Ice Dams - 9/3/2004 - Home Exterior Roofs Siding

Ice Dams

In snowy climates perhaps you have seen thick bands of ice forming along the eves of homes. They may appear harmless, and even remind you of an old Currier & Ives painting, but these formations are ice dams and cause damage to millions of homes every winter. This Q & A will help you better understand how to prevent ice dams in your homes.

Q: Last winter one of my homes had ice dam problems; the attic ventilation in this house is no different than the other homes we build. I heard that ventilation is the best way to prevent ice dams so I am mystified over what is wrong here. Do you have any ideas? What can I do to prevent ice dams in my homes?

A: While it is true that ventilation often plays an important role for the prevention of ice dams, it is not the silver bullet. Let's take this from the top, and start with ventilation even though you may not have a problem here. In troubleshooting ice dam problems, it is useful to understand the causes and look at three areas: ventilation, insulation, and infiltration.

Ice Dam Causes

Ice dams need three things to form: snow, heat to melt the snow, and cold to refreeze the melted snow into solid ice. Ice dams form because heat from the living area of the home works its way into the attic warming the roof sheathing to a point above freezing causing snow to melt. As the melted snow runs down the roof, it refreezes before reaching the edge of the roof. Over time, the refrozen water forms a dam. In some cases, the water from the melted snow works its way back into the attic causing damage to the home.

Ventilation

The idea behind ventilation is to keep the roof cool. Attic ventilation has been part of building codes for decades. Generally, building codes require a minimum ratio of 1:150 net free ventilation of opening area to the square footage of the attic. The exact amount of ventilation will vary depending upon code jurisdiction, climate, and roof design. There are climates, even within the Snow Belt, where ventilation is undesirable because of prolonged humid weather conditions. If a home has ice dam problems, check for ventilation and confirm the ventilation covers as much of the attic sheathing area as possible and does not block soffit vents.

Ventilation schemes include roof, gable, ridge, turbine, and power vents. A preferred design is to use baffled ridge vents with soffit vents because this allows for enough airflow to ventilate a majority of the roof sheathing. Gable vents, roof vents and even power vents often lack the ability to ventilate large sections of the roof sheathing. Baffles for ridge vents are important because they prevent water and snow from entering along the ridge vent opening.

Insulation

If a home has adequate ventilation, check the condition and amount of insulation. Insulation is critical for keeping the warm air inside the home and allowing the attic to remain cold. Follow the model energy code for guidance to minimum insulation levels for your home. The Department of Energy's Insulation Fact Sheet has guidelines and recommendations for the proper amount of insulation throughout the United States, so it is easy to determine the correct amount - even if the local jurisdiction does not have a model energy code.

There are several ways to insulate an attic including blanket batts, blown-in fiberglass or cellulose, rigid extruded sheathing, and foamed-in place polyurethane foam. Ductwork installed in attics should be insulated using any variety of different methods, such as wrapping in foil faced fiberglass insulation, using fiberglass duct (sometimes called "ductboard"), burying the duct below the attic insulation, or using wire helix flexible duct that includes the insulation.

Another problem with HVAC in attics comes from heat generated by the air handler or gas furnace. The manufacturers do not install enough insulation inside to prevent the heat generated from rising. This makes shallow or medium-pitch roofs especially vulnerable for ice dam conditions when the heating unit is located in the attic.

The best practice is to keep all ductwork in the heated or conditioned space of a home and assure that there is enough attic insulation with no gaps in the insulation. In the case of blown-in or foamed-in place insulation, look for light or low spots. Make sure the full R-value of the insulation also covers the top plates of exterior walls.

Infiltration

Air infiltration into the attic is perhaps the least understood element that contributes to ice dams. There are several places where conditioned air from the home enters the attic space increasing the probability for ice dam conditions. Air infiltration trouble spots include; leaky HVAC ductwork, recessed light fixtures, plumbing vents, electric wires, bathroom fans, dryer vents, openings around chimneys and attic access doors/hatches.

Sometimes it is difficult to find the infiltration path. Consider having a blower door test conducted if the house has plenty of ventilation and insulation and yet still has ice dam problems. The blower door test usually either pressurizes or depressurizes a home. In this case, have the house depressurized and get up into the attic with a smoke stick to find infiltration sources and seal them with caulk, foam, weather-stripping, or sheet metal, as appropriate.

Prevention

It is easier to prevent ice dams at the design stage rather than spend hours troubleshooting an existing problem. As mentioned earlier, it is important to have insulation over the top plates of exterior walls. When using trusses, order the type with an elevated seat (energy heel truss or raised heel truss). This provides the extra room for insulation. Use insulation baffles at the eaves to assure an air channel for ventilation and to prevent compression of the insulation. Compressed insulation loses its R-value --remember this when building any platforms in the attic. Builders should discourage homeowners from using the attic as a storage place.

Consider installing bitumen membrane on the roof sheathing. While this material may not completely prevent ice dams, it provides added insurance. This material helps prevent moisture from entering the attic, and it is recommended if your roof design involves valleys or is hard to ventilate by natural means.

Links to Additional Information:

• Insurance4USA.Com
  http://www.insurance4usa.com/ice_dam.cfm
• National AG Safety Database - University of Wisconsin-Madison Extension
  http://www.cdc.gov/nasd/docs/d001401-d001500/d001479/d001479.html
• CertainTeed - Protecting Your Home From Ice Dams pdf
• Grace Construction Products
  http://www.graceathome.com/pages/roof_faq.htm
• Oak Ridge National Laboratory
  http://www.ornl.gov/roofs+walls/articles/icedams/
• Department of Energy's Insulation Fact Sheet
  http://www.ornl.gov/roofs+walls/insulation/ins_01.html