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What is Attic Rain?

During periods of extended cold weather, moist humid air from inside the home escapes into the attic.  This process results in the accumulation of frost on the underside of the sheeting. Once the temperature outside warms up and the sun warms the surface of the roof, the frost melts quickly and the moisture rains down into the insulation. This is called attic rain. 

The moisture from attic rain can cause stains in the ceiling, dripping inside around light fixtures, tops of windows, or cause stains on the exterior siding. 

In newer homes, renovated homes or simply upgrading your furnace can make you susceptible to attic rain – and the first sign is often damage to the interior.

Older Homes have a chimney or “B” vent to exhaust hot combustion gases. When air leaves the house through that chimney, it creates a negative interior pressure (slight suction) which will pull outdoor air into the building. The air outside is dry with corresponding very low amounts of vapour. When the dry outside air comes into the building, it has the effect of absorbing the warm moisture from inside the home, which is then exhausted back out through the chimney.  This convective process acts as a natural drying mechanism every time your furnace runs.

Newer Homes do not have a chimney.  What they have instead is called a sealed combustion system which is more commonly known as a high-efficiency furnace. There is a sealed pipe into the furnace for combustion air, a sealed pipe out for exhaust and a fresh air intake into the return air ducting to ventilate the home. When the furnace runs, it brings air into the house creating a slightly higher interior pressure, which forces the air outward. 


When the moist interior air is pressurized it will try to escape from the home towards the dry exterior air. Sometimes the moist air will find its way into the attic, usually resulting from a hole or penetration of the vapour barrier. This can happen from improperly sealed exhaust vents, light fixtures, bathroom fans, attic hatches, etc.

Once the warm moist air reaches the attic, it cools and accumulates on the underside of the roof sheeting and freezes. When the outside temperature warms up the frost on the underside of the sheeting, melts and rains down causing what is known in the industry as attic rain. 

Older homes can experience attic rain after adding a new furnace. The new furnace creates positive pressures, and the original vapour barrier was not sealed to the same extent as new homes.


An inspection of the attic and vapour barrier can determine where these holes are and seal them up. Attic hatches, pot lights and bathroom fans are the main culprits of air leakage into the attic. 

Attic hatches need to be properly sealed and insulated. Light fixtures and bathroom fans require a vapour barrier boot over the top of them and it must be sealed to the vapour barrier.  Any wires or ducting entering the boot need to be properly sealed. Any dryer or bathroom fan ducting in the attic must be sealed to prevent leakage and be exhausted to the exterior of the home. 

During long cold spells you can reduce attic rain by turning down your humidifier to 20-25% humidity.


You can equalize the interior pressures by turning on the “Ventilation Fan” switch on your thermostat. This activates a fan that removes the same amount of air that the furnace is bringing into the home. You can control or reduce the attic rain in many instances by simply running that fan all winter. Many builders and most homeowners do not know this.  Unfortunately, many mechanical contractors are not even aware of this phenomenon.

Another thing to consider is an HRV (Heat Recovery Ventilator). An HRV will bring fresh air into the home and balance out the air pressure. It also improves the quality of the indoor air.

What to do if you think you might have Attic Rain?

Contact a qualified professional such as GreyRock Exteriors at 780-250-3399 to conduct a thorough inspection.  Following the inspection, they will prepare a comprehensive project plan on how to properly fix the problem to ensure your home can avoid attic rain in the future.


For additional education resources, please visit our section regarding home inspection and relevant information that may affect your property.