Roof Insulation Design with a System Focus

Energy efficiency in a building starts with energy-efficient equipment (HVAC, lights) and an energy-efficient envelope (roof and walls). Because my career is dedicated to roof systems, this blog will be, too.

What makes up most low-slope, commercial roofs? A roof consists of a weatherproofing layer (the membrane), thermal-resistance layer (insulation and coverboard) and the structure (the roof deck and its supports). Sometimes there is a coating or surfacing; sometimes there is a vapor barrier or an air barrier; and sometimes there is a fire barrier. There also are roof hatches, drains, vent stacks and perhaps a few skylights. Many components make up a roof system; it’s not just membrane, insulation and a deck.

The insulation layer, first and foremost, is used to regulate the temperature of the interior by slowing the transfer of energy (heat) from exterior to interior and vice versa. The higher the R-value of the roof insulation, the more effective it is at reducing heat transfer. The 2012 building codes are requiring R-values from 20 to 35 for commercial buildings. As an architect, I ask myself how are roof designers designing roofs to meet these R-value requirements? Another way to ask this is: How do I ensure the building is getting the required R-value?

Thermal-resistance accuracy relies on a number of items; I’ll discuss three:

• Multiple layers of insulation
• No thermal bridges
• A properly calculated R-value

The roofing industry, for years, has realized the importance of multiple layers of insulation and minimizing the number of thermal bridges. Multiple layers of roof insulation make up for imperfections in manufacturing and installation. Sometimes there are gaps between boards where they don’t butt together perfectly. That’s reality. Installing more than one layer of insulation means there is no direct path for air to move from deck to membrane.

Also, most roof decks are steel or wood; therefore, roof insulation is often held in place with metal fasteners. If the fasteners penetrate the entire thickness of insulation, the roof ends up with a large number of thermal bridges that provide an easy path for heat transfer. Adhesive attachment can eliminate the fasteners. Gaps at board joints and thermal bridges reduce the insulation layer’s effectiveness. We need to account for these items as good designers.

What about properly calculating the system R-value? This concept is not discussed much by the roofing industry. I will offer some assistance. The critical characteristics of roof systems—fire resistance, wind resistance, hail resistance—are determined based on the roof “as a system.” Fire, wind and hail resistance are not determined based on individual components. For example, the membrane alone does not determine external fire resistance. A roof’s critical characteristics are determined based on the combination of the roof’s components, as a package deal. The system concept is critical to determining the overall thermal performance of a roof. In addition to the membrane and deck, the components of a roof that affect thermal resistance include skylights, drains, roof hatches, vent stacks, HVAC units and various other rooftop items. Mostly, these additional items (along with the gaps and thermal bridges) reduce the roof system R-value. Again, we need to account for this as good designers.

Determining the R-value of a roof system simply by adding up the individual insulation board R-values means we are overestimating the end result. We want our buildings to work as designed. Our HVAC designs are, in part, based on expected performance of the insulation layer. That means the roof’s insulation layer needs to perform as expected; we must design the R-value of the roof based on the entire make up of the roof.

A designer also must account for the extent of R-value reductions. R-value reductions are based on the anticipated imperfections, thermal bridges and rooftop components (drains, vent stacks) that lower the system R-value. (And, if there’s tapered insulation, a designer should use an average R-value; there’s more than one way to do that.)

If the “board R-value” (BRV) is 6 per inch, it’s probably more realistic to use 5 to determine the system R-value. If the BRV is 5 per inch, a more realistic system value is 4. This, of course, will vary based on the makeup of the entire roof design and the quantity of rooftop components. Appropriately reducing BRVs will account for the realities of installation and the insulation-reducing rooftop components. Building owners should get what they are promised and what they pay for. A properly calculated R-value is a good place to start.