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How Much Insulation Is Needed to Prevent Ice Dams?

Every year, people ask me how they can prevent ice dams. Every year, I recommend home energy audits.

Then I hear from the same people the following year, because – as they tell me with some guilt in their voice – they never got around to the energy audit.

I suspect they think their ice dam was a fluke, and that their home came out of the box with more than enough insulation to protect them most of the time.  I think it’s easier to blame radiant heat from the sun (which is unavoidable and unpredictable for all homes) than it is to set aside the money and the time to have that energy audit and get that insulation up to par.

You probably don’t enjoy being told your home needs repairs, some of which may cost more than a few shekels.  It’s about as pleasant as a TSA finger wave.

But your home almost certainly doesn’t have enough insulation to prevent ice dams.  You can deal with it now, or when you’ve got a multi-ton that causes leaks and makes home repairs (much more expensive ones) unavoidable.

How do I know your home probably lacks the insulation to be fully ice-dam-resistant? Because Minnesota building codes require less insulation than you need to deter ice dams.  Anoka County and Hennepin County building codes require insulation at an R-value of 49. I know of some Minnesota counties that permit an R-value as low as 44.

Meanwhile, energy professionals recommend an R-value of up to 60.

It’s somewhat concerning to note the US Department of Energy only recommended insulation to R-38 values, which is even lower than the Minnesota Building Code recommends. I’m not sure who came up with that number, but we already know that’s not enough because we’re seeing ice dams form even on homes with the required R-49 insulation.  Other factors contribute to ice dams, of course, but a hot attic is the primary cause, and only heavy-duty insulation can keep the heat low.

Meanwhile, The Minnesota Department of Energy focuses more on attic bypasses (places where heat can leak in from your living space).  Those are important to seal as well as possible, but you can’t ignore insulation, or you’ll still get ice dams.

Home energy auditors’ standards are higher.  They’re the guys who see, with their thermal imaging cameras, exactly where the thermal leaks are.  They’re in the best position to know how poorly insulated the average home is, and what yours needs to be more ice-dam-resistant.

The type of insulation also matters.  Most home builders lay down fiberglass bats.  Those are a good start, but not necessarily ideal to prevent ice dams.

Energy pros typically recommend loose-fill, blown-in cellulose insulation.  Why?  Because this insulation gets into all the nooks and crannies the big fiberglass bats can’t reach.  A good insulation pro may add up to 17 inches of blown-in insulation to get it to the proper R value.  Even better than loose-fill cellulose is spray-foam.  (In my experience spray-foam trumps all.)

Until you’ve gotten a home energy audit and if necessary revamp your insulation, your roof will get unnecessarily warm during the winter.  That means you’re prone to ice dams.  When (not if) you get one, it won’t be a fluke, and it won’t be the last one.

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