Attic Condensation Presented by: Joan Maisonneuve Technical Services
Scope of the Presentation How does moisture get into the attic? Blowing snow Moisture from the outdoor air Moisture from side-vents Moisture transfer From the living space How to prevent it Air Sealing Humidity Control Ventilation Homeowner Education
Sources of Attic Moisture Doing the Detective Work
Moisture Source - Outdoor Air How Much Moisture Can Air Hold? 1 000 ft 3 air + 21 C 500 ml 1000 ft 3 air 0 C 150 ml 1000 ft 3 air -30 C 5mL
Blowing Snow Attic vent location Wind How much is too much?
Outdoor Source: Moisture From Vents Furnace exhaust Hot water heater exhaust Gas fireplaces Vacuum exhaust
Air Leakage from the Living Space
Solutions: Part A Attic Venting
Reasons to Vent a Roof: 1. Enhance life span of the roofing material Remove heat from attic Cool underside of the shingles 2. Control ice damming Prevent heating of the underside of the roof sheathing 3. Promote drying 4. Control moisture accumulation Moving air deposits less moisture
Minimizing Blowing Snow Entry Locations of vents Not too close to the peak Opposite prevailing winds Vent Type Good internal baffles or screen Goose necks/whirly birds Eave Vents?
Roof Venting Assumptions The building envelope is tight; the amount of moisture in the attic is minimal Inlet and outlet areas are distributed evenly Side to side Top to bottom A flow exists Complexity of the roof Wind conditions Snow cover Covered vents The outside air is able to absorb moisture Temperature dependant Looking for moisture balance
What Drives Attic Ventilation? The Stack Affect Attic air heated by contact with a hot roof tends to rise. This is natural convection, which pulls cooler air from the soffit up towards the ridge. The steeper the roof pitch the better the stack affect works in particularly cold weather Wind Direction, Speed, Duration While higher wind speeds tend to increase attic ventilation, the relationship is a weak one: Ventilation rates at a given wind speed can vary by a factor of 10. Attic ventilation is only one part of the solution
Internal Framing Issues Compound roofs Roofs built in sections Roof sheathings blocks air movement in the attic between 2 sections of roof
Potential Vent Options Front and back soffit venting Gable end vents Traditional pancake vents, moving ventilators, installed near the eave Off peak vents
Front and Back Soffit Venting Can enough air be introduced with vented soffits to meet the Code? The 1 in 300 requirement The Code minimum 25% at the bottom of the space What are the issues with this solution? balanced intake and exhaust due to insulation height
Front and Back Soffit Venting Slots for Ventilation Dropped Gabel Truss
Gable End Vents
Air Intake Low on the Roof Off-peak air intake vents
Roof Meets a Wall Hip Vents
Powered Ventilation air intake should match air exhaust Check for low temp shut off Maintenance? When it quits working? Are they rated to work in cold temperatures?
Tests for Vents Miami Dade County Wind driven rain test 8.8-inches of water at 110 miles per hour ICC Wind driven snow tests Dust exposure test CAN3-A93-M Largely out of date
Ventilation Must be Balanced Every inch of air exhausted, must be balanced by providing the same or greater amount of air intake at the eaves or soffits.
Attic Ventilation and Cold Temperatures Accumulation/Drying It takes cold air a long time to pick up moisture when it is in solid form (frost or ice) Wind Icicle Attic ventilation is not usually enough to remove moisture at cold temperatures!
Solutions Part B: Air sealing
Indoor Sources Of Moisture New home Room humidifiers Cooking bathing, showers Plants Fish tanks Water features Family type
Humidifiers & Occupants 4 Occupants 1.3 gallons a day (5L) New Construction 1.3 gallons/18 months Flow-through Humidifiers 18 gallons (68 L) of moisture into the air each day
Frozen Moisture Math Exposed length of time + High Relative humidity of air leaking + Exposure to a dew point (cold surface) = Accumulation and build up
Moisture Transfer More moisture moves by air leakage than by diffusion
Seal Air Leakage into the Electrical plugs Envelope Air sealing of poly at window perimeter Vents to outside Joint in window/jamb extension Interior walls wiring chases
Why is There More Air Leakage Pressure at the ceiling (stack effect) Tighter Envelope buildings are easier to pressurize or depressurize at the Ceiling? Diagram from the CHBA Builders Manual
Unbalanced Ventilation Mechanical Systems
Aspirated Furnace Mechanical Systems
Sealed Combustion Mechanical Systems
Seal Air Leakage Points at the Attic hatch Bathroom fans Housings, ducting and connections Plumbing stacks Pot lights Chimneys Chases Ceiling
Proper Poly Laps
Find Hidden Leakage Points: Areas of Missing Vapour Barrier Small Framed Wall 2 Parallel walls
Check Bathroom Fans Moisture escapes at: connections between ducting and vent termination around the housing to ceiling joint
The Duct/Vent Connector Interface
Other Areas of Air Leakage Damage to the poly near a penetration Disconnected piping
Cantilever Over an Entry
Does Frost Always Result in a Leak to the Inside? Many times a melt does not result in a leak because: The water evaporates quickly; The water is absorbed by the attic framing, or insulation; Pools of water cannot find an opening to the interior; The path of water leads to the outside of the building.
Solutions: Part C Reduced Humidity Effective Interior Ventilation
Homeowner Responsibility As the temperature drops reduce humidity Turn down/off humidifier Use spot ventilation Use central ventilation more
Tighter Homes Need Better Average air change rates 3 Ach/hr Central Vent fans How are they interlocked? How much do they run? Passive and Active HRV s Are they effective? Are they balanced? Ventilation
Objections to Lower Humidity Wood Floor damage Short term reduction will not severely affect the floor Comfort/Health short term comfort or long term damage? Cost to run ventilation Use more effective fans Use timers, programmable for humidity or peak demand Short term cost versus long term damage
A Question of Balance Humidity conditions in the home Degree of air leakage from the home Degree of the warming Duration & severity of the cold weather
In Conclusion At Construction Design/discuss details for sealing known leakage sites with your trades Seal all penetrations no matter how small - especially at the ceiling Thoroughly inspect air barriers & penetrations before insulation Blower door test Balance attic air flow Improve interior ventilation operation After Possession Educate Homeowners on their role Turn down or off humidifier as temperature drops Run ventilation more as the temperature drops REMEMBER: There is conflict between indoor humidity for health and hardwood versus the potential for leaks with a severe stretch of cold weather
Resources Building sealing details and theory: Attic Ventilation and Moisture, 1993, CMHC Research Highlight Moisture Technician Course CHBA Builders Manual EEBA Builders Guide for a Cold Climate CMHC Building Solutions Book ASHRAE Handbook of Fundamentals Journal of Light Construction Building Science Corporation
Questions????