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The RIBA seminar Retrofit of Historic Buildings on May 18th was chaired by Carrie Behar of University College London. The context was set with a summary of the landscape of challenges ahead for energy saving in the UK to reduce emissions by 2050. Of the 27 million domestic properties 5 million are of traditional solid wall construction. Heat loss from homes was summarised as: 25% through the roof, 15% via floors, 35% from walls and 25% from windows and cracks. Retrofit is a broad and at times technical subject. The presentations were selective and focused which proved valuable.

Insulating Suspended Timber Floors
Bill Gething shared his experiences on work undertaken, drawing on current research by Sophie Pelsmakers and John Willoughby. The detail of thermally upgrading the floor was reviewed: moisture penetration controlled, insulation installed to minimise thermal bridging, air gaps sealed at floor board level. All so good so familiar….yet the punchline was unexpected. Rather than a tale of a fantastic U-value and caution of damp to the end of the joists, mould in the below floor void was discovered.

Research on mould is in its infancy, however Bill stated that last Friday Sophie Pelsmakers had reported research on this matter which analysed risk thresholds for optimal conditions for mould growth. Conditions require high relative humidity (approximately 76% and above) and high temperatures to combine for extended durations. Conditions in the UK are optimal in the Summer. Mould spores migrate and can travel into inhabited spaces and affect health.

Bill warned structural damage to timber joists may not be present as visual indicator, with mould likely to lurk in the hidden void and remain undetected. To conclude advice is to ensure all organic matter is removed from floor voids to reduce the ‘food’ for mould to propagate – this includes timber off-cuts.

Internal Wall Insulation (IWI)
Caroline Rye of ArchiMetrics Ltd presented research carried out for the Society for the Protection of Ancient Buildings (SPAB). The work compared two installations of IWI to traditional solid wall construction – one with a vapour control layer (VCL) and one without. Material of traditional solid walls is porous, permeable and hygroscopic – able to move moisture around within the construction both drawing it out of a habitable space and releasing it to maintain balanced local environmental conditions. Good practice advises that if installed a VCL should be located on the warm side of the insulation to avoid formation of condensation.

The research measured relative humidity (RH) at different depths within the solid wall construction. Graphically Caroline illustrated that although both constructions saw a general trend of increasing RH over time within the fabric (reasons for which are not fully understood) the insulation with the VCL saw the critical point – the dew point where condensation occurs – being reached. Essentially this results in moisture trapped within the structure, consequences of which lead to accelerated deterioration of the fabric. Caroline pointed out the natural material of woodfibre board used in the installation without the VCL probably facilitated the natural tendency of the wall construction to breathe. It was also pointed out that current research suggests 40-50mm depth of IWI achieves a balance of improved U-value not sufficiently excessive to trigger the dew point.

Retrofit of Traditional Timber Windows
Nicholas Heath ran through options and considerations for upgrading timber single glazed windows. Merits and pitfalls of secondary glazing and double glazing are too numerous to summarise here but news of an ultra thin ‘vacuum’ glazing unit is interesting to note. The very thin profile is useful when considering the upgrade of existing sash windows. It is only made in Japan and is shipped over. But when compared to other double glazed units imported from Europe to the UK the vacuum unit has the least embodied energy! Nick explained this is owed to the fact it does not have gas in its composition.

Nick emphasised the need to consider a whole building performance and not to consider the upgrade of windows in isolation. Key to the retrofit of windows is the treatment of the reveals. Ideally these would be thermally upgraded concurrently to minimise cold bridging and concentration of condensation on the building fabric. The STBA will be releasing research soon on the treatment of window reveals paying particular attention to the coverage and depth of internal wall insulation.

Caroline Rye also highlighted the role of the STBA’s Guidance Wheel and Knowledge Centre in reviewing a whole building approach to retrofit, reviewed on previous blog by Progressive Heritage – click here.

The overall message of the evening was a balanced approach must be taken and it is of paramount importance to work with, not against, the breathable fabric of traditional solid wall construction if accelerated deterioration of historic fabric following retrofit interventions is to be avoided.

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