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Q&A Spotlight

Heading Off Ice Dams

An Ontario homeowner looks for suggestions on how to stop ice damming before another winter sets in

Ice dams are a problem on some but not all parts of this roof. In search of a solution, the homeowner is weighing proposals from several Ontario roofing companies.
Image Credit: Jake Rabe

With the onset of another winter just a few months away, Jake Rabe is looking for suggestions on how to prevent the recurrence of ice dams on his older Ontario home.

“Each winter I have to deal with ice damming along two sides of the roof — nowhere else,” he writes in a Q&A post at Green Building Advisor. “The areas in question are a low-slope roof (3/12 pitch, east and west side of the ridge) and a cathedral style roof (8/12 pitch)… My goal is to stop the ice damming.”

Rabe believes that the house was built between the 1920s and the 1950s. Access to the cramped attic beneath the east and west low-slope roofs is through a tiny hatch on the upper floor of the house. Rabe estimates that there is only about 36 inches from the ridge to the attic floor, with the space now filled with a mix of old and new cellulose insulation roughly 12 inches thick. He has no idea whether the insulation is evenly distributed across the attic floor.

Soffit vents have been installed, although he suspects they have been obstructed by wood or cellulose. There are two exhaust vents at the top of the roof. The roof has board sheathing rather than OSB or plywood.

Rabe has solicited suggestions on how to resolve the problem from several roofing companies. They seem to run the gamut: opening all soffit vents and adding fluffy insulation between rafters; removing shingles and installing metal roofing; adding a new roof deck on top of the existing deck and building in an air gap between the two; removing all existing attic insulation before air-sealing the attic floor and blowing in new insulation; and, finally, removing the existing low-slope and cathedral roofs and replacing them with a new roof of a steeper pitch.

Rabe is leaning toward a plan that would begin with removing the existing board sheathing and the existing attic insulation. Then, he would air-seal all the “visibly obvious problem areas,” open all soffits and add baffles between rafters, and then blow in new cellulose to a depth of 18 inches to 20 inches. The roof could then be decked with OSB and re-shingled. He estimates this approach would cost about $25,000 Canadian ($20,000 U.S.).

“I’m hoping for a guaranteed solution,” he writes.

Is this it? That’s the question for this Q&A Spotlight.

The best solution won’t be cheap

The problem can be solved, writes GBA senior editor Martin Holladay, “but the solution won’t necessarily be cheap.”

If there is enough room in the attic, especially at the eaves, for an adequate amount of insulation, and if a skilled crew of workers can be found, then Rabe’s proposed solution will be successful.

“In many homes, however, this approach won’t work because space at the eaves is limited and because skilled workers are hard to find,” Holladay adds. “Instead, consider installing a new layer or layers of thick rigid foam above the roof sheathing. This approach (a) is expensive, (b) requires you to insulate the gable walls in your attic, and (c) requires you to re-work your exterior trim details at the rakes and eaves — but it will solve your ice dam problems.”

Holladay suggests that Rabe consider using nailbase, a type of roof sheathing that consists of rigid foam insulation glued to a layer of OSB. Nailbase is available in several thicknesses, including a 9-inch-thick panel from ABT that’s rated at R-33.

If he installs rigid foam on the exterior of the roof sheathing, he should not ventilate the underside of the sheathing. This approach requires the sealing of any soffit or ridge vents.

“Some builders prefer to include a ventilation channel above the rigid foam,” he adds. “In fact, some nailbase panels include this type of ventilation channel. But you wouldn’t want to have any ventilation under the original sheathing… You only want to ventilate your roof sheathing when the roof sheathing is on the exterior side of the insulation. Once you bring the roof sheathing into your home’s thermal envelope, you want to keep everything warm and cozy (to limit your energy bills).”

The spray foam option

Rabe has contacted an insulation contractor who recommends that both problem roof areas be insulated with spray foam, creating an unvented “hot” roof.

“The plan, he said, was to remove the sheathing and spay foam the ‘attic floor’ between each joist in order to seal and insulate the attic floor with no more spray foam than the height of each joist,” Rabe writes. “All intake and exhaust vents would be closed off. As for the cathedral, they would also remove the sheathing and spray foam the entire cavity from eave to ridge. No venting. I have no interest in making my attic space into a livable region. I’m all for the cheapest approach to eliminate ice damming.”

He’s concerned, however, that enough heat will bleed through the insulation beneath the low-slope roof to get the sheathing warm enough to melt snow.

“Apologies for the learning process, but is there some advantage to not having ventilation in this scenario?” he adds. “The insulation guy said that spray foam below the sheathing vs. directly on the attic floor are identical.”

Not really, Holladay replies. “Installing spray foam insulation on an attic floor does not create a conditioned attic. This type of attic is outside of the home’s thermal envelope, and it should be vented. There is no reason for the spray foam contractor to seal the soffit vents and the ridge vent if the contractor is installing spray foam on the attic floor. In most cases, the best insulation to install on an attic floor is cellulose, not spray foam, although spray foam is often useful for air sealing.”

A second insulation contractor recommends spraying the attic floor beneath the low-slope roof with foam, blocking the soffit vents but providing ventilation to the attic with Maximum Ventilation intakes. For the cathedral ceiling, the contractor would spray the roof cavity from above, leaving a small air space beneath the new roof sheathing for ventilation. Soffit vents would remain open.

Rabe already uses VMax Model 301 units from the company as exhaust vents for the attic. “I am considering installing ridge vents,” he says, “but the insulation specialist claims that these Maximum Ventilation units (exhaust and intake) are far superior solutions, and even the intakes would serve to draw in air allowing them to fully spray foam the corners tight, maximizing the R-value there.”

Keep minimum recommended R-values in mind, Holladay adds that 5 inches of open-cell foam yields an R-18 thermal barrier, while that much closed-cell foam produces R-32. But in either case, that’s less than the minimum code requirement of R-49 required in the U.S. for even milder climate zones than Ontario.

Consult an expert

Finding a qualified contractor is, of course, key. Rabe has been calling roofing contractors, but they don’t seem to think that air-sealing is very important. So where should he look?

What you want, Holladay says, is a home performance or weatherization contractor. A RESNET-certified or BPI-certified rater would be a good place to look for a recommendation, and Steve Knapp suggests that the Canadian Residential Energy Services website is a place to start. The Building Performance Institute Canada also would be a place to check.

“Keep in mind that spray foam contractors want to sell spray foam,” Knapp says. “Earlier, Martin suggested bringing in a CRESNET rater to check out your home and recommend a strategy. Personally, I think that is the way to go. Otherwise, you may end up wasting funds and possibly making your situation worse, not better.”

The advice Rabe has been getting from spray-foam contractors isn’t sitting well with Alan B.

“That insulation specialist is making me do multiple face-palms,” he writes. “He sounds like one of those stereotypical ‘foam solves everything, R-value myth’ types. For a standard attic the advice is simple: air seal (blower door directed is best) then put R-50 of cellulose on the attic floor. Make sure soffit and ridge venting is adequate. Measure the amount [of venting] you have in square inches and the size of the roof, and post here to get numbers.

“For a cathedral ceiling (meaning no conventional attic, low slope or high slope), adequate rigid foam on top of the sheathing is recommended, and fill the rafters with open-cell foam, drywall, air-seal and paint,” Alan adds. “The needed foam thickness on top of the sheathing depends on rafter thickness and climate zone, so your location in Ontario would be helpful… This should solve the problems at better cost then some of the crazy and wrong solutions you have been given.”

When to vent the attic

There is still some lingering confusion on whether it’s important to vent an attic when the attic floor has been insulated and air-sealed with spray foam.

If the attic is too small to stand up in, and the attic floor is insulated with an adequate amount of spray foam, the attic would not have to be vented, Holladay says. “As long as the spray foam work is not performed in the rain, the assembly will be fine. The spray foam stops humid air from the interior from exfiltrating through the ceiling, so interior moisture won’t be entering from below. The roofing keeps out the rain.”

Vents above the insulation would be a good idea if there is a practical way to install them, Holladay says, but vents that penetrate the roofing are not his personal favorites because they can leak.

Why does the need for ventilation hinge on how big the attic is? asks Alan.

“Low-slope roof assemblies insulated with spray polyurethane foam are often unvented,” Holladay replies. “The fact that there are a few inches of air above the cured spray foam doesn’t necessarily make these roof assemblies risky. That said, venting the air space provides an opportunity for excess moisture to dissipate, obviously, so if Jake can include a soffit vent or a wall vent, he should do so.”

Aesthetic concerns with exterior foam

One potential drawback of using rigid foam above the roof sheathing is what the new insulation would do to the appearance of the house.

“My worry about this approach is how it will look,” Rabe writes. “I would only be applying it to the low-pitch roof and the cathedral roof — nowhere else. I’m concerned the existing, relatively new, gutters will have a, worst case, 10-inch-gap between the shingles and the top of the gutter.”

[Illustration: Building Science Corporation]

If exterior foam is the option Rabe chooses, Holladay replies, it would typically be applied to the entire roof, not parts of it. He refers Rabe to an an illustration from Building Science Corporation showing how a rebuilt soffit and fascia can disguise the thick exterior foam.

“If you are uncertain of the aesthetics of this insulation project, it’s time to hire an architect to help you,” Holladay says.

The potentially “astronomical” cost of placing rigid foam on top of the sheathing has Rabe circling back to a variation of an idea he broached earlier. Working from above, he would remove the existing sheathing and insulation, and air-seal the attic floor. The floor could then be insulated, either with closed-cell foam alone, or with a layer of closed-cell foam and then a layer of cellulose.

“Your plan sounds sensible,” Holladay tells him. “The success of your plan will depend, of course, on the skill of your spray foam installer.”

Our expert’s opinion

Here’s what GBA technical director Peter Yost had to say:

It’s interesting that this home only has problems on certain roofs or roof locations. A blower-door-assisted infrared scan of the roofs would be really helpful in better understanding the thermal bypasses (due to combined air leakage and conductive heat loss) at all the eaves.

All ice dams are the combined result of conductive and convective heat loss at the eaves, at nearby penetrations, or at leaky ducts. In this case it looks like missing air sealing and insulation at the eave (or the top of the exterior wall) is the primary culprit.

It’s not really possible to vent or ventilate your way out of ice dams; the only real solution is making the air and thermal barrier continuous at the intersection of the eave and the top of the wall. You can rake the eaves when it snows, or switch to a less-susceptible roofing, but those aren’t really solutions. For this project (especially with the low-slope shed dormer), attacking the eaves from the outside, along the lines of this project by Paul Eldrenkamp of Byggmeister, is the way to go.

An insulation retrofit beefed up insulation at the eaves.

Paul has done moisture monitoring on ice-dam retrofits like this one — where the total space or depth of the roof assembly really does not allow venting — to show that even if the roof sheathing gets quite wet during the heating season, it will dry well over the rest of the year.

Venting introduces drying potential forgiveness, of course, but you simply can’t move enough air past an assembly wetted by an ice dam to good effect. It is more important to eliminate uncontrolled air movement (leakage) than it is to increase managed air movement (venting or ventilating) to manage ice dams.

Incidentally, Eldrenkamp sent me this photo as part of his work with the Building America team (NorthernSTAR) and their work on ice dams, called Project Overcoat. Go here for more examples of what leading contractors have done to solve ice dams.


  1. Meyer Brendan | | #1

    Does he mean two sides (i.e. west, south) or two parts (i.e. eaves)? If two sides, isn't it just a matter of the sun melting snow on those sides and not others? How would insulation stop the sun from melting the ice.?

  2. Ven Sonata | | #2

    De- icing cable.
    Why not use de-icing electric cable? Zig Zag pattern applied on the exterior roof. About $100. Turn on manually. Common solution in Canada.

  3. User avater GBA Editor
    Martin Holladay | | #3

    Responses to Meyer Brendan and Ven Sonata
    Sun can contribute to ice dams, but if the ice dam is serious enough to concern a homeowner, it's almost always caused by heat flow from the interior.

    Electric-resistance cables are inexpensive, and they definitely melt ice. For that reason, the solution is common. But for energy nerds, this is a deeply dissatisfying solution -- solving a problem caused by heat leakage by increasing one's energy bills still further. Air sealing and R-value improvements are better, because they can solve ice dam problems while lowering a homeowner's energy bills.

    That said, air sealing and R-value improvements usually cost more than $100 -- so I understand the attraction of the $100 solution.

  4. User avater
    Jon R | | #4

    cable sensors
    Interesting, evidently the sensors controlling ice dam heating cables are pretty smart.

  5. Alec Shalinsky | | #5

    Ice Dams
    I don't have a particular solution, but rather some thoughts, some of which have been touched on. I would stay away from a metal roof. I built a home at a ski resort in the BC Rockies, and the architectural guidelines mandated 3 peaks on each house. This means 6 valleys, and we put a metal roof on the house. I did a very good job of insulation and air sealing, all to no avail. Huge snow and a metal roof created ice dams like we never seen. Asphalt roofing fared MUCH better. But valleys are notorious for being difficult to insulate properly and a great breeding ground for ice dams.
    Martin, from an energy point of view, fitting the house with a microFIT array (up to 10KW) of PV cells and heat tape is FAR cheaper (and more energy friendly) than a new roof.
    But seriously, there are other issues, I would suggest. Is the house comfortable? Does the roof need replacing anyway? What is the downside to a few ice dams? How much insulation is up there now. $20-$30K for a new roof is a lot of moo-la.

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