I have a cape cod. The boiler is in the basement and vents to the outside about a foot above ground level. I get a considerable amount of snow melt and ice buildup in the attic section directly above the vent. I have recently been remodeling and I cut a hole to get into this section of the attic (about a 6'x12' area between 2 alcoves). My initial intent in here was to investigate and patch up cold spots on the wall as we had a breeze coming through a corner that borders this.

I found 3 things that could be causing my snow melt and ice buildup.

  1. Spots that are not well insulated, particularly switches, outlets, and a corner section.
  2. 2 Copper radiant heat pipes that go from one end of the attic to the junction box at the other end that were just sitting under the insulation, fully exposed every 16" over the studs.
  3. Steam...coming from the boiler, into the eave vents and into the attic. This is condensing on the roofing nails, then dripping onto the insulation.

I can solve the first 2 with various types of insulation. I could block the eave vents, which is a quick solution. I could extend the pipe out and up past the roof, which with the current ice issue, could just get knocked down, plus I'm not sure how it would look aesthetically. I could also move the pipe from one basement wall to another (likely best permanent solution).

My question is, is it safe or acceptable to block off that section of the eave vents? Is there a solution that allows water to drip out, but not allow steam to come in? If I covered the vents with blown insulation, it should block the steam and allow water to drip through it.

  • I should clarify that the vents are perforated soffit board that run the entire eave.
    – rtaft
    Commented Feb 18, 2015 at 18:03

4 Answers 4


The problem is most likely hot exhaust coming from the vent entering the attic and melting snow on the roof, creating an ice dam and snow melt.

Since you have continuous soffit vents, blocking off any one section would not be the end of the world. You would want to block it with an air barrier material like plywood or foam insulation board, sealed at the edges with caulk or spray foam. Blown cellulose alone won't cut it, since it is air-permeable.

Relocating the vent could work too. Having it exit the house under a gable end rather than an eave would put the exhaust farther away from the roofline and out of the path of a soffit.

  • I did not make any changes and discovered I also had a carbon monoxide problem on the second floor due to this issue. I removed the T exhaust fitting that blew the exhaust straight up and extended it straight out about 12 inches which resolved the CO problem.
    – rtaft
    Commented Feb 20, 2020 at 19:52

My old physics teacher would give you an 'A' for simply creating this question.

It is important to remember that whatever solution you choose must fix the problem, and not just move the problem to a different place - AKA: the Wack-A-Mole paradox.

I think the first step is to get an infrared thermometer,I chose this one,but pick your own vendor and price point


This will allow you to identify the source of the heat leaks, and locate the problem areas.

After you know what problem you are trying to fix, we can move forward from there.


There is only one right answer: build a chimney and vent it above the roof. If the aesthetics don't bother you, just extend the vent duct right to or above the roof line.

If you don't extend it beyond the roof line, you will continue to get the issues with the ice dams since the heated vapor will melt the snow above the exhaust point.

My house is built in the 70's and we have a chimney (no fireplace) which vents the exhaust from my gas water heater.

  • I've owned 3 homes, all built after 95, and all of them vent out the side wall, one home was using PVC for all vents. I may extend the vent temporarily for now to see how it handles it. I haven't dealt with this, too many other projects.
    – rtaft
    Commented Dec 23, 2016 at 12:54
  • My previous house was built in 96 and had a different setup and vented out the side of the house, however, it was vented near the basement of a two-story house so the heat and moisture mostly dissipated by the time it hit the roof 25 feet up. Do you live in a ranch by chance? Commented Dec 23, 2016 at 14:43
  • Cape Cod, the 'attic' section is between two 2nd floor window alcoves, the lowest part of the roof is about 10ft from the ground on the back side.
    – rtaft
    Commented Dec 23, 2016 at 14:55

Can you explain how steam is getting into the attic, your item #3. You said that the boiler is in the basement and vents to the outside??? There should be no steam venting outside the home. My only thought is that there is a vent line coming out the top of the condensate tank (if you have a condensate tank). If this is the problem then you have other problems with the heating system that should be addressed. Can you investigate where the vent line comes from? As for item #2, do you know what the copper pipes are used for, and why are they in the attic? As for the attic problems I would consult insulation contractors and get their recommendations for insulation and attic venting. A wrong decision from you could add to the problem and become a costly mistake in later years.

  • Water is a byproduct of methane burning.
    – rtaft
    Commented Dec 23, 2016 at 12:30
  • I misunderstood the first part of the question, sorry for my misread.
    – d.george
    Commented Dec 24, 2016 at 12:39
  • I just reread my answer and I did not misunderstand the question, as i commented on Dec 24 2016, The heading of the question said "How do you stop steam from entering the attic." And statement #3; steam coming from the boiler ( not vapor or combustion vapor ) entering the attic. So I presumed that the system had a condensate tank vented outside which in pa. is not uncommon.
    – d.george
    Commented Jan 13, 2017 at 11:50

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