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One of the frequent questions that comes up in a home inspection is “What can I do to minimize or prevent ice-damming on my home?”  In northern climates such as that in New England, ice-damming is something that plagues a vast majority of homes during winters when heavy snow starts to accumulate on the roof.  Contrary to the popular belief of many homeowners, ice-damming is not caused by gutters.

Ice-damming occurs when the surface of the roof is just enough above freezing to cause the snow to melt.  The melted snow runs down the slightly warmer roof surface until it gets to the eave or soffit area where it is cold enough to re-freeze.  When interior conditions are not addressed, this process continues unchecked 24/7.  Although a foot of snow or more may be present, this melting and re-freezing process is occurring invisibly.  So what causes the roof to rise above the freezing point in the dead of winter?

The most obvious answer is insufficient insulation in the attic.  According to the EPA homes in climate zone 5, which includes all of Massachusetts, should have an attic insulation rating of at least R-49.  Since fiberglass batt insulation has an R-value of 3.1 – 3.4, using the best case scenario you would need about 14 ½” to make it to this level.  Another popular type of attic insulation, loose-fill cellulose (or recycled paper fibers) has an R-value of 3.2 – 3.7.  Using this material in an ideal application you would need around 13 ¼”.  There are other options for attic insulation, including spray polyurethane foam (SPF), but regardless of the choice these are all designed to prevent conductive heat loss through the ceiling of the living areas below.

Secondly, sealing all gaps and cracks into the attic is critical to stopping a direct path for heat to escape the home and warm the attic to above freezing.  The biggest culprits in this category are openings around chimneys, plumbing vents and recessed light fixtures.  Some homeowners have discovered that you can purchase spray foam in a can, and they like to use it for sealing up these gaps as well as for other applications.  While this product can truly be ‘great stuff’ because it is so easy to use, it should never be used to seal around a chimney because it is combustible.

The final component that is essential to reducing or preventing ice-damming is good attic ventilation.  Having a well-vented attic is important for a number of reasons, including moisture control and promoting longevity of the roof covering.   It also helps keep the attic cooler in summer and colder in the winter.  An attic with poor ventilation can often be identified by rust on the tips of roofing nails and by the presence of dark mold-like stains on the roof sheathing.  The better the air circulation is in an unfinished attic, the healthier the house.

If you want to minimize ice-damming on the roof, the temperature in the attic should mimic that of the outdoors as closely as possible.  34 degrees in the attic might feel cold relative to 68-72 degrees inside the home, but if it’s 10 degrees outside and several inches or even a couple feet of snow are present on the roof you can bet that the snow is quietly melting on the upper part of the roof and that formation of an ice dam is underway.