Insulation type and membrane color affect the performance of conventional roof assemblies including effective thermal resistance and membrane durability.

The Challenge

Conventional low-slope roof assemblies, all known as exposed membrane roof assemblies, are roofs which have the roof membrane on the exterior of the roof insulation and structure where it is exposed to exterior conditions including temperature, solar heat gains, night sky cooling, abrasion, and dirt. The type of insulation used in these assemblies and the color of the roof membrane can both have a significant impact on the performance of the roof. The choice of these materials can impact both the energy transfer through the roof assembly, and subsequently the building energy consumption, as well as durability of the roof membrane.

Recently, the in-service performance of different insulation types has been a prevalent topic in industry, and in particular the temperature dependent insulating properties of some insulation products. While insulation products are rated based on their thermal resistance according to a laboratory testing procedure, this test procedure does not necessarily reflect in-service conditions. Also, the impacts of insulation thermal mass, roof temperatures, and potential latent heat transfer on the actual energy flows through roof assemblies are not considered.

Different roof membrane colors are also used in conventional roof assemblies.  Reflective roof membranes are typically selected to provide increased reflection of solar radiation thereby decreasing roof membrane temperature and urban heat island effect, and often this selection is part of meeting green building program requirements.  However, much of the work with regards to reflective roofs does not consider the effects of soiling as roof membranes age, nor does it consider the actual heat transfers through roofs and the implications for building energy consumption.

Despite the relative large impact that these factors, there is little work available regarding the field performance.

The Project

An industry sponsored research study was conducted by RDH with Soprema Inc., ROCKWOOL (formerly Roxul Inc.), and SMT Research to investigate the durability and thermal performance of conventional roof assemblies with respect to membrane color and insulation arrangement. To perform this investigation and combination of laboratory, field, and modelling work was conducted.

The field portion of the project consisted of a large-scale field monitoring program implemented in the Lower Mainland of British Columbia. A new conventional roof was installed on the facility consisting of 3 different roof membrane colors (white, grey, and black) and 3 different insulation arrangements for a total of 9 unique roof assemblies.  The 3 insulation arrangements were polyisocyanurate (polyiso) insulation, stone wool insulation, and a hybrid combination of the two consisting of a layer of stone wool installed above the polyiso insulation.  Sensors were installed to measure temperature, relative humidity, moisture content, insulation movement, and heat flux at key locations within each of the roof assemblies.

To complement the field investigation work, the effective R-values of the insulation products were measured in the laboratory using the ASTM C518 protocol to measure actual R-values, including temperature and aging effects.

Finally, RDH performed whole-building energy modeling to account for the varying R-values of the insulation (as determined through the laboratory testing) to demonstrate the implications for building energy consumption. This modelling also allowed for the results of the study to be extrapolated to other climate zones across North America.

The Result

The laboratory measurements of the R-values of polyiso and stone wool insulation found that the R-value changes depending on temperature for both insulation types, though in different ways and amounts. When the results of these measurements were then used the calculate the apparent R-values of the roof assemblies it was determined that often then polyiso roof assembly provides reduced in-service performance as compared to the hybrid and stone wool assemblies.

The field monitoring found that both these differences in in-service thermal performance of the insulation arrangements and the different roof membrane colors lead to different heat flows through the roof assemblies. These factors also impacted the temperature of the roof membrane, and consequently building energy consumption and membrane durability.  Lighter color membranes and the more dimensionally stable stone wool insulation will likely provide improved membrane durability due to the decreased rate of roof temperature change and reduced peak temperatures.

The whole building energy modelling of an ASHRAE archetypical commercial building found that more stable apparent R-values provide by stone wool and hybrid insulation strategies provides lower building energy consumption than does the polyiso strategy, but that the optimal membrane color for energy efficiency will depend on climate.  Typically lighter color membranes are most energy efficient in warmer cooling dominated climates, and darker colored membrane are more efficient in cold heating dominated climates. A summary of the results of this building energy modelling are provided in the figure below.

Overall, results of this study will help practitioners to select the optimal membrane and insulation combination for the design of more durable and energy efficient roof assemblies.

Other Resources

Follow these links for more detailed information on the conventional roofing study:

Photography courtesy of SMT Research.

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