Demand Reduction
The first step in making a system sustainable is reducing how much it consumes. In the case of Eigg, we need to reduce the energy used for space heating and the best way to do this is improve the thermal insulation in domestic buildings. This is a process which deals with reshaping of domestic energy demand.
The thermal energy performance of domestic buildings is determined to a large degree by the insulation of the building envelope (1). For this reason building envelope upgrades are proposed. These upgrades include wood fibre board internal wall insulation, sheep wool or hemp loft insulation and double glazing.
Eigg features a range of insulation levels descriptive of the UK as a whole. The UK features an aged building stock with 40% of all houses built before 1944 (2). Nearly all of these buildings can be upgraded to improve energy efficiency. The issue, however, comes in the cost of these upgrades. The balance between capital cost, payback period and life-cycle environmental gains is key.
Previous research has been carried out to classify the housing on Eigg based on the insulation level, heating systems, window type and other factors. The research was done through interviews with the islanders and sight visits by PhD student Russell Pepper.
The homes are grouped into 7 categories, 1-6 for houses and category 7 for caravans. As part of previous research the heating demand for each category of houses was estimated using “Passivhouse Planning Package” (PHPP). In order to estimates costs for the upgrades the Home Energy Model (HEM) software was used.
HEM is a high level software tool that can be used to estimate the costs of retrofitting a housing stock. It also provides carbon rating for a building. HEM gives upgrade costs for 5 levels of insulation, as well as other upgrade options. The insulation levels are as following.
The costs are taken from the Energy Savings Trust and Highland Council. Based on the results of the HEM analysis it was concluded that the most cost effective solution was to upgrade the most poorly insulated homes to "Standard". This is shown in the following table.
Not all homes are upgraded to passivhouse standard. This was not the most cost effective option. Upgrading to higher standards cost disproportionately more, compared to the decrease in heat demand achieved by the upgrade.
The first step done in HEM by taking the cost of upgrading the 6 buildings in category 1 from Poor (pre1983) up to Medium (2003-2007) for a cost of £11,262 per house. (HEM gives the cost directly, and a detached building was chosen)
It is worth nothing that the “renovation category period” in HEM and in PHPP is not the same. As such the closest available year interval was used to make an estimate.
The 10 buildings in category 2 were then upgraded from Standard (1983-2002) up to Medium (2003-2007) for a cost of £8,912.
Finally, the 5 houses in category 3 were upgraded from Standard (1983-2002) up to Medium (2003-2007) for a cost of £8,912 in HEM. This is the same as category 2, but this was done because the year categories in HEM covers a broader range than passive house planning package.
The total estimated costs for all 21 building upgrades was found in HEM and displayed are displayed in the table opposite.
Upgrading the homes in the lowest three categories to category 4.
Cost of upgrading homes in the lowest three categories.
These upgrades were found to ultimately reduce Eigg's space heating demand by 26% when taking into account all the homes. The effect of this over a year is illustrated in the graph below, and included in the cost analysis.
There are two key principles to consider when retrofitting historic buildings: water vapour permeability and ventilation. Any renovation must ensure the existing performance of the building is preserved.
The control of condensation in buildings is governed by British Standard 5250 (3).
Here we are considering buildings typically built before 1919. These buildings are well built with load bearing masonry walls with pitched slate covered roofs, single glazed windows with timber frames. These homes are termed ‘hard to treat’.
Eiggs housing stock has been summarised below along with the results of upgrade modelling.
Details of the construction of the ‘hard to treat’ houses to be renovated can be seen below.
Two Case studies are supplied from Historic Scotland which demonstrate similar techniques being used on in-situ retrofits. Summaries of these are available by clicking the tabs below.
An important aspect to consider with retrofitting is the functional and aesthetic value of homes and buildings which may be lost due to retrofitting. Many retrofitting options can detract from the aesthetic and cultural heritage of buildings. This is discussed at length by Sunikka-Blank (6), concluding with the recommendation that incremental and less intrusive retrofitting be considered to avoid backlash of public opinion. Fouseki suggests that the important issues when retrofitting is considered are ‘what does this building mean for those who use it?’ and ‘what interventions can be implemented that could co-exist harmoniously with those meanings?’ (7).
© University of Strathclyde | TEC Eigg | Sustainable Engineering 2016