Green Roofs - Living Architecture Monitor - Summer 2012 - (Page 43)

ON SPEC gREEN, BLACk AND REFLECTIVE ROOFs GREEN ROOfS VS. REfLECTIVE ROOfS by: ZACH wILLIAMS F or years, the roofing industry has debated a black versus white argument with regard to roofing membranes. Some say black membranes increase performance in certain climates, while others say white membranes can single-handedly lower energy consumption. I’m here to add colour to the debate. White, reflective roofs with high Solar Reflective Index (SRI) values actually cause higher energy bills in cooler environments such as Ontario. The reason for this is there are more Heating Degree Days (HDD) in cities like Toronto, Ontario than there are Cooling Degree Days (CDD). Degree days are a calculation designed to express the heating or cooling demand for a building on given weather days. In the case of HDD’s, a mean indoor temperature of 65° Fahrenheit is generally used. If the high temperature for the day was 40°F, and the low temperature was 20°F, the average of the day’s temperature swing is taken and subtracted from the mean indoor temperature. In this case, the average of 40°F and 20°F is 30°F, so subtracting 30°F from 65°F gives a value of 35 HDD’s for that day. Toronto annually averages 4,500 HDD’s and only 250 CDD’s. Black, non-reflective roofs heat up dramatically when exposed to sunlight. The reason for this is not only because they are a dark color. Part of the formulation of Ethylene Propylene Diene Monomer (EPDM) rubber is Carbon Black. Carbon Black converts ultraviolet radiation directly into heat as a defense mechanism against harmful rays, thereby giving EPDM its high Ultra Violet (UV) resistance. In Toronto, this heating effect is considered good as keeping the buildings warmer throughout the year would reduce heating costs throughout winter and early spring months. By reducing year round heating loads, energy consumption is lowered by approximately 10 per cent when examining a one or two story building. At the end of the day, dark rubber roofs prove to be more energy efficient in Toronto’s climate than reflective roofs. Green roofs have none of these issues. During hot summer months, the surface temperature of a vegetated roof stays within 5°F to 10°F of ambient temperature and the membrane tends to be 10°F under ambient temperature. The hidden problem of reflecting heat energy to another nearby building does not occur with green roofs. So why haven’t we all committed to being good neighbors? Detractors seem nervous about waterproofing issues beneath green roofs since it is a uniformly wetter environment than more traditionally exposed membrane systems. From the green roofs I’ve seen installed, heavier membranes and seam redundancies are used to prevent water issues. Historically, in hot climates, some roofs were constructed with their own water supply controlled by a float valve. This allows a few inches of water on the roof to keep the building cool through evaporation. Today’s advanced membrane technology renders concerns about waterproofing beneath green roofs, even during extreme conditions, a non-issue. aBOvE: reflective and green roofs side by side on Chicago’s City Hall/County Building Photo Courtesy: Steven Peck While more expensive than a straight black or white roof, green roofs have many additional advantages such as: • Reduced costs on heating and cooling from the plants’ transpiration, • More tranquil environment • Relieve stress off sewer systems by absorbing rainwater, and • Extend the roofs’ life. I strongly believe that green roofs offer a lot of advantages over other formal types of roofing of any color. They are far more aesthetically pleasing and allow us to utilize the best aspects of nature to increase the quality of life for everyone, while uniformly saving energy in all climates and managing storm water runoff and urban heat island effect. Zach Williams is a green roof consultant based out of Carlisle, Pennsylvania who formerly managed the Carlisle SynTec green roof program for five years LIVING ARCHITECTURE MONITOR / SUMMER 2012 / 43

Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Summer 2012

Green Roofs - Living Architecture Monitor - Summer 2012
Contents
Letter from the Chairman
From the Founder
New Standard on the Use of Lightweight Aggregate
Urban Agriculture Declaration Caps Amazing Summit Debut
Economics of Biophilic Design
Brad Rowe, David Spalding, Cornelia Oberlander, Paul Kephart
Green Roofing Industry Sees Triple Digit Growth
Green Roof Local Market Symposiums
Industry Professionals Weigh in on Growing Media Quality Control!
CitiesAlive in Chicago Oct 17th – 19th
Green Infrastructure Ontario Coalition Changes our Perspectives on Infrastructure
Improving Tax Abatement
10th Anniversary Commemorative Pull Out Section
Halifax Seaport Farmer’s Market
Leadership Knows No Bounds- Virginia Russell
Policy Profile- A Home for Biodiversity on Green Roofs
New Industry Products
Plant Profile: Chives Thrive on Green Roofs
Half a year, Full-Fledged Growth
In God We Trust – All Others Bring Data!
New Corporate Members
Professional Calendar
GRHC Buyers Guide
Green Supremacy by Zach Williams

Green Roofs - Living Architecture Monitor - Summer 2012