Further, as ceilings are made increasingly airtight, the chances of avoiding condensation (and resulting negative outcomes) are improved. A calculation of relative humidity (RH) is also a valuable tool for predicting the likelihood of sufficient moisture escaping the occupied space. To maintain minimal airborne infection, RH should not be higher than 50 per cent according to American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) guidelines. In Canada, 30 to 40 per cent RH would be more desirable under winter conditions. (See the Canadian Roofing Contractor Association (CRCA) Technical Bulletin: Volume 53, Ventilation, from February 2003. Visit www.roofingcanada.com/technical.html).
If the air pressure inside the roof ventilation area is lower than the relative air pressure inside the building, the negative pressure can pull humid air through the ceiling. While some strategies call for motorized fans to cool hot attics in summer months, these can further aggravate the pulling of moisture into the roof system during winter conditions. Additionally, should the issue of stagnant air zones arise, and the system cannot be balanced equally with intake and exhaust vents, the recommendation is the intake exceed the exhaust.
Proper venting throughout a steeped-sloped roofing system is essential for the durability of the assembly and for control of temperatures above the air space. There are now systems engineered to ensure consistent intake and exhaust airflow underneath the commercial building’s shingled or standing-seam roof.
Heat buildup in poorly ventilated attics places abnormal demands on A/C systems. These and other problems are avoided with airflow volumes engineered to specification using products engineered for proper ventilation.
The key to optimizing temperature on larger commercial roofs is to use a balanced system—one where air intake matches air exhaust. If there is too much air intake or too little exhaust, the temperature cannot remain constant.
An all-metal ridge vent engineered to individual job ventilation specifications can be invaluable. Easily installed with a snap cover, the product should be sturdy (i.e. not compress under stress) while handling heavy snow loads and resisting wind-driven rain and snow. It should feature slotted fastener holes for correct fastener placement.
An energy-efficient polyisocyanurate (polyiso) foam insulation board can be used over sloped, unventilated roof decks. (For more information, see the Polyisocyanurate Insulation Manufacturers Association (PIMA) Technical Bulletins 106, Polyiso Nailbase Insulation and Asphalt Roof Shingles: Design Considerations, and 114, Ventilated Nailbase for Commercial and Residential Sloped Roofs. Visit www.pima.org/contentpage/Bulletins.aspx). Some products combine insulation, a nailable surface, and ventilating air space in one panel, while others include a radiant barrier. Regardless, the material should promote airflow using vent spacer strips to separate 11-mm (7/16-in.) OSB from the foam insulation, creating airways for flow from the eave to the ridge.
An issue solved
Decatur, Ill., has a similar climate to much of southern Canada, including hot summers and snowfall and low temperatures in the winter. After Macon County Health Department buildings had experienced many years of poor ventilation performance, the effects of ice-damming resulted in condensation regularly soaking its fibreglass batt insulation around the inside perimeter of the attic space, ultimately finding its way to the suspended ceiling.
Consequently, the health department enlisted the help of design and manufacturing professionals to help them identify the cause of the condensation issues.
The firm engaged to assist with the project, Architectural Expressions, opted for an engineered ventilation system specifically designed for steep-slope roofs and consisting of three components: an eave vent, a ventilated roof polyiso insulation board, and a ridge vent.
For the Macon County Health Department retrofit, a synthetic underlayment was selected to protect the nailable ventilating insulation throughout the installation process. Such products should be able to withstand long open times before the exterior roof materials are installed. Specifiers should call for compliance with ASTM D1970, Standard Specification for Self-adhering, Polymer-modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection.
A granular peel-and-stick underlayment was specified to provide extra roofing protection for vulnerable areas. In this case, algae-resistant shingles were used to top off the assembly. In Canada, any such products must be compliant with Canadian Standards Association (CAN/CSA) A123.1-05/A123.5-05 (R2010), Asphalt Shingles Made From Saturated Felt and Surfaced with Mineral Granules/Asphalt Shingles Made From Glass Felt and Surfaced with Mineral Granules.
Ventilation’s green attributes
With the advent of increased layers of insulation above ceilings on steep-sloped roofs, roof surfaces are cooler and more vulnerable to the damage produced by freeze-thaw cycles. A properly ventilated commercial steep-slope roof would have the potential to indirectly contribute to Canada Green Building Council’s (CaGBC’s) Leadership in Energy and Environmental Design (LEED) certification due to the system’s R-value and its means of eliminating a potential moisture source for mould and mildew growth, which can be a threat to indoor air quality (IAQ).
LEED’s Energy and Atmosphere (EA) Prerequisite 1, Minimum Energy Performance, requires a 23 per cent cost improvement in the proposed performance rating for new buildings and is based on exceeding the Model National Energy Code for Buildings (MNECB). The program’s EA Credit 1, Optimized Energy Performance, is also based on this code. This energy benchmark can be exceeded with a properly designed ventilation system if it can replace energy-demanding de-icing wires and panels at the roof edge, commonly installed in snowbelt regions to mitigate ice dams.
The metal components of an engineered ventilation system can also contribute to LEED Materials and Resources (MR) credits for recycled content (i.e. MR Credit 4) and regional materials (i.e. MR Credit 5). Further, this type of steep roofing assembly adds durability and potentially improves a lifecycle analysis (LCA) score of the building assembly because airflow beneath the exterior roof covering reduces heat-related fatigue and helps improve material durability. This means engineered ventilation can assist project teams in future editions of the Canadian LEED program.
In addition to LEED, the RoofPoint: Guideline for Environmentally Innovative Non-residential Roofing program could reward project teams in its Innovation in Design (ID) credit category for a properly engineered ventilation roof system. (Developed by the Center for Environmental Innovation in Roofing, this program is a voluntary, consensus-based green rating system to provide a means for building owners and designers to select non-residential roof systems based on long-term energy and environmental benefits. It functions as a checklist, a guideline, an assessment program, and a means of recognition. Visit www.roofpoint.org).
In addition to roof damage in cold climates, ice dams eventually melt and can cause ground-level problems for building owners. Falling ice can injure people along a building’s perimeter or those walking along sidewalks. Fortunately, a roof that melts snow with engineered ventilation and keeps it off sidewalks can eliminate the need for costly removal and sidewalk maintenance.
Paul Nutcher, CSI, CDT, is the director of sustainability services for Think Agency Inc. He has a decade of building industry experience. Nutcher is a speaker, technical writer, and consultant to manufacturers on specifications, training, sustainability, and marketing. He can be contacted at firstname.lastname@example.org.