In-depth moisture risk analysis for retrofit of a Grade II* listed building
Keelmen’s Hospital is a Grade II* listed building in Newcastle city centre. Built in 1701, it originally served as an almshouse until the late nineteenth century, followed by conversions to tenement housing and student accommodation up until 2009 before falling into disuse. The Tyne & Wear Building Preservation Trust, in partnership with Newcastle City Council, Historic England, Architectural Heritage Fund and National Lottery Heritage Fund, is currently seeking to restore the building to create affordable housing.
Thermal upgrades, including internal wall insulation, are being proposed as part of the retrofit project team led by JDDK architects. Any proposals to improve the thermal performance required careful consideration to mitigate any moisture risks and prevent unintended damage to the existing fabric.
Greengauge was approached to assess the moisture risk associated with internally insulating the existing walls at Keelmen’s Hospital.
Strategy
Greengauge’s hygrothermal risk assessment strategy included a number of key elements:
- Determining the key hygrothermal properties of the brick, mortar and plaster samples extracted from the external walls of Keelmen’s Hospital;
- Assessing the moisture risk associated with internally insulating the external walls, comparing several insulation strategies;
- Stress-testing the insulated wall buildups to evaluate their behaviour under potential as-built in-service conditions.
Project team
Client: Tyne & Wear Building Preservation Trust
Architect: JDDK Architects
Heritage Consultant: Maya Polenz
Moisture Risk Assessment: Greengauge
Key data
“When undertaking moisture risk assessments for IWI strategies, essentially we’re trying to find an option where everything works – where all the lights flash green. Greengauge managed to find a cost-effective and pragmatic solution for the Keelmen’s Hospital project. ”
Maria Valkova, Building Physicist, Greengauge
Key features
Material testing
To allow the risk assessment to be as representative as possible, material testing of existing brick, mortar and plaster samples from the existing external walls was undertaken to determine their key material properties and to inform the inputs for the hygrothermal risk assessment. Greengauge found quite a lot of variability in bricks, and a lot of variability in the mortar, from the original historic lime mortar to cement-based pointing mortars. The results from the material testing were used to select the closest representative materials from the material database within the Delphin software.
Modelling 4 different IWI approaches
The assessment compared four potential internally insulated wall buildups:
- Option 1: Adhered Woodfibre (U-value = 0.50 W·m2K)
- Option 2: Insulating Plaster and Adhered Woodfibre (U-value = 0.43 W·m2K)
- Option 3: Insulating Plaster (U-value = 0.48 W·m2K)
- Option 4: Adhered Woodfibre with a variable diffusion resistance air and vapour control membrane. (this option had a number of sub-options with different woodfibre thicknesses and U-values)
2D moisture risk modelling
To ensure a high a degree of accuracy in our moisture risk assessments we used Delphin 6.1.6 software for the hygrothermal risk assessments. Delphin is a two-dimensional hygrothermal simulation tool based on principles outlined in BS 15026:2007. It has probably been one of the more sophisticated types of modelling that we’ve undertaken for internal wall insulation. Typically, a WUFI assessment would be 1D, but Delphin models in 2D, which can include the brick pattern and the variability in brick properties, and also look at the joist ends embedded in the walls, and of course the different internal wall insulation strategies.
Key risk factors
When a building element is designed or retrofitted the moisture risk factors should be assessed. The key risk factors that Greengauge focused on for this project included:
- Mould Growth This includes both mould growth on the surface and within the buildup and which can lead to negative health impacts for occupants.
- Moisture Accumulation This can often be as surface or interstitial condensation and refers to the risk of moisture accumulating at a specific location on or within the buildup which could lead to condensation occurring.
- Timber Decay Elements within a buildup that consist of either pure timber or are timber based (i.e. plywood, woodfibre, OSB etc.) can be at risk of decay and strength loss when their water contents exceed a certain risk threshold.
The impact of orientation on moisture risk
Wall orientation was found to have a significant influence on the hygrothermal conditions within the walls. The Keelmen’s hospital building is built in a square around a central courtyard, so has varying orientations. The SW orientation, , presents the most severe conditions in terms of moisture risk, followed by NW, SE and NE. The risk assessment was therefore split according to high, moderate and low risk orientations.
IWI solutions
The hygrothermal analysis led to different solutions being adopted for different orientations.
- Greengauge proposed an IWI strategy based on Option 4 using varying thickness of wood fibre alongside a vapour control membrane, offered a pragmatic approach that takes moisture risk and orientation into consideration.
- In the most risky and exposed orientation, Option 3 was recommended – use of insulating lime plaster only.
- Option 3 could theoretically have been applied across all elevations. However, it was rejected due to the significant cost and risk that removing the existing internal plaster (which was required for Option 3 but not for Option 4) would have posed to the underlying brickwork. Limiting this approach to only where strictly necessary was key to minimising disruption to the historic fabric.
- In the less risky walls and orientations, the wood fibre insulation was installed at a 40mm thickness.
Removing paint finishes
The paint finishes on some of the external and courtyard walls added complexity to the project. Part of Greengauge’s brief was to assess whether or not the paint needed to be removed before insulation was applied, an undertaking which would have had big cost implications.
The process of removing the paint could have also damaged the historic brickwork. Greengauge’s analysis helped the team minimise damage by identifying that it could be retained, rather than needing to be removed.
Historic England
Historic England has published an Energy Efficiency and Decarbonisation case study on the design-phase decision-making for the repair and reuse project at Keelmen’s Hospital.
It demonstrates how energy efficiency and decarbonisation goals can be balanced with the protection and enhancement of heritage significance while meeting statutory requirements.