How to deal with retrofit risks

The challenge is to improve the thermal performance of historic buildings without a significant impact on their appearance and historic fabric, and at acceptable cost.

Undertaking retrofit has for some time been a hot topic for the UK government. Some of the most difficult buildings to tackle were constructed prior to 1930. These have the difficult challenge of balancing cost and environmental impact. There are additional considerations relating to the aesthetic and cultural significance of the building and place, and its context as part of the built heritage.

The current approach to retrofit starts with an assumption that thermal improvements can only be made by insulation. Some problems are systemic: failure or underperformance due to the process of decision making in selecting the most appropriate materials to be used for thermal upgrades to a building, or the method and decision making process for deciding which elements of the building are to be improved, such as the omission of insulating reveals, heads, eaves and other penetrations for a variety of reasons (such as increased cost) that can be attributed to extending roof lines and gables, isolating walls and fences to allow the placement of insulation behind their location, or relocation/temporary removal of incoming services to provide thermal continuity.

Other problems relate to the assessment of materials and buildings. The current methods of assessing durability and condensation risk are undertaken using unrealistic parameters for risk. These limited methods of testing can provide a level of comfort and assurance which in reality may not be delivered, such as levels of high exposure to wind-driven rain, and high levels of internal and external relative humidity.

Traditional buildings are the most challenging to improve thermally. Due to their use of lime mortar and porous construction they are the most susceptible to the effects of moisture ingress. Better quality control and accurate assessment are needed when choosing an improvement option. The decision to insulate must be taken with the greatest of care, with particular attention to the existing condition of the property, with a focus on fabric integrity and water-tightness, and with a thorough investigation of the likelihood of introducing cold bridges into the building, and how these will be designed out in the installation process.

It is accepted that there is a limit to the extent of changes that can be made to improve a building’s thermal performance without a significant impact on its appearance and on its historic fabric. Improving the sustainability of such buildings results in a tension between the competing demands for heritage preservation and the pressures to reduce the environmental impact of buildings in use.

The problems caused by moisture in building structures are well known. There is a large body of literature addressing issues including rising and penetrating dampness and interstitial condensation. For regulatory purposes the key document is the BS Code of Practice for condensation in buildings, BS5250:2011, which is referenced in the three sets of regulations in the UK. Apart from a discussion on the principles of condensation risk, the main content of BS5250 is prescriptive guidance on how elements such as walls and roofs should be designed and constructed. Many, if not most, designers will refer to this guidance and not carry out any more complex assessments.

If a more detailed analysis is necessary, BS5250 refers to a calculation procedure specified in ISO 13788, which uses a ‘Glaser’ procedure, taking account only of steady-state vapour diffusion. While this may be adequate for some structures such as flat roofs (and even here there are situations where it is inadequate), it does not represent the whole picture for constructions such as masonry walls where very large amounts of water are stored in the fabric. No account is taken of rain impacts and solar gain on the outside surface, liquid water movement, and the effect of moisture on the thermal and moisture transport properties of materials.

A further standard, BS EN 15026:2007 Hygrothermal performance of building components and building elements: assessment of moisture transfer by numerical simulation, was developed to address these issues. This is becoming used in the UK through the German software WUFI (which can run calculations using EN 15026), but there is no robust protocol for using this standard in the UK.

There are a number of difficulties with this approach at present:

WUFI is a very complex programme that needs a good understanding of building physics to use successfully. It can be easy to enter the wrong parameters, producing misleading or inaccurate results.
Detailed data on the heat and moisture transport properties of the materials making up the structure is needed. There is a database, which contains mainly German materials, in WUFI. It is difficult to know whether these are relevant to UK constructions. Many of these properties are complicated and expensive to measure, and would no doubt raise objections from insulation manufacturers.
Detailed external weather data from the location of the building is needed to run WUFI.
EN 15026 describes and models only one-dimensional movement of heat and moisture. The WUFI programme can be two-dimensional but this is currently outside the range of EN 15026, and there is no commercially available version of WUFI for two dimensions.
WUFI does not model air movement in structures sufficiently accurately, and it cannot deal with two-dimensional junctions (such as eaves and reveals).

Incorrect moisture predictions can lead to two types of failure. Predictions that problems may occur, when in fact the risk is negligible, may limit the installation of insulation unnecessarily. Failure to predict real problems and take appropriate precautions will lead to problems such as rot of timbers, frost attack to masonry, damp staining to interior finishes, and bad indoor air quality (due to mould and damp).

The difference in building physics and construction between older and more modern buildings has been ignored in recent years. It is essential to choose the correct intervention so that these principal differences are taken into consideration. With the desire to improve buildings from a thermal performance perspective, the basic principle of ensuring that the building is in a good state of maintenance, and understanding the limitations in construction form, are rarely considered.

It is important that no improvement or intervention restricts the passage of the moisture either to the internal or external surface, without very careful consideration and design. The possibility of unintended consequences must be considered fully.

A maintenance or improvement programme introducing a more modern material into the structure brings a series of risks which are rarely considered. Examples include sand-cement renders, non-breathing paints and silicone waterproofing layers.

Recent research highlights the facts that moisture movement is multi-dimensional, and that any alterations or change in this free movement can result in a build-up of moisture, either on the inner surface of the wall (mould) or within the structure of the building (interstitial condensation). The underlying cause is the isolation of the wall structure from a source of heat and ventilation by a non-breathing element. In the long term this can result in wall failure or in early failure of internal coverings such as plaster. Any signs of disrepair, water ingress, damp or deterioration must be rectified before considering other measures.

The overriding issue that has been identified with current working practices across the industry is an inconsistent approach to assessment. A sound approach is needed, using building pathology, checking and quality control.

The main issues that need to be understood are:

Which areas need to be surveyed before insulation works are undertaken.
The impacts of cold or thermal bridging, and the importance of minimising the risk.
The importance of attention to detail and specification on site.
The impact of saturation on conductivity, condensation risk, temperature gradients, cold spots and convective looping.
Communication between site installation and quality control in a positive feedback loop to ensure that good practice is reinforced and bad practice not ignored.
The need to give talks on the principles of good practice before each project, with any key elements of the insulation system being clearly set out to installers.
More robust and evidenced quality control with well-trained staff.
The creation of more robust details, reducing the over-reliance on sealants and workmanship.

If the effect of thermal bridging is not taken into consideration when calculating potential heat loss from a building, it is likely that the overall heat loss will be underestimated. It follows that if buildings are improved through insulation but thermal bridges remain, heat loss will be concentrated at the point of the bridge relative to the newly improved walls. While it is desirable to minimise this effect to help reduce heating costs, it can also be the cause of physical problems.

Since heat will transfer out of the building more readily at the point of a thermal bridge, the internal surface temperature at that point will generally be reduced relative to the surrounding surface area. When the temperature difference reaches a critical ratio, measured as the temperature factor (fRsi) at the junction, condensation can form, which can lead to problems such as mould growth.

Latest research indicates that when reveals and penetrations are not insulated, the temperature factor and subsequent cold bridge are actually worse than before the walls were insulated. This results in a concentration of risk in a two-dimensional junction that is least capable of dealing with a drop in temperature. Modelling indicates that this area contributes significantly to the heat loss from a building after improvements.

The losses through cold bridging have been identified to be the cause of 19 per cent of heat loss through a building envelope. This lack of attention to detail is understandable in view of the desire to avoid bridging the damp-proof membrane or course, but careful treatment of this area can result in reduced cold bridging and subsequent risks.

This article originally appeared in IHBC's Context 149, published in May 2017. It was written by Colin King, director of BRE Wales.

--Institute of Historic Building Conservation