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How Bad Attic Ventilation Can Cause Ice Dams and Good Attic Ventilation Can Prevent Ice Dams

Most builders don’t ventilate attics properly, even though it’s easy to do right.  We’ve seen under-ventilated attics in homes 100 years old, and we’ve seen them in homes built this year.  Building technology may evolve, but human nature doesn’t.

Contractors’ tendency to “wing it” or do the bare minimum is why attics have too little ventilation or improperly installed ventilation.  One result of that is attics get too hot, eventually causing ice dams to form (more on that in a second).

Believe it or not, that’s a persistent problem even in snow-heavy states like Minnesota.  Two reasons builders haven’t stepped up their attic-ventilation game, at least as it relates to preventing ice dams:

  1. Most people don’t understand ice dams, partly because most of the info online about ice dams (even on reputable sites) is incorrect or misleading.  Most builders don’t realize that attic ventilation influences whether a home gets ice dams. Most homeowners don’t know that, either, which means often they don’t get much or any help on ventilation from contractors.
  2. Many builders just don’t care. Squashing the ventilation problem would cost them a little more, so they adopt a “don’t ask, don’t tell” policy, knowing that homeowners probably won’t notice a problem for a while, if at all.

We’ll admit that attic ventilation is a little counter-intuitive. You might wonder, “Why would you want to let cold air into your home in the winter?”  Well, because you aren’t letting the cold air into the living space of your home: It’s confined to your attic.  Unless you have a finished attic and sleep up there, that’ cold air is a good thing.

A warm attic is like a welcome mat for an ice dam

Ice dams form only under a few specific conditions:

  1. Snow falls on your roof and stays there.
  2. The snow melts.
  3. The snow refreezes.

That cycle repeats until you’ve got a pool of water behind a wall of ice, waiting to work its way through your shingles and into your home.

If you think, “All I have to do is get my roof really hot and the snow will just melt,” we’ll give you points for unorthodox thinking, but you’re still in for disappointment. When the temperature dips down to below freezing (as it often does in December and January) it is impossible to keep snow from refreezing, no matter how hot your roof is.  It’s just too cold.

But no matter how cold it is outside, a hot or even slightly warm roof will still melt some snow.  That will happen even if the roof is a few degrees warmer than the outdoor temperature. If the roof is above freezing at all it will melt some snow. You might not see it melt, but it’s melting under the snow, and will continue to melt.

One easy way to tell if you have an attic ventilation problem is to look outside on any winter day.  Do you see icicles on your roof?  If so, inadequate attic ventilation may be partially or entirely to blame.  Ice dams still can form if you don’t get icicles, but an icicle is a good indication that you either have an ice dam or will get one in the near future.

That means your goal is to get the temperature in your attic as close as possible to the temperature outside. 

How to fix insufficient or bad attic ventilation 

It’s probably not practical for you to have your entire roof ripped off so you can have the ventilation redone from scratch. But the good news is you don’t have to, because you have a few choices.

One option is to install additional roof vents and soffit vents. Those can move the hot air from your attic to the exterior of your home. Simple vents are an inexpensive and relatively effective solution.  (Don’t concern yourself too much with attic fans, which aren’t as helpful as you might think.)

If your ductwork and vent lines run through your attic, get your ducts and vent lines insulated while you’re at it.

You also want to find and seal all of your attic bypasses. Those are all the places in your walls and ceiling where heat escapes and seeps up into the attic. The most common attic bypasses are gaps around your outlets and switches (but behind the bezels where you can’t see them).  Attic bypasses included “anywhere” your walls or ceilings were penetrated (e.g., ceiling fans, ceiling lights, smoke detectors, etc.).  If a hole was cut in your ceiling or wall to install something, that’s a potential air leak into your attic.

Also, improve your attic insulation. That’s the #1 thing we recommend to our customers, because it’s inexpensive, and it will lower your heating bill, because it keeps all your heat in the living areas where it belongs. We usually recommend blown-in insulation, because you can get it into more of the small spaces where batts of fiberglass can’t go.

If you’re not sure you can do any of that, at least turn your heat down a few degrees and keep up with raking snow and ice off your roof. We recommend the latter anyway, because allowing snow and ice to accumulate on your roof can damage it. Snow gets really heavy really fast. By the time you’ve let three inches of snow accumulate up there, you’ve got the weight equivalent of a pair of SUVs sitting up there.

If you’re custom-building your home you can nip the problem in the bud by paying close attention to the ventilation. It doesn’t matter whether you go for a soffit-and-ridge ventilation system or a soffit-and-gable vent system, as long as the end result is you get 1 square foot of net free ventilation per 150 square feet of attic floor. “Net free ventilation” is the total area of your vent openings.

While none of these measures will completely prevent ice dams, together they can prevent up to 90% of ice dams you’d get otherwise. That annoying 10% of ice dams can form when Mother Nature decides to give us 33-degree weather during the day and below-freezing weather at night. There’s nothing you can do about those, unless you’re a roof-raking Rambo who mows down every flake of snow on sight.

Is roof-raking a hassle?  Sure, but if you don’t want to or can’t improve your attic ventilation just yet, then the alternative is to get ice dams (which we guarantee you’ll enjoy even less).  In that case, we look forward to meeting you very soon.

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