by Michael Wright, Associate
There is the weight of private sector expectations through investors seeking greater rental values, more efficient and adaptable floor plates, and lower maintenance and running costs. Then there is the weight of public sector compliance requirements through building regulations, life safety systems, and sustainability targets through increasing energy efficiency. Cumulatively this imposes a heavy burden on what a property is required to achieve in our modern age.
So, what happens when the unstoppable force of private and public sector expectations of ‘vorsprung’ in the context of sustainability measures of property, meets the immovable object of a traditional listed building?
The answer, increasingly being advocated within the Heritage sector today, is that of retrofitting of traditional buildings. The Bristolian’s Guide to Solid Wall Insulation, 2015 succinctly defines retrofitting as “…the improvement of a building’s energy use through technical interventions.”
Various options can now be advocated to tackle the real, and perceived, notions of cold, poorly insulated, and damp traditional buildings. Practitioners are now able to consider external wall insulation systems, internal wall insulation systems, hybrid wall insulation systems, and sensitive repair options to windows, such as draught-proofing strips, secondary glazing systems, and low-E films.
The market and governmental forces compelling existing traditional buildings to better their sustainability credentials are colossal. Between 2003 and 2013, there was a 128% real term increase in gas and electricity bills, it is estimated that 2.3m households in England alone live in fuel poverty, and health issues associated with people living in cold housing is estimated to cost the NHS approximately £1.36bn per annum (The Policy Exchange). Further, the fuel poverty strategy for England currently is that as many fuel poor homes as is “reasonably practicable” achieve a minimum energy efficiency standard of Band C, by 2030. Existing traditional listed buildings are an obvious target for areas of improvement.
The principle and process of retrofitting as a means to achieve balance between the need of UK property to become more sustainable, and the need to safeguard the cultural value enshrined within our traditional buildings (which can’t be replaced), appears, and is, a good idea in theory. However, the challenges in the practical application are akin to racing a fast German automobile; too much speed and intervention, and the risk of impact damage to your carefully crafted entity (which can’t be replaced) becomes unsustainable; not enough speed, and your entity will fail to keep pace with modern world occupier demands and will swiftly be resigned to the scrap heap.
An understanding of external wall insulation systems and internal wall systems, or a mixture of both – hybrid wall insulation systems – is of particular importance in respect of retrofitting proposals.
The installation of an insulation system to either the internal face or the external face of a traditional load bearing masonry wall to help retain heat would appear to be common sense. Practically, and externally, what happens where there are service penetrations that penetrate through external wall construction? What happens where there is wall head gutter, where there is a wall fixed downpipe, where there is an external wall mounted street level gas box, none of which can easily be temporarily removed and set aside to permit the installation of the insulation? The results will be a myriad of cold spots in the wall, where the risk of heat loss, condensation and mould growth is thermal and pressure differentials now created by well- meaning proposals.
Traditional single glazed sash and case windows, as an example, can be an effective target element when trying to mitigate energy loss, albeit we must recognise that windows hold intrinsic heritage value to a property. In order to illustrate that we are applying good building conservation philosophical principles we should, therefore, consider measures that are reversible and pose minimal intervention. Some such measures include the installation of draught proofing strips to the inside frame of the windows, the installation of secondary window systems and the application of Low-E coating films to the glazing. With these measures, it should always be remembered that timber windows, being vapour permeable, permit a certain degree of ventilation through a room. Will enclosing these on the inside face with aluminium glazing systems reduce air flow through the frame and increase the condensation risk? The installation of timber window shutters (or bringing existing shutters back into function) is a positive option.
Research carried out by Historic Environment Scotland found that timber window shutters can reduce heat loss through windows by 50% (HES, 2016).
In respect of the above – wall insulation systems and interventions to existing traditional windows – simply proceeding with works with no research data is not sufficient. So how do we understand the risk associated with our proposals? A good starting point is ‘BS 5250:2011 Code of practice for control of condensation in buildings’, which encourages the reader to understand how building elements – such as retrofit internal wall insulation systems – can be designed and constructed to avoid moisture problems.
Importance of Data
As a development from this, and if a practitioner is being asked to consider and propose measures, detailed modelling software should be utilised, as detailed in ‘BS EN 15026: 2007 Hygrothermal performance of building components and building elements; assessment of moisture transfer by numerical simulation.’ If this sounds complicated to the reader, that’s because it is, but the value in being able to achieve whole building simulations illustrating the movement of heat and moisture within a space, is critical to understanding whether proposals are going to be effective for your client.
Of course, technology is not just about improving thermal performance; increasingly issues of indoor air quality need to be considered as well – the modern practice of eliminating air leakage can spell disaster for historic building fabric – as well as the occupants of the buildings.
Many condensation risk tools are based upon a calculation known as the Glaser method. However, the results need care as they are really intend to model a design and are not necessarily reflective of actual performance. To gain a more realistic picture Watts use German software WUFI®. WUFI® – an acronym for Wärme Und Feuchte Instationär, which translated means heat and moisture transiency – is a software tool that uses the latest findings regarding vapor diffusion and moisture transport in assessing proposed building materials. In the absence of such hard data, how can a practitioner illustrate that their proposals won’t result in cold spots, condensation risk, and mould growth, and perhaps even a claim for negligence?
In light of the complexities surrounding this area, one need not search far to find a litany of failed retrofitting traditional efforts across the UK in the last few decades. In one instance at a residential estate in England, and within weeks of the completion of retrofitting measures, the designer looked on aghast as the property slowly began to succumb to rampant black mould growth, interstitial condensation, penetrating water ingress, collapsed ceilings, and respiratory difficulties amongst some of the elderly occupiers. The local newspapers were quick to pick up on the story.
The principle of retrofitting traditional listed buildings is one which strikes a balance in safeguarding the cultural heritage for which we all hold collective responsibility. However, like the design of a German car, the process of retrofit design to a traditional building must adopt a rigorous and methodical process that utilises bespoke software to identify the critical line between success and failure. Any intervention should only be contemplated with the assistance of independent qualified chartered building surveyors, experienced in the process, with access to the specialist software, and who are best placed to swerve the varied and complicated hazards posed, and ultimately strike ‘Vorsprung durch Retrofit’.
The Watts Heritage Building Consultancy team enjoy decades of experience in helping our clients maintain, repair and retrofit their traditional buildings. If you consider you would benefit from our expertise, please get in touch.
Michael Wright is an Associate Building Surveyor at Watts Group and Chair of the Watts Heritage Consultancy Department.