Solid-wall Construction

Measuring and Improving Thermal Performance

Tim Yates

Monitoring equipment is attached to a section of the interior face of a wall from which the wallpaper and plaster has been removed to expose the stonework beneath  
Figure 1 In situ measurement of thermal conductivity and interstitial moisture behaviour using heat flux monitors and in-wall gradient sensors (Photo: Caroline Rye/Archimetrics Ltd)  

The pressure to address the threat of climate change by reducing the emission of greenhouse gases, particularly carbon dioxide, is growing steadily. The UK has committed to an 80 per cent reduction in CO₂emissions by 2050.

Early in discussions it was acknowledged that buildings are a major contributor to these emissions, so efforts to improve the energy efficiency of our existing buildings are intensifying. But for many people the interest in building refurbishment is also, and perhaps primarily, driven by a desire to improve the living conditions in older houses.

Most new buildings have cavity walls, but many pre-1919 buildings are constructed of solid natural stone or brick. These buildings present a particular challenge when it comes to improving thermal performance as any improvements are likely either to change the appearance of the building or reduce space inside. Nevertheless, because around 35 per cent of heat from dwellings is lost through solid external walls, improving the thermal performance of this type of wall is seen as key to reaching the challenging emissions target set by the government.

Research has shown that the potential for reducing emissions (and saving money) in older homes is considerable. Analysis of the latest English House Condition Survey data (2007) shows that 4.8 million dwellings in England (21.5% of English housing stock) were built before 1919 and the proportion in the rest of the UK is similar. The current average Standard Assessment Procedure (SAP) rating of the pre-1919 stock is 40 (a low energy efficiency or ‘E’ rating on the Energy Performance Certificate (EPC) scale). This is approximately ten SAP points below the stock average of 50, and 50 points below new-build which meets Code for Sustainable Homes Level 3 (a ‘B’ on the EPC scale).

IMPROVING ENERGY EFFICIENCY

The key to improving energy efficiency is to take a ‘whole-house’ approach. This means considering the type of construction, exploring all the appropriate energy-efficiency measures, examining the renewable energy options, and implementing water-saving and waste-reduction measures. Many simple improvements, such as draught-proofing, can be made but evidence shows that for solid-walled dwellings to achieve significant CO₂ savings, the thermal performance of the external walls must be improved.

Where traditional and historic buildings are concerned, careful consideration is needed if performance is to be improved without compromising their heritage value, damaging historic fabric or undermining the wellbeing of occupants by changing the way buildings breathe and respond to their internal and external environments.

WALL INSULATION

Traditional solid-wall construction is probably the most difficult and often the least cost-effective building element to insulate. For listed buildings, any form of wall insulation is likely to require listed building consent and for the majority of buildings external insulation will usually require planning permission.

External insulation can be particularly difficult to incorporate into existing buildings as costly ancillary adaptations such as changes to the eaves and verges of roofs are often required. The potential benefits from installing internal or external insulation should be considered carefully, along with the planning constraints, the potential impact on the fabric of remedial works and the impact on internal conditions.

Internal insulation is usually applied directly to the inner face of the external wall, followed by a finish such as plaster. It is often necessary to relocate plumbing and electrical services and to adjust skirting boards, door architraves and fitted furniture. Cornices will also need to be modified which may result in the loss of original plasterwork.

Whatever insulation material is used, the improved wall will normally need to achieve a U-value of no more than 0.30 W/m²K, although a lower standard may be acceptable depending on the building. (U-value is a measure of heat transfer through a building element, so the lower the value the better.) Thicker internal insulation systems may significantly alter the sizes of rooms, corridors, etc.

External insulation systems usually comprise an insulation layer fixed to the existing wall and a protective render or cladding installed on top to protect the insulation from the weather and mechanical damage. The increased depth of an external render or insulation system will require adaptation of roof and wall junctions, window and door openings and rainwater goods. Decorative details such as string courses and quoins may also be affected, and natural materials such as stone or brick will be hidden, effecting a significant change in character.

As most suitable external insulation systems will also need to be protected from rain and mechanical damage, they should normally be considered as a two-component system where all layers must work together. Materials are available which can be used as a single coat, such as insulating lime renders which contain expanded vermiculite, but these tend to give significantly lower insulating values. They can, however, sometimes be applied in circumstances where other types of external insulation would be unacceptably detrimental to the character of a historic building. Again, whatever insulation material is used, the improved wall will need to achieve a U-value of no more than 0.30 W/m²K.

PREDICTING THE RISKS AND POTENTIAL BENEFITS

It is necessary to understand from the outset how the proposed changes are likely to alter the behaviour of the building. The biggest area for concern is moisture, both in the wall and in the building as a whole. The addition of insulation material to a wall is likely to alter the way moisture moves through it. For example, if non-breathable materials are added to an older porous wall, its ability to breathe and regulate moisture is compromised. Dampness, and even structural damage could result. Even if the walls are dry when the work is carried out, there may be evidence of past problems and a risk of them recurring. The main sources of external dampness will include run-off from gutters and downpipes, defects in the fabric such as roof flashings, and penetrating damp from driving rain. Rising damp may also be a problem, but only at the base of the wall. Internally, condensation is the mostly likely problem.

   
  Figure 2 An example of a more complex model showing predicted temperature (red) and dew point (purple) changes through a sandstone rubble infill wall (Image: Dan Browne/ SPAB Project)  

It is also necessary to consider the state of repair of the walls as dampness is often associated with salt crystallisation or efflorescence. Together, these processes can accelerate the deterioration of stone or brick surfaces, internally or externally.

The moisture content of a wall depends on the prevailing weather conditions, time of year and exposure of the site. If the wall is damp or in a poor condition, these problems need to be overcome before installing wall insulation. If there are any doubts about the condition of the existing walls, they should be professionally surveyed before any improvement works are considered.

Before any work is undertaken, it is also important to estimate the potential savings in terms of energy use, CO₂ emissions and reduced heating bills. The most common method of predicting the benefits is to model the performance of the building before and after improvements. The usual model is a SAP or RDSAP (Reduced Data Standard Assessment Procedure; further information on these procedures is available at www.bre.co.uk).

SAP uses a series of input values for the thermal conductivity of the different building elements, a series of accepted values and a set of equations that represent the environmental physics of the building. Accepted values are provided as part of the assessment method and these are used when specific performance information on the product or system is not available. However, when specific performance information is available for walls, floors, roofs and other elements, it should be used in preference to data from the tables.

MOISTURE

Moisture movement and its measurement in walls has been studied for many years. Various measurement methods have been developed and tested but the most reliable seems to be to collect a small sample by drilling a hole and then weighing, drying and re-weighing the sample. This is a good way to determine the moisture content, but to start mapping moisture and to measure change over time requires too many drill holes to be practical.

However, advances in computing have allowed the development of complex models in 1 and 2 D, which can be validated by a limited programme of on-site intrusive measurements. The most frequently used model seems to be WUFI (Wärme und Feuchte Instationär or Transient Heat and Moisture), developed by IBP in Germany. This model is validated using data derived from outdoor and laboratory tests. It allows realistic calculation of the transient hygrothermal behaviour of multi-layer building components exposed to natural climate conditions by modelling the coupled heat and moisture transfer in building components.

There are a number of projects applying WUFI modelling to traditionally constructed solid-walled buildings. One of these is the EU-funded SUSREF Project (Sustainable Refurbishment of Building Facades and External Walls). This project includes modelling of buildings in Wales by BRE Wales and Cardiff University to examine the distribution of moisture in a solid wall and the changes that occur when improvements are made to the wall’s thermal performance.

Another project is being co-ordinated by the Society for the Protection of Ancient Buildings (SPAB). The results were reported at a meeting held in June 2011. An example of the equipment used is shown in Figure 1 and one of the outputs can be seen in Figure 2, which shows changes in temperature, water content and relative humidity.

Understanding the distribution of moisture and temperature in a wall is important if changes such as the addition of internal insulation are to be made to it. Such changes may alter the point at which interstitial condensation occurs or the minimum winter temperature. (Interstitial condensation occurs when relatively warm moisture-laden air diffuses into a vapour-permeable material – if it is relatively warm on one side and below the dew point temperature on the other, the moisture-laden air can reach ‘dew point’ within the material and deposit water there). These changes can affect the long term durability of the wall materials and of any timber or steel frame materials, which may be particularly vulnerable. The research undertaken by SPAB has also shown that moisture content can affect the measured U-value of the wall by between 10 and 40 per cent.

THERMAL CONDUCTIVITY

Most thermal performance calculations rely on a series of assumed values, often based on measured values and then extrapolated to cover a wider range of building materials. In the case of natural stone the values used are often based on the density of the material and assume that the wall is solid stone whereas, of course, stone walls are rarely solid and include varying numbers of voids which may or may not be filled with rubble and/or mortar.

Two recent studies, The SPAB Research Report 1: U-value Report (2010) and Historic Scotland’s U-values and Traditional Buildings (2011) have focused on U-values as an indicator of thermal performance and involved the comparison of in situ measurements with U-values calculated with software programs and often-used ‘default’ U-values. A key objective was to help construction professionals and building energy performance assessors make more informed and balanced decisions when assessing and improving the energy performance of traditional buildings.

   
  Figure 3 Plot of calculated U-values (using BR 443) versus measured U-values for a range of solid walls from The SPAB Research Report 1: U-value Report (‘BR 443’ refers to the Building Research Establishment document BR 443: Conventions for U-value Calculations, which describes the calculation methods for determining the U-values of building elements based on British Standards)  

U-values are normally calculated with computer programs developed with modern non-traditional construction in mind which follow the conventions set out in BR 443: Conventions for U-value Calculations. For the studies, in situ U-value measurements were carried out mostly of uninsulated solid walls but, for comparison, some cavity walls and building elements retrofitted with insulation were also measured. The non-invasive measurements were generally taken of building elements with their internal and external finishes intact.

The studies then compared the U-values measured in situ with their calculated equivalents using the software program BuildDesk U v3.4. SPAB’s research suggests that 73 per cent of the traditionally built walls sampled (including timber, cob, limestone, slate and granite construction, 59 walls in total) actually performed better than expected (see Figure 3). A particular focus of the Historic Scotland comparison was the impact of the lime and stone core of a traditional solid stone wall on thermal performance.

This research is not a criticism of the calculation methodology or U-value modelling software, but it does highlight the difficulty of modelling and calculating the thermal performance of traditional walls using conventional techniques. These studies demonstrate that software programs for U-value calculations tend to overestimate the U-values of traditional building elements. In other words, traditional building elements tend to perform better thermally than would be expected from the U-value calculations. Furthermore, it is suggested that the in situ measurement of U-values is a useful tool which can aid in the assessment of the thermal performance of traditional building elements.

POTENTIAL PITFALLS

Although historic buildings are often too precious to alter by adding solid wall insulation, it is suitable for many unlisted or converted pre-1919 buildings. In these cases it is important to first establish that the alteration will be of significant benefit and then to understand its probable impact.

Overall, the recent work on moisture and on thermal values has shown that in order to make good and reliable plans for the thermal improvement of solid walls we need to have a good understanding of how the walls of a building are performing now: how the moisture is distributed, how and how much the walls are breathing, and how good the current thermal performance is. Only then is it possible to make good, low-risk plans for improving the thermal performance of walls, which will improve the building’s thermal efficiency without threatening its fabric.

 

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Acknowledgements

This article draws on the work of many at BRE and in the wider research community. The author is very grateful for access to their work and their ideas on thermal improvements for solid-wall houses.

Useful information

Building Research Establishment www.bre.co.uk

Climate Change and Your Home (English Heritage) www.climatechangeandyourhome.org.uk/live

The Energy Saving Trust www.energysavingtrust.org.uk

Historic Scotland technical papers www.historic-scotland.gov.uk/technicalpapers

The National Refurbishment Centre www.rethinkingrefurbishment.com

The Refurbishment Portal www.rethinkingrefurbishment.com/portal

The Society for the Protection of Ancient Buildings research reports www.spab.org.uk/advice/energy-efficiency

Sustainable Refurbishment of Building Facades and External Walls http://cic.vtt.fi/susref

 

 

The Building Conservation Directory, 2012

Author

TIM YATES PhD is technical director of the Building Research Establishment Ltd with responsibility for projects on heritage buildings. He has been involved in the built heritage for more than 35 years and is currently chairman of the British Standards Committee on Cultural Heritage.

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