Cool Roofing

July 2015

by Bryce Given

Choosing a roofing type specifically for a flat roof is not an easy task for the uninformed. Over the past several years, there has been a push for lighter colored (cool) roofing.  Cool roofing is a hot topic when it comes to designing or replacing your home’s or commercial building’s roofing.  It is often considered because:

•       designers and owners like the many color options beyond dark colored asphalt roofs
•       many quality cool roofing products are offered
•       ease of installation
•       easily repaired
•       long term material warranties
•       relief of heat island effect above roofs

So, what is not so cool about cool roofing?  Through testing, field observations and published articles, roofing consultants and manufacturers are aware of a higher probability of condensation as a result of greater air temperature differences between joist cavities and the surface of the roofing.    Under these conditions there is an increased and prolonged ability for moisture to form under the roofing and in joist cavities.

Articles on roofing instruct that solar radiation is partially deflected from roofing surfaces, with the remaining radiation being absorbed into the roof and structure. With darker roofing, more heat is retained and greater drying in the joist cavity can occur.  Under cool roofing, less heat accumulates from absorption in the joist cavity.  This lower heat retention in the cavity space may lead to less drying and over time could cause greater buildup of moist air, which in turn condenses when it contacts a cooler surface such as the cool roofing or cooler roof sheathing.  Sufficient ventilation is critical to roofing and roof design, along with selection of the appropriate materials for the project.

The negative effects of cool roofing are most often seen on older buildings with insufficient roof ventilation, but can also be an issue in new construction if its unique properties are not properly taken into account in the design.  The roof membrane and underlayment are often specified along with building ventilation requirements, durability and aesthetic considerations. As a critical component of an integrated building system, roofing must meet building code and manufacturer specifications. 

Attic Ventilation

July 2012

by Bryce Given

When it comes to ventilating your home's attic, the right amount of air flow and air volume can make a significant difference to the comfort and durability of your home.  There are four primary reasons to ventilate, two of which relate to the comfort of your home and all of which can affect its durability.  The four reasons for ventilating are:

1. Reduction of summer time heat in attics: Proper ventilation will improve flow of the sun-heated air and direct it out and away from the attic space, reducing heat that radiates into living areas.  For flat roof designs, it is critical to cross ventilate through perimeter vents and unobstructed air movement channels within the roof's joist cavity.  For steep sloped roofs, ventilation is typically achieved through eave and ridge vents with at least 50 percent of the ventilation capacity located high on the roof's ridge.  Your local Building Department and current Building Code will provide specifics regarding Code compliant installations.
2. Reduction of winter time moist air and condensation in attics:  Attic moisture as vapor can condense to water droplets when coming into contact with a cooler surface, and then can drip onto attic insulation, gypsum board ceilings and wood framing causing them to stain or deteriorate.  Proper attic ventilation can help to eliminate moisture and discharge stagnant attic air, reducing the potential for condensation.  
3. Ice Dams:  An ice dam is a buildup of ice at the edge of a roof, which prevents melting snow water from freely draining off the roof.  Roof temperature imbalances cause the dam to form as melting snow flows down the roof but freezes again at the point where the roof is below 32°F.  The ice builds up over time as more water collects at the roof edge and freezes again.  Then the water above that has not yet reached freezing backs up behind the ice dam.  Cracks in the exterior roof covering allow this water to flow under the roofing and protective felt layers and into the attic, potentially causing damage to roof sheathing, framing, ceilings, insulation, and other areas of the structure.  Natural roof ventilation along with increased ceiling/attic insulation can help maintain uniform roof temperatures in cold weather to minimize or prevent formation of ice dams.
4. Prolonged service life of building materials:   Wood sheathing, framing and roofing materials are moisture and heat sensitive.  Materials can break down more readily through heating, cooling and moisture cycles.  Proper attic ventilation can reduce the highs and lows of these cycles to create a more consistent environment, thus prolonging material durability. 

Currently, the 2009 International Residential Code (IRC) is the generally accepted minimum standard for roof ventilation in most local jurisdictions.  Incorrect air ventilation design, blocked vents and non-sealed ductwork are often observed as contributors to water problems in attics.  Before undertaking ventilation and roofing projects, contact your local Building Department or qualified Design Professional to verify the requirements for the work you intend to accomplish.