Buildings

Icicles in the Attic

Q: I have icicles forming on the rafters in my attic, and I can’t figure out why. I know I don’t have a roof leak, since I just had the house re-roofed after having several leaks that soaked the insulation and stained the ceiling. Several contractors said I obviously need more attic ventilation, although I did have them install ridge and soffit vents with the new roof. They suggested installing a powered attic ventilator. My neighbor Sherman, who seems to know what he’s talking about most of the time, says powered attic ventilators are bad ideas. What should I do?

A: As with any moisture problem, the key is to figure out where the moisture is coming from. To have icicles forming on your rafters, either water must be coming in from the outside in the form of rain or snow, or water vapor is getting into the attic and then condensing on the cold rafters. Assuming your roofers did a good job, it's unlikely that rain or snow is the culprit.

If you were in Buffalo or Missoula or Anchorage, you might be having ice dams form at the eaves, causing melting snow to back up into the attic & then re-freeze. But since you're in North Carolina, and we haven't had enough cold weather or snow this year to create that problem, I'll have to vote for water vapor as the source of your problem.

So where's water vapor likely to be coming from in your house? The obvious sources are the kitchen and bathrooms. Do you have vent fans in the bathrooms? Do you run them when you take a shower? How about the kids? Are the vent fans ducted all the way to the outside? I've been in lots of houses where the ducts from the vent fans are simply run into the attic, or have come apart and are now dumping the moisture from the bathrooms or the kitchen into the attic. The homeowner never knows until the ceiling falls in.

How about a range hood in the kitchen. Do you have one that is ducted to the outside, or is it one of those that simply filter some of the grease and then dump the air and moisture right back into the kitchen? If it is ducted to the outside, is the duct intact?

In many houses, even if the vent fans are properly ducted, they are rarely used. What happens to the moisture? Typically, it will go to the first solid, cold surface and condense. Single pane windows, uninsulated walls, and metal door frames are favorite spots.

To some degree, if you don't have cold surfaces handy in your house, the moisture will remain in the vapor state and increase the relative humidity of the air, which, within limits, tends to make the house more comfortable in the winter, less comfortable in the summer.

If, however, air is leaking from the house into the attic, the moisture in that air will condense on the first solid, cold surface it comes to, which will usually be the rafters or the bottom of the roof decking. How does air leak from the house into the attic? Aside from being pumped in by improperly ducted vent fans, air leaks into attics through leaks in heating ducts and equipment, from around pull-down stairs and attic hatches, around and through recessed light fixtures, through holes made for wiring and plumbing and ductwork, and through the cracks between the sheetrock and the top plates of interior walls.

One way to find out where the leaks are is to go up into the attic and look for extra-dirty places in the insulation. This is often a good indication that air has been flowing through there, being filtered by the insulation. Lift up the insulation, and you'll probably find a crack or hole that needs to be sealed.

The best way to find the leaks is to hire someone to use a blower door on the house. This is simply a large fan, which can pressurize the whole house and enable the technician to find the major leaks and fix them.

Needless to say, running a powered attic vent fan will simply increase the amount of warm, moist air flowing into the attic from the house. It will certainly increase your heating bill, and is unlikely to help with the moisture problem.