Solid masonry with external insulation

This in theory is the easiest method to attain the Passivhaus goals for walling, and can be achieved with a wide choice of rigid and semi rigid insulation materials. The structure, usually of concrete or blockwork, is inside the thermal envelope providing good thermal mass. The airtightness layer is usually applied to the outside of the structure in the form of a flattening render coat, in preparation for the adhesive layer for the insulation. Thermally broken mechanical fixings are usually required. The rainscreen is commonly a render system, with integral keying membrane.

It is crucial to inhibit air movement between the insulation and the structural walling, as any thermal bypass would seriously diminish the thermal performance. It is important to have a flat surface on which to fix the insulation and to apply adhesive evenly with a notched trowel.

Advantages and disadvantages of solid masonry with external insulation

Advantages Insulation and structure are completely separated, meaning that thermal bridging can be minimised. With this system, quality of workmanship is, on the whole, visible, making quality assurance easier. There is also no need for cavity trays with this method of construction. All trades can proceed within the building without the worry of damaging the airtightness layer.

Disadvantages External rendering can be easily damaged because there is no structure behind it. If heavier cladding (such as brick slips) is used there may be significant thermal bridging problems if supported off the internal structure. Contractors in the UK do not appear to be experienced yet in the care needed to minimise thermal bypass, and external insulation systems to reach Passivhaus standards seem expensive in the UK at the present.

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