It is impossible to achieve an indoor environment that can be categorised as comfortable by international standards (e.g. ASHRAE 55 or ISO 7730) with an indoor air temperature of only 18°C.
Despite the myth, there is no indication that Kiwis are more tolerant to colder temperatures than people elsewhere in the world, which should not come as a surprise, as many of us were born overseas, or grew up in far away places. In fact, several NZ studies suggest that if Kiwis are capable, technically and financially, to heat their homes to the degree Americans and Europeans are accustomed to, they will (Fyfe, 2005; Howden-Chapman et al, 2009).
Comfort is often viewed as “nice to have” but not essential. There are even those who argue that leaving one’s thermal comfort zone is beneficial. But: there is little support for this stance in science.
Poor thermal comfort has implications for the well-being of occupants, as it adds stresses to the body. Temperatures that are either beyond or above a tolerable spectrum play a particular role, as we all need to maintain a very narrow range of body core temperature to survive.
A statistical analysis in 1985 examined seasonal mortality rates in Britain, and concluded that about 80% of variance in winter deaths can be explained by changes in temperature. For every degree change in the average winter temperature the number of winter deaths is estimated to vary by about 8,000 (Alderson, 1985). Based on an analysis of excess winter mortality rates – the extra number of deaths occurring during the winter season – and indoor climate of various countries, Boardman (1991) calculated a decline of the excess winter mortality rate of 1% per indoor temperature increase of 1°C over a range of 14-21°C. A third of excess winter deaths were attributed to respiratory disease, and over half to cardiovascular disease (Curwen, 1991), in line with changing risk factors for cardiovascular conditions, like arterial blood pressure or clotting factors, which vary with temperature.
New Zealand has one of the highest excess winter mortality rates in the world (Davie et al., 2007), and notoriously low indoor temperatures in winter.
While low temperatures are widely accepted as a key factor for seasonal variation in deaths, there is some dispute over whether cold internal or external temperatures dominate in their health effects. Clearly, there is at least a strong correlation between low indoor temperatures and excess winter mortality in European countries, whereas a correlation to outdoor temperatures could not be found (Boardman, 1986; McKee, 1989). Wilkinson and Armstrong (2001) found that people in poorly heated homes were significantly more vulnerable to winter death than those living in well-heated homes. Healey (2003) compared excess winter mortality rates in Europe, and discovered that countries with the mildest winter temperatures of the sample, Portugal, Spain, Ireland and England, exhibited the highest rates of excess winter mortality. This was explained by the lack of thermal efficiency of the housing stock in these countries. It is a fair assumption that vulnerable parts of the population will spend more time indoors and thus be predominantly affected by indoor temperatures. Cold indoor environments will moreover invoke increased relative humidity, and decreased air change rates, which in turn are likely to impact negatively on the well-being of occupants. In fact, Healey (2003) recognised a significant relation between the overall level of relative humidity and excess winter mortality.
A WHO report concluded 18°C to be the minimum air temperature for healthy adults, yet saw 20°C as the minimum air temperature for sick, handicapped, pre-schoolers and elderly persons. In addition, it diagnosed air temperatures below 16°C with relative humidities above 65% as introducing additional health hazards to the indoor environment (WHO Regional Office for Europe, 1987). Considering that most households will have members who are not healthy adults in their prime, and most certainly have members who are not comfortable at 18°C, calling for 18°C as the indoor air temperature we should be aiming for, introduces unnecessary suffering. 20°C as a minimum needs to be achieved in all habitable rooms of a house. Passive House energy balances are calculated with 20°C as the minimum temperature.
- Alderson, M. R. (1985). Season and mortality. Health Trends, 17, 87–96
- Boardman, B. (1986). Seasonal mortality and cold homes. In Unhealthy housing: a diagnosis (pp. 1–21). Coventry, England: Institution of Environmental Health Officers and Legal Research institute, University of Warwick.
- Boardman, B. (1991). Fuel poverty : from cold homes to affordable warmth. London; New York: Belhaven Press.
- Curwen, M. (1991). Excess winter mortality: a British phenomenon? Health Trends, 22, 169–175.
- Davie, G. S., Baker, M. G., Hales, S., & Carlin, J. B. (2007). Trends and determinants of excess winter mortality in New Zealand: 1980 to 2000. BMC Public Health, 7(1), 263–272.
- Fyfe, C. (2005). Heat pump survey – Report 2: Winter 2004. Christchurch: Community Energy Action Charitable Trust.
- Healy, J. D. (2003). Excess winter mortality in Europe: a cross country analysis identifying key risk factors. Journal of Epidemiology & Community Health, 57(10), 784–789.
- Howden-Chapman, P., Viggers, H., Chapman, R., O’Dea, D., Free, S., & O’Sullivan, K. (2009). Warm homes: Drivers of the demand for heating in the residential sector in New Zealand. Energy Policy, 37(9), 3387–3399.
- McKee, C. M. (1989). Deaths in winter: Can Britain learn from Europe? European Journal of Epidemiology, 5(2), 178–182.
- Wilkinson, D. P., & Armstrong, B. (2001). Cold Comfort: The Social and Environmental Determinants of Excess Winter Death in England, 1986-96. Policy Press.
Photo and Graphs used with permission, (c) eHaus