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.


Modelling upgrades with HEM tool


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.


      • Poor (pre1983)
      • Standard (1983-2002)
      • Medium (2003-2007)
      • Good (2007 – now)
      • Super (passivhouse)


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.

Methodology in HEM




The challenges of upgrading historic buildings


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.


  • Water vapour permeability: Often referred to as a breathable construction. Historic buildings are made from natural materials which absorb and release water, referred to as ‘hygroscopic’. If a building becomes impermeable to moisture condensation, damp and mould will follow.


  • Ventilation: The draught typical in historic buildings forms the natural ventilation system. This despite having negative effects on the buildings thermal insulation reduces the moisture levels. Inappropriate ventilation will also cause damp and mould growth. The damp in the air is caused by condensation. Warm humid air leaving a humid or inhabited space will condense releasing moisture when it meets a cold material.


The control of condensation in buildings is governed by British Standard 5250 (3).


Eigg's housing stock


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.


case studies


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.


  • Case Study 1 - Kildonan, South Uist (4)

    This case study conducted by HS concerns a building of similar dimensions, construction and in very similar climate to the houses on Eigg. The driving rain and wind are significant factors making the vapour permeability of the upgrades critically important. The upgrade project is undertaken to demonstrate how upgrades can be carried out on historic buildings. In this case the renovation involved improving the permeability of the cement and masonry. Wall upgrades - For the walls 100mm of wood fibreboard insulation is added between timber strapping fixed onto the masonry. The fibreboard is then painted with a clay finish (without the vapour barrier) to ensure entire vapour permeability. On the north facing wall calcium silicate board insulation is applied directly to the masonry using a permeable adhesive. Ceiling insulation improvements- The coom ceiling is insulated with 50mm thick wood fibreboard. A 10mm void is left behind the board to allow moisture circulation between insulation and the sarking board . Window Improvements- Two window improvements were implemented. Timber double glazed sash and case windows are added. Secondary glazing – polycarbonate which is removable. Secondary glazing is fitted due to the exposed location of the building. The secondary glazing is a polycarbonate sheet fixed to the inside of the window using magnetic strips which allow removal by the occupant. Improvements to floor- The existing floor is a thin layer of concrete laid directly onto the beach sand base. An Aerogel backed board 30mm thick is applied as the floor is not cracked. The thermal performance improvements are summarised below
  • Case Study 2 - Leighton Library (5)

    This case study is used to demonstrate the options for loft insulation and the need for ventilation. In this case study loft insulation is used to reduce thermal loses. Insulation at ceiling level creates a cold roof space. This results in lower temperatures which increases the risk of condensation without sufficient ventilation. This means that the natural ventilation of the slates is not appropriate. To combat this large louvered vents are added to the roof space. The building faces limitations to the amount of internal space thus making internal wall insulation not an option. Loft Insulation- Wood fiber board is used due to its hygroscopic properties and preference is given to natural materials derived from renewable sources. The roof space is insulated using 200mm of this material. Usually with slate roofs there is sufficient ventilation to remove moisture, however in this case a previous refurbishment with bituminous felt has meant that this is no longer sufficient. To compensate large louvered vents are fitted. Post intervention humidity readings for March 2012 showed a relative humidity of 57% at an average tempt of 15 °C. The external humidity is 69% at an average temp of 1°C.

Reality Check


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