With the increasing cost and extensive delays in the supply of timber building materials, it is increasingly common to see steel framing adopted in domestic construction. However, the question remains: can steel be a direct substitution? This article explains why a direct substitution of material can pose a problem with condensation under some climatic conditions.
Background
Due to COVID-19 supply chain disruptions, there have been major price hikes in the cost of building materials, particularly with timber and engineered timber products. The cost of timber framing has risen markedly in Victoria and is now equivalent to that of steel framing. To have a more reliable supply of framing materials without additional cost, steel framing is increasingly substituting timber framing in domestic construction.
Although the structural adequacy of steel framing can be determined to be equivalent to, or in excess, of that for timber framing, it may not be a suitable substitute in every instance.
Steel has a much higher thermal conductivity than softwood (pine) and will result in thermal bridging if insulated using the same method as timber frames, i.e. to insert insulation batts between studs. The appropriate design and installation when constructing with steel framing is to run rigid insulation (such as EPS or rock wool) external to the structural framing, or as some would say, ’outsulation’. This would require thicker walls and thus a redesign of the floor plans.
The problem with condensation
If this problem is not arrested expeditiously, there will be many new houses built which may arguably be code-compliant, but which will invariably develop condensation and mould problems, especially during winter when cold thermal bridging is most severe.
Condensation provisions were introduced into the National Construction Code (NCC) only recently, in 2019. The condensation management provisions only apply to the cooler climates (Climate Zones 6, 7 and 8). These provisions are a start, but a small part of the recommendations which the building scientists recommended in their ABCB Condensation Scoping Study report in 2016. It is understandable then that the NCC does not yet have a comprehensive response to condensation risk, but this is where building practitioners must decide if adhering to the NCC as a minimum standard is still sufficient for a building to be assumed to be fit-for-purpose.
Thermal bridging in a Class 1 building, as of NCC 2019, is not considered when evaluating a building envelope’s total R-value. This allows new buildings to be constructed with steel framing and bulk insulation, when in reality the envelope performs practically no different to one that is uninsulated, see Figure 1 below for numeric quantification.
In each case, the wall insulation has an R-Value of 2.0 m2 K/W. Based on the NCC Vol Two definitions, in every one of these cases, the insulated wall frame will be deemed to have a Total R-Value of 2.0. However, when calculated, the system R-Values of the insulated wall frame (i.e. excluding plasterboard, air cavity and cladding) tell a different story. Determined using calculation method described in NZS 4214:2006, referenced under AS/NZS 4859.2:2018. (a) Softwood framing (90x35mm), double top plate, bottom plate and one noggin per panel. (b) Similar to (a), with 10mm construction tolerance still air gap between window reveal and frame by backer rod. (c) Similar to (b), with the addition of jack stud beside the jamb stud, as commonly constructed. (d) Replace jack & jamb stud with 89mm steel SHS (3mm wall thickness), common with large span windows.
Source: Law, T. (2021). An increasing resistance to increasing resistivity. Architectural Science Review. https://doi.org/10.1080/00038628.2021.1916428
Beyond minimum building standards
It is hence needful that state building regulators, or building practitioners themselves, go beyond the minimum standard of the NCC, as the standard is dangerously low in that it allows buildings to be code-compliant when there is an immediate and definite risk of condensation in the building fabric.
Unsuspecting consumers are not aware that they are being supplied a new house that can be deemed code-compliant but, due to ill-informed material substitution, will have interstitial condensation in walls resulting in mould on the paper-faced plasterboards. When direct substitution occurs, there will be a designed-in and built-in problem in new houses which will be very costly to rectify. Furthermore, it is likely that insurers will refuse compensation since the house may be deemed to be code-compliant.
By adopting external rigid insulation, the primary benefit for consumers will be the prevention of condensation in steel-framed houses. The secondary benefit is that when the thermal bridging problem is resolved, the houses will also be more energy-efficient.
A caveat around substituting timber with steel
As thermal bridging is not presently considered in the definition of Total R-Value in NCC Volume Two, the industry around domestic construction is generally not well informed about thermal bridging.
The Building Regulation 2018 of Victoria requires an RBS (relevant building surveyor) to require testing of materials so as to “prohibit the use of any material that is found to be unsuitable or unfit for the purposes for which it is intended” (reg. 120). It thus falls on the RBS, for the public good, to require a method of installing steel frames appropriate to the climate that exceeds the requirements of the NCC in order to be fit for purpose.
When in doubt whether it is appropriate to do a direct substitution of timber framing with steel framing, it would be best to check with the RBS, drawing his/her attention to the risk of condensation with thermal bridging. Building surveyors/certifiers, if in doubt as to how to determine the appropriate levels of rigid external insulation, should seek advice from their respective state building regulators.