Attic Rain - Causes and Solutions

What is attic rain?

As expert roofing contractors in Calgary and Edmonton, we care to educate home owners on what may cause water damage to their roofs and within your homes. Attic rain is a buildup of humidity in your attic space. It's difficult to spot as it is not caused by faulty shingles, and occurs naturally. When the temperature drops at night, the humidity in the air freezes and builds up as a layer of frost on your pipes and the underside of your roof deck. Once the temperature rises in the morning, this frost can thaw out and cause leaks and damage to your ceiling. This can be seen through water staining around light fixtures and vents, or mold growth in the attic space and inside your walls. The buildup of humidity can originate from either inside the house or from the outside air.

What causes it?

You may be asking yourself now, what might be some more specific cases where attic rain is happening? Various vents from inside your house, such as the bathroom fan or kitchen fans can leak air into the attic space. Leaks from the bathroom and kitchen fans leave an abnormal amount of humidity in the attic when compared to outside air. 

With that being said, if you find yellow spots or staining in your attic it’s not necessarily due to an air leak. If the vent duct leading up through the attic is not insulated properly, condensation can build up because the air inside the duct is warmer than the air in the attic.

Another likely cause of attic rain, especially in Calgary, comes from chinooks in the winter. When snow on a roof melts very rapidly, humidity increases in the attic space. This humidity freezes and thaws overnight, which will cause a sheet of frost on the underside of your roof deck.

What can we do about it?

The first fix is to make sure your attic is properly ventilated. The intake ventilation (soffits or intake vents) and the exhaust ventilation (standard roof vent) should be an equal ratio. If the intake ventilation is lower than the exhaust, air could be pulled from inside your home instead of from outside.

It’s also possible that the too much insulation in your attic is preventing the intake ventilation from performing properly. If the insulation is blocking the soffits or intake vents, and no baffle system is set up, air cannot travel easily into the attic. A baffle system uses panels between trusses to keep an open space between the roof deck and insulation for air to travel through.

If there are no issues with the ventilation ratio, and the insulation does not seem to be blocking the intake ventilation, it’s time to look at the exterior. It’s possible that snow is covering the exhaust vents on the roof, thereby blocking the path for air to exit the attic. This is more common on roofs with a pitch of 4/12 and lower as snow can build up and cover the entire roof. A possible solution is to invest in higher profile vents such as a Ventilation Maximum, or a Duraflow Turbo. Both of these options will sit above the top ridge of your roof, allowing them not to be impeded by snow and for air to continue to exit from the attic.

Some homes, especially ones built within the last decade, are very tall and built close together. These types of homes will usually have intake ventilation with vents near the eaves of the roof, instead of soffits. This is because fire spreads easier between soffits of homes in close proximity when compared to intake vents. Unfortunately intake vents can be blocked by snow whereas soffits do not have this problem. If your roof is 8/12 or steeper, it is unlikely that enough snow can build up to actually block intake vents. If you have a roof pitch closer to 4/12 or less, then snow build up could be blocking your intake vents as they are on the bottom third of the roof (where snow tends to build up).

The next thing to check is whether your vents have insulated ductwork. It is standard to have insulated ductwork in modern homes, but older houses might not have it. Insulated ductwork gives a barrier between the cold winter attic air and the warmer air coming through the duct from the house, preventing condensation from forming.

Finally, you should verify that your vapour barrier has not been deteriorated, and that you have proper insulation in your attic. Older and thinner vapour barriers have a higher chance of being deteriorated and therefore not protecting from moisture. Older styles of insulation such as wood chips or asbestos do not absorb moisture as well as modern blow in insulation. These last two fixes should be done after all other possibilities have been looked into.

At Roof-Ex, our expert residential roofers provide roof maintenance services and inspections for damage caused by attic rain. If you're a homeowner in Calgary, Edmonton, or surrounding areas and you believe that your home is susceptible to attic rain, contact our team to determine the cause and possible solutions.