Modeling efficient building design: a comparison of conditioned and free-running house rating approaches

Architectural Science Review, June, 2007 by Richard Aynsley

Modeling efficient building design: A comparison of conditioned and free-running house rating approaches

by Maria Kordjamshidi, Steve King, Robert Zehner & Deo Prasad, Architectural Science Review, 50(1), March 2007, 52-59.

This is an extremely important paper as it could be used to argue for better treatment of energy efficient, free-running housing in Australia under house energy rating schemes. The Australian Building Code provisions for energy efficiency and its associated home energy rating scheme have resulted in a majority of new houses being air conditioned, particularly in warmer climate regions of Australia (Miller & Ambrose, 2005). After a study of contemporary housing subdivisions in Queensland, including the numbers of air conditioned houses, Miller and Ambrose reported that in 2001 around 28% of dwellings were air conditioned. By 2004, this number had increased to 36% with the expectation of a further increase to 56% in 2005 (see Figure 1). They raised the question: "Is climatically inappropriate design a factor behind the increase in energy use for cooling in Queensland?"

The definition provided in the Kordjamshidi et al., paper for free running includes the words "does not use any mechanical equipment to maintain or improve its indoor thermal condition." This is not the case at least in north Queensland, where protestations resulted in the benefits of ceiling fans being included in assessment of indoor conditions during summer conditions.

A typical 84 [m.sup.2] 3-bedroom, brick veneer, slab-on-grade house with awning shaded windows and an insulated metal deck roof in Townsville (north Queensland, Australia) rated at 2 stars by NatHERS in the 1990s consumed 295 MJ/[m.sup.2] or 23.6 GJ per year in electrical energy representing production of 6,844 kg of C[O.sub.2]. This is more than 10 times the energy used by an attic fan and about 100 times that by ceiling fans supplemented by natural ventilation from wind (Aynsley, 1997).

A similar uninsulated no-star house would consume around 350 MJ/[m.sup.2] or 28 GJ and produce 8,120 kg of C[O.sub.2] or 19% more than the 2-star version. A 4-star version of the house with better glazing etc would consume around 200 MJ/[m.sub.2] or 16 GJ and produce 4640 kg of C[O.sub.2] or 32% less than the 2-star version. The same house with 6 ceiling fans operating at medium speed (40 watts) for 12 hours per day for 4 summer months (December through March in Australia) would consume approximately 0.2 GJ of electrical energy representing 58 kg C[O.sub.2]. If natural ventilation by wind is not utilized and fans were operated 24 hours per day, this amount would be less than double as fans in the bedrooms are likely to be used mainly in the evenings and fans in the living areas are unlikely to be operated all night.

[FIGURE 1 OMITTED]

From Table 4 in the Kordjamshidi et al, paper, observations regarding the influence of insulation in the building envelope reflect the findings of a study by the University of Melbourne (1981). For a well ventilated, free-running house with more than 30 air changes per hour, the indoor air temperature is equal to outdoor air temperature. It takes only a few seconds for air to pass through the house. The function of envelope insulation in free running houses is to limit indoor surface temperatures to less than 4K above air temperature with an upper limit of 38[degrees]C. This prevents the surfaces becoming significant sources of radiant heat gain to occupants (Koenigsberger & Lynn, 1965). This is most important with respect to ceiling insulation as people are more sensitive to radiant heat from ceilings than other room surfaces (ISO 7730, 2005). The total thermal insulation needed for roof and roof space to ensure that ceiling temperature does not exceed air temperature by more than 4K under maximum sol-air temperature conditions can be calculated using Equation 2.

The temperature differences across each element in a low-mass multilayer roof are proportional to the thermal resistance of each element. For a given rate of heat transfer, the ratio of the total resistance of a roof construction to the temperature difference across the roof, [R.sub.t]/[DELTA][T.sub.R], is equal to the ratio of the resistance of the indoor air film under the ceiling to the temperature difference across the air film, [R.sub.iaf]/[DELTA][T.sub.iaf].

[R.sub.t]/[DELTA][T.sub.R =][R.sub.iaf]/[DELTA][T.sub.iaf]. (1)

This equation can be rearranged as:

[R.sub.t] = [DETLA][T.sub.R][R.sub.iaf]/[DELTA][T.sub.iaf]. (2)

By setting [DELTA][T.sub.iaf] to 4K so that the ceiling temperature is 4K above air temperature (outdoor air temperature) in the free running house, and setting [DELTA][T.sub.R] as the peak difference between the sol-air temperature on the roof surface and indoor air temperature (outdoor air temperature in a highly ventilated free running house) one can calculate the appropriate total resistance for the roof construction.

In winterless climate locations such as north Queensland, it is important to utilize reflective air space insulation in roofs. This takes advantage of the lower resistance to heat flow up of a 100 mm reflective air space during nighttime, around 0.48 [m.sup.2]K/W, to heat flow down during daylight hours, around 1.42 [m.sup.2]K/W. This characteristic speeds house cooling after sundown (Aynsley & Su, 2005). Other types of roof insulation trap heat stored in walls and floors during the day indoors, slowing radiant cooling from the roof at night. Metal roof temperatures at night in Townsville, north Queensland, cooled by radiation to the sky have been measured at up to 8K below air temperature.