“Free heat” from roof space insufficient to heat typical wooden house



New University of Otago research suggests that the amount of heat generated in the roof cavity of a typical pre-1940s wooden house is insufficient to heat homes when people would most need the warmth; in winter, on cloudy days and at night.

In peer-reviewed research published on-line in the international journal Energy and Buildings, University of Otago Energy Studies graduate Warren Fitzgerald found that only small potential heating and cooling benefits were possible at certain times from pumping air from the roof space into the living areas of some New Zealand houses.

However, those potential benefits were not large enough to significantly alter the average indoor air temperature.

His study used a computer model of a typical older-style New Zealand weather board house with a pitched iron roof, and ceiling insulation, that was located in Dunedin. The experiments transposed the computer-model of the Dunedin house to three other main centres – Christchurch, Wellington and Auckland – then added weather data from the NIWA website for these centres.

Running the experiment over continuous ten-day periods in January, April, July and October to represent the four seasons – he was able to estimate the useful space heating that could be extracted from the roof space in each city during spring, summer, autumn and winter.

The highest average heating potential in the house was only 0.52 kW, which is approximately equivalent to the heat output from five 100-watt light bulbs. The highest average cooling potential of pumping cool air down from the roof cavity was around 1 kW.

“The majority of the time, it was calculated that pumping air from the roof space into the house would provide little heating or cooling benefit. In fact, this would often actually act to push the internal temperature in the house further away from the desired level rather than closer to it,” Mr Fitzgerald, whose research was part-funded by an Energy Efficiency and Conservation Authority (EECA) bursary, says.

Data collection and computer modelling were carried out between 2008 and 2010. The model assessed heating and cooling potential with an assumed desired internal house temperature of 18 degrees.

Project leader and University of Otago Physics Research Fellow Dr Inga Smith says the study aimed to assess only the heating and cooling potential for houses using positive pressure ventilation systems which draw air from the roof cavity. This is the most common type of mechanical whole-house ventilation system currently on the New Zealand market. The research did not assess the ability of such systems to control moisture or humidity in houses.

“Based on the measurements and modelling work detailed in this report, we therefore recommend that existing positive pressure mechanical ventilation systems should not be promoted and marketed as heating and/or cooling systems,” Dr Smith says.

“In essence, this research shows that there is a very small amount of free heat available, but this is often not available when it is needed the most during winter.”

Dr Smith stresses that the implications of using roof space air as a means of household humidity control have not been investigated in this research.

In addition to recommending future research on humidity control, she also recommends future research into potential concerns with interchanging roof and living space air, such as fire and smoke hazards.

Broken down centre by centre, the study shows that in best-case scenarios, the useful space heating that could be extracted from the roof cavity of an Auckland house of the type used in the study would be 310 watts on average in July; the useful space heating that could be extracted from the roof cavity of an equivalent Wellington house would be 200 watts on average in July and in October; the useful space heating that could be extracted from the roof cavity of an equivalent Christchurch house would be 340 watts in July; and the useful space heating that could be extracted from the roof cavity of the same Dunedin house would be 520 watts in October.

The project team also included University of Otago post-graduate students Muthasim Fahmy, undergraduate students Michael Carruthers and Bonar Carson, as well as Dr Zhifa Sun, a Senior Lecturer in the Department of Physics energy studies programme. It also included Mark Bassett, a Principal Scientist at BRANZ.