The Ugly Truth about Those Beautiful Icicles and How to Prevent Them

Updated: Mar 27, 2018

Here are a few tips from my decades as a home inspector, energy auditor & property manager.


Icicles are caused by your home’s heat escaping to the attic and melting the snow faster than the environment can absorb the moisture. It then refreezes when it meets the cold eaves. Cold, unheated garage roofs never have icicle/ice dams. Look around and you’ll see it’s a fact.


Heat loss from the home comes mostly from air leaks, though duct leaks and missing insulation can contribute significantly. The most cost effective attic treatment is to SEAL all the gaps around light boxes, top plates, pipe penetrations, , etc. and then spread out your existing insulation to be even all around.




Insulating without air sealing is a great way to end up with a moldy attic. Adding insulation makes your home warner, but the roof gets colder. If the air continues to leak to the attic, it will condense on the now colder roof and the mold buffet begins. Most insulation will not stop air leaks. AIR SEAL FIRST.


Attic ventilation is like a bilge pump for a boat – if moisture creeps in, ventilation gets it out. With enough holes, even a huge bilge pump will not keep the boat from sinking. So make sure you have good ventilation, but seal the holes that leak warm moist air.

So make sure you have clear ventilation. Air seal (caulk, canned foam or one part foam, foam board) all leaks – gaps, holes, drop soffits, penetrations attic hatch, and don’t forget the duct seams. Make sure bath fans exhaust outside, not into the attic. Insulate last, after all other items are addressed. 



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