Ice damming and Attic ventilation explained

Ice damming is a big problem in the Chicago area, as well as most of Northern North America. Ice damming and frost accumulation in the attic space are both caused by interior heat build up when there is snow sitting atop a shingle roof. This happens when an attic is not properly ventilated and/or insulated. The solution to ice damming or interior frost accumulation is 3 part.

The ice dam forms by interior heat from the building melting the snow, especially melting the snow from the bottom side. When the snow freezes, as the temperature drops at night, the ice expands and pushes under the shingles. In addition this ice creates a bump which restricts further run off, and with additional freeze-thaw cycles the ice damn becomes larger. Eventually the ice has nowhere to go except under the shingles and inside your building structure.

First your attic ventilation must be assessed for it’s overall effectiveness. Your attic ventilation should be balanced, and many roofs have undersized and/or imbalanced ventilation. Not only will an under ventilated attic cause ice dams, but it is also a major contributor to attic mold growth, de-lamination of your roof plywood substrate, premature decay of your roof shingles, and condensation build up.

A properly balanced ventilation system will preferably have an intake as well as an exhaust. The attic ventilation works on a principal of fresh air in, hot air out. Some common mistakes made by improperly trained contractors is the installation of exhaust vents low on the roof to be used as an intake. This causes more harm than good. A good intake will usually be in your overhang, also called soffit. When no overhang is present a behind-the-gutter fascia vent may be used. to create the air intake.

When it comes to the ventilation requirements, the typical rule of thumb is one square foot of net free area per 150 square feet of attic floor space. However this is only to be used as a rule of thumb, and the formula for determining proper ventilation is actually more complex based on the architecture of your building.

Exhaust vents can vary in size, type and style. Keep in mind there is NO ONE SIZE FITS ALL VENTILATION. A very common mistake made by improperly trained contractors is to install a ridge vent when the ridge is too short, such as many hip roofs. Most, not all, hip roofs have too short of a ridge line for proper exhaust ventilation. If a ridge vent is used when the ridge is too short, the ventilation is actually crippled preventing proper exhaust.

Another common ventilation problem is kitchen or bathroom exhaust vents dumping hot and moist air into the attic space. These kitchen and bathroom vents should be connected directly to a baffled and dedicated exhaust vent. The baffle in the vent is key in preventing backup of cold air. Typically a flex hose will be used to connect the exhaust fan in the kitchen or bathroom directly to the baffled roof vent.

Even with a properly ventilated attic space ice dams can still form on your roof. This is because of excessive conditioned hot air escaping from inside your building structure and accumulating within the attic. Most buildings with steep slope roof systems are very under insulated if built before the 1980’s. The suggested R value by the US Department of Energy, for the Chicago area, is about an R 49. Most local building codes require a minimum of an R 38 for steep slope roofs. Having said that, many older buildings with steep slope roof assemblies actually have an R value of about an 8. This means an extreme amount of heat is escaping into the attic, since the higher the R value means the greater resistance to thermal transfer.

The 3rd approach in preventing ice dams is perhaps the most common, but does not actually prevent ice dams from forming at all. It is actually a precaution from keeping the ice dam from getting inside once it has formed. This is an ice shield membrane sometimes called Grace Ice & Water shield, or Certainteed Winterguard. There are many ice shield membranes available on the market, what they do is seal down to the wood roof substrate (plywood or nominal lumber) and also seal around any nails. This ice shield should extend from your gutter line, at least 24″ past the exterior wall.

Please note: if you have 3′ of ice shield but a 2′ over hang you do not have enough ice shield. This is a common mistake made by many unqualified contractors. It is also suggested that ice shield be installed in all the problem areas, not just the gutter lines. These problem areas include valleys, where the roof meets the wall and all penetrations on the roof which are equally as prone to ice backup. When you replace the roof is the perfect time to install ice shield, and it’s a very cheap upgrade to do the roof right the first time.

In summary, your steep slope roof is a system which should be properly protected with ice shield, properly ventilated with adequate intake and exhaust ventilation, and properly insulated in the attic to block thermal transfer.

Frost In Your Attic

Did you know that Frost in your attic is caused very much by the same symptoms that cause Ice dams? A very common problem is improper roof ventilation. Mixing and matching roof ventilation can often cause more harm than good! Yes too much roof ventilation can be a bad thing if the styles and types of vents are mixed and matched. Roof ventilation is a science that is unique to the architecture of your building. There are no one size fits all solutions.

Another common mistake which will cause frost to collect in the attic is inadequate insulation. Most buildings with steep slope roofing assemblies built before the 1980’s are severely under insulated. Another problem with insulation is improper installation often blocking the attic intake ventilation, or uneven gaps and uneven layers in the insulation allow thermal transfer into the attic. Hot air holds moisture and when that heat migrates into your attic so does the moisture.

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