Marc G. Allaire Enercorp Inc. Scarborough, ON Abstract Special requirements are required for cold climate design. Considerations such as structural expansion joints, added weight caused by snow loads, vapor retarders, air barriers, insulation requirements and drainage are discussed. Also addressed are system selection, installation guidelines and special specification criteria. Marc Allaire Marc Allaire is an Associate with Enercorp Inc., a sales and marketing company representing Johns Manville, Carlisle, and Owens Coming in Canada. He graduated as a Civil Engineering Technologist and has been involved in the building science industry since 1980. Allaire furthered his education in building and roofing sciences at the University of Wisconsin with the Roof Consultants Institute and with the Roofing Industry Education Institute (RIEI). He has related experience in the roofing and build¬ ing science field as an engineering representative, technical sales representative, systems development manager, product manager, and consultant. Allaire specializes in roofing/building evaluation and assess¬ ment, repair program development, specification writing, construction administration, and project man¬ agement. Allaire—5
As the temperature in the northern climates of North America begins to drop in October towards the very frigid months of winter, design and installation of roofing systems take on new parameters and challenges. Snow, freezing rain, wind chill factors and below sub-freezing temperatures are all factors which can have a detrimental effect on the long-term performance of roofing systems. Cold climate design must incorporate the control of heat flow, moisture flow (vapor and liquid), air flow, snow accumulation, and ice formation. It must also consider the proper assembly and construction of the design. Some manufacturers address cold weather installations of their roof systems while others do not mention it. Knowledge of product installation limitations is vital to designers considering the application of roofing systems dur¬ ing cold temperatures and snowy conditions. Structural Considerations Two items come to mind when considering cold climates and structures. One is the additional weight to be con¬ sidered due to live loads. The other is the movement created by the expansion and contraction of materials due to temperature swings experienced during cold weather periods. Positioning and inclusion of expansion joints for a given structure will be dependent upon temperature variations and the type of materials used.. Since it is not uncommon to see summer highs of 100°F (38°C) and winter lows of – 20°F (-29°C), it can be expected that materials will move significantly. Roof composition and the location of the dif¬ ferent components within the roof system will have an effect on the extent of movement expected to occur at any given point in the roof structure. Conventional insulated roof assemblies will tend to move less because the deck is located below the insulation layer. The spacing of expansion joints may differ from those required on parking or plaza decks which have no insulation and are subject to extreme temperatures from both the bottom and top side. There are no hard and fast rules in most building codes for the location, size, and quantity of structural expansion joints. Structural designers draw on their experience and education to incorporate them in the design to accommodate anticipated movement. Other design factors which form an integral part of the structure’s design may provide a logi¬ cal location for the inclusion of an expansion joint. This is quite frequently the case when a low roof area meets a high roof area. The break between these two areas provides an excellent opportunity to include an expansion joint. Additional weight is the other factor to take into consideration when designing roof structures in a cold climate. Snow will accumulate on roofs and add a significant amount of load to the structure. Freshly-fallen snow has a differ¬ ent density than snow which has crusted over for a period of a month. Snow which turns into ice is even heavier. Drifting against high parapets and high roof areas must also be accounted for, since their strata will be different and heavier than the flat area of the roof. Code requirements for snow loading differ within North America. The National Building Code of Canada (1990), for example, uses a formula which incorporates various coefficients to determine the total snow load. It takes into account the basic roof snow load factor of 0.8, rain loads, wind exposure, slope and accumulation. The Uniform Building Code (1991) provides a table and requires the designer to refer to the building official for the determination of the snow loads. As a designer, these added load calculations which are incorporated in a roof structure’s design may have some effect on selecting the most economical support system and deck type for a project. Vapor Retarders/Air Barriers The use or omission of vapor retarders in a roof assembly is a sensitive issue. Based on their experience, Canadian designers tend to specify vapor retarders more often than not. As a matter of fact, their motto is “when in Allaire—7 doubt, specify one.” The reason for this fact is that because our exterior winter temperatures are very low (average – 4°F or -20°C for Ontario), there exists a natural moisture drive from the higher vapor pressure interior to the lower vapor pressure exterior. Dew point calculations show that condensation always has the potential to occur at the mem¬ brane level when it is located towards the exterior or cold side. In order to prevent condensation from taking place, a vapor retarder is used on the warm side of the insulation. In the case of retrofit roofing where the existing membrane remains in place and where no vapor retarder exists in the original design, the design calculations become even more important. This problem is not as critical in climates where the exterior temperature hovers around the condensation temperature. The potential for condensation may exist one day, followed the next day by a drying cycle if the temper¬ ature warms up a few degrees. Wayne Tobiasson’s article on “Vapor Retarders For Membrane Roofing Systems” explains in detail the logic on the use of vapor retarders in the U.S. The recognition of moisture diffusion and its control has been well documented and practiced. The National Research Council of Canada (NRCC) and the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), however, have both determined that uncontrolled air leakage will cause more harm than uncontrolled moisture diffusion. Holes, gaps and openings in the roof assembly can allow large volumes of moisture¬ laden air to enter the assembly and potentially condense on the first surface they encounter that is below the interior air saturation temperature or dew point. In order to put into perspective the relationship between vapor diffusion and air leakage, let’s examine the case depicted by Rick Quirouette in N.R.C.C.’s Building Practice Note No. 54. Assume a wall section with a vapor barrier having a water vapor permeance of 5 ng/Pa.s.m2 (0.087 grains/h.ft2.in Hg), is exposed to room conditions of 21°C (70°F), 30% RH and an exterior temperature of -20°C (-4°F). If these conditions were to prevail for a month, approx¬ imately 6 grams (0.2 oz) of moisture would accumulate in the wall cavity. This would likely create a thin layer of frost on the surface of the sheeting within the wall. Taking the same example, assume that an opening exists in the interior wall, say at an electrical outlet, which penetrates into the cavity and is equivalent in size to a 625mm2 (1 sq. inch) hole. Under a pressure difference of 10 Pa (0.2 Ibs/ft2), equivalent to a 15 km/hr (9.3 mph) wind, 2,600 m3 (91,818 ft3) of air would enter and exit the cavity over the same month period. This translates into approximately 3,000 kg (6,614 lbs) of air and 14 kg (30.9 lbs) of water. Assuming that only 10% of the air condenses out in the cavity, then air leakage has deposited 1.4 kg or 1,400 g (49.4 oz) of moisture into that cavity. This means that air leakage has deposited 233 times the amount of moisture that passed by diffusion only. It is therefore critical to create air seals within a roof assembly. Although vapor retarders can tolerate some imperfections without drastically affecting their overall performance, air barriers, on the other hand, must be continu¬ ous, have low air permeability, must be able to structurally support the pressure difference across them, and must be as permanent as the structure and roof system into which they are being installed. Air barriers can be both the vapor retarder and air seal or they may incorporate a combination of materials in order to withstand air pressure. Thermal Efficiency Trying to manage and control heating and cooling loads within a building always is a great challenge for a designer. In cold climate design, we tend to want to control heat loss more than heat gain since this condition is prevalent for more months throughout the year. In order to put that into perspective, the average of degree-days below 18°C (64.4°F) in Ontario would be in excess of 4,000. Insulation is one of the materials used within a roof assembly to control heat loss. The quantity of insulation used on any structure is dependent on many factors, including code requirements, building use, and the location and type of building. Energy conservation quite often plays a factor in the final design. A warehouse will not have the same thermal efficiency requirements as a temperature-controlled museum. Once a designer has made the decision on the amount of R-value that is required, several materials are available to suit the design. These include low R-value materials such as fiberboard and perlite; medium R-value materials such as fiberglass, mineral wool, and expanded polystyrene; and a high R-value insulation such as extruded poly- Hawn-Abremski—8 styrene and polyisocyanurate. Most of these materials can also be tapered to facilitate construction and improve drainage. Phenolic foams, which were also a high R-value material, are no longer marketed because of the problems associated with corrosion of steel decks. Regardless of the type or thickness of insulation required, the standards of adequate support, staggered joints, and tight fitting boards need to be adhered to, whether the design is for cold or hot climates. Since condensation can potentially occur at the underside of the roofing membrane in a cold climate environment, the preference tends to be towards a two-layer system of insulation. This allows the joints in the top layer of insulation to be staggered from the bottom layer and minimizes the potential of thermal bridging of fasteners installed in the base layer. Thermal bridg¬ ing of fasteners is an interesting phenomenon; however, there is not a significant amount of test data available to designers. These data would help to substantiate designs, which incorporate mechanical attachment of insulation and/or membranes, in facilities having high relative humidity. Drainage Drainage must meet the requirements of the prevailing building code. Interior drainage is often preferred in cold climates because water in drain pipes and leaders will not freeze when it is tempered by the interior heat of a build¬ ing. In areas of high interior relative humidity, condensation can form on piping carrying very cold water run-off from the roof. A simple solution is to insulate the piping. Another benefit of interior drainage is that the natural heat loss at the interior drains helps them to remain clear and free of ice and snow. Cost is often an issue raised by the building owner. This can lead to poor design considerations on the part of the designer. Perimeter drainage in a cold environment can lead to some serious problems. Gutters and downspouts are often specified to collect the water at the perimeter edge. When used, they must be heated with heating cables in order to prevent the formation of ice. The gutters must also be well anchored to the structure to resist the tremendous amount of stress caused by the additional loading resulting from snow and ice accumulation. At times, designers may choose to allow water to run off to simply flow over the edge of the roof. Perhaps the facility is heavy manufacturing which is located in a desolate area where control of water run-off is not a concern. The results of the choice can be devastating. Ice columns created by the water run-off during warming spells can form to become as large as 2 to 3 feet in diameter across the face of the building. This creates a potential hazard to the occupants as well as the roofing system itself and the equipment located on the roof. System Selection Most roofing systems marketed in the northern portion of North America can withstand the rigors of the changing climate to varying degrees. The practical considerations of the installation of a roofing system in cold weather can pose significant problems which will ultimately reduce the long-term performance of a given system. The manufac¬ turer’s literature seldom addresses the potential installation problems experienced when installation takes place at temperatures of -15 degrees F and possibly -25 degrees F with the wind chill factor. It is important for designers to consider all aspects of a system as well as the manufacturer’s printed limitations for the system prior to specifying it for the project. The usual design criteria, including cost, code requirements, performance, weight, appearance, main¬ tenance, and so on all apply. The decision process for the selection of the roofing system, however, must take into account the added criteria of cold weather installation. The question must be answered on whether the selected sys¬ tem can be adequately installed under the prescribed climatic conditions. This will surely have an effect on the final selection. The method of attachment, for example, may preclude some systems from being considered. If a fully bonded system is being considered, then fully adhered single plies utilizing weather-sensitive adhesives (which can become very viscous or freeze at low temperature) cannot be used. Perhaps one may consider torch-applied systems, since they tend to tolerate cold temperatures better. If snow loads are very heavy in a particular area, a design incorporates a smooth surface roof with no ballast may benefit the owner since snow removal throughout the winter may be required. Allaire—9 All aspects of the design must be considered prior to making the final decision on a system. This will ensure that the proposed system can be properly installed during the expected weather conditions. Specification Considerations Presuming that the roof assembly has been selected and the specification is taking form along with the details for the project, are there any special requirements which should be included in the document to deal with winter condi¬ tions? Yes. The following suggestions do not deal with the physics of the roof assembly, but rather with the practical considerations which help make the installation a success. Where snow accumulates during the installation of the roof system, it will need to be removed prior to installa¬ tion. The consultant should specify who is responsible for the removal of snow. If it is to be the roofing contractor, then have a section dedicated to the costs associated with snow removal. It is also important to specify the degree of cleanliness desired as it relates to snow removal. Steel decks are natural traps for ice and snow because of their shape. Designers must insist that snow and ice be removed from the flutes of the deck in order to prevent trapping moisture within the new assembly. Wood decks can become saturated from prolonged exposure to the elements. It must be made clear that the decks be dry prior to the start of roofing. Concrete decks can surface freeze during cur¬ ing, thereby causing a frail and dusty surface. These surfaces must be properly prepared by sandblasting or shotblast¬ ing prior to roofing. Concrete also has the ability to hide a thin frost film in the early morning, which will change to water as the day warms. This will result in poor bonding of the roofing materials. Storage of materials is always a contentious issue with the contractor. Be specific on where and how all materials should be stored for the project. Most insulation materials will be affected by moisture; therefore, they must be pro¬ tected from the elements. Specify proper tarps that will protect against snow and rain, yet breathe to prevent sweat¬ ing. Polyethylene and factory-applied wrappers do not provide adequate protection. There seems to be a belief that snow is not water. If snow-covered surfaces are roofed in, problems will occur in the future. Snow is simply another form of water. Asphalt kegs and rolls of felt must also be stored to prevent moisture intake. In the case of asphalt, ice and snow can create a significant safety risk when introduced into a hot asphalt kettle. Applying wet rolls of felt or modified bitumen will only build in moisture into the system. This can cause blistering and reduce the long-term performance of the system. Modified bitumen rolls are considerably thicker than felt and will become stiff and boardy. Their installation becomes more difficult and the potential of causing wrinkles and fishmouths is enhanced. Adhesives, sealants and certain flashing sheets used on single ply membranes should be kept in a heated trailer on site or in a hot box on the roof until ready for use. This procedure keeps these materials at the right viscosity and temperature, ensuring a better installation. Certain single ply manufacturers recommend the use of heat guns to assist in the installation of flashing materials, but not to speed up the cure of adhesives. Adhesives must be allowed to have solvents flash off at their own pace in order to perform properly, thereby increasing the time required to execute a proper seal. Most manufacturers recommend that surfaces which are to receive the roof system need to be free of dirt, dust, oil, grease, snow, ice, and any other contaminants. These will have an adverse effect on the adhesion of the components in the system. Night cut-offs are always risky when they are not properly installed. In the case of a snowy area, a night cut-off could become a monthly cut-off and be subjected to a tremendous amount of snow. It is imperative that cut-offs be stringently specified and properly executed in the field. Many squares of otherwise good roofing could have to be replaced due to a failed night cut-off. Discussion about design considerations and the treatment of materials and systems for low temperature installa¬ tion have been expressed in this article. It is the writer’s opinion that the ability of the roofing mechanic to install the systems at low temperatures is as critical as the considerations given to material selection. If the roofer cannot be rea¬ sonably comfortable in the installation procedure, then all the precautions in the world will not help in obtaining a successful installation. One manufacturer has made recommendations regarding the installation of roofing systems during cold weather. The suggestions pertain to the installation of materials; however, they can also be applied to the workers. They are as follows: Allaire—10 • “Minimum working temperatures: It is recommended that when temperatures remain below -10 degrees C (14 degrees F) for mopping applications and below -18 degrees C (0 degrees F) for thermofusible applications, operations should be suspended. Minimum working temperatures should take into account a factor for wind chill.” • “Wind Chill temperature (° C) = air temperature (° C) – [windspeed (km/hr)/2].” • “Daily forecast should be followed to determine commencement of work or to anticipate possible suspension. For example, if forecasts indicate temperatures will be dropping quickly below the minimum, no work should commence. Conversely, with indication of rising temperatures during the day, slightly lower temperatures on starting are acceptable.” These recommendations help the installation of a roofing system during cold weather because they provide defi¬ nite guidelines to work within. SUMMARY Since designing roofing systems in cold weather is different than the norm, the designer must be aware of all the forces which are acting on the building envelope. Recognition and understanding of the effects that snow, ice, cold winds, high interior humidity, and cold temperatures can have on a roof system will help to avert the problems com¬ monly associated with cold climate roof designs. The practical considerations of the installation of the system must also be considered along with the design. The assembly as a whole must be able to resist all the forces to which it is subjected, but also be able to be built efficient¬ ly, properly, and in real time. REFERENCES Baker, M., Roofs:Design, Application and Maintenance, Multiscience Publications Limited, Montreal, Quebec, 1980. Hutcheon, N.B., “Requirements for Exterior Walls”, Canadian Building Digest #48, National Research Council Canada, 1963. Laaly, H.O., “The Science and Technology of Traditional and Modem Roofing Systems,” Volume Two, Laaly Scientific Publishing, Los Angeles, California, 1992. Quirouette, R.L., “Building Practice Note #54, The Difference Between a Vapor Barrier and an Air Barrier,” National Research Council Canada, 1985. Tobiasson, W., “Vapor Retarders for Membrane Roofing Systems,” U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, N.H., publication unknown. Allaire—11
