Do you ever wonder why some residential buildings in Canada develop a large buildup of ice at the edge of their roofs? Although it can look magical during the holiday season, this is both a safety hazard and a building envelope concern. The accumulation of ice at the edges (also known as eaves) of a sloped roofing system is called “ice damming”
Have you ever looked into your attic space during the winter? If not properly designed and ventilated, it can look like the inside of a snow globe. We are referring to a process called “attic rain,” which is the accumulation of water or moisture within an attic space and is commonly shown by frost build-up.
Ice damming and attic rain go hand-in-hand, and there are many factors which contribute to their formation and severity.
The problem and causes
Ice damming and attic rain can be detrimental to residential buildings as their presence can lead to water infiltration into the building structure, building envelope, and living spaces. Excess water at locations not designed to handle it will accelerate the deterioration of the building components. Other potential issues include increased roof loading, obstructed roof vents, the promotion of organic growth, and reduction of building materials’ performance (insulation, air barrier, etc.).
Ice damming is the result of heat loss from living spaces into the attic space. This raises the surface temperature of the roof sheathing and melts the bottom layer of snow on the exterior of the roof. The water will then run down the slope of the roof and freeze when it comes into contact with cold air at the roof eaves, creating an ice dam. Left uncorrected, the ice dam will increase in size up the roof slope, exasperating the issue.
Attic rain is the result of moisture laden air within the attic condensing and freeing on the underside of the roof sheathing. When the exterior temperature rises, the frost will melt resulting in “rain” within the attic. Warm air has the capacity to hold more moisture than cold air and therefore the primary sources of moisture entry into an attic come from improperly sealed bathroom exhaust fans or air leakage in homes with high levels of humidity.
A typical residential attic can be broken down into two sections. The first being the attic floor which consists of drywall, a vapour barrier, insulation, an attic hatch, and soffit venting at the eaves. The second being the attic space which consists of roof ridge venting, bathroom exhaust ductwork and plumbing stacks which penetrate through the attic floor and exhaust through the roof sheathing, possible fire separation walls, and possible chimney framing.
Although intended in modern construction design, no building is 100 per cent efficient. Warm air and moisture will leak into all attic spaces at varying degrees, and therefore the goal is to minimize this leakage as much as possible. The primary factors that affect the formation of ice damming and attic rain are the amount and condition of the attic insulation and roof ventilation (both soffit and ridge). Secondary factors include the construction and detailing of the attic vapour barrier, attic hatch, and exhaust ventilation.
Attic insulation will limit the amount of heat loss from living spaces into the attic, and the vapour barrier will block any moisture within the air from entering the attic. Proper roof ventilation promotes air flow within the attic and expels any moisture or warm air which penetrates through the attic floor system. Attics are typically designed for air to enter through the soffits, move upward, and expel through the ridge vents. It is imperative the soffit vents are not obstructed by insulation as this will restrict the airflow pathway and reduce the ventilation capacity of the system. Baffles (shown in blue in the adjacent picture) are used between truss spaces to block any insulation from covering the soffit vents.
Principles of good design
The following design guidelines will help mitigate the severity and frequency of ice damming and attic rain formation by addressing the primary and secondary factors listed above:
A. Ideally exterior temperature = interior attic temperature
B. Insulation – ABC 9.25.2 – minimum R34 Insulation value (10-12”). Best practice is R50 (15-17”). Install baffles between all trusses to avoid insulation blocks the soffits vents (Picture 6)
C. Vapour barrier - 6 mil poly sheet, continuous over entire attic floor, seal to all mechanical & electrical penetrations (Picture 8)
D. Exhaust fans - Install polyethylene pans around bathroom exhaust fans and seal all edges to the vapour barrier. Seal electrical penetrations through the pan with sealant (Picture 1 and 2)
E. Ductwork - No lengthy ductwork runs (Picture 3), insulated, avoid multiple elbow connections, seal properly to roof sheathing penetrations
F. Attic hatch - Weather stripping around the perimeter, insulation over top (Picture 7)
G. Roofing system – ABC 9.26.5 – Eave protection (Ice and water shield), min. 900mm up roof from edge, min. 300mm inside inner face of exterior wall. Best practice is 1800mm up roof from edge and at wall transitions, around sheathing penetrations, and roof valleys
H. Roof venting – ABC 184.108.40.206 – 1/300 rule for required ventilation, min. 25% at ridge, min. 25% at eaves (soffit). Don’t install ridge vents within 1800mm of each other (Picture 4 – Short Circuiting). Choose vents that cannot be blocked by snow (low-profile vs. high-profile). Install vents properly on sheathing opening (Picture 5)
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