Key design principles for high-efficiency rainscreen walls

December 13, 2013

Photo © BigStockPhoto/Vladitto[1]
Photo © BigStockPhoto/Vladitto

By Gareth Christopher, BSc, and Richard Roos, MASc
Great strides have been made in understanding the complex interactions between heat, vapour diffusion, and air movement in the design and operation of high-efficiency rainscreen wall systems.

Increasingly demanding building codes and voluntary programs have driven designers to focus on the deleterious effects of thermal bridging on the overall energy performance of structures. Moisture dynamics are equally important design concerns, and industry experience has shown a building envelope’s resilience is central to achieving ongoing performance levels.1[2] Resilience is established through ensuring long-term thermal stability and simultaneously maintaining the ability to manage wetting within the enclosure.

Changes in construction methods can be coupled with failures, as well as mould growth in glass fibre insulation. Advances in building science research have exposed the underlying causes such as the necessity for drainage planes, continuity of air barriers, and the risks association with the incorporation of low-permeance materials into building envelope designs. Responsible designs consider the enclosure’s vapour profiles and intentionally plan for optimum drying potential through the use of vapour-open insulation products such as stone wool or glass fibre.

Although not adequately addressed in regulations, moisture is a central design concern alongside thermal performance. Judicious use of continuous insulation (ci) provides synergistic benefits to both thermal performance and moisture management. This article outlines the principal thermal- and moisture-related design considerations regarding resilience and continuous insulation in rainscreen wall systems and high-performance architectural panels.

Keeping up with changing regulations
International sustainability objectives focused on curtailing the effects of global warming are being addressed by federal government initiatives that manifest in increased insulation levels in building codes. The operation of buildings accounts for approximately 40 per cent of North America’s energy use and, as a result, thermal requirements in building codes have become more demanding. The movement toward performance-based building codes—such as recent changes to National Building Code of Canada (NBC) and voluntary programs such as Passive House—recognize the effects of thermal bridging, and have been devised such that the overall energy performance of new buildings and major retrofits are within acceptable limits.

Energy modelling plays an important role in parametric analyses that guide designers toward building performance optimization. Proprietary modelling tools contain sophisticated algorithms accounting for thermal bridging losses in envelope designs. Continuous insulation has quickly become a necessity for designers required to meet building code requirements, voluntary programs, and overall thermal performance targets.

Thermal control
Recent editions of American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings and National Energy Code of Canada for Buildings (NECB) have significantly increased the minimum requirements for exterior wall insulation levels. These requirements for increased thermal efficiency necessitate the use of continuous insulation in exterior wall assemblies.

ASHRAE 90.1, NECB, and NBC consider climatic conditions when specifying the minimum prescriptive thermal performance for the building envelope.

This thermal image demonstrates the moisture content in a conventional building at –15 C (5 F). Image ©[3]
This thermal image demonstrates the moisture content in a conventional building at –15 C (5 F). Images ©

Building design is strongly influenced by Climate Zones as design considerations for building envelope components vary greatly for hot-humid, warm-temperate, or cold climates. Holistic assessments of the energy consumption due to building envelope thermal transfer and mechanical system efficiencies are central to overall design decisions.

International Energy Conservation Code (IECC) Climate Zones 5 through 8 exist in Canada. These zones require lower-permeance vapour control layers, and higher levels of thermal resistance in the assemblies; they would also benefit from the incorporation of heat recovery ventilators.

As building code thermal requirements increase, inefficiency due to thermal bridging losses becomes an increasingly important consideration. High-performance building envelopes are made possible by the adoption of exterior continuous insulation. The industry has responded by developing numerous options for continuous insulation products and fastening systems. Rigid and semi-rigid insulation board systems have been developed to permit relatively few localized penetrations that serve to support the cladding systems on the exterior of the insulation.

The term ‘effective thermal resistance’ accounts for the amount thermal bridging reduces the insulation’s function. Thermal bridging is of critical importance in the design of modern wall assemblies and is particularly significant within steel-framed wall assemblies. The material and design of the fastening systems used for attaching continuous insulation can have a wide range of impacts on effective thermal resistance depending on the fastening system design.

In cases where continuous metal channels penetrate the insulation boards, the thermal bridging losses can reduce the insulation’s effectiveness by as much as 50 percent. The ability of the cladding attachment system and the insulation fastening to minimize thermal bridging has been well-documented, and is a critical element in the wall’s thermal efficiency.2[4]

ASHRAE 90.1 defines this continuous insulation layer as:

Insulation that is 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 envelope.

Lightweight architectural metal and composite panel manufacturers have developed proprietary fastening and cladding support systems providing thermal breaks and separating the girt and metal furring channels from the backup wall to maximize the effective R-value, improving thermal efficiency.

ASHRAE 90.1, and some building codes such as NBC, provide guidelines and prescriptive tables for the determination of the effective R-values in typical wall constructions.

Further, voluntary programs such as EnergyStar for New Homes, employ Natural Resources Canada’s (NRCan’s) Table for Calculating Effective Thermal Resistance of Opaque Assemblies, to account for the thermal bridging losses in the residential applications.

Continuous insulation
Manufacturers have responded to the need for improved thermal efficiency by providing continuous insulation boards and prescriptive measures to meet the R-value requirements through a combination of insulation batts installed within wall cavities and exterior continuous insulation. The insulation batt market has traditionally been dominated by fibrous batt insulation materials consisting mainly of fibreglass and mineral wool. Continuous insulation requires a rigid board type product, manufactured from high-density fibrous board products such as glass fibre and mineral wool, as well as foam plastic materials like extruded polystyrene (XPS), expanded polystyrene (EPS), sprayed polyurethane foam (SPF), and polyisocyanurate (polyiso).

In cold climates, use of exterior continuous insulation provides the combined benefits of enhancing the enclosure’s thermal performance, as well as reducing the risk of condensation within the envelope component. Although moisture risks are reduced by its use, careful design of building envelope resilience is critical to long-term success.

Moisture management
Building science has evolved into a discipline enabling designers to predict building envelope performance. Understanding the interdependencies between heat, air, and moisture plays an increasingly important role in the construction and renovation of buildings to meet the energy efficiency requirements and ASHRAE-prescribed guidelines.

An assembly’s drying potential is an important factor in assessing its vulnerability to moisture problems. Moistureremoval mechanisms can dry an enclosure assembly.[5]
An assembly’s drying potential is an important factor in assessing its vulnerability to moisture problems. Moisture-removal mechanisms can dry an enclosure assembly.

The responsible design of building enclosures accounts for realistic eventualities beyond idealized conditions. Envelope components experience punishing conditions from the elements, stresses due to the improper detailing and construction, as well as unintended mechanical and plumbing failures. The proper use of vapour-permeable building products allows for resilient building envelope performance, capable of managing design moisture loads as well as unplanned occurrences.

Optimal drying rates are obtained through the use of vapour-permeable insulation products—including stone wool or glass fibre boards—having permeances of 30 to 40 perms. Drying rates can be reduced to dangerously low levels when using low-permeance products such as foam board materials with permeance ratings as low as 0.03 perms.

In the event moisture makes its way into the building envelope, it is critical the enclosure components can dry, so as to maintain their physical integrity and performance properties. The risk of moisture-related damage or biological growth increases with longer drying periods.

In idealized conditions that only consider vapour diffusion as a moisture source, these materials may be shown to be suitable options. However, it is rare vapour diffusion will be the sole moisture source. When additional moisture loads are introduced, such as bulk-water entry and humidity contained in air, the inclusion of low-permeance products—such as XPS, EPS, and polyisocyanurate (polyiso)—may restrict the drying of envelope components resulting in mechanical damage or biological growth. The key to resilient building envelope design is the intentional planning for rapid drying of the building envelope components.

In cases where unplanned moisture sources—such as rain from leaky glazing units, damaged sealants, burst pipes, or air leakage—are introduced, low-permeance products may resist the assembly’s ability to dry to the exterior. A vapour control layer on the interior may also limit the drying potential to the interior as well. In these cases, resilience of the envelope may be insufficient for effective moisture management.

Vapour-permeable insulation works in a complementary fashion alongside vapour-permeable water-resistant barriers (WRBs) to regulate moisture levels. Effective moisture management requires vapour-permeable insulation be water-repellent, and allow moisture dispersion by gravity flow and evaporation; it should be non-porous, non-hygroscopic, and not support surface diffusion or capillary flow.

Designing for durability includes considerations for the permeances of the materials in the enclosure. Specifying vapour diffuse insulation materials such as mineral wool or glass fibre allows for effective moisture management and lead to durable building enclosures that provide long-term stable R-value, improved fire barriers, and moisture management.

Moving forward, continuous insulation will persist in playing a critical role in high-performance buildings. Although the overall thermal performance of building enclosures can be increased through the use of exterior insulation, important components such as fastening systems can significantly influence the effective thermal resistance of the assembly. Component materials and fastening system designs are important considerations for the minimization of thermal bridging losses.3[6]

Risks of moisture-related damage and biological growth are inadequately addressed in building codes and require additional knowledge and design experience. Low-permeance materials such as foam boards, can introduce a problem for designers and building owners due to limited drying potential. Idealized design conditions may prove inadequate for the realities of long-term durability. Unplanned moisture loads not within the control of designers—such as those introduced by improper construction, worn sealants, air leakage, or system failures—are not uncommon, and may cause serious damage to building envelope components.

Resilient building envelope designs can be achieved through the use of vapour-permeable insulation products such as those made from mineral wool and glass fibres. Durability can best be achieved through responsible application of building science principles in specifying vapour-permeable insulation and building envelope materials.

1 For more, see the book High Performance Enclosures–Design Guide for Institutional Commercial and Industrial Buildings in Cold Climates by John Straube. (back to top[7])
2 Natural Resources Canada’s (NRCan’s) Construction Technology Update No. 9, Evolution of Wall Design for Controlling Rain Penetration, G.A. Chown, W.C. Brown and G.F. Poirier. (back to top[8])
3 Other resources include the book Building Science for a Cold Climate by Neil Hutcheon and G. Handegord, and NRCan Construction Technology Update No. 17, Pressure Equalization in Rainscreen Wall Systems, by M.Z. Rousseau, G.F. Poirier, and W.C. Brown. Finally, BSC Insight 005, Thermal Bridges—Steel Studs, Structural Frames, Relieving Angles, and Balconies by Joe Lstiburek can be referenced for more information. (back to top[9])

RR Head shot2[10]Richard Roos, MASc has 11 years of professional experience in building science as a professor, consultant, general contractor, and property manager. As at member of Roxul’s technical team, he provides building science expertise as well as leadership in product development and materials testing. Roos’ consulting expertise ranges from moisture safety and heat transfer analyses in high-performance building enclosures, to sustainability programming for such clients as the U.S. Department of State, Enbridge Gas Distribution’s Savings By Design program, architects, and builders. He can be reached by e-mail at[11].


GarethChristopher- bio picture[12]Gareth Christopher, BSc, has worked at Roxul as a technical product specialist for the building envelope segment, responsible for application development and technical management of the North American roofing and commercial businesses. He earned his bachelors of science degree as a chemistry specialist from the University of Toronto, and has also received professional designations from the University of Toronto in management and administration and building science. Christopher can be contacted by e-mail at[13].

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