It was very cold, -20-30 C (or well below zero F) for a few weeks, but the deep freeze has ended, overnight it warmed up to well above freezing, everything is melting. And there is this huge new water stain on the ceiling… If this is your house, keep reading.
Here are some clues. Your house has a humidifier and you use it, you do a lot of cooking, there are more than three people living in the house, you have a lot of house plants, someone, or maybe more than one someone, likes their long, hot, morning showers, and maybe forgets to use the bathroom fan, or maybe there is no bathroom fan. Maybe you live in a older house with new insulation, or maybe your house has a cathedral ceiling.
The picture above was taken on a very cold day notice the ice (frost) built up inside the attic hatch opening.
The next picture, from another house was taken a few days after a big melt. This was an older house with upgraded insulation stuffed into the eaves, blocking ventilation. A brand new rotary ventilator was spinning merrily on top of two year old shingles. Insulation and rafters were very wet, and there were indications of mold growth in the attic. Water was leaking everywhere.
What is happening?
Air from inside the house leaked into the attic. Warm inside air holds more moisture (humidity) than cold air. When the inside air enters the freezing attic the moisture condenses as thick frost. Because the frost does not evaporate readily, it builds up like the snow in your yard. The more inside air leaking into the attic, and the longer and greater the cold spell, the greater the build up of ice. If it warms up suddenly, the ice melts, there is too much water to evaporate, so it runs down, if it can collect somewhere, or it finds an opening, you have your leak, and a water stain. Water may even run down the back (inside) of sloped roof sheathing and rafters and trickle into the walls.
If the air inside your house had been dryer, the moisture would have been absorbed by the dry air inside the attic before it could condense. If the attic was warmer it would also be drier because the air in your attic came from outdoors and able to hold more moisture, the leaked humid inside air might have dried or been absorbed before there was enough of it to leak. If the ceiling and vapor barrier were intact the inside air would have stayed inside.
If you have this problem, there are a number of things you can do to prevent it. The first thing is to reduce the humidity inside the house. In all the cases above, the humidity indoors was over 30%. Even though 30% is recommended for comfort and even more for fine wood furniture or hardwood floors, it is too high for very cold winters. If you can see condensation on your windows, your humidity is too high.
Use your exhaust fans, you should have a kitchen exhaust to outdoors if you do a lot of cooking, you should have a bathroom fan in every bathroom that is used for showering. If you have no fans, you should open a door or window (this may be tough – be strong :-)) If these methods do not get indoor humidity below 30% you may need a de-humidifier.
Check the attic for adequate ventilation. A well ventilated attic is a healthy attic. If your house has upgraded insulation, it may not have been properly installed. Air needs to be able to flow in from the eaves and out the ventilators on the roof. Mixing static and rotary (turbine) ventilators is a bad idea, use one or the other. A rotary ventilator can ‘short circuit’ attic ventilation by drawing air out of a nearby static ventilator instead of from the eaves. Many pros only recommend static ventilators, rotary ventilators sometimes work too well, drawing air into the attic from inside the house.
Check the ductwork inside the attic, in Canada the only duct work normally found in attics are exhaust ducts from the bath and kitchen fans. All ductwork in unheated attics should be insulated. Straight pipe is better than flex, and uninsulated white plastic dryer vent is not appropriate for bath exhausts. All exhausts must be vented outside through the roof, not inside the attic. Exhaust ducts should be as short as possible. It is not difficult to install a ‘gooseneck’ for an exhaust duct on a normal asphalt shingle roof, there is no need to run duct from one side of the attic to the other.
You also need to prevent air from inside the house leaking inside your attic. The picture above shows a badly sealed light fixture in a cathedral ceiling, it has allowed air to leak into the attic space as well as water back through the ceiling. Air may also be able to leak through the wire opening (knockouts) (circled). The area between the fixture box and ceiling should be caulked so that air cannot leak into the attic. A small amount of foam sealant at box openings will seal it. (Do not fill the box with foam, the electrical wires could overheat and cause a fire.). All openings in the ceiling between house and attic should be checked and sealed if necessary, including the exhaust fan openings.
New homes have a heavy (6 mil) plastic sheet vapor barrier between the unheated attic space and exterior walls. The sheets are taped where they are joined, and molded plastic coverings for electrical fixtures are also taped into the vapor barrier. Older homes built between the 1960’s and 1990’s used lighter plastic (3 mil), lapped joints instead of taped joints, and no covers for the fixtures. Older yet homes may have lapped building paper under the plaster, or nothing at all. Whatever you may or may not have, it is not necessary to remove your ceilings and walls to upgrade your vapor barrier. Vapor barrier paint applied to exterior walls and ceilings will work just as well or better.
Cathedral ceilings can be a problem because they may not have any ventilation and their attic, if they have one, often cannot be accessed. If you have a cathedral ceiling it is particularly important that all ceiling penetrations are well sealed, and if necessary, a vapor barrier paint is applied.
And don’t forget to weatherstrip the attic hatch!