Hybrid Roofing Systems: An advantageous solution
Roofing systems play a critical role in a building’s overall performance, and they represent a significant investment. When it comes to commercial roof design, there are plenty of elements to consider, and there is no such thing as a one-size-fits-all solution. Yet, the long-term durability and function of a building depends on the type and quality of roofing materials installed.

“Designing the perfect roof can be a challenge,” says Mike Benetti, Senior Segment Manager, Building Insulation-Commercial with Roxul Inc. “When it comes to insulation, in particular, roofing designers are faced with a wide array of options. No one commercially available rigid-board insulation possesses all the optimal properties designers look to address. While choices remain highly customized to meet specific project conditions, stone wool insulation has proven to provide a number of distinct advantages.”

The design alternatives possible with stone wool insulation give designers greater options to consider in meeting specifications and improving a roof system’s overall performance. One such option explored by building science leaders is a hybrid roofing system that combines two different and unique roofing materials to take advantage of the distinct properties of both.

In this case, a hybrid system using a layer of polyisocyanurate and a layer of stone wool was examined. The result is a high-performance roof system at competitive cost compared to the long-established industry default—a two-layer polyisocyanurate insulation system with coverboard. The cost benefit is due to the reduction in layers and labour required. Not only are the hybrid system’s performance benefits considerable, but they also address a number of common issues that can develop in systems using foam plastic insulation alone.

Higher published R-values have made foam roofing insulation, particularly polyisocyanurate, the favoured choice among designers and specifiers for years, although recent building science research has shown a reduction in thermal performance as temperatures drop. This was illustrated in Information Sheet 502, “Understanding the Temperature Dependence of R-values for Polyisocyanurate Roof Insulation,” published in 2013 by engineering and architecture firm Building Sciences Corp. of Somerville, Massachusetts. The variation is due to blowing agents that may condense (changing from a gas to a liquid) in cold conditions, resulting in a decreased R-value for some closed-cell foam roofing insulations. In fact, the long-term thermal resistance of polyisocyanurate can be affected as its insulating gases, such a pentane, diffuse as the material ages.

“Stone wool insulation does not use blowing agents to achieve R-value as foam plastic insulation materials do,” notes Benetti, “but rather, relies on trapped air between its fibres. As a result, stone wool insulation’s long-term thermal stability remains more consistent over time, despite temperature fluctuations. In fact, the thermal performance of stone wool insulation has actually been shown to increase at low temperatures, making it an advantageous material in a hybrid roofing system.”

By incorporating an equal or thicker top layer of stone wool insulation on top of foam plastic insulation, designers can keep the lower layer’s temperature closer to the designed thermal gradient. Ultimately, this leads to greater comfort for the occupants, as well as better overall energy efficiency throughout the life of the building.

A hybrid roofing system is more effective than a system using polyisocyanurate alone at addressing the issue of climate-driven R-values or the understanding that thermal performance is affected by different temperatures, environmental conditions and local climates. After all, how a roofing system performs in true field conditions is an arguably more valuable metric than ASTM testing of roofing materials in isolation.

A hybrid roofing system, incorporating stone wool insulation, also benefits from stone wool’s lower rate of heat transfer within the material itself, as compared to polyisocyanurate insulation—again supporting greater thermal comfort and energy efficiency. This is upheld by 2013 field monitoring research conducted by RDH Engineering of Vancouver, British Columbia, which was presented at RCI’s 2013 Symposium on Building Envelope Technology (“Monitored Field Performance of Conventional Roofing Assemblies—Measuring the Benefits of Insulation Strategy”). It found stone wool insulation had lower heat flux measurements, interior temperatures, and cap sheet surface temperatures. The resulting decrease in peak membrane temperature is also thought to slow the deterioration of the membrane.

“Comfort, durability, and energy efficiency are improved with the addition of stone wool, as part of a hybrid system, versus a system that utilizes primarily foam plastic insulation,” says Benetti. “Its unique properties shore up some common issues that can be present with a traditional foam plastic application.”

For example, foam plastics have a much larger rate of expansion and contraction than stone wool. How much and how quickly insulation expands and contracts due to changes in temperature can also affect its overall thermal performance. Why? Expansion and contraction can lead to permanent material deformations, leaving considerable gaps that result in heat transfer and energy loss. On the other hand, stone wool products remain dimensionally stable and virtually unaffected by expansion and contraction as a result of temperature fluctuations, ultimately resulting in greater long-term thermal resistance. Stone wool is also moisture-resistant, further contributing to exceptional stability which eliminates stresses on the roofing membrane and extends the overall service life of a roof.

“Dimensionally stable products, like stone wool, minimize heat loss through insulation gaps, which can significantly reduce a roof’s R-value,” says Benetti. “For example, a one-inch gap in a traditional foam plastic roof can lower thermal resistance by 10 to 15 per cent. Stone wool has one of the lowest expansion co-efficients among all types of insulating material, while polyisocyanurate foam plastic remains at the high end of the scale. Thus, a hybrid system that capitalizes on the dimensional stability of one material to compensate for the tendencies of another, results in a more effective system when compared to polyisocyanurate alone.”

Sound absorption is another important consideration roofing system designers must take into account. Factors such as metal roofs, HVAC units, and close proximity to airports and schools will necessitate using products with excellent sound reduction properties. In fact, building codes in some local municipalities require builders to meet strict acoustic specifications. Using stone wool over foam plastic insulation can considerably increase a roof assembly’s sound absorbency, due to stone wool’s density and multi-directional fibre orientation.

Fire safety and the building codes enacted to protect occupants remain of paramount importance in building design and construction. Roofing assemblies are a component worthy of consideration in a building’s overall fire protection plan. Fire-resistant materials, like stone wool, that do not add fuel to an existing fire or contribute to toxic smoke, are optimal. Installing stone wool insulation over foam plastic insulation can provide valuable safety benefits.

The 2011 edition of the National Roofing Contractors Association’s NRCA Roofing Manual: Membranes Roof System notes “compatibility with bitumen and other adhesives, component compatibility, impact resistance, fire resistance, moisture resistance, thermal resistance, stable R-value, attachment capability, dimensional stability, and compressive strength” are the gold-standard characteristics of an ideal roofing system. Designers must balance these goals with solutions that are best-suited to specific project conditions.

“Stone wool commercial roofing products check many of the boxes that architects, designers, and specifiers look for when it comes to a quality roofing system,” explains Benetti.

Stone wool insulation is made from basalt rock and steel slag—materials that are naturally fire-, mould-, and mildew-resistant. They are also water-repellent, impact-resistant, and sound-absorbent, offering good dimensional stability and stable R-value in real-world conditions. The beneficial qualities of stone wool insulation contribute to a hybrid roofing system that is resilient, energy-efficient, and environmentally conscious.

Combining high performance with the advantage of a higher R-value per square inch, a hybrid roofing system now provides a competitive option for designers looking to achieve reliable results in an industry of evolving demands and choices.

This article was adapted from the original work of Saverio Marzella, CDT, RRO, RCI, a U.S. national sales manager for roofing at ROXUL® Inc., in Milton, Ontario. For more information, visit: www.roxul.com.

All information listed in this section was submitted by Roxul Inc.
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