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Volume 4
ANNEX B: THERMAL INSULATION POLICY POSITION

Introduction

B.1. This short paper explains how Communities Scotland developed the framework for the guidance on thermal insulation. It sets out the background, summarises the options we considered and explains how we reached a decision on a recommended approach.

Background: The Housing Improvement Task Force

B.2. The 2006 Act adds a new element to the tolerable standard. To meet the tolerable standard a house must have "satisfactory thermal insulation".

B.3. This new requirement has its origins in the work of the Housing Improvement Task Force ( HITF). The HITF discussed three broad approaches:

1. a specific level of overall energy efficiency performance based on an established technical measure such as NHER or SAP;

2. a minimum level of thermal efficiency for specified elements based on the U-value (the rate at which heat is transmitted through the fabric); or

3. a qualified performance indicator such as "reasonably" energy efficient or "insulated to a basic standard".

B.4. The HITF considered each option and concluded that the most flexible, robust and practical approach would be option three. The final report highlighted the need for detailed guidance and the importance of constructing guidance which allowed the tolerable standard to target the very worst properties ²⁵. The new provision in Section 11 of the Housing (Scotland) Act broadly reflects the HITF recommendation.

Guidance: Options

B.5. The 2006 Act does not specify a definition of "satisfactory thermal insulation". There is no 'off the shelf' tool that defines and measures thermal insulation.

B.6. We have considered this issue in depth, with an emphasis on producing guidance that is practical to work with and which helps local authorities to target the worst-performing houses.

B.7. We identified a variety of potential methods of assessing thermal insulation and considered the pros and cons of each. The initial options we considered were:

1. the presence of single / multiple insulation measures (roof insulation and cavity wall insulation);

2. mean internal temperature;

3. condensation analysis using U-values; and

4. an integrated approach with Energy Performance Certificates.

B.8. We concluded after an initial review that options one and two had the greatest potential to provide the basis of practical guidance.

B.9. We examined these options in more detail. We worked with the Scottish House Condition Survey ( SHCS) team to analyse data from the 2004/05 Scottish House Condition Survey to produce a profile of the housing stock and to understand the potential impact of adopting various options ²⁶. We used the mean internal temperature model to examine the impact of a range of property characteristics, including roof insulation and wall insulation, on the temperature achieved.

Analysis

B.10. The SHCS team used the Mean Internal Temperature ( MIT) model to analyse data from the 2005 national survey. We provide further information on the mechanics of the mean internal temperature option towards the end of this chapter. The SHCS team ran three sets of analysis of MIT, using three benchmark temperatures. The percentage of houses that do not achieve these temperatures across all housing in Scotland is as follows:

• Temp A - 1.3 per cent of all houses
• Temp B - 5.0 per cent of all houses
• Temp C - 20.3 per cent of all houses

B.11. For each of these three temperatures, we analysed the Scottish housing stock by a number of variables. We wanted to examine the impact of the two principal insulation measures - roof insulation and additional wall insulation. We also analysed the data by wall construction type, house age and location (urban / rural), to establish if these factors had a significant impact on MIT.

B.12. The table below summarises the results of this analysis, listing the indicators that show an above-average failure to meet each of the three temperatures. Across all three temperatures the presence of roof insulation has by far the most significant impact of any of these variables. At Temp B, for example, the percentage of houses with no roof insulation that fail to meet the temperature (62%), is more than 12 times the level of failure across all houses and almost 5 times the failure rate of the next most significant variable.

MIT: Profile of houses most likely to fail to achieve temperature

source: SHCS 2004/05 data

* This analysis includes only houses which could reasonably have roof insulation. It excludes flats with non-heat loss roofs ( i.e. where the flat is directly below another flat).

Conclusion

B.13. The absence of roof insulation is a significant variable in the failure to achieve each of the three temperatures. The analysis does not show definitively that the lack of roof insulation is the cause of the failure to achieve the temperature. But the relationship with lower temperatures is markedly stronger than that of solid walls or no additional wall insulation. We have concluded that, for the purpose of the tolerable standard guidance, the presence of roof insulation is a reasonable measure of whether a house has satisfactory thermal insulation. Identifying houses with no roof insulation and targeting action will make a significant improvement to heat loss in some of the most poorly-insulated housing in Scotland.

B.14. Across Scotland, 93 000 houses (4 per cent of all housing) could reasonably have roof insulation but have none. The final paragraphs in this annex set out a summary profile of roof insulation across all housing in Scotland, using data drawn from the SHCS.

B.15. Each option we considered has strengths and weaknesses. Overall, we believe that roof insulation is the most practical measure of thermal insulation for the purpose of the tolerable standard.

B.16. Any assessment framework needs to enable assessors to identify the worst houses in a practical way. The approach set out here has the advantages of being:

• supported by an evidence-base;
• understandable for assessors and homeowners;
• straightforward to measure; and
• easy to identify the remedial action owners need to take.

B.17. We recognise that this indicator will not capture all houses with poor thermal insulation, for example cases where wall construction is the main cause of heat loss. We also know that the relative importance of roof insulation as a measure of thermal performance varies across house type. A top floor tenemental flat is likely to lose a higher proportion of its heat through its roof than a detached house. As with all chapters of our guidance on the tolerable standard, assessors should use this framework as a guide alongside their professional judgement, taking account of individual circumstances, rather than as a prescriptive framework. The legal test is 'satisfactory', and in each case this is the local authority's decision.

ANNEX B: FURTHER INFORMATION

Mean Internal Temperature

B.18. This approach uses data modelling to produce an estimate of the mean temperature inside a house over a 24-hour period. The temperature achieved is a measure of the thermal performance of the house.

B.19. The assessment takes account of all the energy characteristics of an individual property (such as age, wall and roof construction, floor and wall area, windows, doors and ventilation).

B.20. The approach uses the BRE's Domestic Energy Model ( BREDEM) that underpins both the SAP and NHER scales. It uses a set of standard assumptions covering factors such as the outside temperature and the number of hours the house is heated. It also uses a standard heating regime (based on full gas central heating with combi boiler). One effect of this approach is to remove the variable of heating type from the assessment.

B.21. The temperature achieved over a 24-hour period is a measure only of the thermal performance of the fabric of the house. It is not an indicator of the temperature that any room in a house should achieve when it is occupied.

B.22. The temperatures we used in the analysis are as follows:

• Temp A: 14°C
• Temp B: 14.5°C
• Temp C: 15°C

ANNEX B: FURTHER INFORMATION

Roof Insulation Profile of Housing in Scotland

B.23. We analysed data from the 2004/05 Scottish House Condition Survey to produce a profile of the roof insulation characteristics of housing in Scotland ²⁷:

• 568 000 (25 per cent) of Scotland's housing has no roof insulation
• 472 000 of these are flats with non-heat-loss roofs ( i.e. non-top floor flats)
• 96 000 with heat-loss roofs have no roof insulation
• 3 000 of these are 'hard to treat'

B.24. Therefore 93 000 houses (4 per cent) of houses could reasonably have roof insulation but have none.

B.25. Hard to treat: 32 000 houses (1 per cent) of Scotland's housing are defined by SHCS as 'hard to treat' for roof insulation ( i.e. flat roofs or mansard roofs with rooms in roof space). But more than 90 per cent of these do in fact have some form of roof insulation.

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