Understanding the performance of solid walls
Our ability to make changes to solid walls has probably never been greater and our motivation, driven by the desire to save energy, has never been stronger. We can now effect radical change in structures that have, in some cases, stood for hundreds of years. Many of the means by which we may make these alterations are unprecedented. An understanding of how solid walls perform is essential if we are to make genuinely effective changes that will not cause harm to buildings or their occupants.
Towards the end of the noughties, in response to growing public debate concerning climate change and carbon emissions, the Society for the Protection of Ancient Buildings (SPAB) saw an increase in enquiries seeking advice on improving the energy efficiency of older buildings. In the absence of official guidance specifically for these buildings, the society decided to undertake its own programme of research to underpin its advice and information services. Central to this research was the need to understand how both heat and moisture performed in solid walls.
The SPAB U-value research measured heat flows through 77 solid walls of traditional construction in historic buildings. This work was carried out between 2009 and 2012 by ArchiMetrics, with guidance from Paul Baker of Glasgow Caledonian University. Following the method set out in BS EN ISO 9869, it used a heat flux monitor in combination with measurements of internal and external temperature, taken over time, to provide an in situ U-value (an estimate of heat loss) for each of the walls. The types of walls measured were deliberately diverse, including different examples of stone, cob, brick and timber-frame constructions.
Some measurements of more modern materials were taken where these materials had been retrofitted. Some of the walls had historic or more modern dry-linings. As well as surveying the range of heat losses from these different wall types, the study undertook a comparison of these in situ U-values with those produced via the standard U-value calculating method BS EN ISO 6946 – with startling results. The research found that in 77 per cent of cases the calculated U-value overestimated the heat loss of the solid walls in comparison with the heat loss that had been measured in situ.
While undertaking the U-value research it became clear that issues of moisture within solid walls were closely linked to those of heat loss and were of equal, if not greater, importance. In order to answer questions about the behaviour of moisture in solid walls, ArchiMetrics developed a method called interstitial hygrothermal gradient monitoring (IHGM), by which temperature and relative humidity could be measured through wall sections. Monitoring equipment was installed in solid walls that were to undergo insulation. Later these measurements were joined by others to determine the moisture content of materials through the wall sections. This continuing work is part of the SPAB Building Performance Survey, a project which has measured not only heat loss and moisture in walls, but also the effect of other changes within buildings before and after refurbishment.
The Building Performance Survey, started in 2011, measures moisture in three walls made of brick, granite and cob. The value of long-term measurements of moisture behaviour can not be stressed enough. Changes to the moisture profiles of walls can occur slowly. As the study progresses we have been able to identify both short and longer-term trends that reach beyond the changes attributable to individual seasons or annual weather patterns; underlying trends which are impinged upon by the material characteristics of the walls themselves, including those of refurbishment.
We find that a relatively thin, south-facing brick wall internally insulated with a small (40mm) quantity of woodfibre insulation is performing satisfactorily with regard to moisture risk. That is, while the wall becomes quite wet over the winter, it seems able to evaporate this moisture during the spring and summer so that over repeated annual cycles relative humidity (RH) within the wall is, on average, below an 80 per cent threshold. (Eighty per cent is the commonly used measure of RH risk above which mould growth and fungal decay can flourish, particularly on organic substrates.) In contrast, on a thicker north-west-facing granite wall internally insulated with 100mm of polyisocyanurate (PIR) insulation we find a rising trend of RH. Shortly after insulation RH in this wall began to increase. It is on average greater than 90 per cent, and continues to increase year on year, indicating that the wall is accumulating moisture.
The cob wall is different again. This wall is externally insulated with a thick (50mm) layer of insulating lime render. As part of the process of applying this new render the wall was wetted down, resulting in very high measurements of RH through the section. Over the past five years we have watched as measurements of RH have gradually reduced in the centre of the wall as the structure rids itself of excess moisture. However, RH remains at 100 per cent in proximity to the new render, and there is a question as to whether this material is retarding the drying process.
Valuable and interlinked lessons may be learned for retrofit from these two research studies. Applying insulation to the internal or external side of a solid wall will alter not only heat flow through it but also how moisture behaves within it. The extent of the change will depend on the material characteristics of the original wall, its relative permeability, porosity and moisture-carrying capacity. It will also depend on the location of the wall and its aspect, what kind of weather it is subject to, quantities of rain and particularly rain driven by prevailing winds and the amount of heat it receives from direct solar radiation. Crucially the quantity and type of insulation added to the wall will also have an impact.
Thinking around the insulation of solid walls has become more informed over the past few years, partly as a result of research by the SPAB and others. However, too often retrofit decisions are still based solely on reducing heat losses without consideration of the effects this will have on moisture. The official (BS 6946) U-value calculation procedure suggests that solid walls lose far more heat than that measured from real walls. In retrofit situations this encourages the use of large quantities of insulation material. In addition, as large quantities seem to be required, the thickness of the insulation becomes problematic, steering people towards higher-performing, less thermally conductive materials, most of which are vapour closed.
Often a measured in situ U-value shows a solid wall to have a heat loss of around two-thirds or half that estimated by a calculation. In these circumstances reducing the amount of insulation applied to a solid wall, and/or using a less high performing, more vapour open material will still result in meaningful reductions in heat loss. This is important from the point of view of the long-term performance of moisture within the wall. Insulation applied to the internal face of a solid wall means that less heat passes through the wall. The temperature through the wall section is lower and thus, during the colder winter months, dewpoint conditions can occur more frequently, leading to a possible increase in interstitial condensation.
While a solid wall may be able to tolerate the removal of some heat input, this will need to be in proportion to its ability to evaporate moisture over an annual cycle in order to avoid moisture accumulating within the fabric. In reality this is something which is difficult to estimate and likely to be particularly significant for thick, shaded, exposed and more porous walls. By using smaller quantities of insulation we are more likely to maintain a balance between the desire to cut heat loss through the fabric and continuing to allow sufficient heat to pass through the wall to avoid moisture accumulation.
If we abandon a race to the bottom for wall U-values on the grounds that these walls do not lose as much heat as we previously thought, then the use of smaller quantities of less-high-performing, more vapour-open materials, becomes more attractive. These materials are likely to be critical to the wall’s ability to move and lose water. The aforementioned brick wall is an example of a wall insulated internally with a small quantity of vapour-open material. Prior to insulation, this wall measured an in situ U-value of 1.48 W/m2K. Following the application of 40 mm of woodfibre board this U-value was reduced to 0.48 W/m2K. This represents a significant improvement, being a 68 per cent reduction in heat loss.
With regard to moisture, this wall is performing satisfactorily. On average it is below the critical 80 per cent RH threshold, indicating that moisture which accumulates within the fabric is able to evaporate away sufficiently over an annual cycle. However, this is not the case for the granite wall. Here we see average RH at levels well beyond the 80 per cent risk threshold. There has been a trend of increasing RH measured every year since the wall was insulated, which suggests that moisture is accumulating and that the internal wall insulation is a likely cause. The PIR insulation material is 100mm thick, vapour closed and sheathed in impermeable metallised foil. The in situ U-value prior to refurbishment was 1.20 W/m2K. Post-refurbishment it is now very low, 0.16 W/m2K.
Conditions within the wall have moved closer to dewpoint, increasing the likelihood of interstitial condensation. In addition, the insulation forms a barrier to the movement of water within the wall, preventing access to an evaporative surface. Moisture is accumulating as this treatment has increased opportunities for moisture formation within the wall fabric and reduced opportunities for evaporation. The cob wall is externally insulated with a relatively thick 50mm render. This again is a wall that demonstrates the importance of using materials that can allow excess moisture within a structure to evaporate. It is also a caution regarding the use of large quantities of water as part of refurbishment processes, and an as yet unfinished lesson in just how long drying processes can take.
For many years the SPAB has advocated against the use of less permeable or vapour-closed materials on traditional or historic buildings. We now have measured evidence that these materials might prove damaging in some circumstances when used in the retrofitting of older solid wall buildings. With regards to saving energy and safe moisture profiles, we also have an example of an approach that appears to work. While the circumstances of each one of these walls may be different, it would be unwise to dismiss these case studies as isolated or extreme examples.
Given the complexities of moisture performance in relation to both materials and building context, a precautionary approach should be adopted. This would discourage the use of excessive quantities of wall insulation and vapour-closed materials for solid walls. In relation to buildings of greatest significance, the measurement of U-values should be promoted in order to improve the careful specification of insulation quantities to avoid over-insulation and its associated moisture risks. Calculated U-values for solid walls should be treated with some scepticism.
Where measurement would not be appropriate, tables of existing measured walls could be consulted (see references below). Proposals for changes to solid walls should be carefully scrutinised, with particular attention paid to moisture performance and this question uppermost: when the water gets in, how does it get out again?
 Related articles on Designing Buildings Wiki
- Bonfield Review.
- Energy efficiency of traditional buildings.
- Energy-related retrofits of buildings and urban areas, a comparison between Germany and the UK.
- How to deal with retrofit risks.
- IHBC articles.
- Retrofit and traditional approaches to comfort.
- Retrofitting traditional buildings.
- Solid wall insulation.
- The Each Home Counts report and traditional buildings.
- The Institute of Historic Building Conservation.
- Thermography for traditional buildings.
- Understanding dampness.
 External references
- SPAB Research Report 1: The U-value report 2010. Revised 2012
- SPAB Research Report 2: SPAB Building Performance Survey 2011–16, interim findings
- SPAB website: http://www.spab.org.uk/advice/energy-efficiency/.
- BS EN ISO 9869:2014 Thermal insulation: Building elements – In-situ measurement of thermal resistance and thermal transmittance.
- BS EN ISO 6946:2007 Building components and building elements: Thermal resistance and thermal transmittance – Calculation method.
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