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My roof is plagued with icicles—why?

QuestionWe are building a new two-story house and the tile roof has R-factor 38 insulation. The house is not heated above 65°F, but the roof is plagued with icicles. I am concerned that we are losing expensive heat out the roof. Why is this happening and is there a remedy?

AnswerI appreciate the time your family spent with me on the phone to share the details of your new home and your husband's experience acting as a first-time general contractor.

Sometimes Mother Nature provides perfect weather for icicles and ice dams. If temperatures are near freezing when the first snowfall hits, there will be some meltdown. Then, if temperatures quickly drop while attics still contain relatively warm air, there will be additional melting. When the water reaches the eaves, it will freeze and form ice dams and icicles. This can happen to houses with good insulation and ventilation and even to unheated structures.

Your home may not have an ice dam/icicle problem for years, but if the weather is right, almost everyone will get them.

Having said that, I believe your home's open design, second floor cathedral ceiling and high entryway, and first floor living room that vaults into the arched ceiling will create heating and air movement issues that will affect the exterior side of the roof. These issues can contribute to icicle formation and affect overall mechanics inside the home.

Generally, good attic ventilation keeps the attic cooler in the summer and reduces damaging humidity and helps prevent winter ice dams. With your home's engineered I-joist cathedral construction, it's hard to achieve balanced airflow compared to a structure with an attic. In new construction passive ventilation using soffit vents, ridge vents, and attic baffles is probably the most effective to remove hot air in attics and cathedral ceilings.

It's important to achieve proper airflow from soffit vents through the I-joist cavities up to the ridge vents. I'm not sure if your Hardisoffit predrilled vented panels with 5.0 square inches of net free ventilation per lineal foot deliver adequate airflow. I am unable to determine from the photo the type of ridge vents under the tile ridge caps, or if they are even vented. The International Residential Code requires 1 square foot of attic ventilation per 150 square feet of attic area. Alternatively, if ventilation is provided at both high and low levels, it can be reduced to 1 square foot per 300 square feet of attic area. This is supposed to be net free areas of ventilation.

I am also not sure if the I-joists have been drilled with at least 1-inch holes above the insulation for cross ventilation between cavities. I also have concerns about the size of the I-joists. Since R-38 insulation is 12 inches thick, 16-inch I-joists are needed to achieve at least 1 inch of open space above the insulation and to allow for a 1-inch hole in this area. The top and bottom members of the I-joists total 2 3/4 inches, leaving a 13 1/4-inch web. The design of the I-joist allows 1 1/2-inch holes to be drilled anywhere in the web except in the 6-inch zone outside where the I-joist is supported, but definitely not through the top or bottom member.

One solution to the ventilation problem would be to install a raised section of roof at the ridge, or at the highest section of the roof, for cross ventilation at least near the top of the roof. A vent could also be put at the ends of the raised roof section.

I'm glad air baffles were used at the ends of the I-joists to help air move above the insulation. It would have been better if baffles had been attached the entire length of each I-joist. Ask your architect how he complied with ventilation requirements for the arched roof system since it doesn't appear to have ventilation.

The dark brown roof tiles exposed to the sun will heat up this area, even in winter. Also, the heat ducts in the I-joist cavities will transfer heat into those cavities and to the roof above them.

There is also a problem with some mixing of cold air returns and heat registers in the cathedral ceiling and the floors, which will create heat imbalances. Because the cold air returns draw warm air instead of cold, the furnace will be heating warm air and distributing it at ceiling level, keeping the underside of the roof warm. If the roof system doesn't have enough ventilation for this extra heat, you may always experience icicles and/or ice dams.

I suggest you replace the ceiling fans on the cathedral ceiling with remote-controlled heavy-duty fans to help push heat down toward the floor. Also run the furnace fans 24 hours a day to even out any temperature differences.

Finally, consider a roof-mounted ice-melting system. Heat cables may work if you can attach them to the roof tiles, but they would spoil the roofline's aesthetics. Additionally, an ice-melting system would allow you to install a waterproof membrane (if one has not been installed already). No matter what system you select, because your home is nearly finished, you may have a problem getting the electrical up where you need it.

I wish your question had come in before the walls and ceiling were finished. It would have been a lot easier—and less expensive—to address the issues.

Copyright ©2006 & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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C.R.S., Inc. · Spokane, Washington · USA

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