BRANZ Build (Oct / Nov 09) A Concrete Piece in the Affordability Puzzle

01 Oct 2009

A Concrete Piece in the Affordability Puzzle

Rob Gaimster – Cement & Concrete Association of New Zealand (CCANZ)

Debates about affordable housing need to consider efficacies over time, not just initial cost. Then options become much wider and include concrete as a material for affordable, low cost housing.

During the past 20 years, home ownership rates have fallen significantly, from 74% of households in 1989 to 67% in 2006. This is expected to fall to 62% of households by 2016.

Initiatives designed to increase the supply of affordable, low-cost housing should not be restricted to light-weight construction options more heavy-weight solutions are also possible.

Is concrete up to the challenge?
Does the New Zealand concrete industry have an off-the-shelf affordable housing solution to-offer? If one takes the cost criteria of the Department of Building and Housing's (DBH) Starter Homes Design Competition as a de facto definition of an affordable home, this challenge is still to be met.

The competition specifies 'a good quality affordable home' as having a maximum of 120 m2 of habitable floor area, arranged as a simple stand-alone single-storey dwelling (excluding garage). The maximum target cost is $1,400 per m2 (including GST) for the design constructed on an urban site. This criteria is challenging for concrete structures, but not necessarily insurmountable, for example, by leveraging on tilt-up construction technologies.

Concrete offers whole-of-life benefits
When the concept of affordability is considered beyond initial cost, concrete construction for residential housing offers a range of medium and long-term efficiencies to save overall.

Thermal Mass
The thermal mass of concrete structures is the key to their affordability from a whole-of-life perspective. If temperatures continue to rise as a result of climate change, concrete's thermal mass will play an increasingly significant role in ensuring that New Zealand's housing stock is affordable throughout its life.

The vast majority of the total energy used in operating buildings is for heating and cooling. The capacity of concrete to absorb, store and later release heat minimises internal temperature fluctuations. A comfortable temperature can be maintained and the need for energy-intensive air-conditioning is reduced, even removed.

The recent introduction of NZS 4218:2009 Thermal insulation - Housing and small buildings could potentially increase costs to homeowners. With higher insulation levels in New Zealand houses, new dwellings (the majority of which will be built in the warmer climes north of Taupo) may overheat during summer if no cooling measures are provided. This will place greater demands on energy intensive air-conditioning units.

By constructing houses that incorporate a concrete mass element, whether an exposed area of floor slab or an exposed wall, there is the potential to absorb excessive heat gains. However, providing high mass materials alone will not guard against overheating. Summertime surplus heat needs to be removed or it will be admitted later into the cooling space and tend to increase the temperature again. Thus, it is very important that this thermal mass is cooled usually by passing large amounts of cooler night air over its surface.

A good example of sustainable passive design using exposed concrete elements in a residential setting is the Waitakere NOW Home®.

Fire-resistant, durable and quiet...
Concrete also has a range of other attributes that makes it affordable over time. For instance, concrete is fire-resistant, impervious to wilful damage and provides sound insulation. These are increasing relevant if high-density housing is considered as an option to alleviate the shortage of affordable housing.

When concrete's durability and low maintenance are also accounted for, the cost savings over time are significant.

Getting serious about affordable houses
Other issues besides construction material need to be addressed if New Zealand is serious about achieving affordable housing. These include removing local zoning restrictions to increase land supply for new residential developments and the intensification of existing residential developments, particularly hi the Auckland region. The rationalisation of local authority consent processes to reduce compliance costs and improve housing affordability would also be welcome.

The issue of housing affordability is closely related to the complex requirements of the Building Act and Burl I Code DBH's on-going review processes to streamline compliance is a step in the right direction. CCANZ would also like to see an Acceptable Solution under E2 External moisture for concrete and concrete masonry systems.

The labour costs associated with construction can impact dramatically housing affordability. Efforts to coordinate a sector-wide strategic approach to combat the current skills shortage and poor productivity need to continue.

There seems an over reliance on lightweight construction - exposed concrete elements should become part of the mix. If New Zealand is committed to facilitating the widespread availability of affordable housing any accepted definition of what it constitutes needs to include efficiencies over time.