Old Walls

My previous article described potential retrofit solutions for historic buildings and discussed the difficulties and challenges involved in the process. One of the most common retrofit options for historic buildings is insulation. Given the fact that the majority of loft insulation has already been realised and a lot of the cavity wall insulation has been installed, it is now solid wall insulation (SWI) that is of most significant importance. Nevertheless, SWI and especially its installation in traditional buildings, are challenging and there are numerous barriers preventing its broader implementation as a retrofit measure. Other than the heritage status of buildings, which might be a complicating factor, there are considerable practical and technical difficulties to consider. This article focuses on the SWI in historic buildings as well as on the relevant technical and practical challenges.

Solid wall is a wall that has no cavity, usually made of brick or stone. The Appendix S of the SAP 2009 document used in the evaluation of buildings’ energy performance suggests U-values of 2.4 & 2.1 W/m2K for stone and 2.1 W/m2K for brick walls of existing buildings. U-values are most commonly used as the basic criterion defining the heat losses through walls and hence buildings’ energy performance. Therefore, U-values’ accurate calculation is a significant factor in estimating buildings’ initial energy performance and consequently their potential for energy performance improvements through building fabric retrofit.

Nevertheless, recent research by Historic Scotland, English Heritage and SPAB, conducting in-situ and laboratory measurements of traditional buildings’ U-values has resulted in completely different figures. The calculated average is approximately 1.4 W/m2K, as opposed to the 2.2 W/m2K which is currently broadly used. One of the main reasons for this is the fact that the models typically used for these calculations are based on modern types of wall assemblies, which act completely different to the traditional ones in terms of air movement and thermal performance. Therefore, the laboratory or in-situ measurements provide different values, which would be expected to represent the actual heat losses more accurately. The research was conducted on a limited number of samples and on various types of solid walls; therefore, these results cannot recommend a new U-value for broad use. However, they seem to suggest that traditional building elements perform thermally better than expected and they do make us wonder how accurate our assumptions are when estimating the heat losses and the performance of existing walls and historic buildings.

The two potential solutions for retrofitting solid walls are external (EWI) and internal wall insulation (IWI); in these cases, an insulating layer is fixed to the existing wall externally or internally, respectively. EWI makes it easier to avoid thermal bridging and doesn’t involve any space loss in the interior of the building. Moreover, it causes minimal disruption for the occupants and can improve the appearance of ageing facades. It obviously involves a considerable higher level of intervention on the building’s exterior, which might make it inappropriate for historic buildings or environments. Furthermore, it does require the extension of window sills and possibly of roof eaves. On the other hand, IWI is typically easier to install in a historic environment, although it decreases the interior building area and it usually causes a significant level of disruption for the building occupants due to the need to refit kitchens, bathrooms or heating systems. The fact that both types of insulation require other complimentary works to be performed, increases the cost compared to the one initially estimated. These are the so called hidden costs, which can increase the payback time from an average of 6 years to 37 for IWI and from an average of 13 to 27 years in the case of EWI.

The installation of SWI can involve considerable technical difficulties, one of them being relevant to humidity and moisture barriers. Traditional buildings are typically constructed in a way that allows them to ‘breathe’ through stone or even brick walls that allow some level of ventilation. Modern SWI systems use materials that aim to high thermal resistivity and limited air-permeability, thus contradicting the existing breathable structures. Therefore, the existing wall cannot perform as its original design dictates, which can lead to moisture build-up, especially when the internal and external conditions are favourable. Hence, this type of intervention creates a risk for condensation and mould growth, deteriorating indoor air quality conditions.

Finally, the insulation industry and the complexity of drivers and actors make the implementation of SWI even more challenging, especially for historic buildings. For example, the insulation industry aims to the minimisation of time spent on site and has no direct financial gain from improved construction details that might enhance a building’s energy performance. Historic buildings are particularly challenging and require bespoke solutions which sometimes limit the margin for profit for the industry. Moreover, SMEs and corporate entities, government-led economic instruments, industry professionals, clients, occupants and the technologies themselves create uncertainties and difficulties. Relevant research has identified the insulation industry, financial pressure, poor technical understanding and limited training of the installers as key factors that hinder the broader implementation of SWI in existing buildings in the short term.

It is up to policy incentives for clients and the insulation industry to reshape the landscape of SWI for historic buildings, while research can have a significant contribution to this transformation.

Image Credit: Handwerker via Wikimedia Commons


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