Sense and sensibility
THE ART (AND SCIENCE) OF SAVING ENERGY IN HISTORIC BUILDINGS
THE CAREFUL REFURBISHMENT OF AN HISTORIC BUILDING CAN SAVE UP TO 90% OF HEATING COSTS - BUT THIS REQUIRES A PRAGMATIC AND BALANCED APPROACH WHICH INTEGRATES FABRIC, BUILDING SERVICES AND HUMAN BEHAVIOUR
There are many reasons for wanting to improve the energy performance of an old building. These might include the desire to reduce carbon emissions, to save money, to improve comfort and health, to reduce worry about fuel bills, or to improve the value of a property. Some people are also obliged to carry out improvements because of legislation or building regulations.
However, there is increasing evidence that many recent retrofitting projects have not led to the expected reductions in energy use. More worryingly, they may have actually harmed the building fabric and even the health of building occupants.
By following predetermined energy targets or by using inaccurate base data and simplistic assumptions, owners and professionals may be tempted to adopt disproportionate energy saving interventions that may not only be costly and unnecessary but also overly invasive. Well-intentioned improvements – like adding lots of insulation or sealing up of the building to eliminate drafts can impact on the way a traditional building behaves by increasing the amount of moisture in the interior and structure. This is not only detrimental to the building and its occupants but might eventually lead to the unnecessary loss of important historic fabric – or to put it another way, the architectural character we admired so much in the first place.
A pragmatic approach, based on a clear picture of how the whole building performs, should be developed before the work starts. In reality, historic buildings can be much more energy efficient then many people think. Recent monitoring of a number of buildings by the Society for the Protection of Ancient Buildings (SPAB) has shown that the actual measured thermal performance of a traditionally constructed wall could be up to three times better than the theoretical assumption. To help build up a real-life picture, on-site technology can be used to obtain accurate detailed technical information. This might include data logging (to establish moisture content in the structure, humidity, and temperature), thermal imaging, dampness testing, fan pressurisation or smoke tests to identify the precise location of draughts in difficult-to-see areas. As well as assembling the physical data, it is also important to understand the building’s history, it’s cultural significance, current, and future use.
A thorough understanding of how a building is working (or not!) can be used to inform the most appropriate strategy for improving a building’s environmental performance. There are three interrelated things to think about- the approach to ventilation, the amount and type of insulation and the heating strategy. Sadly, there is no “one size fits all” solution. We generally look at three options, which are graded in terms of increasing complexity, intrusiveness, capital cost and the approach to moisture control. Each takes into account overall energy consumption, the practicality, cost and complexity of the work, varying loan rates, repayment periods and the potential return on capital investment.
Light Green- is the most conservative approach and is the least expensive. It can substantially improve performance by reducing excessive air infiltration and ensuring that the heating and ventilation systems are efficient. A good idea might be to install “through the wall” ventilation with heat recovery in kitchens and bathrooms, avoiding the need for obtrusive ductwork. Windows and internal shutters are repaired to ensure that air gaps are minimised and simple measures like adding curtains to improve thermal comfort. Without the disruption and cost of installing insulation, the annual energy use could be reduced by over 40%.
Mid Green- this involves carrying out a reasonable level of internal insulation using permeable materials to ensure that moisture levels are managed and increasing draught-proofing measures such as secondary glazing. If space can be found for ductwork, full mechanical extract ventilation (MEV) with the possible addition of demand control ventilation (DCV) can help reduce heat loss. As the building is made increasingly airtight, whole-house mechanical ventilation or a passive stack system could be considered but these require space for extensive ductwork.
Deep Green – this uses a high level of insulation to minimise energy demand but has the most impact on the fabric and character of the building. In order to save internal floor space, it often precludes the use of natural “breathing” insulation materials and requires a full mechanical ventilation system (with the option of heat recovery) to be installed. This approach is generally only recommended if the building is not historically significant, a major overhaul is being undertaken and practically all drafts can be eliminated. Annual energy use could be reduced by up to 90%.