Optimum IWI build-up for a solid natural stone (Granite) wall

[Grant McCowan]

Assessment: We have a customer in Aberdeen with a property that has been constructed in granite circa 1900.  The property consists of a duplex flat on the first floor with a loft conversion to form a bedroom and communal living area on the landing.   There is considerable dampness and mould growth throughout the first floor rooms despite significant infiltration (measured air permeability of 14.4 m³/m².h @ 50Pa).  Remedial works have been carried out to the northwest facing walls in the two adjacent bedrooms: the lath and plaster has been replaced by plasterboard with a single vent offering approximately 10,000 mm² ventilation to the habitable space.  There is no insulation to this wall.  This has not resolved the issues.  The lath and plaster on the other walls on this floor all show signs of mould growth.

Retrofit Strategy:

Insulation: Diathonite and wood fibre to granite walls and between and over rafters

Airtightness: Target: 2 m³/m².h @ 50Pa; Lime plaster to walls and intelligent membrane to rafters

Windows: Existing uPVC double glazed units to rear to be modified (remove trickle ventilation) and retained existing aluminium framed dg units to front to be replaced with triple glazed; Velux roof windows to be retained.

Ventilation: MVHR

Proposed treatment of solid granite walls: Replacement of linings and application of lime plaster as the internal airtightness layer on 80mm wood fibre boards (bonded with adhesive and mechanical fixings) on a 15mm Diathonite as a thermal levelling layer ended up with u-values for different walls in the 0.33-0.35 W/m²K range and satisfied WUFI analysis on a project in Preston.  The same construction is being considered for this property in Aberdeen.

Granite is less permeable than sandstone.  Does the impermeable nature of granite compared to sandstone warrant a different treatment i.e. does the lower thermal transmittance value and permeability permit a relaxation on the rule of thumb to keep it between 0.3 and 0.4W/m²K in general?

 

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