Understanding Winter Leaks
Our roofs can leak when it is literally freezing with arctic like temperatures outside, a phenomenon that has gained further attention now from Building Science Engineers, Architects and Manufacturers of home building products. Winters that cause ice damming leading to interior leaks are typically rare. Climate change, however, may have something to say of that trend.
Remember the winter storm of 1999? That was the time our military was called in to Toronto to help keep our city moving. We were under siege with snowfall.
It was also a time, according to the Insurance Board of Canada, that $50 million worth of claims were received by insurers in Southwestern Ontario. Many of those claims were directly related to property damage caused by ice damming.
The storm of 1999 inspired a symposium by Canada Housing and Mortgage Corporation. Invited guests and speakers to the symposium included Research Engineers, Architects, Engineering Firms, Meteorologists, Insurance Companies, Roofing Consultants and Roofing Contractors.
Meteorologists provided statistics demonstrating global climate change. Also noted was change to how we live in our homes. Since it is a new technological age we are simply using more energy in our homes than we did before. This combination of change to our interior and exterior environments raised concern for the possible need to rethink how we design future buildings as it relates to Winter Leaks.
If conditions that cause ice damming continued on a regular basis and to the degree witnessed in1999; design change would be imminent. Financial costs to insurance companies would be unsustainable.
What is an Ice Dam? Ice dams are masses of ice on our roofs that usually collect on the lower edge of the roof or in the gutters. Accumulation of ice and snow can also be found in complex roof designs with hips and valleys.
How does an Ice Dam form? Ice dams usually form when there is significant depth of snow. They occur when the outside air is freezing but the attic air temperature is above freezing. The warmth from the attic will melt the snow above. As the melt water runs down it refreezes at the cold edge of our roofs, the roof overhang.
What causes a Winter Leak? As the cycle of melt water continues to run down the roof, the wall of ice prevents it from draining, hence the name Ice Dam. This snow melt has nowhere to go so it will usually find its way under the shingles and into your home.
What can we do about it?
Basic Building Science Principles are to control the flow of heat, air and moisture in our homes. Cool attics usually result in cool roofs which will prevent the snow melting that creates the ice dams in the first place. This is the first line of defense.
The attic generally becomes the first place that comes to mind when we think of adding insulation. It has relatively easy access and few obstructions that make the attic a favourite starting point for homeowners.
Adding more insulation, however, is only half the battle.
Heat wants to move from any area of warmth to areas of cold. Many people believe that because hot air rises, most heat loss will be through the ceiling. Not necessarily so. Heat moves in any direction, up, down, or sideways – as long as it is moving from a warm spot to a colder one. Warm air that leaks into the attic accounts for substantial heat loss, especially during the winter when our houses operate like a big fat chimney whereby air tends to enter at the lower levels and exit at the upper levels.
Air leaks are usually found at penetrations or discontinuities. The best line of defense is a systematic identification and sealing of as many of the air leakage paths as possible. Comprehensive air leakage control cannot be understated.
Some examples of common air leak areas are electrical outlets, bypasses around chimneys and plumbing stacks that can channel air directly from the basement to the attic, recessed spot lights, gaps under baseboards, pocket doors, exhaust grilles, attic hatches, cold air returns, ducting for fans or heating systems, bulkheads, top plates and any breaches in the air barrier system.
In Building Sciences, it is important to understand too that your house works as a system. Each part is related to all other parts. Any change in one place causes an effect elsewhere. For example, reducing air leakage makes the house more comfortable and protects the envelope from moisture damage, but it will also increase humidity levels in your home, since less water vapour escapes. This may create an increase of condensation on the windows.
If you find your home has more than average winter leaks (all homes are unique from each other) you may want to consider a professional air sealer. Professional air sealing companies often use a depressurizing fan test to identify and measure the air leaks in a house.
What can your roofer do for you?
Roofing contractors can mitigate the potential for Winter Leaks by adhering a composite membrane (ice and watershield) consisting of modified bituminous coated material at the eave. It is important to understand, however, that this is not a guaranteed solution as it only deals with the symptoms of the problem and not the real problem. That is, a cool attic by preventing heat loss.
The ice and watershield should extend past the inside of the bearing wall a minimum of 300mm. More is always better. In areas of valleys, install ice and watershield first into the valley before installing the metal valley. Then, install an additional strip of ice and watershield on the leading edges of the metal valley sandwiching the first ply of ice and watershield. Also, install ice and watershield around all roof penetrations and at vertical wall intersections for added protection.
Many people are led to believe that ventilation is a solution. Not necessarily so, and it must not be relied upon as such. Ventilating the attic may reduce the potential for ice damming but it has its limitations. Ventilation is driven primarily by wind, and effective ventilation requires circulation, air intake at the soffit vents and air exhaust at the static roof vents. Imagine trying to sail a boat when there is no wind. That scenario is similar to cooling your attic with ventilation. Your home likely has other obstacles to contend with, such as tall buildings and trees limiting air flow or snow covered exhaust vents.
The primary purpose for vents in the winter is to remove excess moisture from the attic. If excess moisture is not removed quickly, water will condense onto the wood surface and insulation. That will cause mould and reduce the thermal efficiency of the insulation. In the summer, the primary purpose of ventilation is to reduce the attic space temperature, which can reach 160 degrees Fahrenheit. That will help to ensure the shingles reach their effective service life while keeping the house cooler. The rule of thumb is one square foot of net open area for every 300 square foot of attic floor area. The venting should be split evenly between the soffit and the peak.
Heating cables can also be effective but they do have their drawbacks. They should be monitored throughout the winter season. Many people forget to turn the cables off. Control boxes are available now that fasten under the troughs. These are designed to act as a sensor and automatically switch on or off as the need arises.
If you experience a Winter Leak, the only thing that can be done at that time, is to have the ice removed from your roof to clear a pathway for the melt water to escape. We call this the bang and smash method. There is always a liability for property damage here since there is no delicate way to do this.
-Tim Mulroy TORONTO ROOFING INDUSTRIES LTD.