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Community and Q&A

Ice dam woes

mg6467 | Posted in GBA Pro Help on

Gents, I am an Electrical Engineer by profession. Your site is awesome, thanks… it is a shame that the home design and building industry can’t incorporate this stuff so that homes are done right the first time around. Sorry… preaching to the choir.

We purchased a 2 year old home that we had inspected prior to purchase. The inspector either was ignorant of the issues in the attic or didn’t look at all.

We started to suffer from ice dams in several locations after snowfall. West facing over porch and North facing garages. (Bonus rooms are over garages) For temporary relief I installed heat cable. After researching your site I decided to tackle the real problems myself. The home is located in Utah (6B zone). It has blown insulation to R38 in the flats and then what appears to be mainly R13-15 fiberglass batts on the vertical runs (knee walls and vaulted ceiling walls). Many places it was not fit right or installed correctly. Seems like there should be a better way to do vertical walls in attics.

Initial inspection of the attic revealed horrible sealing issues which I am remedying. All of the bathroom vents tubes(4x) were barley run to the soffit, which I am running out the side walls now. The plan is to get everything sealed up well and then add additional insulation + to meet zone recommendations.

Initial Blower door test revealed leakage area (ASTM 779) at 1.23 ft^2. For the size of home (5600sqft.) I plan to get this well below the target of 0.98 ft.^2.

My fear is that after all of this sealing and additional insulation, I will still not be able to overcome the sin of the air handler and duct work in the attic. 🙁 This should be outlawed in the codes!! So far I have totally enclosed and sealed the air handler itself as part of the conditioned space (closet). It was left wide open on the backside and top. (They just stuffed fiberglass batts around it)

I have sealed up the air handler supply duct metal work. From the supply coming out the top of the air handler, they have used flexible 7″ (R8.0) duct that is black!! Would you suggest replacing the black with reflective? I know we will still be dealing with the flexible duct in the unconditioned space but moving it all in the conditioned space is not an option at this point without major renovation.

I did read on your site that the duct could be sealed with SPF. I was considering just burying what I can with blown insulation. We are in a very dry climate, would this still be an issue with condensation? Not sure what else to do to minimize radiant heat through the (R8.0) flex duct.

The other concern is the duct that runs the full length of the very small attic space above the bonus room. I am going to seal the duct and register but the radiant heat (and opposite in summer) is my real concern. Any suggestions? Maybe it is time to concede to increasing air flow.

See attached photo #1,2 (octopus monstrosity from air handler), #3 (flexible duct running above bonus room 2 sets of duct run east and west the entire length)

The floor of the bonus room over the garage has no insulation between floor joists, the openings at the knee wall are not sealed in any way and I can actually see the carpet poking through the edge. Photo #4. I am assuming I will either need to blow insulation down in between the joists and seal the ends. Any additional suggestions would be great. Maybe just SPF the whole thing?

Sorry. One last thing. The wind blows fierce from the east and drifts the snow up the roof to the turtle vents. Yeah go figure, they put the vents on the non-show side of the home. What happens is the snow blows in the vents and creates nice piles of snow that sit on the insulation until warmed and then we have leakage. I tried filter media which worked but it seemed to restrict the airflow too much. Any suggestions. I don’t have adequate ventilation according to the S.F. of the attic.

Many thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Your attic is a mess, but you know that.

    The best solution would be to convert your vented unconditioned attic into an unvented conditioned attic by installing rigid foam above the roof sheathing, followed by a second layer of roof sheathing and new roofing. If you do this, you'll need to seal the soffit vents and ridge vent. Here is an article that describes this approach: Creating a Conditioned Attic.

    The solution is expensive, but it's hard to imagine that you'll get a handle on your ice dam problem any other way.

    Option 2: sell the house and buy a house built by someone who reads GBA.

    You have my sympathy. Good luck.

    -- Martin Holladay

  2. user-228058 | | #2

    We did some work on a home like this here in Connecticut, these pictures remind me of it. What we did is took Martins approach in some areas( creating a conditioned attic) and left some areas unconditioned.( It would have been impossible in a practical sense to convert it all to conditioned space.) Where the air handler and main distribution trunk was we created a room with a gasketed door using rigid foam walls and some cutting and cobbling against the roof. In areas simaler to your second picture we created a dam along the sides of the flex duct again using rigid foam for the dam, then we filled up the area on top of the duct with loose cellulose.
    Imho however the biggest fix is when the roof is ready to be replaced to possibly add rigid on top to a level that meets your climate, (then creating the conditioned attic, you need a plan to block the vents then) , also see the article on rigid foam on here somewhere, but most importantly to strap the roof with 2x4 sleepers and a new layer of plywood before the roof. This circumvents the vast majority of the issues with the ice damming ...

  3. mg6467 | | #3

    Martin and Scott. Thanks a bunch for the feedback on this mess, much appreciated.

    Scott, so as roof renovation will not be taking place for some time, I am wondering about your suggestions of creating insulating structures to bury the duct work. If I were to do this, would you suggest getting rid of the black flexible duct and replacing it with reflective or is that even going to matter if it is buried in cellulose?

    Best Regards,

  4. user-228058 | | #4

    I do not know wether the reflective type of flex would be better, I have heard that some HVAC contractors are using a sort of glue that they atomizer in the ductwork. My understanding is that this finds the leaks and seals them up. I would think that having sealed ductwork would be a big step in the right direction.

  5. GBA Editor
    Martin Holladay | | #5

    You won't see any advantage to the use of ductwork with a reflective jacket (especially if the ducts will be buried in insulation). A radiant barrier is only useful if it faces an air space.

    To improve the performance of your ductwork, you need (a) to do a very careful job of sealing the duct seams to prevent air leakage, and (b) to increase the R-value of the duct insulation. Creating side dams near the ducts and piling on cellulose insulation above and around the ducts is one approach.

    -- Martin Holladay

  6. mg6467 | | #6

    Martin and Scott,

    Many thanks again for the expert guidance.

    Best Regards,

  7. mg6467 | | #7

    Hello Martin and Scott,

    I plan to seal the flex duct, registers and light fixtures in the narrow attic area above the bonus room as you have outlined on your site. Just to be sure about improving the performance of the flex duct, I added a few graphics to the narrow attic view. (See attached) I plan to put foam board (yellow) as the dam. Position the duct (Red) and then fill with insulation (green) to the top. Is that correct?

    Many Thanks,

  8. GBA Editor
    Martin Holladay | | #8

    You have the right idea. Good luck.

    -- Martin Holladay

  9. user-228058 | | #9

    Micheal, Also consider how the roof is vented. Most likely the rafters that the green arrow points towards lead to a ventilated soffit (at the bottom of the rafter).You will need some manner to create a vent space up against the underside of the sheathing, this also needs to in some manner close off the area where the rafter notches over the plate( the birdsmouth cut) . Otherwise the loose fill insulation will fill up the soffit blocking off the vent.

  10. Jon_R | | #10

    It would be interesting to compare the cost and effectiveness of heating cables to adding powered attic ventilation (pressure balanced and only operating under ice dam conditions).

  11. mg6467 | | #11


    As it turns out the bonus room wing shown in the photo, with my graphic additions, is not vented on the top at all. There are no baffles creating an air channel to the soffit below. :-( This is the wing over the garage where the majority of the ice issue exist. If I implement the measures already mentioned previously, (Flex duct sealing and increasing duct R-value) would it be wise to ridge vent that portion of the roof to further combat the effects of radiated heat from the duct?

    Best Regards,

  12. GBA Editor
    Martin Holladay | | #12

    It sounds as if the section of roof with ice dam problems is unvented. It's possible to detail the roof assembly to be unvented, but you have to do it right. Fluffy insulation isn't good -- you need rigid foam above the roof sheathing, or closed-cell spray foam below the roof sheathing.

    More information here: How to Build an Insulated Cathedral Ceiling.

    -- Martin Holladay

  13. user-228058 | | #13

    While Martin is right on the bigger picture ,I find in the real world these are not always practical. You have already ruled out adding on top of your roof. Also often spray foaming in a retrofit, while not only pricy I wonder if it would be effective. ( getting down around the plate and such)
    This is difficult because I can't see the way the roofs come together and other factors. Are there soffit vents? Could you create propervents that are dammed with fiberglass or such to prevent the material from filling the soffit? Would this create a uninterrupted flow or cool( or hot in the summer) air to a ridge vent?
    Keep smiling..

  14. mg6467 | | #14

    That is if a gale wind came along and tore just those sections of the roof off then we would get it done right away. As for now, I am just trying to make things as efficient as possible and reduce the ice dam issue. As mentioned previously, the plan was to seal the ductwork and attic spaces and then bury the ductwork. The final combating measure was to ensure venting into the attic space. When you brought up not filling in the soffit when I buried the duct that raised some questions. See attached photo. As you can see the top triangular attic portion of both wings is a distance from the soffit. The yellow areas are where the ice is forming. As explained in the notes, even if I seal everything and bury the duct, I have an issue with airflow to the top west attic area in particular. First the air does not have a direct path to that attic space due to the fiberglass batts on the sloped areas. Second there is no vent on the West facing ridge. See my crude cross section drawing for further reference. I am assuming I need to get airflow through the slanted areas under the roof to the top attic area. Then perhaps ridge vent the top. Yes?
    Best Regards,

  15. GBA Editor
    Martin Holladay | | #15

    Your builder made lots of mistakes. As a result, you have ice dams. Solving the problems won't be cheap.

    If you want to save money, you can implement some half-measures. It sounds is if that's what you are doing. But these half-measures won't necessarily solve your problems.

    Here is a partial list of your builder's errors:

    1. Ducts were installed in an unconditioned attic.

    2. Fiberglass batts were installed in a sloped ceiling assembly without the code-mandated ventilation gap between the top of the insulation and the underside of the roof sheathing.

    3. Your builder attempted (sort of) to try to vent rafter bays even though one of the roofs has a valley that makes this approach impossible.

    4. Your builder failed to include an air barrier adjacent to the insulation later.

    The best solution is to install one or more layers of thick rigid foam above the roof sheathing, followed by a new layer of sheathing and new roofing. I know this work is expensive, but it's the best way to go.

    -- Martin Holladay

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