What Is Attic Rain? A Homeowner’s Guide to Preventing Winter Moisture Damage in Canadian Climates

Attic rain is a roofing phenomenon where interior moisture freezes on attic structures and melts during warm Chinook winds, causing water damage.

Attic rain describes warm moist air that collects in interior attic spaces, freezes on roof structures forming ice and frost, and melts during warm spells. Post warm Chinooks, the frozen water seeks a way out of the attic. The melted ice runs down walls, soaks through OSB plywood in the roof, and collects around lighting fixtures, bathroom vents and piping.

The water often leaves a brown stain from the chemicals picked up in the attic’s building materials. Homeowners and property managers often misdiagnose attic rain as roof leaks, leading to unnecessary shingle replacement costs. According to a 2023 Canada Mortgage and Housing Corporation study, the average household releases 10 to 50 litres (2 to 10 gallons) of moisture into the home every day.

Attic rain causes water damage that homeowners often mistake for roof leaks, costing thousands in unnecessary repairs

What Causes Attic Rain?

Four primary factors contribute to frost accumulation and attic rain, though identifying the primary source requires professional inspection:

• Inadequate roof air intake & air pressures, poor ventilation and soffit venting
• Inadequate or poorly installed insulation
• Moist air from the home’s interior escaping into the attic. Cooking, bathing, laundry, & humidifiers add moisture to the home’s interior
• New home energy efficiency standards leave homes airtight leaving moisture no escape route

What Are Some Signs Of Moisture Damage?

Inspect walls, ceilings, light fixtures, windows and vent stacks monthly for moisture beads, brown staining, or water marks. Condensation on windows indicates indoor humidity levels exceed 50%, a primary contributor to attic rain.

Any black areas may mean toxic mold, and there are special precautions to remove this. A certified mold remediation specialist with IICRC certification should remove and dispose of mold-affected materials.

Take photos of any suspected water damaged areas. Photographs of damaged areas help roofing specialists identify moisture sources and estimate repair costs. Dated photographs also provide documentation for insurance claims related to moisture damage.

Inspect the attic for signs of ice buildup, moisture stains or wet insulation. Again take pictures of frost, ice buildup, vents, and moisture signs. Contact a Calgary roofing company with proven track record certified by the to inspect your attic and roof.

If your home is a new build, you may be qualified to apply for assistance through the Alberta New Home Warranty Program.

What Can I Do To Reduce Moisture In My Home?

Homeowners can reduce attic moisture immediately by taking five specific actions:

The furnace circulation fan balances air pressure by cycling fresh air intake every 20 minutes, preventing moisture accumulation in enclosed spaces. There’s no exact data, but many Canadians turn off their air intake fans because of noise they generate, or they forget to turn it on again in the fall. Don’t forget to turn the furnace fan back on to improve air ventilation.

If you are unsure about the air pressure regulation and ventilation processes in your home, talk to a furnace and roofing specialist.

HVAC specialists may recommend installing a Home Recovery Ventilator (HRV), which exchanges indoor and outdoor air while retaining heat, and a Principle Ventilation Fan (PVF) to maintain balanced air pressure.. Set the HRV controls to continuous, and the PVF switch is usually on or near the furnace. Turn it on.

Make sure all your appliances’ vents are connected. Dryers are a big culprit. Temporary venting can come off the back of the dryer and all that moist air filters into the house. Beyond odour control, run bathroom exhaust fans for 20 minutes after showers to remove up to 2 litres of moisture before it migrates to the attic.

The stove fan should vent to the exterior of the home. Check if it is and use the fan when cooking. Range hood fans remove cooking smoke and up to 400 cubic feet of moisture-laden air per minute during meal preparation.

Turn Off The Humidifier

Check your walls and floors for temperature differences. Make a note of them for the professional roof inspection.Cold spots on walls or floors indicate air leaks into the attic or un-insulated voids that require sealing. Note down insulation levels, grade, and quality for possible improvement.

If you have leakage around electrical fixtures, cut the breaker to the switch if you can. Otherwise, tape the switch or plug off for safety purposes until a contractor can come and fix the leakage. Caulk or seal around fixtures and pipes leading into the attic. The small spaces around these areas can let moisture into the attic.

If water saturates drywall across more than 1 square feet of ceiling area, contact a roofing company immediately.. Keep family members and pets away from the area, in case the ceiling drywall falls.

Additional Measures to Protect Your Roof and Attic:

1. Ice Dam Prevention: Be aware of icicles forming along the edges of your roof, as they may indicate ice dams. According to the National Roofing Contractors Association, ice dams force water under shingles, causing rot in roof decking within 2-3 freeze-thaw cycles.
2. Snow Removal: Use a long-handled rake from the ground to clear snow from your roof. Avoid using a shovel or getting on the roof yourself, as on study found a significantly higher number of falls on days with more than 12 inches of snow compared to days with less.
3. Proper Attic Insulation: Verify attic insulation reaches R-50 value (approximately 16 inches of fiberglass) at the roof ridge, as recommended by Canadian building codes. Replace any wet insulation, as it loses its insulation value.
4. Seal attic bypasses—gaps where warm air escapes into attic spaces—at these eight common locations:
   • Chimney
   • Tops of walls
   • Plumbing & electrical entry
   • Light fixtures
   • Dropped ceilings
   • Knee walls
   • Fans
   • Heating ducts
5. Outdoor Precautions: Knockdown overhanging snowdrifts and shovel snow away from the walls of the house to help prevent ice dams and reduce home moisture levels.

Know When To Call A Professional

Schedule a comprehensive attic inspection with a certified roofing contractor to measure ventilation rates, assess insulation R-values, and identify moisture entry points. Professional roof inspections assess seven integrated systems—ventilation, insulation, moisture barriers, HVAC integration, structural integrity, drainage, and building envelope sealing.

Ice dams can also be a cause of major roofing headaches, like attic rain. Check out our article on ice dams and why you don’t want to let them buildup. If you need help checking your attic for potential damage, give us a call or fill out our convenient automated online contact form.