by David Stassi
Throughout history, mothers have admonished their children to “wear a hat or you will catch a cold.” While we now know this is not true, mom’s advice provides building professionals with an important reminder why roof insulation is crucial: buildings lose about 25 percent of their heat through the roof when it is cold outside. (For more information, read “The Two-Part Secret to Minimizing Energy Loss on Your Commercial Rooftop” here.) On hot days, the roofing membrane can hold heat for hours, which puts a burden on air-conditioning equipment.
To mitigate against heat loss and gain, the International Energy Conservation Code (IECC) requires insulation throughout the building envelope, including in roofs. In addition to reducing energy consumption, like a hat for a building, roof insulation helps create a more comfortable indoor environment for building occupants.
Specifiers have numerous product options available for insulating the roofs of commercial buildings. Three common types of rigid foam insulation are polyisocyanurate (polyiso), extruded polystyrene (XPS), and expanded polystyrene (EPS). All have been installed in roof assemblies throughout North America for decades.
While it is virtually certain project teams will include roof insulation of some type, the choice of product greatly impacts the material and labor costs. This article discusses performance attributes of rigid foam insulations, cost factors to be aware of, and product make-up options helpful for lowering costs.
Measuring what matters
Specifiers have available to them many metrics when it comes to choosing roof insulation. Chief among these is R-value per inch of thickness. It is self-evident the higher the R-value per inch, the more efficient the insulation is at blocking heat flow. While this figure might be important if the goal is to build the thinnest roof assembly possible, it does not provide meaningful insights on the insulation’s benefits vs. costs.
Specifying insulation based on R-value per inch alone is comparable to buying milk based on its calories per gallon. The figure might be useful in some circumstances, such as for a weightlifter looking to maximize calories per volume consumed, but it is largely irrelevant to what most people care about: price per gallon.
Similarly, to determine which insulation provides the best benefit for a project’s available budget, it is important to consider R-value per dollar—in other words, the insulation’s “bang for the buck.” As polyiso, EPS, and XPS insulations come in numerous product make-ups (e.g. various facer types and panel sizes, and tapered or untapered), it is not possible to provide a single R-value per dollar figure for each of these three insulation types. In general, though, they rank from highest R-value per dollar to lowest R-value per dollar as follows:
Building professionals often choose EPS insulation for its performance benefits, but given the material’s high R-value per dollar, it also helps reduce project costs. Additionally, product options are available for reducing costs even further.
Cost-saving product options
Here are three roof insulation applications, with rigid foam insulation options for reducing material and labor costs.
Creating a positive slope for drainage on a flat or low-sloped roof typically requires stacking multiple layers of insulation. As XPS and polyiso insulations are available in boards no more than a few inches thick, they require crews to haul and install many layers. In contrast, EPS insulation is available in blocks up to 1016 mm (40 in.) thick. Manufacturers will cut these blocks to virtually any slope or shape to fit roof crickets, saddles, valleys, and ridges. Since it removes the need to stack many layers of thin insulation, tapered EPS speeds up installation and can reduce roof insulation costs up to 30 percent compared to other tapered insulations (Figure 1). Additional cost savings result from reduced dumpster fees to dispose of insulation cut-offs.
Given roofs’ exposure to the elements, a typical roof requires one or more re-covers during a building’s time in service. EPS panels pre-folded into bundles are an option with the potential to save labor and materials in roof re-covers requiring a separator board. Such “fanfold” bundles comprise EPS with polymeric facers on both sides. The bundles typically are made up of 25 panels measuring 0.61 x 1.2 m (2 x 4 ft) each, and unfold to cover up to 18.6 m2 (200 sf/two squares). A typical 18.6-m2 bundle weighs less than 5 kg (11 lb), so it is easy for one person to carry.
EPS fanfold bundles require fewer fasteners per square meter than most roofing insulations, and cost less than most other re-cover boards. They require about 60 percent less person-hours to install compared with working with individual insulation sheets. Material costs are also lower than wood fiber, perlite, or gypsum board. On large projects, the total savings can add up to tens of thousands of dollars (Figure 2).
With a savings of $19.40 to $33.40 per 9.3 m2 (100 sf/one square), EPS fanfold saves approximately $20,000 to $34,000 on a 9290-m2 (100,000-sf/1000 square) roof re-cover, compared to other cover boards.
Metal roof re-covers
As metal roofs become increasingly common in North America, building teams must be prepared to re-cover such surfaces when the roof assembly reaches the end of its service life. The standing seams of the metal roofing make it challenging for work crews to create a flat, stable surface for the new roof membrane system. Typically, crews must cut and place many pieces of insulation to fill the channels between the standing seams.
An easier alternative is EPS flute fill insulation, which is factory-cut to fit any metal roof flange profile (square, tapered, or profile-cut). By filling in the channels, contractors do not need to worry about their insulation’s ability to span the raised seams without failing. Flute fill insulation also minimizes the use of insulation above the flutes, thereby reducing the possible need to raise curbs and other rooftop systems.