Heritage retrofits pragmatically ensure sustainability

Recognising that retrofitting historic buildings to ensure their sustainability entails a range of pragmatic considerations, building surveyors and conservation surveyors will benefit from seeing how these work in practice.

A pair of recent projects – one domestic and the other commercial – offer useful illustrations of the principles discussed in our previous article.

The ground floor had a lounge, small kitchen, utility room and bathroom, while the first floor had a bedroom and a large landing that had been used as a dressing area.

Given its rural location, the property was served by a septic tank that it shared with the adjoining number 2 Red House Cottage. Both properties were also connected to mains gas.

Although not listed, the historic nature of the cottage placed some restrictions on what could be done physically within the available budget while reflecting the character of the wider estate.

There were also financial restrictions in terms of how much could be spent by the National Trust's surveyor to ensure an adequate return on investment.

The small budget meant it was not possible to carry out any in-situ energy tests to determine the building's energy performance before refurbishment.

The property did however have a registered energy performance certificate (EPC) with an E rating; the vacancy and refurbishment provided an ideal opportunity to improve this.

The aims for the refurbishment project were agreed between the estate manager (the client) and the building surveyor who undertook the project. These aims were to:

• improve the overall energy efficiency of the property
• improve its layout to suit modern living
• prepare it for further upgrades, such as the adoption of an air-source heat pump, without further disruptive fabric alterations in future
• replace the septic tank with a sewage treatment plant
• redecorate externally and repair timbers, including overhauling the roof coverings and chimney stack.

These aimed to deliver three main outcomes: first, to ensure the property was compliant with statutory regulations including the Minimum Energy Efficiency Standards (MEES); second, to provide attractive living standards; and third, to repair or replace defective components.

Such an approach prioritises modifications to the physical fabric, enhancing the envelope, insulation and ventilation systems to reduce energy consumption, rather than relying on renewable energy or other technology.

In this case, work included installation of internal wall insulation, secondary glazing and increased loft insulation. Where they were not replaced, external doors were draught proofed. The insulation means that an air-source heat pump can be fitted in future when budgets allow without requiring further upgrades.

This also provided an opportunity for preparing the building for future upgrades to further stretch the budget, and over the long-term guidance such as the Sustainable Traditional Buildings Alliance's Guidance Wheel was also referred to.

The property was also rewired and a new central heating system was installed. Due to the restricted budget, the building surveyor specified a new gas central heating system, including an A-rated gas combination boiler with heat recovery and external weather compensator, alongside a wood-burning stove.

In addition, the interior was reconfigured so the bathroom could be moved to the first floor to introduce a more practical living arrangement downstairs.

The new sewage treatment plant also meets the general binding rules. This ensured the property met prospective tenant expectations for modern living and that the cottage was in a lettable and, therefore, economically sustainable condition. 

This appears to be a common feature for cottages on the estate, in an attempt to resolve persistent damp issues without previous custodians of the estate fully understanding that the permeability of traditional materials allows moisture movement.

However, due to the highly porous nature of the brickwork and the relative thinness of the solid walls, the initial attempt at removing this sand–cement coat proved difficult.

The removal of this coating caused unacceptable damage to the walls, and there was a risk of compromising the structural integrity if this continued.

The building surveyor therefore decided to leave the sand–cement internal render in place and change the internal wall insulation specification, previously intended to be wood fiberboard insulation system attached to the internal wall.

To mitigate the risks of condensation, the building surveyor specified that small, vented cavity battens be attached to the internal face of the external walls, each backed with a damp-proof membrane.

A 50mm insulated, class 1 fire-rated foil-backed plasterboard was then installed. The inclusion of a vented cavity aimed to avoid trapping moisture and encourage any buildup of moisture to be removed through ventilation.

This change in the approach to internal wall insulation had limited impact on the internal floor area and proved a cost-effective means of insulating while mitigating the risks of interstitial condensation that the previously specified insulation – a wood fibreboard system – would have presented had this been installed directly to the non-permeable sand-cement surface.

Following the refurbishment, the cottage achieved an EPC rating of C, and a new tenant has already moved in.

The list entry refers to the exterior of the property but there are also many historic internal features, most notably the ornate second-floor ceiling. The listing description also includes the jetty to the front elevation facing the main street.

Originally built in 1600, the building has three principal storeys, with accessible attics and a basement as well.

Historic mapping of the location and physical evidence confirm that the present property comprises three different structures, likely built at differing times.

Significant modifications were previously made in the mid-18th, 19th and 20th centuries, including the installation of a steel beam at first floor level.

After the National Trust acquired the property, on what was once considered York's prime shopping street, it was leased for many years. However, like many high-street premises in recent years, trade declined and the shop fell into near-dereliction.

In 2017, the property became vacant when the last long-standing tenant moved out following declining trade. At the end of 2018, the property was let to a pop-up social enterprise, the tenancy terminating in 2023.

Both tenancies included only the ground floor, resulting in the upper floor remaining untouched for approximately 40 years. A full refurbishment of the whole property was necessary to relet it, improve its condition, sustainability and conservation.

At Coney Street, we worked within the National Trust's formal Project Management Framework and with the internal client to develop the client brief and identify the main project aims, which were to:

• provide a lettable shell for a new tenant to lease as a commercial premises
• improve the energy efficiency of the building, reduce carbon emissions and meet the MEES
• prevent disruption to tenants if additional improvements were needed in future
• ensure the building was structurally sound
• renew the mechanical and electrical infrastructure, services and fittings
• conserve and restore the second-floor ceiling.

The brief also considered aspects such as heritage significance, the property setting, likely rental income and the current market. The brief was subsequently discussed and signed off by the National Trust's internal project board.

The narrowness of the building meant that it was preferable not to lose internal floor area. The building surveyor worked with the client to determine that insulating these elevations and the party walls would not have a noticeable impact on building performance relative to the cost of doing so and resultant loss of floor area.

Parts of the roof did not include a loft space or were inaccessible. Due to the sensitive nature of the second-floor conserved historic plaster ceiling, no insulation was retrofitted above to avoid unintended consequences such as physically disturbing the plasterwork, and changes to the moisture profile of the roof space.

The property is not on the mains gas network, and before refurbishment the ground floor had a combined air conditioning and heating unit while the first and second floors had no heating system.

Despite this, the building surprisingly achieved a commercial EPC score of 71 before work started, placing it well within the EPC band C (69–80). The National Trust hoped that this could be improved, and as the property already had a three-phase electrical supply – although it was not connected at the time – this enabled renewable energy generation to be installed.

An M&E engineer also carried out an options appraisal for the new heating system, and they determined that the most viable option was a 14kW air-to-water heat pump which serve wall-mounted radiators serving the first and second floors of the property.

The kitchen and WCs were replaced and lime plaster repaired, and the property was fully redecorated internally and externally. Major structural stability works were also required after discovering that the first-floor jetty was not adequately supported.

A specialist conservator carefully undertook the conservation of the second-floor ceiling, removing hessian-backed gypsum plaster and modern paint coverings, repairing cracks in plaster and applying a soft distemper. The ceiling was consequently successfully restored to its former glory.

Where appropriate, modest measures such as draught-proofing and LED light fittings were installed. As works neared completion, the National Trust commissioned a draft EPC, which found the building would likely achieve a B rating (26) – a mere point away from an A rating.

Following consultation with the EPC assessor, the National Trust's building surveyor decided to perform an airtightness test to inform further possible interventions.

Some additional minor improvement works were then carried out to achieve a permeability of 17.4m³/(hr.m²)@50Pa, better than the default of 25m³/(hr.m²)@50Pa assumed in the Simplified Building Energy Model (SBEM) underpinning the EPC.

This is still a permeable building but slightly mitigated draughts and maintained good ventilation. This meant a small improvement in draughts while still maintaining a good level of passive ventilation and enabled a final EPC rating of A.

From a building surveying perspective, a space that is heated throughout is less likely to suffer from more rapid deterioration than unheated space, helping to avoid further unnecessary interventions and associated embodied carbon.

The trust also hopes that the refurbishment of the shop has played a small part in the overall revival of Coney Street.

The examples of Coney Street and Red House Cottage demonstrate that it is possible to embed sustainability into building refurbishments where a pragmatic approach is adopted.

Both projects faced budgetary constraints and different physical limitations, but through understanding the aims of each project, the surveyors were able to identify practical solutions. This included improving the EPC ratings, creating functional spaces for prospective tenants, improving the condition of the assets and contributing to their longevity.