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Icicles on foam insulated house

GBA Editor | Posted in General Questions on

We just moved into our house in W. MD in mid Dec. The house is completely foam insulated, walls and roof system. The insulation job was performed as well as it could have been done. After 30 days of below 25F temperatures and several feet of snow, we are getting icicles around the entire house, some places worse than others but I suspect roof area in the worse spots as the reason. I think am getting heat transference through the 8-10 inches of roof foam and probably more likely through the 2×12 roof rafters that is causing melt under the snow and then icings as it hits the 2ft overhangs. Does this sound like a reasonable deduction and does anyone have suggestions other than heat tape on the overhangs? Thanks Derek

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Replies

  1. Expert Member
    Michael Maines | | #1

    Derek, are the icicles worst on the south side?

  2. Derek | | #2

    No. There doesn't seem to be any solar effect. I still have 12 inches or more of snow on the entire roof. No bare spots at all, perfectly even melt on top of the snow cover.

  3. GBA Editor
    Martin Holladay | | #3

    Derek,
    One way to determine whether your icicles are forming due simply to your weather, or are due to heat loss through the roof, is to compare your icicles with those on unheated buildings like garages, barns, and equipment sheds.

    If indeed your icicles are worse than those on nearby unheated buildings, your roof is leaking heat. That would indicate a need to call a building performance contractor to determine how the heat is leaking through the roof.

  4. Derek | | #4

    Hi Martin,

    It is definitely in the heated portion of the house as there are no icicles over the bonus room above the garage which we keep at about 55F. The icicles are pretty uniform around the heated house. I am doing a blower door test and shooting the house with a thermal camera tomorrow but I don't think I will find much as the house was well built and insulated. I think it is a performance characteristic of the foam that no one has done any research on that I can find. Over time, the heat from inside has to be making its way through the foam and raising the roof temp. The snow is insulating the exterior of the roof so there is not a lot offset of the freezing temperatures keeping the roof cold. In addition, the 2x12 rafters are a week point.

  5. Expert Member
    Michael Maines | | #5

    Snow is actually a pretty good insulator, something like R-2 per inch, or R-24 for 12 inches. 8-10 inches of open cell foam is, let's say R-39. That means that 38% of your R-value is OUTSIDE your roof surface right now. More if you factor in thermal bridging through the rafters.

    With an outside temp of 25°F and an inside temp of 70°, you have a 45° differential. 38% of 45° is 17°. The surface of your roof is (in theory) 25° + 17°, or 42°. Obviously, warm enough to melt snow.

    Meanwhile, your 55° bonus room only has a 30° differential. 38% of 30° is 11°. 25° + 11° is 36°. Still above the melting point, but you get the idea. The snow is raising the temperature of your roof surface. That's why in heavy snow areas, vented roofs are prefered. Washing the underside of the roof surface with cold air keeps the snow frozen.

    Your options are:
    Keep your roof relatively free of snow--there is a special tool for that purpose. It's hard on the roofing though. The plastic version is not as bad as the metal version.
    Use metal roofing so the snow pack does not get very deep before sliding off.
    Vent the roof.
    Use heat cables at the eaves.
    Increase the amount of insulation you have inside the house.

  6. Derek | | #6

    Hi Michael,
    That makes sense. Do you think installing the radiant bubble foil to aattic ceiling would be enough to solve it? I was also kicking around celulose on the attic floor?

  7. Expert Member
    Michael Maines | | #7

    Is there an echo in here? ;-)

    Derek, if you stand in your attic now, what are you looking at? Rafters and foam?

  8. jbmoyer | | #8

    Derek,
    Michael is right...
    My knowledge of Maryland geography is not so good, but don't the Appalachian Mountains run through West MD? I'm assuming your have about 6,000-ish HDDs in your area?
    You said you have 8-10 inches of open-cell spray foam in the attic. So about R-29 to R-37. I don't think this is sufficient for your climate.
    From your description of the project, I'm assuming the house is new. Thus a new roof is not an option. Neither is venting. Your only option appears to be adding more insulation.

  9. Derek | | #9

    Sorry about the triple post, I am on my phone doing this.
    Michael, yes rafters and foam.

    James- Yes we are in the mountains at about 3100 ft. I wa4 told by several foam contractors that after a certain amount of foam there is no additional benefit? I guess I could fill the rest if the cavity with closed cell? There are still some area that would be tough to get to due to the roof system geometry.

  10. Expert Member
    Michael Maines | | #10

    I think you're in a mixed wet-humid climate, which is tricky. The roof and wall assemblies need to be able to dry to both sides, so you don't want closed cell foam on the inside. More open cell might be the answer. No such thing as "no additional benefit" to additional insulation, just diminishing returns. Martin's suggestion of getting an energy specialist to look at your house is probably the best route to a solution for you.

  11. Dan | | #11

    I have sprayed homes and diagnosed such homes. I would recommend an energy home performance expert that has many years of hands on experience in the energy field to look at your home and do a blower door test as well. With the attic hatch open and with it closed.
    THe way your home was sprayed indicates that the heating cooling system is in the attic. Ecspecailly open cell foam expands so quickly that it sometimes does not fill all the voids but leaves pockets and holes that allow heated air into and out. Attention to detail has to be done. Most homes even if spray foam is done had ridge vent and soffit vents added. THese can and will be direct paths for any air leakage to contribute to your problem. Ductwork in the way along the soffit could have caused a less than ideal spray job that is allowing air leakage. A good connection has to be made from the soffit/roof deck to the top-plate of the exterior wall. By the way if you do have more open cell spray foam added make sure its not the "I" word brand becasue it does not stick to itself. If you have a blower door and even an infrared done on the roof deck from the inside, with the blower door running and then not running I think you will see your problems. You can attack them with a 16lb tank of minimal single componet expanding foam. Curious what is your fuel usage and square footage? Dan A+ Energy

  12. Derek | | #12

    Hey Dan, thanks for the input. Our heat system is all radient floor and I made sure the HRV duct did not intrude into the roof system. No AC as it very low humidity in our area and if it hits 85F in the summer it is a heat wave. Our heated space is aproximately 4000sq/ft. We heat with a munchkin boiler that is propane. We use propane for instantaneous hot water and cook top in the kitchen. Fuel use so far in a very cold month (high temp never above 25f) we used 200 gallons.

  13. Riversong | | #13

    Derek,

    Unfortunately, you've fallen victim to people like Dan who push spray foam where it's not the appropriate option, and use sales propaganda (such as adding more won't offer additional benefit, when in reality it will make the job so costly that the customer will balk).

    There is good research on "hot roofs" and you've discovered one of several problems with them. You've got considerable thermal bridging through your rafters and less than the minimum IECC prescriptive R-value for zone 4. With as little as 8" of snow on the roof at 25°, you'll have melting at the insulated cavities and melting at the rafters as low as 10° outdoor temperature.

    If the ice damning results in roof leakage, the open-cell foam can hold a good deal of moisture against the roof deck, causing saturation and potential moisture problems when temperatures rise.
    In all but the most humid climates, a vented roof will outperform a "hot roof" and eliminate a whole raft of potential moisture and heat problems.

    The only simple solution to your current situation is to add at least one inch of rigid XPS foam board to the underside of your rafters to increase the R-value and reduce thermal bridging. Either that or use a roof rake to remove the snow at the eaves before it can melt.

    Snow-melt cable rarely takes care of the problem.

  14. adkjac upstateny | | #14

    Wow... I just built a home in Upstate NY. Cold area. Icynene sprayed right to roof sheathing and over trusses. 5 inches. Walls 5 inches. floor joists 5 inches. Has a cellar. Roof is not vented. Attics are conditioned space.

    No snow melt at all. 3rd winter. The house is tight. The long direction is East West and amples Southern windows... fewer to the North.

    Propane 95% efficient basic furnaces. 3000 plus square feet. two 500 gallon propane tanks. When I go to the home to work on it... once the sun comes up.. the heat stays off for the day as far as I know.

    I am over the top happy with this build. And the Icynene is making me very happy. The main reason I went with low density foam.... is because it does breath.... I like the fact that it is more permeable. I really feel moisture trapping could be a huge problem with 2lb foam.

    I now have an interest in the 1lb foam.. of flash and cellulose.... with super high levels of cellulose dense packed.

    Also... the guy making water filled windows beat me to my idea... I love the idea of a window being a net plus btu producer.

    aj

  15. Derek | | #15

    Thanks again for all the input, pretty interesting stuff.
    Adkjac- It does not sound like you are full time at this house? I would look at some of the suggestion if you are still in construction phase as the icicles really did not start on my house until about two and half week of constant indoor temperatures of 68-70F.

    I am going to take some remedial action but the icicle proplem I have is not terribly bad even under what I consider a pretty bad weather month. I will continue to experiment.

  16. Gerry | | #16

    Perhaps the difference in A from upstateny roof and yours is the truss construction of his roof. Typically a roof truss will have a 2x4 or 2x6 top chord. So his spray form encapsulates the top chord and there is no themal bridging of the roof structure.

  17. Riversong | | #17

    adkjac,

    If you're in climate zone 6, your 5" of open-cell foam doesn't even come close minimum IECC standards, which are R-49 ceiling, R-20 walls, R-30 floors. If you're in northwestern NY, zone 5, the requirements are 38/20/30.

    You've got, at best, R-18 all around.

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