Energy payback of conservatories: offsetting the energy embodied in the materials with solar gains

Conservatories are one of the most popular of all home improvements, with many thousands being built each year, mostly as retrofits to existing houses. As potential collectors of solar energy, they are often specified in low energy houses however, before the solar energy can be harvested, capital, or ‘embodied’ energy must be expended to bring the conservatory into existence. This paper reports on the initial investigation of energy balance on how much energy is embodied in a conservatory and how much useful solar gain is collectable. It examines the four main construction frame materials: PVC-U, aluminium, and hard and soft-woods. It considers the embodied energy (EE) of not only these four main frame options, but also the many other building components and it establishes the overall EE for a variety of options. The paper examines the potential for solar gain, from basic irradiation data and, by use of computer simulation models, considers the usefulness of this potential solar gain in reducing the overall energy use of the main building. The results show that glass is the major component of a conservatory, which only contributes less than 20% of the EE; whereas the floor and dwarf-walls make up at least 48% of the total EE. Wood, particularly softwood, is clearly the best option from an EE perspective. Solar gains vary significantly with design and studies using the best options of low EE and high solar gains showed a ‘payback’ time of the EE of just less than 10 years. However, fitting low-transmission glass designed to reduce unwanted summer heat gains has the penalty of reducing beneficial winter heat gains. With the accumulation of relatively high EE of the most commonly used PVC-U frame material, the high EE of the walls and floor and the fitting of low-transmission glass, the payback extends significantly beyond 10 years. A variety of alternative options, in design, construction methods, and materials (to maximise solar gain and minimise EE) are suggested for further study, to ensure that a conservatory can be specified to provide a significant long-term energy gain as well as an attractive amenity.