Fastener thermal bridging in low-slope roofs: A loss of energy, money
Mechanically attaching low-slope membranes and insulation can lead to significant costs. AIA partner GAF outlines ways to reduce or even eliminate the use of metal fasteners in low-slope roof assemblies.
When you go outside on a cold winter day, do you leave your jacket open? Of course not; you zip it shut so you are enclosed by insulation to stay warm. The same is true for buildings.
Steel studs, fasteners, and, even worse, actual penetrations that bridge across or through a building’s insulation layer allow for heat flow inward during warm periods and outward during cold periods. If insulation isn’t continuous, heat can flow into and out of buildings, reducing energy efficiency and potentially wasting money. This article summarizes a large modeling study published elsewhere suggesting ways to significantly reduce thermal bridging.
What is continuous insulation?
Building designers are now familiar with the use of continuous insulation. ASHRAE 90.1 defines continuous insulation (ci) as “insulation that is uncompressed and continuous across all structural members without thermal bridges other than fasteners and service openings.” For walls, this has led to the use of foam sheathing installed outboard of any studs or other structural members.
Is low-slope foam roof insulation continuous? Maybe not!
For low-slope roofs that meet the ASHRAE definition, many assume that above-deck insulation is continuous. However, its definition implies that thermal bridging due to fasteners is not significant or is unavoidable and, therefore, acceptable.
Wall systems use relatively low numbers of fasteners per unit area of continuous insulation (typically, around six fasteners per 4x8’ board), and the impact is therefore relatively small. However, attached single-ply systems installed above fastened polyiso boards can contain a staggering numbers of fasteners.
- A typical big box roof has an area of 125,000 square feet.
- Roofs like this, to achieve a standard wind uplift resistance, would have >49,500 insulation and membrane fasteners if mechanically attached.
- Each of those fasteners is highly conductive, leading to a modeled R-value of 25.7 versus a design R-30. This is a 14.3% reduction!
The true cost of mechanically attaching low-slope roofs
By mechanically attaching low-slope membranes and insulation, the building owner can incur several significant costs:
- An R-30 insulation level may perform at R-25.7 as shown in modeling studies due to fastener bridging. They’ve lost R-4.3 of the installed insulation, which carries an economic cost.
- A lower-than-designed R-value means decreased energy efficiency. It’s worth noting that a good TPO membrane is expected to last more than 25 years, meaning that energy efficiency losses are incurred over a long time period.
- Fastener costs, while lower than adhesives, need to be factored into any analysis of thermal bridging costs.
Alternative low-slope attachment methods
There are several ways to reduce or even eliminate the use of metal fasteners in low-slope roof assemblies:
- Use induction-welded insulation plates to reduce the total number of fasteners. These plates are used to attach insulation but they also fuse to the membrane, eliminating traditional membrane fasteners.
- Attach the membrane and top layers of insulation with adhesive. In this way, only the bottom layer of insulation would be mechanically fastened. Such practice reduces the total number of fasteners and “buries” those that are used, reducing the thermal bridging effect.
- Eliminate mechanical fasteners so that all the layers of the assembly are adhered. This would represent the design intent of ci, whereby the specified insulation value is not compromised by fastener thermal bridging. Note that adhering insulation directly to a steel deck is not normally considered code compliant with respect to wind uplift resistance, but is included for comparison purposes to represent an ideal.
These approaches are shown schematically below:
- Initial installation costs: While mechanically attached systems are generally regarded as lower cost compared to adhered systems, a simple analysis of construction material costs (i.e. metal fastener versus adhesive costs) often does not take into account lost insulation costs and reductions in long-term energy efficiency.
- Design versus effective R-value: While a designer may specify and the building owner may pay for a specific R-value, the final effective R-value could be lower due to thermal bridging. That “lost” R-value carries an economic cost. Also, HVAC systems are specified based on the design R-value even though modeling shows the effective R-value could be significantly lower.
- Energy efficiency: Thermal bridging reduces the insulation value, thereby leading to lower energy efficiency. Space heating and cooling costs would then increase, potentially over many decades.
Modeling of the total costs has identified the potential true long-term cost of thermal bridging across a wide range of cities. Results for a 20-year period are summarized here for a 125,000-square-foot building in two cities, Chicago and Miami, which represent cases where HVAC costs are dominated by either heating or cooling.
By modeling in this way, we can identify opportunities to reduce or eliminate thermal bridging to optimize cost and performance. The cost difference between attachment approach, when totaled, indicates the cost-neutral opportunity to reduce thermal bridging.
- For HVAC use in geographies dominated by heating, such as Chicago, costs totaling $75,390 would be eliminated to be put toward the buried fastener approach.
- For HVAC use in geographies dominated by cooling, such as Miami, costs totaling $84,765 would be eliminated to be put toward the buried fastener approach.
This article summarizes a large modeling study published elsewhere. GAF provides a wide range of attachment choices so that the design professional can configure the optimal system for each building. The insights offered here might help you to ask the right questions, and GAF’s Building Science Team is here to answer any questions and help design your project with energy efficiency in mind.
Author Thomas J Taylor, PhD, is building & roofing science advisor for GAF. He can be reached at Thomas.Taylor@gaf.com.
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