Ice dams are caused by roof surfaces that are too warm. More attic insulation and ventilation is one cure for the problem.
Photo credit: Steve Maxwell

Ice dams are caused by roof surfaces that are too warm. More attic insulation and ventilation is one cure for the problem. Photo credit: Steve Maxwell

Houseworks: What can we do to stop ice from building up on our roof?

Q: What can we do to stop ice from building up on our roof? We live in Ottawa and our two storey house was built in 1986. We had a contractor come in and he discovered no insulation around the perimeter of the attic. He added ventilation baffles and blew in enough insulation to bring the entire attic up to R60, but we still get ice. The contractor wants to blow in more insulation, but I’m not sure it will solve the problem. Any ideas?

A: Ice dams can be difficult to get rid of, as you’re discovering. As you and your contractor probably know, the cause of large ice build ups is a roof that’s too warm. This can cause rooftop snow to melt even when outdoor temperatures remain below freezing. As this melted snow runs down the roof it eventually hits the eaves which are the same temperature as the ambient air because there is no heated space underneath them. Cold eaves cause the meltwater to refreeze at the eaves in an ongoing process. The longer the air temperature remains below freezing, the larger the ice dams will get because they have more time to grow.

Making the roof surface colder is the ultimate cure for ice dams, and that was the reasoning behind adding more insulation.

With a total of R60 in your attic, it sounds like you’ve taken things as far as you can go in that regard. But even with lots of insulation on board, lack of ventilation to the outdoors can still cause excess heat to build up under the roof. The rule of thumb for attic ventilation area is 1/300 of the floor area of the attic. In my experience you often need to double this amount of vent area to keep an attic cool enough in winter to stop ice damming.

Even with all these details in place, ice dams can still keep forming, especially where the roof slopes down near the eaves. In cases like this, rooftop heating cables can deliver a solution. I’ve had a lot of experience with this kind of thing, and you can see a video I made about eliminating rooftop ice dams at baileylineroad.com/ice-dam-solution.

Insulating walls in a 1940s home

Q: Is it possible to blow insulation into the wall cavities of one of those veteran homes built right after WWII? Iím afraid to shop around for this work because I donít want to be taken advantage of.

A: Blowing insulation into wall cavities is tricky because you never really know how congested those cavities are. The veteran houses I’ve seen are double brick construction with a small air space between the bricks. Does this sound like your home? If so, the challenge is two-fold. First, thereís not a lot of space between bricks in this kind of construction – only about an inch. This translates to only about R5 of insulation at the most, and even then only if you can get the space completely filled with insulation. The second issue is the fact that blobs of hardened mortar are usually found between the layers of brick, making it difficult to get consistent insulation flow through the space. The best approach I’ve seen uses slow-rise expanding poly urethane foam injection along with an infrared camera to determine coverage. The best expanding foams give off heat as it cures, so the infrared camera will show missed spots as being colder than areas that are warm because of the curing foam.

A better option than blowing or injection is to apply two inches of rigid expanded polystyrene sheet insulation to the inside face of exterior walls, followed by new drywall. This is a lot of trouble, but it’s often the only way to make a practical difference.

Steve Maxwell loves older houses even though they’re usually full of trouble. Visit BaileyLineRoad.com and join 31,000 people who get Steveís free Saturday morning newsletter each week.

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Slow rise, expanding polyurethane foam is one option for insulating hollow wall cavities. For this method to work the wall needs a completely empty internal cavity. (Photo by Steve Maxwell)

Slow rise, expanding polyurethane foam is one option for insulating hollow wall cavities. For this method to work the wall needs a completely empty internal cavity. (Photo by Steve Maxwell)

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