially shaded. See the "Boston" row of Table 3A, and the "Full depth ocSPF" column:
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
>"Above the porch with the dormer is a master closet- no insulation in the floor and I believe just batts with plastic in the knee wall- that room is cold in winter and hot in summer."
If it's completely trapped dead-space (not a triangular storage space), filling it with cellulose is fairly low-risk (particularly since it's on the sunny side of the house), which could be done as a DIY with a rental blower. If blowing from indoors is too complicated or messy, drilling & filling from the porch ceiling works.
Definitely seek out borate-o/sulfate-free cellulose if you go that route. In most of Massachusetts Kamco is a pretty convenient source for borate-only cellulose, but they don't rent blowers. (They have locations in Woburn, Needham & Avon, and several others outside of the R128 region.) The box stores with rental blower equipment don't usually keep borate-only goods in stock.
>"Between the dormer and bumpout is a small laundry room- leaky as well as dryer exhaust runs to the porch ceiling with a crappy access panel door."
A dryer vent passing in through an uninsulated micro-attic is DEFINITELY going to contribute to ice damming problems. It's important to keep both the dryer vent and drain on the conditioned space side of any new insulation, which may preclude just packing the whole shebang full of cellulose. Some pictures from inside that space would be a useful starting point for that discussion.
Dana- here are a couple pictures. Looks like they were able to get open cell in the knee wall in the master closet/dormer area. The roof pitch with existing plaster and sidewall were filled in with blown-in this past spring when we did 2nd floor walls.
Tiny laundry room pics are pretty poor. Insulation company didn't touch the knee wall so safe to say little to no insulation there. With the access door I might be able to pump in blown-in for the entire porch ceiling if it shoots far enough.
An aside, but does anyone know what you'd call this house? 1940s- I've tried searching to find a similar home, mainly for color schemes if we were to paint it. It has a center colonial feel on the first floor but huge roof and porch make it unique on the street as most are brick Tudor.
>An aside, but does anyone know what you'd call this house?
The roof angles & proportions as well as the gable facia returns at the soffits are a bit Greek Revival-like, but the architects may have a better take on that. Most Greek Revival houses don't have a brick veneer in some parts, clapboards in others. Most (but not all) true Greek Revival houses would also use round rather than square pillars for the porch.
I'm pretty sure it is full depth stone in the front and not a veneer- I measured my indoor wall temp on a sun blazing afternoon and the dining room indoor walls behind the stone was 3-4 degrees cooler than my son's bedroom right above it.
There are clapboards behind the aluminum- I'd be nice to pull it back but a project I'm just not ready for $$. I've seen good results painting aluminum if it is done correctly.
I'll dig up a couple rehab photos tonight when I get home- I'm sure I've got a couple from those rooms. Thanks Dana.
Kevin