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Community and Q&A

Preventing Ice Dams

DustinReed | Posted in Energy Efficiency and Durability on

Hey, I have a 1.5 story home (second floor is finished attic space) with a couple dormers and a couple valleys. Half the house is 2×6 rafters with what looks like r-15 foil faced fiberglass insulation (foil face down) with a ventilation gap above insulation venting from the attic under eaves space. The other newer half of the house is 2×6 rafters with r-19 fiberglass and I don’t believe there is an adequate ventilation space above fiberglass. I know what kind of insulation there is in the newer half because there was a leak from a leaky ridge vent and I tore out some drywall in a closet.

Bottom line, I don’t want to/can’t fur down and insulate because of lack of head room. My roof is in good condition (there’s a spot in attic under eaves I’m keeping an eye on to see if it leaks, there is evidence of past leaking) except for the low quality ridge vent that I had to glue down all the exposed nail heads. I haven’t spent a fukl winter in this house yet but I believe this roof if very vulnerable to ice dams. My utilities aren’t that high so I’m hesitant to spend all the money to insulate externally but I do want to prevent ice dams. A bonus would also be to keep rafters at more constant temp because I have some nail popping on ceiling drywall.

Would re-roofing and adding better ventilation solve the ice dams?
I’ve considered leaving existing decking and building up 2 inches and re-decking and adding a ice and water shield to have a major first line of defense against it.

Cathedral style roofs are tricky.

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  1. GBA Editor
    Kiley Jacques | | #1

    I can't help but think you should spend a winter in the house to see what evidence of ice dams you have before tearing apart the roof and starting all over. Studying patterning on the roof in winter can help identify air leaks. You might find air sealing those leaks will be your solution. I suggest reading Martin’s article on the subject: Prevent Ice Dams with Air Sealing and Insulation.

    1. DustinReed | | #4

      Kiley, yes it would make sense to live in it through winter first. Of course, I'm afraid of water damage inside which I think may have happened before. I'm considering buying a roof rake so that snow doesn't sit on the roof for very long and create lots of ice.

  2. Expert Member
    BILL WICHERS | | #2

    Ice dams are caused by snow melting higher up on the roof, then the water running down and refreezing out on the colder eves. To stop the ice dams from forming, you need to minimize the amount of thawing that occurs higher on the roof. How do you do this?

    First, you want to have a good ceiling air barrier, since air leaks from the living space are a BIG source of "wasted heat" going up into the attic/roof area. If you can, try to fix this first. Recessed can lights can be a big problem here in cathedral ceilings (I have two left in my own home that I still need to fix).

    Next, try to make sure you have a good amount of insulation in place. R19 is not a "good amount" -- it's not even up to code. If there is any way to add more, you should.

    Last, make sure you have good ventilation. I like to think of ventilation as a safeguard that carries away any little remaining leaks that may have snuck by your air sealing efforts. In your case, this is probably the easiest option. Make sure you have soffit vents in EVERY rafter bay. Oftentimes you see vents every so often on the soffit, but that's not really enough for proper ventilation. You can cut in a strip-style vent, or use the small ones that go in with a hole saw. Keep in mind that while the small round ones are easy to install, they offer much less ventilation area compared to the strip vents or larger rectangular vents.

    Next, be sure your ridge vent is in good shape. It sounds to me like you may have used one of the foam style ridge vents that go under shingles? Those really don't work very well. The molded plastic style (something like this: are much better. It's relatively easy to add or replace a ridge vent.

    Insulation and air sealing helps to limit how much heat can makes its way to the roof. Ventilation helps to cool off the roof. When used together, you can usually eliminate ice dams, or at least minimize them.


    1. DustinReed | | #5

      Bill, thanks for the reply. I do see opportunity for some air sealing (attic under eaves access doors, outlets, holes from previous cables, etc). I have only one recessed light which I would like to get rid of. I actually have the ridge vent that is one complete piece made out of aluminum or some type of metal, with nails in every other slot or so. I went up there a couple months ago to pound them down and caulk the heads because a lot of them were raised. It appears my most practical and affordable option would be exterior rigid foam for the insulation but the tricky part will probably be finding a contractor to do that near Albany, NY. Zone 5 I believe.

      1. Expert Member
        BILL WICHERS | | #7

        Commercial roofing contractors are likely to be more familiar with insulating a roof with rigid foam (it's much more common in the commercial world), so you might try calling some of those.

        BTW, I very much agree with Kiley's suggestion to look at winter frost patterns on the roof. Winter frost patterns can show you a lot about where and what is leaking or is insufficently insulated. What you want are cold mornings where there is a light, but clearly visible, layer of frost. Light layers are the most sensitive. Dark spots indicate warm things, and this will help to direct your air sealing efforts.


        1. DustinReed | | #8

          Great, info. Thank you.

  3. Jon_R | | #3

    > dormers and a couple valleys

    You will probably have to use alternative vent designs to vent these areas. For example, "shingle-over vents".

    1. DustinReed | | #6

      Jon R, I looked up "shingle over vents." It appears you can vent under layers of roof decking? Interesting product.

  4. walta100 | | #9

    Air sealing is almost always the first step with any old house.

    Getting old half story building anywhere close to adequately air sealed is a herculean task at best.

    Your current insulation is totally inadequate by today standards. Given you unwillingness to give up head room seems your only option left is to add the necessary insulation on the exterior not cheap or easy.


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