Preventing ice dams on steep-sloped roofs

September 1, 2012

Photo © BigStockPhoto/Kenneth Sponsler[1]
Photo © BigStockPhoto/Kenneth Sponsler

By Paul Nutcher, CSI, CDT
Without a properly engineered ventilation system on the roof, ice-damming can threaten a building’s health. An ice dam is a ridge of ice forming at the eaves of a roof, cricket, or valley that prevents melting snow and water from draining off. The water backing up behind this obstruction can refreeze, creating an ice dam. These dams can be the cause of roof failures; once significant amounts of moisture enter the building, there is a high probability mould and other negative impacts can occur.

Ice dams can push apart shingles or standing-seam metal roofs and compromise the building envelope. When shingles are moved during freeze-thaw cycles, moisture penetrates the building envelope through cracks and openings, causing damage to walls, ceilings, insulation, and even interior areas. Condensation on metal roof assemblies can result in rust and thermal movement, which combine for potential structural problems due to the ice’s weight on a weakened roof. With moisture behind the weatherproofing layers of the building envelope, mould can grow. This creates the potential for sick building syndrome (SBS) and threatens an otherwise healthy indoor environment.

Ice dams and icicles are also extremely heavy objects that can cause severe bodily harm or even fatalities when they slide or fall off a roof onto pedestrians. (For more on the associated dangers, see the article, “Beware of Falling Ice and Snow: A Winter Perspective on Building Design,” by Mike Carter, CET, and Roman Stangl, CET, in the January 2012 issue of Construction Canada. Visit www.constructioncanada.net[2] and select “Archives.”) There are remedies for this situation, but some come with their own complications. For example, zigzag de-icing wires may negatively affect a building’s environmental impact due to melting water accumulating in pedestrian areas and refreezing, along with presenting the need for undesirable maintenance practices. These wires can also be extremely energy-inefficient and increase the building’s power consumption. A better way to combat the danger of ice dams involves strategies for engineered ventilation.

Determining proper ventilation
In comparison to residential homes, commercial buildings have:

These characteristics all influence the provision of roof ventilation. There has been a distinct lack of definitive data within the industry explaining the interaction of these variables, making product recommendations difficult.

Typically, the specifier considers the stack effect caused by wind and temperature differences with the understanding that, generally speaking, for every square inch of exhaust air there should be equal or greater intake space to ensure a balanced system. However, for cathedral ceilings (i.e. no attic spaces), a ventilated nailbase product is frequently used to create an air space. Essentially, the ventilated nailbase allows air to pass through a channel between the roof’s outer surface and the insulation of a cathedral-style ceiling with the intent of cooling the outer surface. In this case, the general rules for attic ventilation do not apply, making ventilation recommendations more complicated. Manufacturers have sponsored third-party studies to address this issue.

This engineered system provides ventilation for steep-sloped roofs, helping ensure consistent intake and exhaust airflow underneath the roof covering of commercial building applications. Proper venting throughout a steeped-slope roofing system is essential for durability and for controlling temperatures above the air space. Images courtesy Atlas Roofing[3]
This engineered system provides ventilation for steep-sloped roofs, helping ensure consistent intake and exhaust airflow underneath the roof covering of commercial building applications. Proper venting throughout a steeped-slope roofing system is essential for durability and for controlling temperatures above the air space.
Images courtesy Atlas Roofing

Wind engineering and air quality consultants, Cermak, Peterka, Peterson (CPP) of Fort Collins, Colorado, produced a study for the roofing industry dealing with the subject of ventilated nailbase insulation airflow. (Visit www.cppwind.com[4]). The research allowed development of a net free area (NFA) calculator for roofing ventilation product manufacturers. Designed for steep-sloped roofs with a minimum 1:6 pitch, this tool advises designers as to how to specify the added benefit of the ventilation system.

Airflow through ventilating nailbase products is often rendered ineffective because the eave and ridge vents are not matched in their ventilation capacity. Inadequate volumes of properly directed ventilation cause problems to a roof, especially in extreme climates.

Over time, the most common factors prematurely weakening a steep-slope roof are:

The calculator needs the roof measurements and thermal information to provide climate appropriate results. To enter the roof design into the tool, one must have:

For the calculator, the thermal information is viewed from the top down, and includes the colour of the roof surface and the R-value of the ceiling/wall insulation, along with the outside temperature.

The air gap—the most influential variable to achieve the correct NFA—is within the designer’s control. Depending on the climate, both summer and winter ventilation should be calculated to maintain the proper air gap temperature. To avoid premature shingle degradation and baking, an air-gap temperature of no more than
66 C (150 F) should be maintained during the summer. To avoid ice-damming, the temperature must be kept below 0 C (32 F) to help prevent freeze-thaw cycles.

According to the report:

In the winter, this ventilation can prevent snow from melting on the surface of the roof. In the summer, it can prevent the roof surface from overheating from solar radiation. The amount of air flowing through the gap will determine how effectively these goals are accomplished.

There are known relationships between air flow rates and viscosity, heat transfer and conductance, and radiation and emittance. Conduction occurs when heat is transferred through solid material, or between two materials in direct contact. Convection is the transfer of heat energy within a fluid (gas or liquid) by movement of currents. Radiant heating consists of radiant energy being emitted from a heat source and is absorbed by surrounding objects. Understanding the balance helps predict the airflow rate and air temperature rise in the ventilated nailbase insulation air space.

The standards for commercial roofing apply to ventilated assemblies, including American National Standards Institute/Single-ply Roofing Industry (ANSI/SPRI) ES-1, Wind Design Standard for Edge Systems Used with Low-slope Roof Systems—the most rigorous testing standard for wind uplift protection on perimeter edge metal. When using an engineered ventilation system in coastal areas, additional considerations include water-infiltration testing and ensuring compliance with Testing Application Standard (TAS) No. 100(A), Test Procedure for Wind and Wind-driven Rain Resistance of Discontinuous Roof Systems.

The roofing contractor attached the panels with specified 178-mm (7-in.) nailbase fasteners. At left, the polyisocyanurate (polyiso) insulation boards were covered with a synthetic underlayment.[5]
The roofing contractor attached the panels with specified 178-mm (7-in.) nailbase fasteners. At left, the polyisocyanurate (polyiso) insulation boards were covered with a synthetic underlayment.

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[6]).

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.

Ventilation solutions
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[7]). 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.

Ventilation considerations are crucial for sloped roofs, whether standing-seam metal or shingled. This eave vent facilitates air intake below the rake edge of the roof to execute an engineered ventilation system for the Macon County Health Department. At right, the roofing contractor loaded these panels on the roof for installation; the products were installed at the specified 51 mm (2 in.) of air space.[8]
Ventilation considerations are crucial for sloped roofs, whether standing-seam metal or shingled. This eave vent facilitates air intake below the rake edge of the roof to execute an engineered ventilation system for the Macon County Health Department. At right, the roofing contractor loaded these panels on the roof for installation; the products were installed at the specified 51 mm (2 in.) of air space.

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[9]).

Conclusion
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 paul@thinkagency.com[10].

Endnotes:
  1. [Image]: http://www.constructioncanada.net/wp-content/uploads/2015/11/bigstock-Icicles-1342218.jpg
  2. www.constructioncanada.net: http://www.constructioncanada.net
  3. [Image]: http://www.constructioncanada.net/wp-content/uploads/2015/11/Techni-Flo-Model-V6.jpg
  4. www.cppwind.com: http://www.cppwind.com
  5. [Image]: http://www.constructioncanada.net/wp-content/uploads/2015/11/1n2photo.jpg
  6. www.roofingcanada.com/technical.html: http://www.roofingcanada.com/technical.html
  7. www.pima.org/contentpage/Bulletins.aspx: http://www.pima.org/contentpage/Bulletins.aspx
  8. [Image]: http://www.constructioncanada.net/wp-content/uploads/2015/11/3N4photo.jpg
  9. www.roofpoint.org: http://www.roofpoint.org
  10. paul@thinkagency.com: http://paul@thinkagency.com

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