An Ice Problem

May 02, 2010
May/June 2010
A version of this article appears in the May/June 2010 issue of Home Energy Magazine.
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Credit: John Noble
Q: I was just reading an article you have online regarding ridge vents and soffits (see “Out, Out Dammed Ice,” Nov/Dec ’96, p. 21). Our house was built in 1997. We have some soffit venting and some louvered vents. Last spring we installed a new roof and had a ridge vent put in with new louvered vents. I recently went up into the attic and found ice formed under the new louvered vents and light snow coming in. Each louvered vent that had ice formation had water dripped below in the attic.

I read that the attic could be unbalanced, causing the snow to enter. Not sure why I have ice formation. I was wondering if I needed more soffit ventilation?  Is there a certain spacing, like every so many feet?  I have a two-story house that has six soffit vents on the back of the house and none on the front. The builder put two vented windows that have louvers with screen behind, one in the attic space above the garage and one at the other end of the house. Do you not put soffit vents on the front of homes?

I have another problem. I have had ice damming problems in the front of the house for the last 14 years. There is a downspout from the roof which has no soffit ventilation. Granted, the underhang is only 3 feet, but could that cause the ice?  The valley that runs into that downspout is above part of our garage. Its floor is uninsulated; the walls to the inside rooms are insulated.

Any suggestions you might make would be greatly appreciated. I have fought water in the attic and ice dams for years.

via e-mail


Greg Labbé is the Director, Energy Retrofits and Training at Green Saver, in Toronto, Ontario, Canada. (Image credit: Jens Lohmueller)

A: It seems like you have two separate problems. The first is moisture (snow) coming into the attic through the vents causing leaks on your ceiling. The second is that your house has ice dams yearly. Overall we should assume that your house has enough ventilation to have met current local building codes, and we should agree that adding more ventilation will rarely solve a true ice dam; and as stated by the good folks at,  ventilation in some climatic zones may contribute unintentionally to the moisture load in an attic. (See BSD-102, Understanding Attic Ventilation, p. 5.) To this point you seem to have found evidence of snow entering the attic through the new vents.

The first problem—snow entering your attic from the louvered vents—seems to suggest that the ventilation is responsible for snow coming into the attic and melting into the insulation. As an experiment, block the snow from entering the vent from  the outside and check for snow in the attic after a snowstorm or after snow has drifted in the wind. If the snow doesn’t enter, you’ve solved your problem and you can open the vents again in the spring, though the rain may come in just as easily, causing similar problems.

As for the ice dam, the cause is usually an attic filled with warm air that melts snow on the rooftop (see “Roof Topography and Ice Damming,” HE Jan/Feb ’10). Ideally the attic air should be the same temperature as the outside air. The symptoms of an ice dam are unusually large icicles on the eaves. The symptoms of an excessively warm attic are uneven snow or frost melt patterns on the roof before the sun has come out. The Canada Mortgage and Housing Authority has a free Research Highlight titled  Diagnosing Attic Performance by Snow- and Frost-Melt Patterns. (To download the highlight, go to Click on Research Highlights and find it under the heading Technical Series from 2004.)

Inside the attic, symptoms of ice damming include wet wood surfaces, mold, and frost or ice buildup—especially at the outside perimeter of the attic on the sheathing. A great way to check the attic floor for heat and air leaks is to scan the attic floor with an IR camera while pressurizing the house with a blower door. This can be done by a qualified building consultant both before and after the remedial work of professional air sealing and insulating is completed.

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