The potential benefits of retrofitting thermal mass into New Zealand houses

Abstract

This article reports on a study undertaken to investigate the effectiveness of thermal mass to moderate indoor air temperatures and to reduce energy demand when it is introduced into existing timber frame residential buildings. The research investigated characteristics of the existing New Zealand housing stock and how energy is used in those dwellings. Six different house typologies were defined and computer models created. Different combinations of thermal mass retrofits were assigned to each house type with variations also involving heat pumps. In all, the investigation produced 72 different scenarios, each of which was modelled for thermal performance using IES VE software. General trends indicate that thermal mass retrofits are more effective in older houses, with more recent dwellings generating little improvement on average annual indoor temperatures. This is most likely a consequence of the higher insulation levels required in the newer houses as well as lower infiltration rates. In a sense, these newer houses start from a higher performance level and the introduced thermal mass provides relatively smaller improvements. The research concludes that, while thermal mass is an integral part of passive solar design, insulation is much more important, particularly in houses where smaller areas of glazing limit the extent to which the sun can charge the mass. A useful outcome of the project is a set of cases that can help inform residential owners about different passive retrofit options and the associated benefits to costs.

Keywords:

• thermal mass
• retrofit solutions
• solar energy
• New Zealand houses
• thermal simulation

Acknowledgements

The author acknowledges the funding provided by the Cement and Concrete Association of New Zealand and Victoria University through the Victoria Summer Scholars Scheme. The software used to analyse the different cases was provided by IES and Rawlinson's helped analyse the construction cost of the retrofit options.