Hygrothermal Analysis Using WUFI to Safely Upgrade a Beautiful Town House

Carlisle Terrace is a row of town houses in a Conservation Area in Dublin. The new owners of one of the houses were keen to improve the energy efficiency of the building, and Duggan Architecture proposed a scheme that involved internally insulating the front elevation, with external wall insulation to the side and rear which are less sensitive. This type of scheme is unusual in the area, and the planning officer asked for more evidence that the proposed insulation would not risk damaging the structure.

Working with Joseph Little, we undertook WUFI calculations and provided a hygrothermal risk assessment of the proposed wall insulation, giving the design team and owners comfort and satisfying the planning officer’s query.

Following best practise established in a Historic Scotland paper authored by Joseph, the granite rubble and lime wall was modelled as a ‘sandwich’ in WUFI Pro (a one-dimensional software tool). A climate file was created, and a ‘pre-conditioning’ simulation was run to establish the likely existing hygrothermal conditions in the wall. This is a process of simulating the existing structure for a number of years until an equilibrium is reached that will be more representative of the existing moisture content and temperature profiles than the default settings of the materials straight from WUFI’s database.

A selection of insulation scenarios were then modelled to establish the risk associated with both the IWI and EWI. Again, as per best practise, the system was ‘stress-tested’ by applying additional moisture loads due to rain ingress (e.g. from cracks in the render or compromises around window reveals), and from air circulation behind the insulation, due to imperfect airtightness.

A direct-adhered woodfibre system was assessed for the IWI case. A concern had been raised that the system did not incorporate an Air and Vapour Control Layer (AVCL). Although this is standard practise, it is based on theory that ignores liquid water movement and storage (the Glaser Method). A more comprehensive approach reveals that it is important to allow walls such as this to dry to the inside as well as the outside, which a conventional AVCL prevents. We were able to show via simulation that the wall remained drier at all times using a simple lime plaster without an AVCL.

Using WUFI Pro we were able to show that with suitable materials and techniques, the proposed IWI and EWI could be undertaken safely, and the planning officer was able to accept the insulation proposals.

 

Photo credit – Colm Duggan, Duggan Architecture

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