Designing for Exterior Continuous Insulation

By Alejandra Nieto

The fact that we have to use insulation in our building enclosures is not new to the building professions. Codes and standards have been prescribing specific R-Value and U-Value requirements for the last few decades as a means to increase energy efficiency and to improve overall building performance. Good design shows that the best performance will be achieved when the insulation is placed on the exterior of the building, as opposed to the interior in between the structure. Most recently, codes and standards also have figured it out, and are prescribing the use of continuous insulation.

There are several benefits with using continuous insulation, which include increasing the durability of the assembly, reducing the risk of condensation, reducing thermal bridging, and increasing performance. However, due to vague understanding of what is considered “continuous,” the benefits of using continuous insulation often are times significantly reduced.

Temperature of sheathing layer (condensation plane).

What do the codes say?

Thermal resistance requirements will differ between codes. Enclosure code compliance will depend on where the project is located, the climate zone, and the type of building being constructed. Typically, codes will have two compliance methods: prescriptive and performance.

As it relates to thermal performance, the prescriptive method will indicate the assembly specific R-Value requirements of the insulation to be used within the wall assembly. Newer codes typically require both interior and continuous insulation (i.e., insulation between the framing and exterior insulation).

The performance method will indicate maximum overall U-Value to be reached or total effective R-Value. With this method, all layers of the assembly are considered; reductions to the prescribed R-Value are assessed, depending on the structural components within the assembly.

Designing with exterior insulation

For optimal energy performance, it is best to go the performance path when designing the building enclosure. Since it takes into account all the factors within the assembly, it encourages the use of exterior insulation. As it relates to temperature and heat loss, you want to wrap the building in a warm layer as opposed to stuffing it. Along with reducing thermal bridging, exterior insulation significantly reduces the risk of condensation within the assemblies, resulting in a more durable and resilient building enclosure.

Different types of building enclosures will have different critical layers. For example, in a typical steel frame wall assembly, the exterior sheathing is the critical layer within the wall assembly. This is so because it is the first “cold” interface surface within the wall assembly, which translates to being the plane for potential condensation to occur. The design strategy to prevent condensation at this plane is to use exterior insulation to keep the sheathing “warm,” and ideally, above the dew point temperature. For example, as noted in the figure on page 19, when using R8 of continuous insulation, the temperature of the sheathing layer is higher than the assembly without continuous insulation. During colder temperatures, this is a significant difference to reduce risk of condensation.

There are several different types of insulation that can be used as exterior insulation, including semi-rigid mineral wool, extruded polystyrene (XPS) rigid board, expanded polystyrene (EPS) rigid board, closed-cell or open-cell spray polyurethane foam (SPF), and polyisocyanurate (polyiso) rigid board. Each of these insulation types has different performance characteristics and added benefits. The thickness of the exterior insulation that should be used will depend on code requirements, the thermal resistance value of the insulation, and the vapor permeability of the insulation.

• Semi-rigid mineral wool has an approximate R-Value of R4/inch
  (RSI 0.70/25.4mm), and is highly vapor permeable.
• XPS rigid board has an approximately R-Value of R5/inch
  (RSI 0.78/25.4mm), and is vapor impermeable.
• EPS rigid board has an approximately R-Value of R4/inch
  (RSI 0.70/25.4mm), and is vapor impermeable.
• Closed-cell SPF has an approximate R-Value of R6/inch
  (RSI 1.05/25.4mm), and is vapor impermeable.
• Open-cell SPF has an approximate R-Value of 3.5/inch
  (RSI 0.61/25.4mm), and is vapor permeable.
• Polyiso rigid board has an approximately R-Value of R5.6/inch
  (RSI 0.98/25.4mm), and is vapor impermeable.

When is continuous insulation actually continuous?

ASHRAE defines continuous insulation as insulation that is uncompressed and continuous across all structural members without thermal bridges other than fasteners and service openings; it is installed on the interior or exterior or is integral to any opaque surface of the building. Although the definition is pretty clear, there seems to be misconceptions on different attachment methods being classified as continuous insulation.

Table 1: Thermal model results , U-Values , R-Values and Reduction (%)

Different insulation types will have different fastening requirements. Most commonly, they are fastened using long fasteners/screws, or can be adhered. Although fasteners will have an impact on performance, the effects are minimal and considered negligible. Deflection and other concerns come into effect only when using fasteners to attach continuous insulation—especially with heavier cladding systems, such as terra cotta panels, and with taller buildings that will experience elevated wind loads. This can lead to more fastener requirements or the use a grid system (z-girts) to attach the claddings.

This is when the grey area for the term “continuous insulation” begins. How many fasteners does it take until the thermal bridges are no longer negligible? If using a grid system or clip and rail system, is the insulation still “continuous,” and can it still follow the prescriptive path for compliance? Although often not seen as priority since it doesn’t affect code compliance, the critical question is how do the significant thermal bridges affect the durability of the building enclosure? These questions can be addressed using thermal modelling to determine where and how significant the thermal bridges are, and whether or not they increase the potential for condensation.

Figure 2: Thermal model results,  R20 exterior insulation installed with z-girts.

Figure 3: Thermal model results,  R20 exterior insulation installed with fibreglass clip system.

Figure 4: Thermal model results,  R20 exterior insulation fastened with long screws (5 screws per board).

As identified in the table and figures below, it is evident that a clip and rail system has more significant effects than fasteners, and that a z-girt system is the worst performer. The areas of high thermal bridging will decrease the temperature of the critical areas (note the temperature gradient through the assembly), which can lead to increased risk of condensation within the assembly. These thermal bridges also increase the heat transfer through the assembly, which then affect the energy performance of the building enclosure.

It is critical to note to all of these reductions are considering perfect workmanship, and do not consider other parts of an assembly that can cause thermal bridging such as openings (windows and doors), metal flashing (if applicable), floor slabs or balconies. It also is important to note that similar thermal reductions are going to occur independent of the type of insulation and its nominal R-Value. The final overall effective performance may differ, but the amount of reduction always will be at par because the steel (or structure) always will be the driving factor.

With masonry veneer buildings, the exterior insulation does not have to be fastened to the substrate. Rather, brick ties can be used to secure the insulation in place. However, brick ties—and more importantly, shelf angles required for masonry veneer—can significantly affect the thermal performance of the assembly. The material of the brick ties and shelf angles will change the performance. With regular ties, switching from galvanized steel to stainless steel will make a significant difference in reduction (Finch & Higgins, 2013). Additionally, other solutions exist where the brick ties are thermally broken to reduce thermal bridging.

What does the science say?

Although we still may not know what the codes consider too much thermal bridging to no longer consider continuous insulation “continuous,” we can at least take away the following:

• Thermal bridging matters!
• The performance compliance path for codes typically will give you a more efficient building enclosure.
• The more exterior insulation, with minimal thermal bridging, the better.
• Grid systems (i.e., z-girts) should never be used as an exterior insulation attachment.
• The fastening material conductivity makes a significant difference when it relates to brick ties and angles.
• You can’t account for workmanship deficiencies.

About the author:
Alejandra Nieto is an Energy Design Centre (EDC) specialist at Roxul. She is a graduate from the Master of Building Science program at Ryerson University, with a background in construction science and management, and architectural technology from George Brown College. She has experience in design and research of the methods and materials involved in energy-efficient buildings and systems. As a specialist for the EDC, she provides expertise in building envelope design, and moisture and heat transfer analysis.

Works cited
Finch, G., & Higgins, J. (2013).
Masonry Veneer Support Details: Thermal Bridging. 12th Canadian Masonry Symposium. Vancouver.

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