Ever decreasing circles

Recent increases in the insulation requirements called for in Part L of the Building Regulations are unjustified according to research at the University of Glamorgan.

A study into the optimum level of thermal insulation for an extension to a domestic home at the university has concluded that further insulation increases are not cost effective and could mean that the government has to rethink the way compliance with Part L1 of the Building Regulations is achieved.

The scenario modelled at the university involved the addition of a two-storey extension to a detached three bedroom dwelling situated in Cardiff, South Wales. The main house was assumed to have been constructed circa 1960 and was occupied by two adults and two children.

Environmental Design Solution’s dynamic thermal modelling software TAS was used to undertake a theoretical analysis of the carbon dioxide emissions and energy cost savings resulting from incremental changes to the fabric U-values for the dwelling’s new extension in response to regulation changes.

A control model was set up for the complete dwelling without any insulation. This created a benchmark to allow comparisons between this model and successive upgrades in insulation levels to the extension. Each scenario was modelled for boiler efficiencies of 95% (SEDBUK Category A) and 75% (SEDBUK Category E).

The building’s construction was based on a combination of historical regulations and the robust details recommended in ADL1. A worst case external temperature scenario was used for the 24-hour simulations.

Table 1 outlines how the thickness of building insulation has increased as a result of UK legislation as well as current government best practice for the entire house over the last 30 years.

Simulation results

Figure 1 compares the construction of the uninsulated extension with the increasingly insulated extension constructed to comply with the different amendments to the Building Regulations.

The figure illustrates the negligible savings in energy cost and CO₂ emissions gained by insulating the extension and typifies the diminishing returns gained from increasing insulation levels. More than half the total savings of current levels are generated by the introduction of 1976 standards; these typically consisted of 15 mm of expanded polystyrene cavity wall insulation; 55 mm of glass fibre wool in the roof space and zero insulation in the solid floor. The findings show that benefits resulting from further increases in insulation are minimal.

The greatest savings in energy consumption and CO₂ appear to have been made by 1991 and any further improvements are small by comparison. In particular, the introduction of insulation in the floor for the 1995 simulation results in a minor percentage saving of CO₂ (equivalent to an 0·08 kg reduction of CO₂₎ regardless of boiler efficiency.

It is also interesting to note that a requirement of U-value of 0·6 W/m^2K was introduced for solid floors in 1985. However, the TAS database indicates that no insulation is needed to satisfy this requirement.

Future changes

The research for this study was undertaken prior to the latest revision to Part L of the Building Regulations in April 2002. However, simulations were carried out to consider the effects of the then possible changes to ADL1. These simulations contrasted from those previously in that they looked at incremental changes to individual elements, such as the roof, floor and walls as opposed to the cumulative changes for the entire extension of previous standards. The control model used for these comparisons is the current ADL1 standard.

Solid floors were the first area modelled. Figure 1 indicates the level of CO₂ savings generated to date. Figure 2 shows that to reduce the CO₂ by a further 0·076 kg from 2002 levels (at 75% efficiency) requires a fourfold (55 mm-206 mm) increase in insulation thickness equating to a saving of £0.01 in energy costs. At this rate of payback, bearing in mind that this is for the coldest day of the year, any requirement for insulation to solid floors is economically and environmentally unsustainable – even for the current ADL1 requirement which equates to 55 mm of polyurethane board.

Next the research looked at cold roofs. The simulation results in figure 1 show there is no justification for the most recent increase in insulation thickness to 239 mm as the improvements illustrated is partially due to changes to cavity wall insulation introduced at the same time. The argument that optimum insulation levels in roofs have been surpassed is reinforced by figure 3 which shows the change of thickness required to produce an additional daily saving of 0·109 kg of CO₂ at 75% efficiency is an increase of 248 mm. This equates to an energy saving of just £0.01, and shows that the economic optimum has been exceeded.

The final step was to look at cavity walls. Figure 4 gives the impression that worthwhile improvements can be made by increasing insulation levels further with the maximum simulated diurnal saving in emissions at 75% efficiency equating to 0·9 kg. However, the reason for this digression from the previous trend is attributable not just to the increase in insulation, but to the improved thermal performance and thickness of the internal blockwork.

This digression brings further complexity to the assessment of optimum insulation levels, since increasing the thickness of the blockwork will undoubtedly have both economic and environmental consequences due to the high embodied energy in some insulation products and the embodied energy associated with excavating wider foundations for increased cavity widths. Furthermore, technical limitations of cavity widths currently make this level of thermal performance difficult to achieve economically.

Finally, as the optimum insulation thickness has been surpassed, the increased U-value penalties imposed for the use of electric heating are therefore ineffective and unnecessary if the aim is to provide some compensatory measure for the increased carbon intensity of the energy source.

The effect of boiler efficiency

Figure 5 outlines just how important boiler efficiency is. It puts into perspective the comparison between increasing the levels of insulation as opposed to increasing boiler efficiency.

The figure shows that by upgrading the insulation in the whole building from zero to the ADL1 standards is only marginally better than increasing the boiler efficiency to 95%. The difference between the two is only 0·332 kg of CO₂ emissions and £0.02 in money on the worst case day. This indicates an attractive and more easily achieved alternative to the elemental changes recently made in ADL1.

Implications of the research

This study shows that further increases in insulation requirements are unjustified. Indeed, what is required is a complete rethink of the way in which compliance for buildings and extensions is achieved. It is not suggested that zero insulation is required but that workable thicknesses, which do not cause technical or construction complications can be used. The research also shows that as an alternative to increasing insulation thicknesses a ‘trade off’ against more efficient heating systems should be allowed, since these produce energy savings that dwarf those achieved by increasing insulation thickness.

It is clear from this research that any future requirement for increased levels of insulation would fail, which calls into question the government’s proposal, published in July 2004, to further amend Part L to include energy efficiency measures on existing properties providing they have a payback period of seven years.

In addition, current regulations ‘penalise’ the inclusion of electric heating by requiring greater a further 11% increase in insulation levels to achieve the SAP rating target for the forthcoming revision of Part L. On this basis, there are those who speculate that electric heating is destined for extinction. However, calculations based on the study’s findings so far indicate that this assumption is somewhat misguided: results show that a 375% increase in solid floor insulation produces CO₂ savings of just 0·5%. At this level of improvement an 11% increase in insulation equates to an energy saving of just 0·01%.

As a result of these findings, the team at The University of Glamorgan is planning to explore these issues further, for example to look at the current and proposed insulation penalties relating to the use of electric heating. Although well intended, it may be the case that these penalties do not compensate for the higher carbon rating of electricity and are therefore unnecessary. The team also plan to investigate whether the controllability and responsiveness of electric heating compensate for the higher carbon rating when used in high thermal specification buildings.

• Mike George graduated in 2004 from the School of Technology at the University of Glamorgan. Dr Andrew Geens and Dr John Littlewood are senior lecturers in the School of Technology at the University of Glamorgan.

DTLR (2002a) Limiting thermal bridging and air leakage: Robust construction details for dwellings and similar buildings. Amendment 1. The Stationery Office.

DTLR (2002b) Approved Document L1 Conservation of Fuel and Power. The Stationery Office.

GIL72 (2002) General Information Leaflet 72/72 Advanced standard. BRE Publications.

Smit, J, ‘Precious heat’, Homes, the Building housing supplement, September 2004.