Steel SIPs for Residential Building Construction: Lessons from Air Leakage and Thermography Analysis of Australian Houses

Globally, the energy efficiency of housing continues to be a major policy focus, as exemplified by the International Energy Agency (2011), the European Union Energy Performance of Buildings Directive (European Commission 2010), and the International Energy Conservation Code (ICC 2012). This trend commenced in cold and cool temperate zones in the Northern Hemisphere where there are high space-heating requirements. The incorporation of energy efficiency into building regulations for housing is variably driven by a combination of social issues (e.g., fuel poverty, health), resource scarcity or security (e.g., where do the fuels for heating come from), and environmental concerns (e.g., climate change). Based on prescribed or assumed comfort levels for human occupancy, regulations may stipulate a maximum allowable energy budget for mechanical heating or cooling (MJ/m²/year) or require specific performance requirements for building elements (e.g., minimum U-values) or a combination of these plus a requirement for building services (space heating/cooling, hot water, and lighting) to be met by on-site renewable energy (e.g., net-zero-energy buildings).

Air leakage (uncontrolled air infiltration) is one important factor in a building's energy consumption, potentially introducing moisture loads on the structure, contributing to poor indoor air quality through condensation and airborne pollutants, and negatively affecting occupant thermal comfort and energy costs (Ojanen and Kumaran 1992; Hagentoft and Harderup 1996; Guyot et al. 2016; Seppaenen and Kurnitski 2009; Younes et al. 2011). Thermal leakage, through breaks or bridges, also affects the energy efficiency and indoor environmental quality (IEQ) of a dwelling and may or may not be related to air leakage sites. Air leakage and thermal leakage can be a result of inferior design, low-quality or inappropriate construction materials, incorrect utilization of materials, poor workmanship, structural damage, or local climate (Taylor et al. 2014; Fox et al. 2016; Karagiozis and Kuenzel 2009; Üllar et al. 2015). Airtightness testing and thermography, used individually or together, are established nondestructive methods for identifying such building defects (Balaras and Argiriou 2002; Grinzato et al. 1998; ATTMA 2010; BECP 2011).

The extent of air leakage problems and the potential energy-efficiency benefits of well-sealed buildings are arguably dependent on the climate in a particular location, the nature of construction in that location (in terms of building form, materials, and construction practices), whether a dwelling is intended to be naturally or mechanically ventilated, and the nature of the energy supply (in terms of cost, availability, and environmental impacts). For example, Australia’s current minimum standards for space heating and cooling range from 39 to 349 MJ/m²/year (climate dependent), with aspirational goals (highest performance ratings) ranging from 0 to 119 MJ/m²/year (Department of the Environment and Energy 2016).

Australian detached houses, similar to those in the United States, are typically stick-built—that is, constructed using 90-mm structural wall frames (either timber or steel) that are either constructed with on-site techniques or made into walling sections off-site and erected with on-site techniques. These frames are covered with an external skin (typically brick or lightweight materials such as timber, fiber cement, or metal sheeting) and an internal wall lining (typically plasterboard). Houses are constructed either on a cement slab or on a suspended floor raised off the ground on stilts (i.e., with a crawl space). Roofs are typically framed/trussed (steel or timber) with metal sheeting or cement tiles. To meet current thermal performance requirements for the different climate zones, variable types of reflective foils and bulk insulation are installed under the roof, the ceiling cavity, and the wall frames, depending on local practices. Various studies have highlighted deficiencies in this construction practice that contribute to high rates of air and thermal leakage in Australian houses and the energy-performance gap between buildings as designed and as constructed (Ambrose and Syme 2015; Harrington 2014).

Compared with site-built framed housing, the use of structural insulated panels (SIPs) in the United States has been reported to reduce air leakage rates by as much as 85% (Malhotra and Haberl 2006), reduce job-site time, and provide stronger, more durable dwellings (Hodgson 2003).

To date, the typical SIPs utilized in Australia have been sheathed in metal [i.e., utilizing 0.42–0.6-mm metal sheets bonded to a core of expanded polystyrene (EPS), polyurethane (PUR), or polyisocyanurate foam (PIR)]. The panels have been used predominantly for industrial purposes (e.g., cold-storage facilities and warehouses) and to some extent in commercial buildings. They have both structural and semistructural (e.g., used as a cladding over a minimal frame) properties. Buildings using these panels do not require additional external or internal linings or claddings and can be assembled without the need for joining adhesives. Within the Australian residential market in the last 20 years, these metal SIPs have been limited mainly to patio roofing (providing insulated roofing for the outdoor living spaces often found in Australian homes). For the past decade, a small number of products offering fully or substantially SIPs have tentatively crept into the housing market as a walling material. The two main types of products are metal-sheathed EPS and PUR sheathed in fiber cement (FC). Unlike oriented strand board (OSB) SIPs (also relatively new in the Australian housing market), steel- or FC-sheathed SIPs are complete walling systems that do not require the addition of breathable membranes, timber battens and cladding, or internal plasterboard (drywall). These walls can be easily rendered or, in the case of steel SIPs, left in their raw state [e.g., if the steel sheath has a COLORBOND (BlueScope, Melbourne, Australia) coating, i.e., a patented coating system common for Australian steel used for residential construction]. Steel SIP roof panels do not require additional cladding internally or externally. All SIP products (steel, FC, and OSB) for detached housing in Australia are in the early stages of market diffusion, battling against an industry renowned globally for its struggle to incorporate sustainability and innovation in construction methods (Lutzenhiser 1994; Lovell and Smith 2010).

Although steel SIPs conceivably have an advantage over OSB SIPs in terms of construction time and reduced material requirements (because there is no need for additional internal and external claddings), a comparative analysis of different SIP products was not the purpose of this study. The purpose of this study was to determine to what extent stick-frame builders can successfully adopt the new steel-SIP construction method to reduce the typical issues with air and thermal infiltration that affect the energy efficiency and indoor environmental quality of homes.

Structurally, the five houses are similar in that they are frameless (i.e., the SIP wall and roof panels provide the structural integrity usually provided by steel or timber frames and trusses, in addition to the insulation and internal and external cladding), and therefore there are no wall or roof cavities. Table 1 compares the key building attributes of each house, showing the characteristics of the main construction materials and design variations in the area, volume, and glazing. Table 2 gives the location of each house, a climate description, and the seasonal conditions under which each house was expected to perform.

Semistructured interviews were conducted with each builder and owner (separately) to examine the key motivations and experiences of utilizing steel SIPs as the main construction material. Interviews (typically 1 h in length) were recorded and transcribed. The main goal for each house (as articulated by the owners) and the key motivations for the owners and builders in using this material are shown in Table 3.

Each house was subject to a building depressurization test [ASTM E779 (ASTM 2010)] in combination with infrared scanning and smoke tracer [ASTM E1186-03 (ASTM 2009)]. Equipment utilized included a blower door with a Retrotec (Everson, Washington) DM32 dual-channel digital gauge and an FLIR (Wilsonville, Oregon) E60 thermal imaging camera. These tests were undertaken on completion (or near completion) of the construction phase. Particular attention was paid to SIP panel joints; the joints between walls, roof, and floor; and envelope penetrations (e.g., windows and doors, power points, and other building service penetrations). The test conditions for each house are shown in Table 4. International performance standards for housing were used to compare with the measured results. Case-study building documentation (e.g., house construction plans) and manufacturer product documentation (printed and online) were used to help interpret quantitative data.

Air leakage results for the case-study houses (Table 5) were compared with international regulations (Table 6). In examining these figures, it is important to note although Australian regulations require windows and external doors to be weather-stripped and air leakage to be minimized, acceptable levels of air leakage are not quantified. The vast majority of homes in Australia are also naturally ventilated (i.e., there are few mechanical ventilation units). ASHRAE Standard 62.2 (ASHRAE 2010) specifies that mechanical (forced) ventilation is required in housing with a natural infiltration rate (not at 50 Pa) of less than 0.35 air changes per hour (ACH). A comparison of Table 5 and Table 6 clearly shows that although each of these houses compares very favorably with reported typical leaky houses in Australia (Ambrose and Syme 2015; Biggs and Bennie 1988), they vary in their ability to meet various international (Germany, United States, Spain) standards (Chan et al. 2013; Feldmann et al. 2013) if these performance measures were to be adopted by Australia.

Few air leakage points were detected between the floor and the SIP walls, between SIP walls and roof, or between SIP walls and window/door frames. One noticeable exception is shown in Fig. 7. This high level of consistency across all five homes would seem to indicate that the SIP product and method of construction have been successful in limiting air infiltration (i.e., success is not dependent on the skill or experience of the main contractor). The most frequent locations for observed air leaks were in the typical aluminum window and door frames and in general power outlets; they were not directly related to the SIP product itself. The actual size of each individual leak identified was very small. However, the equivalent leakage area ranged from 905 to 3,340 cm² (refer to Table 5). One home did show evidence of air gaps between the wall and roof panels (detected by visual inspection and later rectified). However, this location was not detected by the smoke tracer because these joints were not physically accessible (i.e., walls >3.5 m).

By far the largest thermal breaks in each house were in the openings, particularly the single-glazed windows and doors of three of the houses (refer to Table 1 for glazing specifications). No thermal breaks or holes in the insulation were observed in the wall panels, indicating that the wall product and its construction method can overcome common construction faults, even for builders who had not used the product previously. [Wall and roof panels are constructed with a patented slip joint so that when two panels are joined, the metal sheaths overlap, and the insulation is abutted. No tapes or seals are required (Fig. 1).] One of the houses, however, showed relatively minor thermal breaks between a few roof panels. Some of these breaks seemed to indicate that the builder had not taken care to ensure that the roof panels were fully abutted during construction (Fig. 2). This may be an indication of a practical construction challenge given the long roof spans and skillion roof architecture that SIPs can achieve. In one instance, thermographs seemed to indicate that some EPS insulation had been removed from the roof panel to install a skylight and electrical cabling (Fig. 3). Such thermal breaks could easily be overcome with the use of spray foams and the like, provided the builder is aware that any insulation that is removed should be replaced. During construction, the same house had visual evidence of an air gap between the walls and roof of the main living area; these were sealed by the main contractor. Unsurprisingly, the largest area of thermal loss was through glazing, as shown in Fig. 4.

Thermal bridging was a much more common occurrence. Thermal bridging was seen in all homes, in particular at the wall–roof intersections (Figs. 5 and 6), wall–floor intersections (Fig. 7), and aluminum window/door frames (Fig. 8). The construction method requires use of a Galvabond (BlueScope, Melbourne, Australia) base channel plate and top channel wall plate (Figs. 9–11) (Bondor 2017). Heat appeared to be conducted from the outside wall/channel, along the channel, to the inside of the house, including traveling some distance up the wall/along the internal roof section. Heat may also have traveled along the underside of the roof panel and into the house. These bridges exhibited temperature differences of 1–3°C, as shown, for example, in Fig. 12. In this particular house, the thermal bridge shown in Fig. 7 could also be attributable to an exposed concrete slab edge. The SIP base channel could have assisted in heat conductance from the exposed slab into the house because of its placement directly on the slab. Thermal conductance through window frames was not unexpected because the industry standard in these locations is for aluminum frames (not thermally broken) even if double glazing is specified.

The risk of condensation and mold from thermal leakage can be calculated by applying the temperature factor as follows:

For the two houses in the cool-temperature zone, the results were 0.25 and 0.625 for Broadford and Mount Gambier, respectively, outside of the U.K. recommendations for residential buildings (0.75) and Finnish regulations (0.65) for healthy housing (Kalmees et al. 2007). Using the maximum 4°C temperature difference recorded at the time of testing, the results for Freemantle and Toowoomba were –0.125 and 0.25, respectively. Internal surface temperatures were not recorded before and after depressurization, so the relative decrease in surface temperature could not be calculated. Numerical modeling was not conducted to quantify the impact of these thermal bridges, but a method of doing so has been proposed by Taylor et al. (2014). Therefore, based on the temperature factor and consideration of regulations and guidelines for cold European climates, this would seem to indicate that thermal bridging was severe in each of the homes, raising concerns about structural risks and health hazards (e.g., mold growth). The validity of this test method for quantifying thermal leakage in these climates and for this material is questioned in the next section. Since construction, indoor condensation in these homes has neither been quantified nor reported by the occupants.

The following section discusses the implications of these results for performance-testing practices, for housing regulation, and for the housing supply chain.

ASTM E1186-03, pertaining to building depressurization and infrared scanning technique, “relies on the existence of an indoor–outdoor temperature difference of at least 5°C” and assumes that this condition could be met in most geographic locations for a large percentage of the year. Thermography standards assume climatic conditions where the internal/external difference is at least 10°C (Fox et al. 2016). One of the key guidelines for good thermography is to ensure a temperature difference of at least 10°C between indoors and outdoors for at least 4 (RESNET 2012) to 24 hours [ISO 6781 (ISO 2015)] prior to the test. In warm climates where houses are naturally ventilated, these temperature differentials (either 5 or 10°C) cannot be achieved for much of the year, and there may be no mechanical heating/cooling appliance to precool or heat the home for testing purposes (because the homes are passively heated/cooled). This means that the quality of thermographs, and hence their usefulness, may be limited and that the ideal conditions for conducting the testing are often not available for the construction industry (if such testing was required for regulatory purposes). Furthermore, the methods used to determine the potential impact of thermal bridges or air leakage (e.g., the temperature factor and relative decrease of surface temperature) may have limited application in these climates and/or for this type of product. Further research is required to determine if there is a role for thermography for housing in warm climates, and if so, what testing techniques, equipment, and impact assessment calculations can provide the useful outputs within the climatic and practical site-access conditions.

In addition, Australian practice for air leakage testing differs from that specified by ATTMA (2007) in that air conditioning (AC) (heating/cooling) systems (ducted or split systems or window box systems) are usually not sealed during testing. This is because houses are typically operated with AC systems open to the conditioned space of the house (i.e., no closable dampers), and the purpose of the testing is to gauge air leakage under typical conditions. This difference in practice, combined with the fact that most air conditioners are not ducted, makes it challenging to compare air leakage results with other nations with similar construction practices (e.g., the United Kingdom and the United States).

The results also raise the question of what, if any, airtightness standard should be applied to naturally ventilated homes in warm climates. Some countries have different standards depending on whether homes are mechanically or naturally ventilated (e.g., Germany), whereas others appear to have different standards depending on the climate zone rather than the nature of ventilation utilized in the home (e.g., United States, Spain). The stringency of the standards appears to be somewhat correlated to a country’s targets for energy efficiency (and associated reductions in greenhouse gas emissions); however, benefits to occupant health and building operation and maintenance are also sometimes raised. A minimum performance standard of 10 ACH at 50 Pa has recently been proposed for Australia (Ambrose and Syme 2015), representing a 30% improvement on assumed average performance, yet double the requirement for warm climates in the United States (refer to Table 6). A lifecycle benefit–cost analysis for each climate zone could provide the evidence required to make informed decisions for either regulatory or market-driven responses.

From a market perspective, this research has also shown that the SIP houses provide a very high level of airtightness (2–9.6 ACH at 50 Pa) compared with 134 houses tested recently by Australia’s Commonwealth Science and Industrial Research Organisation (CSIRO) in seven capital cities (1–39 ACH at 50 Pa) and in comparison with the air leakage rates assumed in the building simulation software used for assigning energy ratings to houses as designed (15 ACH at 50 Pa) (Ambrose and Syme 2015). The CSIRO report found that 30% of houses tested were ≤ 10 ACH at 50 Pa, demonstrating that higher airtightness levels are possible in the current market. However, both the CSIRO report and this paper raise the question of how high-performance houses can be differentiated in the housing market. This issue has also been raised in the United States by Chan et al. (2013), who propose that as-designed and as-built certification may be required and note the possibility of linking certified air leakage results with building simulation results. This is an issue of quality assurance that has also been raised for the Norwegian wood-framed housing market (Relander et al. 2012).

Lastly, the results strongly point to the need for a whole-systems approach if even higher performance standards (e.g., as evidenced by lower air leakage rates and elimination of thermal breaks and bridges) are to be achieved. The information guide and technical manual available on the website of one SIP manufacturer promotes the product as a “fully insulated domestic housing application,” yet makes no reference to how the joints in the building are to be insulated, how to address thermal bridges, or the importance for occupants/designers to consider the U-value of doors and windows, as they make up a considerable portion of the building envelope. None of the construction drawings for the case-study houses detailed how air and thermal leakage would be handled either, perhaps indicating that the design/construction industry itself does not know about, or will not address, these issues. It is conceivable that the product manufacturer could play a role in education, training, and knowledge dissemination by highlighting the importance of these issues to the energy performance of a house as a system and in providing practice notes about how the industry can limit air and thermal leakage during construction. (The manufacturer’s technical manual is quite explicit in specifying how to meet structural, cyclonic, and bushfire requirements.) The importance of well-specified details and drawings, in addition to instructions for sealing joints and eliminating or reducing thermal bridges, has previously been highlighted for SIP construction, including the role of manufacturers in providing this information (APA 2007). SIP manufacturers may also consider strategic relationships with high-performance glazing manufacturers, given the strategic role of glazing in green building (Koebel et al. 2015).

This research has shown that there is a need for a holistic approach to the house that takes into consideration all of the components required in the construction methods and quality, the local climatic conditions, and the different roles of supply-chain agents, including owners/occupiers. This is consistent with the findings of Australia’s CSIRO noting that build quality and attention to detail were significant factors in building performance (Ambrose and Syme 2015), Swedish housing industry findings that commitment to high-performance outcomes and a lifecycle approach were both required (Persson and Grönkvist 2015), and proposals for building quality improvement through testing during construction (Taylor et al. 2013).