Ice Dams and Soffit Vents

Ice Dam

Ice Dam and Soffit Vents


Right now in Massachusetts people are going crazy because of the ice dams on their roofs!  A company has arrived from Minnesota that has steam generating devices and personnel that will climb up on your roof and melt the amazing amount of ice that is collecting in the gutters, weighting them down, backing the water up the roof, and leaking into the ceilings below.  Companies have produced melting products like salt that can be thrown up on the roof to melt the ice.  Why is this happening?

Obviously because there is an enormous amount of snow that has fallen on roofs in Massachusetts!  But this really only highlights a problem that festers every year.  Before we insulated attics we didn’t have ice dam problems.  Heat from the interior of the house passed through the ceiling, heated the attic, and melted the snow evenly.

To save energy (and money) we now insulate our attics so much less heat escapes from the house which is great for a whole lot of reasons.  If air sealing was done prior to the installation of the insulation, reducing the amount of air that moves through the holes in the ceiling, transferring warm, moist air from the house into the attic AND if the insulation is installed perfectly from one edge of the ceiling to the other, there wouldn’t be ice dams either because the entire attic would be almost the same temperature as outside and the snow would melt evenly.

Soffit and ridge vents were the solution to the imperfect installation of the insulation.  The idea is that cold air pours in through the soffit vents and sweeps up under the entire underside of the roof deck and pours out of the ridge vents.  Soffit and ridge vents are there to prevent ice dams!  The building code says that you have to have attic venting.

Soffit and ridge vents are designed to solve the ice dam problem only.  They are NOT going to cool the attic.  The building code is the same in Florida as it is in Massachusetts.  They haven’t had much of a problem with ice dams in Florida.  They do have a problem with hurricanes.

For ridge and soffit vents to do their job inducing an airflow up the entire underside of the roof deck, they have to have an unimpeded path from the soffit to the ridge.  Companies make baffles or chutes to guide the air from the soffit.  These are not easy to install effectively particularly on a retrofit basis.  And when the air sweeps in from the soffit and passes through the fiberglass insulation or blows back the loose fill cellulose, it reduces the insulation value at one of the most critical points in the thermal boundary: the tops of the exterior walls.  Consequently, heat flows up the exterior walls into the attic, melting the snow on the roof immediately before the roof extends out beyond the house where it is exposed to the outside air and cold on the top and on the bottom.  The snow melts, runs down the roof, hits the cold surface and freezes.  More water from melting snow moves down the roof, and collides with the ice dam.  The dam forms a lake above it and the water works it way back in, under the shingles, and drips down onto the ceiling and into the house.

How do you fix this?

Well you can’t do much inside the house until the ice and snow is gone from the roof.  Once that wonderful day arrives, you should try to resolve the problem so that it doesn’t happen again.

  • Before adding insulation to the attic, make sure that as many of the holes between the house and the attic are sealed.  It is a lot easy to do that when they aren’t buried under lots of insulation;
  • Make sure that the soffit and ridge vents are actually open and allow air to flow through them;
  • Install effective air baffles to guide the air from the soffit vents up the underside of the roof deck.  It is very hard to see because it is a narrow, triangular space at the eave and there are nails sticking through the roof deck, but it is vital that the baffles make contact with the soffit and seal the entry air into a pathway to the ridge;
  • Then add insulation to the attic.  Blowing in insulation is like painting the walls.  The hardest part in painting and insulating is the preparation.  Once that’s done, blowing in the insulation is quick and cheap.

You could consider insulating the underside of the roof deck with spray foam.  That is a more expensive process but can be quite effective.  Any of these solutions is cheaper than hiring a company from Minnesota to keep steaming your roof time after time!



Please visit our website at

Still to come: the BPI Home Energy Professional QCI Handbook


Tags: , , ,

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: