How hot is your roof?

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Designed to Beat the Heat

GraceBy: Brian Impellizeri, product manager, Grace Construction Products

Building owners today, more than ever, are seeking roofs that are durable and energy efficient, while still boasting a lower total cost of ownership.  With this increasing demand for high performance roof designs, it should not come as a surprise that in recent years usage of metal has outpaced all other types of roof covering materials.  In fact, market share for metal roofing materials has jumped from 3 to 8 percent and continues to climb (Metal Roofing Alliance, May 2010). While the benefits of a metal roofing system are many – long service life, excellent durability, better weather resistance, lower energy and life cycle costs – the means used to attain these benefits attribute to hotter roof assemblies that conduct significantly more heat onto the roofing underlayment than traditional assemblies.  To fully realize the benefits of metal roofing that building owners expect, it is essential that the most appropriate, heat-resistant products are specified to withstand thermal stress and prevent premature roof failure.

There are a multitude of factors that impact rooftop temperatures and, although it is difficult to accurately predict what that value might be, knowing these can help gauge a metal assembly’s thermal profile.  Some of the more important factors to consider are the type and color of metal used, insulation, ventilation and project region.  The type and color of metal directly impact temperature, with high-end metals such as copper and zinc conducting more heat when compared to stainless steel.  The color of the roof covering is also a factor as darker colors have a tendency to absorb heat while lighter ones tend to reflect it.  The type and quantity insulation used can impact thermal load.  Insulation systems with a higher R-value such as spray foam, structural insulated panels and rigid roof insulation boards will transfer more heat to the underlayment than a system that uses fiberglass or no insulation at all.  The amount of ventilation within a given roof design can also be a factor as the heat generated by a highly insulated roof can be somewhat dissipated when an air space is incorporated between the roof covering and the underlayment.   The physical location of the project should be considered; projects located in sun-filled regions of the country or those in the higher elevations can see increased levels of UV exposure.  Additionally, other factors that should be considered when specifying an underlayment include tie-ins, chemical compatibility, aroma and project status. 

With these factors comes a hotter roofing assembly that places more thermal stress on the underlayment, the last line of defense against roof failure due to heat.  The results of extreme heat can vary from an aesthetic issue at best, to water leaks at worst.  If an underlayment that is not designed for extreme temperatures is specified, heat can be conducted to the adhesive, causing it to melt and oil to seep out of the failing underlayment and down the side of the building, leaving residue and stains.  However, once the adhesive melts and dries out, the problem goes beyond aesthetics.  The underlayment, now without the essential lubricating oils, can become brittle and crack.  This causes the adhesive gaskets that create a waterproof barrier around nail and fastener punctures to fail. 

Specifying an underlayment with high temperature stability and waterproofing protection is the best way to ensure that building owners achieve the expected return on their metal roofing investment.  There are specialized underlayment products on the market, such as Grace Ice & Water Shield HT, a rubberized asphalt-based adhesive, and Grace Ultra, a butyl-based adhesive backed by a layer of high density cross laminated polyethylene, that are designed to protect against high temperatures without compromising on waterproofing protection.  There are key differences to note when comparing the two categories of high temperature underlayment technologies – ones that use rubberized asphalt as their base adhesive and those which are butyl-based – to each other.

Underlayments comprised of 100 percent butyl-based adhesive are modified with synthetic rubbers to provide superior temperature and durability performance.   They are thermally stable up to 300 degrees Fahrenheit and will not degrade over time, are chemically compatible and nearly odorless.  They are ideal for assemblies with high R-values, that incorporate high-end metals such as copper or zinc or ones that require a tie-in to EPDM or TPO.  Because of its exceptional durability and longevity, 100 percent butyl-based underlayments offer insurance against leaks and roof failure in marquee projects such as hospitals, schools and airports.

High Temperature underlayments comprised of rubberized asphalt-based adhesives are enhanced to provide thermal stability as high as 240 – 260 degrees Fahrenheit.  These underlayments typically have excellent initial adhesion and are appropriate for the majority of standard metal roof designs where high-temperature and waterproofing protection are required.

Beyond the heat resistance both types of underlayments provide, there is the additional benefit of extended exposure time during construction–the underlayment can be left exposed for up to 120 days, compared with the standard 30 day exposure time for traditional underlayments.  With the often unpredictable construction cycles for metal roofs, the extended exposure time provides flexibility during the building process and helps avoid added costs that come with reapplying underlayment due to overexposure.

By understanding the specialized products on the market that guard against the ever-expanding temperatures of today’s metal roofs, architects can continue to design roofs that deliver the full promise of lower life-cycle costs and greater building owner satisfaction. 

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