Using Spray Foam to Create Air Barriers on Exterior Walls
By John Broniek
When selecting air barrier products to employ in building designs, architects have a broad range of choices. Spray foam insulation products are often selected because they can provide superior performance and also contribute to greater energy and operating cost savings in buildings. In particular, closed-cell (medium density) and open-cell (low density) spray-applied polyurethane foam (SPF) can insulate and air seal wall assemblies in commercial buildings quite effectively, economically, and efficiently. Its air-sealing characteristics, along with the ability to minimize moisture and temperature-related issues, and capacity to enhance the performance of a wide variety of building types makes it ideal for commercial projects. At the same time, more design freedom because of the custom, field-applied nature of the system is realized. As a result, these products are becoming a popular alternative among architects when designing exterior walls for airtight construction.
The need for air barriers in exterior walls is well understood in the building community. Effective air barriers have positive impacts not only on heating and cooling energy costs, but also on moisture problems, indoor air quality (IAQ), and acoustics. This is because moving air can carry unwanted moisture, contaminants, or noise into a building. Overall, proper air sealing with an air barrier system results in more sustainable and durable buildings addressing numerous green design issues such as more efficient use of construction resources. There is a realization among the design community an air barrier system is a combination of components within the building enclosure, which typically includes a primary air barrier material. The system components are designed, installed, and integrated in such a manner as to stop the uncontrolled flow of air into and out of the building enclosure.
Air barrier material performance
To be a qualifying air barrier material, the air permeance of a product must be equal to or less than 0.02 L/(s·m²) (0.004 cfm/sf) when tested at an air pressure of 75 Pa (1.57psf). The testing basis for this rating is ASTM E2178, “Standard Test Method for Air Permeance of Building Materials.” Closed-cell, medium-density, SPF products are required to have this testing conducted to meet the requirements of the Underwriters Laboratories of Canada (CAN/ULC) S705.1-01, Standard for Thermal Insulation – Spray Applied Rigid Polyurethane Foam, Medium Density – Material – Specification. The National Building Code of Canada (NBC) notes in Appendix A generic, medium- and low density 25-mm (1-in.) thick polyurethane spray foam have air permeance characteristics that would result in both qualifying as an air barrier material. Medium-density spray foam insulation normally is installed at a nominal 32 kg/m³ (2.0 lb/cf) density while low-density spray foam is installed at a nominal 8 kg/m³ (0.5 lb/cf) density. In addition, in the American National Standards Institute/American Society of Heating, Refrigerating and Air-conditioning Engineers/Illuminating Engineering Society of North America (ANSI/ASHRAE/IESNA) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, both SPF product types are recognized as air barrier materials. Product manufacturers should be consulted for ASTM E2178 test results that indicate the thickness of a specific product for it to qualify as an air barrier material.
Air barrier system performance
The Canadian Construction Materials Centre (CCMC) has developed a Technical Guide for Air Barrier Materials describing the criteria for the assessment of mechanical and physical properties of air barrier systems for exterior walls of low-rise buildings for the purpose of obtaining a CCMC Evaluation Report. Although low- and medium-density SPF products are eligible to apply for a CCMC Air Barrier Systems Report, currently just a small number of medium-density SPF products have obtained it and only for exterior continuous insulation (ci) wall applications. According to the technical guide, an air barrier system must:
- have an acceptable low air leakage rate;
- be continuous and durable;
- have sufficient strength to resist the anticipated air pressure load; and
- be buildable in the field.
A significant installation benefit of medium-density spray foam insulation is its ability to conform directly to the wall (sheathing or structural) surface regardless of shape, geometry, or irregularities. It can be employed on the exterior or interior side of a wall and fully covers and seals the underlying construction to provide a truly continuous uninterrupted insulation layer. The result is, for example, curved buildings have traditionally needed a very labor-intensive process where rigid foam boards had be cut to shape and then excessively taped at each joint, but this can now be replaced with a quick, smooth, and far less labor-intensive installation. For exterior as well as interior ci applications, medium-density spray foam insulation has the key attributes needed for wall performance. These include:
- an integral air barrier based on material testing as mentioned earlier;
- high long-term thermal resistance (LTTR) insulation values per CAN/ULC-S770, Standard Test Method for Determination of Long-term Thermal Resistance of Closed-cell Thermal Insulating Foams, testing;
- a vapour barrier, typically at a 50-mm (2-in.) thickness or greater, based on water vapour permeance testing per ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials, thereby eliminating the need for a polyethylene (PE) vapour barrier at the inner portion of the wall; and
- a water-resistant barrier (WRB) since it is capable of shedding water (when integrated with flashings around penetrations and key wall transition points a durable water-resistant system is formed).
Air barrier systems
In the CCMC Air Barrier System evaluations, the medium-density SPF product is considered as the principal material in the plane of airtightness for the air barrier system. To qualify as an air barrier system, the wall needs to accommodate movements of building materials, and provide connections to adjacent materials to prevent air leakage at all critical locations including connections, joints, and penetrations. An extensive number of tests are conducted to receive the air barrier system designation including air barrier system testing. To achieve air barrier system compliance, it is recognized the use of medium-density, spray foam products in exterior ci wall applications requires membrane materials (self-adhered or liquid) to provide optimum air barrier system performance. Consequently a great deal of effort in field evaluations and product compatibility testing has been made by manufacturers to define the components of such a system and where they should be used.
Design and performance considerations
The use of medium-density spray foam insulation in exterior ci applications provides architectural firms and commercial building contractors a great opportunity to positively impact their building schedules, budget, and overall performance quality. Additionally, medium-density spray foam insulation in exterior ci insulation applications offers many benefits including:
- can cover the widest range of exposed surfaces to prevent thermal bridging;
- features an approach to air- and water-resistive barrier creation not dependent on extensive use of joint sealants in exposed situations;
- has a much better ability to bond to the entire length of wall penetrations, such as brick ties and fasteners, thereby better ensuring thermal and airtightness effectiveness;
- is less likely to have air barrier discontinuity at transitions and control joints;
- has very low water absorption and is capable of shedding water; and
- can be applied in cold temperatures as low as -10 C (14 F).
There can be challenges associated with SPF exterior ci, most of which can be overcome with attention to details and pre-application planning. Dry and non-windy weather conditions are necessary to facilitate optimal product adhesion and limit over-spray situations. The application area should be isolated and other construction trades restricted from entering. Confirm the substrates the spray foam will be applied to are compatible with it. Develop a plan to ensure wall penetrations including windows, doors, anchors, and attachments are covered or will be remediated so the spray foam does not affect their long-term operation.
Using open-cell, low-density spray foam insulation
Although it has most often been associated with residential construction, low density spray foam can also be used on the interior side of a commercial building. Its air-sealing characteristics allow it to qualify as an air barrier material. Many spray foam manufacturers have the appropriate test information to confirm that ability. Therefore, using open-cell spray foam within a wall cavity, with either metal or wood studs, can constitute the main component of an air barrier system. Its use also means expensive air membranes on the exterior of the wall sheathing can be avoided or scaled back in scope, thereby reducing construction costs. Other benefits of using open-cell spray foam include being better able to be sprayed on cold substrates and employed in cold weather applications. It can be used for applications on wood-based substrates, and it is also compatible with concrete, masonry, and steel. Open-cell spray foam can be cut fairly easily allowing for changes, such as adding an electrical box within a wall stud cavity, to occur more simply. Open-cell spray foam insulation effectively blocks heat transfer with a tested thermal performance of approximately 0.66 to 0.72 (m2•K)/W (R-3.7 to R-4 per in). Its lower density gives it a comparatively softer make-up than denser, closed-cell insulation, meaning it can also seal around the edges and perimeter of stud cavities and any penetrations in a flexible manner. Some of the other benefits of open-cell insulation are tied to this lighter, softer, and more flexible make up. Acoustic control, for example, is enhanced in wall assemblies due to its flexible and absorptive properties, more so than with rigid insulation. Should water infiltrate the assembly for any reason, its vapour permeability means the material can dry both toward the interior and the exterior as may be preferred. As a material it does not provide a food source for mould. The cost of open-cell spray foam insulation is generally very attractive and competitive when compared to labour and materials for other types of insulations. Installing spray foam insulation within building interior assemblies are typically less challenging than exterior applications. Isolating the application area and having the substrates ready for spraying are still necessary. Further, the building construction should be far enough along so the installed SPF will not be subject to weather—if windows are not in place it is good practice to cover their openings. As with all construction products, installation is critical to the performance of the material and development of a good performing wall air barrier. spray foam installers who have successfully undergone training at the hands of manufacturers or a trade association are more likely to deliver the installation needed. Specifically, the Canadian Urethane Foam Contractors Association Inc. (CUFCA) has an installer certification process that includes training and licensing. The advantages of using spray foam insulation to create an air barrier on exterior walls are numerous. Using medium-density SPF products in an exterior ci wall application has been proven to create an airtight wall assembly and is becoming widely specified. Meanwhile, low-density SPF products are a cost-effective approach to creating airtightness within wall stud cavities. In both cases, the result is a more energy-efficient building.
John Broniek is senior engineer for Icynene-Lapolla. where he provides product use recommendations to all external audiences including designers, applicators and building owners. He also acts as a building science technical resource to Icynene-Lapolla’s sales team, dealers, and community. Broniek has been involved in improving the energy efficiency and durability of buildings throughout North America since 1990. Broniek can be reach at firstname.lastname@example.org.