How this warm weather mixed in with rain in the dead of winter can overwhelm homeowners with and without appropriate coverage

I’m bringing up the weather not because it's everyone's favourite topic of conversation but because Mother Nature's mood are sometimes temperamental, posing the possibility of a sudden rise in property water damage for homeowners. 

This week's weather-related risks, the ice dam risk that keep can keep you up at night

An ice dam is caused when heat from inside the home melts snow on the roof, and the resulting water runs down the slope until it reaches the edge and freezes. Then when the snow further melts, the water backs up beneath the shingles and drips down the walls and creeps across ceilings. This water damage can also lead to another problem: black mold caused by retained moisture. While icicles hanging off the roof can be a pretty picture, they are also a sign that the home has an increased potential for damage due to ice damming.

Damage

It’s easy to understand that allowing water to leak into your house is a bad idea. Ice dams cause millions of dollars of damage every year. Much of the damage is apparent. Water-stained ceilings, dislodged roof shingles, sagging ice-filled gutters, peeling paint, and damaged plaster are all easily recognized and usually repaired when weather or budgets permit. But other damage is not as obvious and often goes unchecked.

Ice dams usually develop along roof eaves, above the plate line of exterior walls. Heat lost from homes at this point aggravates snow melting and ice-dam development. There are two reasons for increased heat loss at this point: Rafters on most homes sit directly on top of exterior walls leaving a shallow space for insulation between the top of the wall and underside of the roof sheathing: Low R-value = heat loss! And secondly, builders are not particularly fussy when it comes to air-sealing this point to prevent the movement of warm indoor air up to the underside of the roof surface. Air can leak through wire and plumbing penetrations here. Also warm indoor air can leak from the wall cavities rising upward and passing between the small cracks that exist between the wall top-plate and drywall. Ice dams also form below skylights because they are typically low-R assemblies that melt water that subsequently runs down to a cold section of roof below the skylight.

Roof leaks wet attic insulation. In the short term, wet insulation doesn’t work well. Over the long term, water-soaked insulation is compressed so that even after it dries, the insulation in the ceiling is not as thick. Thinner insulation means lower R-values. It is a vicious cycle. The more heat lost – the more ice dams form – the more it leaks – the more the insulation gets damaged – and so on. As a result you pay more to heat (and cool) your house. Cellulose insulation is hygroscopic and particularly vulnerable to the hazards of wetting.

Water often leaks down within the wall frame where it wets wall insulation and causes it to sag leaving uninsulated voids at the top of the wall . Energy dollars are again robbed, but more importantly, moisture gets trapped within the wall cavity between the exterior plywood sheathing and interior vapour barrier. The result: smelly, rotting wall cavities. Structural framing members can decay. Metal fasteners may corrode. Mould and mildew can form on wall surfaces as a result of elevated humidity levels. Exterior and interior paint blisters and peels. And the well-being of allergy-sensitive individuals is compromised.

Peeling of wall paint deserves special attention here because its cause may be difficult to recognize. It is unlikely that wall paint (interior or exterior) will blister or peel when ice dams are visible. Paint peels long after the ice and all signs of a roof leak have evaporated.

Water from ice dams infiltrate wall cavities. It dampens building materials and raises the relative humidity within wall frames. The moisture within the wall cavity eventually wets interior wall coverings and exterior claddings as it tries to escape (as either liquid or vapour). As a result, interior and exterior walls shed its skin of paint.

So the message here is to check your home carefully when ice dams form. Investigate even when there doesn’t appear to be a leak. Look at the underside of the roof sheathing and roof trim to make sure they haven’t gotten wet. Check the insulation for dampness. And when leaks inside your home develop, be prepared. Water penetration often follows pathways difficult follow. Don’t just patch the roof leak. Make sure that the roof sheathing hasn’t rotted or that other less obvious problems in your ceiling or walls haven’t developed. And then detail a comprehensive plan to fix the damage. But more importantly, solve the problem.

Solutions

The damage caused by ice dams can be controlled in 2 ways: Maintain the entire roof surface at ambient outdoor temperatures or build a roof so that it can’t leak into sensitive building materials if an ice dam forms.

Cold roofs make a lot of sense. Here you let the cold outdoor air work for you. Keep the entire roof as cold as the outdoor air and you solve the ice-dam riddle. Look at the roof of an unheated shed or garage, a pile of lumber or an abandoned home. Ice dams don’t form on these structures because there is no uneven melting and freezing!

For new construction it’s easy. Design the house to include plenty of ceiling insulation, a continuous air barrier separating the living space from the underside of the roof, and an effective roof ventilation system. Insulation retards the conductive flow of heat from the house to the roof surface. An air barrier retards the flow of heated air to the underside of the roof. And a good roof-ventilation system helps keep the roof sheathing cold. In an existing house this approach may be more difficult to follow. Often you are stuck with less than desirable conditions. But let’s look more closely at all the issues that will guide your strategy.

Insulation: Houses should be equipped with ceiling insulation of at least R-38 (about 12 inches of fiberglass or cellulose). The insulation should be continuous and consistently deep. The most notable problem area is located above the exterior wall. Raised-heel trusses or roof-framing details that allow for R-38 above the exterior wall should be used in new construction. In existing structures, where the space between the wall’s top plate and underside of the roof sheathing is restricted, install high R/inch insulating foam (R-6/inch). Be sure to seal the insulation at this point to prevent warm-air leakage from the living space.

Ventilation: A soffit-to-ridge ventilation system is the most effective ventilation scheme you can use to cool roof sheathing. Power vents, turbines, roof vents and gable louvers just aren’t as good. Soffit and ridge vents should run continuously along the length of the house. A baffled ridge vent (like the one sold by Air Vent) is best because it will exhaust attic air regardless of wind direction. The exhaust pressure created by the ridge vent sucks cold make-up air into the attic through the soffit vents. A 2-inch space or “air-chute” should be provided between the top of the insulation and the underside of the roof sheathing in all applications. The in-coming “soffit” air washes the underside of the roof sheathing with a continuous flow of cold air. CAUTION: Be sure to install insulation baffles above the exterior wall to protect the insulation from the air that blows in through the soffit vents.

Whatever plan you decide to follow, focus on the cause. Ice dams are created by the heat lost from the house. Develop a strategy that is centered around this fact whenever possible. Ventilate, insulate well and block as many air leaks as practical. There are no excuses for new construction. However, cures for existing structures are often elusive and expensive. In some cases you have to treat the symptom. The payback is damage prevented.

What is certain is that change is inevitable, and Mother Nature may be more unpredictable than she has shown us in the past.

Should you have a question please do not hesitate to contact us at [email protected] or call us at 613 736 9222

Source: BUILDING AND CONSTRUCTION TECHNOLOGY

UMass Amherst | Department of Environmental Conservation