When air from the house leaks into the attic, it condenses and freezes

That could be caused by

  • High interior humidity
  • High interior air pressure from stack or mechanical pressure
  • Inadequate air sealing techniques 
  • Disconnected Vents - Fans, Dryer, Kitchen Hood, Etc
  • Exterior high humidity entering the attic space through soffiting - IE Front door under large vented soffit area

To stop the build-up you need to prevent the humid air from entering.


Most common sources

It is common to have some condensation or frost in attics and inside walls, but it should be small amounts. This is not an issue,  as these small amounts it will usually sublimate (change directly from frost to vapour) before it melts.

At times there can be significant accumulations of frost. When this melts, it causes stains in the ceiling, dripping inside lights or in windows, and the most common giveaway is stains on exterior cladding.


Attic rain is caused by the rapid melting of  excessive frost in the attic or walls. It is caused by a poor understanding of building science. 
There are two main factors. Holes & Pressures.

Newer homes, renovated homes or if simply upgrading your furnace make you susceptible to attic rain.  WHY?

OLDER HOMES had a chimney or “B” vent to exhaust hot combustion gases. When air leaves the house through that chimney, it creates a negative interior pressure. That creates a slight suction, which will pull outdoor air into the building. The air outside is dry, having very low amounts of vapour, therefore as is comes into the building it absorbs moisture, acting as a natural drying mechanism every time your furnace runs.
(as air warms its relative humidity goes down. Same amount of moisture, but warmer air can hold more of it, hence it is relative to temperature)

NEWER HOMES have no chimney. They use what is called a sealed combustion system. You have a sealed pipe into the furnace for combustion air and a sealed pipe out for exhaust. No Pressure change. You do however require a fresh air intake into the return air ducting to ventilate the home. Now, every time the furnace runs, it brings air into the house creating a slightly higher interior pressure, which forces air outward. That is where the holes come in. Limiting holes equals limited condensation. Big holes, which never mattered in older homes, due to the drying when the furnace ran, now are critical to keep small.

The irony is, that you can reduce this by turning on the switch under the thermostat labeled “Ventilation Fan”. This activates a fan that removes the same amount of air that the furnace is bringing into the home, hence equalizing the pressures. You actually can control the attic rain in many instance by simply running that fan all winter, but many builders and most homeowners don’t know this. Even mechanical contractors are not aware of this phenomenon. (we test over 50 new homes a year for this problem)

The other irony is that if you run that fan 24/7 through the winter your heating bill will outpace the savings of that high efficient furnace. 

A less expensive operating system is an HRV (Heat recovery Ventilator) that operates balanced and saves money. (other than a small upfront cost to install during construction) An HRV brings in equal amounts of air as it discharges, preheating the incoming air with the heat from the exhausted air. It provide higher indoor air quality as well.


First you must find the holes (air leakage locations). To do that you need to induce air movement. This can be accomplished by using a blower door to depressurize the home. Under a negative pressure, cooler air drawn into the home through the holes will leave a signature that can be found using an infrared camera. These holes should be verified by wetting the back of your hand and placing it next to the opening. You can then also feel the cool air. In the summer, this air will often be warmer, since it may be hotter in the attic than the house.

Now you can seal the holes.

If you can keep the total square inches of total openings into the attic to less than 4 square inches, you should not have issues. (as long as there are no abnormally high interiro pressures)  Potlights and bathroom fans are number 1 leakers.

Next you need to equalize the interior pressures. Test the mechanical system as a whole, with the ventilation fan on and then also off, as the furnace runs. Also do a pressure test on each bedroom. If there is insufficient return air to the furnace, a room can have too much supplied air and not enough return, causing a higher pressure. A transfer grill or undercut door can increase the flow. Use a diagnostician that understands these issues and how to test for them.

These stains are actually resins from the OSB sheathing.

Dust can look similar but it will wipe off easily using your hand.

They are sticky and don't wipe off easily
Click to enlarge what the Alberta Building Code says about Condensation
See what CMHC states about Attic Ventilation
Attic rain is when you have an accumulation of frost from air entering the attic from the house.

Usually takes extended periods of cold weather

Once the air warms or the sun shines on bare shingles, the frost melts quickly, raining down into the insulation and sometimes leaving staining and dripping from lights.

Adding ventilation WILL NOT PREVENT it.

Newer homes with higher insulation levels have limited  radiant heat loss. Warm air is required to absorb moisture from the attic. Cold attic means air can't absorb much moisture. Meaning ventilation wont stop it.    ** ventilation will help dry the attic after 

Older homes can experience attic rain after adding a new furnace or additional insulation.  New furnace creates positive pressures and added insulation eliminate radiant heat loss needed to absorb moisture.

There are two types of attic rain.

  • Usually from a larger hole or penetration in the attic space.
  • Unsealed exhaust vents (bath fans and dryers) let air escape into the attic.
  • If you check your attic during cold weather, you will see frost accumulating around the air leakage area.

  • Usually occurs throughout the attic
  • High interior relative humidity exacerbates the issue
  • Also often affects the walls, although you may not see the evidence in all cases
  • OSB resin staining often related to excessive air leakage into walls and attic spaces during cold weather, forming frost and when melting, absorbs the resin from the OSB sheathing

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Attic rain is NOT the result of lack of roof vents.

Over ventilating the attic can cause the wind to "suck" air from your house, into your attic, making the problem worse.

If attic rain  was caused by lack of venting - why would your car windows frost up in the winter sitting in your driveway - Lack of ventilation?
LOCALIZED - Note the frost only in the two spaces above the exhaust vent - No frost in left space

LOCALIZED - Note the frost only in the two spaces above the exhaust vent - No frost in left space

LOCALIZED - Close up of above photo shows the leakage at exhaust connection to roof sheathing