Looking further than the initial dollars

This Issue This is a part of the Life cycle costs feature

By - , Build 101

With home ownership in New Zealand changing on a 7–9 year cycle, long-term considerations may seem irrelevant to some homeowners. But taking a long-term view could have positive benefits for our health, the environment and our pockets.

Figure 1: Life cycle energy comparison for common construction types with all day heating (excludes embodied and operating energy for appliances).
Figure 2: Life cycle energy comparison for common construction types (includes embodied and operating energy for appliances).

Construction decisions are generally driven by initial cost first and resource use second. But houses last a long time and need to cater for changing requirements, like young children or elderly parents. These changes and ongoing maintenance mean the cost and resource use continue throughout a building’s life.

Increasing public interest in global warming, sustainability and reducing the detrimental environmental impacts of human activities requires that construction actions consider long-term performance and implications. ‘Lifetime consideration’ could lead to a New Zealand housing stock that is healthier, more energy and resource efficient, and cheaper to maintain.

Construction options

This concept of lifetime consideration can be explained by looking at common construction options for a simple house (the Building Industry Advisory Council (BIAC) house design with a floor area of 100 m2) over a long period. Building Code requirements for component durability are based on ease of access after installation and are therefore not a true representation of useful life. For the best use of resources embodied in the house, it needs to be maintained for the optimum period possible – about 80–100 years in New Zealand.

Table 1 shows the three common construction types considered.

User behaviour important

Comparison of life cycle energy use (the energy used by the house over its lifetime, for construction, maintenance and to maintain 18°C internal temperature during the day) for these three construction types in Auckland is shown in Figure 1.

Light and heavy construction types perform similarly, although life cycle embodied energy is lower with heavy construction due to its lower maintenance requirement. Superinsulated construction has slightly higher embodied energy, due to its additional construction materials, and provides notable savings.

Although operating requirements of appliances, water heating, cooking and lighting are dependent on user behaviour, their inclusion in the life cycle energy comparisons shows their importance in lifetime performance (see Figure 2).

Greenhouse gas emissions

Floor, walls and roof constitute a major fraction of the mass of a building and could be expected to contribute extensively to life cycle energy and greenhouse gas emissions.

In the light and superinsulated construction types, however, floor construction reduced the greenhouse gas emissions by 13% and 11%, respectively. Superinsulated walls also recorded a 3% reduction in emissions. This is due to carbon locked in timber framing and other timber-based products used in the construction of these elements. Heavy roofs also reduced greenhouse gas emissions by 5%. Light and superinsulated roofs, however, contribute 17% and 20% respectively to the life cycle greenhouse gas.

The figures differ so much because of the emissions resulting from the use of long-run steel sheets as a roof cladding on the light and superinsulated houses. The heavy construction house has a roof of concrete tiles, which have much lower emissions, even allowing for the emissions associated with cement manufacture. The high value for the floor of the heavy construction is because of the cement content in the concrete slab.

Figure 1: Life cycle energy comparison for common construction types with all day heating (excludes embodied and operating energy for appliances).

During the useful life of the building, finishes are replaced many times and contribute 34%, 19% and 33% of the life cycle building greenhouse gas emissions for light, heavy and superinsulated construction types, respectively. The replacement cycle is therefore of utmost importance.

Table 1: The three common construction options considered and their R-values.  
  Light construction   Heavy construction   Superinsulated  
Flooring particleboard floor on raised framing with double-sided foil R1.5 concrete floor slab  with perimeter insulation R1.5 particleboard floor on raised framing with 200 mm glassfibre insulation on a plywood layer R4.4
Walls 94 mm glassfibre insulation within softwood timber wall frame R1.9 94 mm glassfibre insulation within softwood timber wall frame R1.8 200 mm glassfibre insulation within softwood timber wall frame R4.4
  fibre cement external cladding and plasterboard internal lining   external brick veneer and plasterboard internal lining   fibre cement external cladding and plasterboard internal lining  
Roof and ceiling pitched softwood truss roof with corrugated steel roofing R1.95 pitched softwood truss roof with concrete tiles R1.8 pitched softwood truss roof with corrugated steel roofing R4.4
  flat plasterboard ceiling with 100 mm thick glassfibre insulation   flat plasterboard ceiling with 100 mm thick glassfibre insulation   flat plasterboard ceiling with 200 mm thick glassfibre insulation  
Windows single-glazed aluminum windows R0.18 single-glazed aluminum windows R0.18 double-glazed aluminum windows R0.33
Table 2: Life cycle greenhouse gas emission factors for a Building Industry Advisory Council standard house. (Some values are negative because of carbon locked into materials like timber.)  
Building element     CO2 emission factors (kg/m2)      
  Light construction   Heavy construction   Superinsulated construction  
Foundation 2 1% 6 2% 2 1%
Floor –32 –13% 61 23% –28 –11%
Walls 7 3% 13 5% –7 –3%
Roof 42 17% –13 –5% 51 20%
Joinery 20 8% 20 8% 29 11%
Electrical work 14 6% 14 5% 14 5%
Plumbing 111 44% 111 42% 111 43%
Finishes 85 34% 51 19% 85 33%
Total 251 100% 262 100% 257 100%
Table 3: Life cycle cost comparison for Building Industry Advisory Council standard house.  
Category       Life cycle cost (NZ$/m2)  
  Year 0 Year 25 Year 50 Year 75 Year 100
Light construction          
Construction cost 973 1,100 1,207 1,224 1,228
Space heating energy cost 0 57 74 79 80
Total 973 1,157 1,281 1,303 1,308
Heavy construction          
Construction cost 1,177 1,292 1,374 1,395 1,397
Space heating energy cost 0 54 70 75 76
Total 1,177 1,346 1,444 1,470 1,473
Superinsulated construction          
Construction cost 1,148 1,301 1,381 1,407 1,410
Space heating energy cost 0 30 39 41 42
Total 1,148 1,331 1,420 1,448 1,452

A comparison of life cycle greenhouse gas emissions for the three construction options is shown in Table 2.

Figure 2: Life cycle energy comparison for common construction types (includes embodied and operating energy for appliances).

Life cycle costs

Life cycle costs calculated with a 5% discount rate for the above construction types are shown in Table 3. These costs involve the building works only. Activities such as preliminaries and site works that would be similar for all constructions types are omitted. No GST is added to the initial construction cost, but 12.5% GST has been added to replacement work. Space heating energy use only is considered in terms of operating requirements. Line charges, which are applicable to all domestic electricity uses, have been omitted.

Both heavy and superinsulated constructions perform similarly. The superinsulated construction is approximately 11% more expensive than the light construction in life cycle terms, with about 18% increase in initial cost; the heavy construction type is 13% more expensive in life cycle terms and 21% more expensive at the initial stage.

But the use of discounted cash flows, as in this case, generally tend to focus on the initial investment rather than the continued operating cost savings which may result from the use of increased insulation.

Comparative analysis useful

Such modelling should not be used to predict the life cycle performance of a particular design, as the predicted performance will seldom be matched by the actual performance. This is because of the influence of the building’s users, in the same way that the habits of different drivers affect the petrol consumption of apparently similar cars.

This type of analysis can be useful during the design stage to compare alternative designs, and assess possible improvements to a design. Naturally the earlier life cycle techniques are introduced, the easier it is to inform the design process and the greater the benefit to the final design.

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Articles are correct at the time of publication but may have since become outdated.

Figure 1: Life cycle energy comparison for common construction types with all day heating (excludes embodied and operating energy for appliances).
Figure 2: Life cycle energy comparison for common construction types (includes embodied and operating energy for appliances).

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