Helen Watts Engineering PLLC

Hints for Homeowners #4 - Ice Dams

Home
About HWE
Structural Engineering Services
Residential and Condomium Structures
Contact Us
Builder Tools
Hints for Homeowners #1 - The Spring Inspection
Hints for Homeowners #2 - Live Green
Hints for Homeowners #3 - Hire a Contractor
Hints for Homeowners #4 - Ice Dams
Hints for Homeowners #5 - Landscaping

“What PRETTY and HUGE icicles on our eaves!” “WHAT is that “Drip – Drip - Drip? UH-OH!”

 

The time to prevent ice dams is now, before those big icicles form. An ice dam is a sign that some of the expensive oil burned for heat is being wasted on the underside of the roof. The heat will melt any snow that lands on the warm part of the roof, then the water will flow down the roofing until it gets cold enough to freeze again. Where it freezes, more water lands and builds up, then that dammed water freezes too – then more. When it freezes, the water can freeze under roofing and flashing, pushing the roofing and flashing up – which lets more water in. Water doesn’t just travel down in the winter in Maine.

 

So, is your house a candidate for ice dams? I often look at houses (my girls will say I always look at houses) and when there is a frost I look at the roofs. The roof of a heated house, in an ideal, no-ice dam world, will have the same frost coating as an unheated building, such as a garage, that is pointed in the same direction. So, before the sun melts off the frost in the morning, have a look at your roof. Can you see where the roof rafters are? Is there no frost near the roof ridge (with no ridge vent)? Does the garage (or your neighbor’s house) have frost, but not your house, or some part of your house? This will show where the heat leaks are - one cheap house X-ray!

 

There are two methods for keeping the underside of the roof cold. One is to prevent the heat from the living space from getting to the roof sheathing, and the other is to ventilate away any heat that does escape to the roof, and you need to do BOTH. Sources of heat in attics include 1) air leaks from the living spaces – like uninsulated sidewalls facing living areas, openings for ceiling lights, gaps between the chimney and the ceiling, insulation that has been moved or crushed or wetted, and the attic access panel, which needs to be insulated AND sealed; 2) inadequate insulation in the attic floor – go for a total insulation of R-38 or better, and if the first layer of batt insulation goes between the joists, take the second layer across the top of the joists; 3) bathroom fans or dryer vents that vent right into the attic space, or into a kneewall and from there into the upper attic. These not only put lots of heat into an attic, they add moisture, causing causing “interesting” mini-floods when the water condenses on the cold underside of the roofing then warms up and melts as the sun hits the roof, draining the water to places far from the source of the problem. The outlets for these vents should go completely to the outside, not just rest on the inside of the soffit vents. So Seal, then Insulate, with Ventilation. I go in a lot of homes with 4" batt insulation in the attic. That's a lot of heating dollars that could be saved, easily.


If you're going to add insulation, FIRST do any structural repairs, then do air sealing, THEN do the insulation. 

 

If any heat does enter the attic, it should be removed by the roof ventilation. There should be a clear path for outside air from the soffit vents up to the ridge vent or gable vents. The insulation added should not block the ventilation of the roof sheathing. This is often done with air chutes, which are molded polystyrene baffles that hold the insulation away from the underside of the roof sheathing and make a clear path to the soffit vents. Air chutes are especially needed at the point where the insulation on the attic floor meets the eaves, and where you have a cathedral ceiling or living space under a sloping roof.

 

Notice that I haven’t talked about gutters as a cause of ice dams? They aren’t. If the ice dams form, however, the gutters can help make the dams worse, or, more likely, the ice will damage the gutters. Keep the gutters cleaned and properly attached – and use them only where needed. 

 

And what if you missed the opportunity to insulate and ventilate and an ice dam forms? Well, first, going up on a snowy, icy roof is not safe. There are safe methods (think ropes, tied - off ladders, and working with another person who practices safety as if your life was at stake), but climbing on a sloping snow pile with ice below, when asphalt shingles are most brittle, is less fun than talking to political pollsters. You are likely to damage your roof shingles, as you will see when you look at the roof when it is cleared off. The rake or whatever can crack and pierce the roofing, and your weight can break off tabs from the shingles. You may also put holes in your flashing.

 

IF you do go, clear from the bottom up, for the best footing, and clear the snow from the whole roof – including the top part of the roof that is supplying the melt water, and the lower part of the roof where the snow insulates the ice and prevents melting. I have one client that paid $250 to have her roof professionally cleared – each storm. That’s a lot of money for insulation.

 

The people that take the snow off the lower three feet of the roof may have another ice dam start four feet up, which would be ABOVE the three-foot typical strip of Grace® Ice and Water Shield membrane that goes down at the eaves under the shingles, there to prevent damage inside if ice dams occur. 

 

Some people install a zig-zag of heating tape at the eaves. This is more energy cost, and no answer to the primary problem, which is an expensive heat loss to start with. It also can put the ice dam above the membrane. You can put the heat tape on a switch, and power it only as needed. I have one spot above my entrance with heat tape.

 

When you get to the ice dam, there is a method for clearing it. Take magnesium chloride (salt) and “break the dam” by salting on the ice heavily in a 6-12” strip, every four to six feet along the length of the dam. Then, when those breaks are melted, go for the rest of the dam, and when the dam melts the water will have a convenient place to go, down and away. Magnesium chloride is better on the environment than calcium chloride, and at a slightly higher cost.


Oh, and one more thing. If you have a home in Maine, then you have a thick layer of snow expected on your roof. So even if you are air-sealed, insulated well, with good roof ventilation, you may still get ice dams because the snow is an insulator and on a warm day the underside of the roof sheathing will get warm. So the best you can do is prevent all the other possible causes.

 

So, look at your roof on a frosty day - where is the frost melted? Stop any air leaks and make sure the bathroom and dryer vents go outside. Make sure your roof vents are not blocked. Insulate the walls and ceilings of your living spaces. And then, enjoy having all the heat you paid for, inside your home.

Updated 10/14/11

Helen Watts Engineering PLLC * 455 Litchfield Road * Bowdoin, ME * 04287 * hcwatts@gwi.net * (207) 522-9366