Retrofitting external wall insulation update

This is a BRANZ research update on retrofitting external wall insulation.

In Build 136, BRANZ said:

Injected or blown-in insulation can only be retrofitted if there is undamaged wall underlay or building paper installed (older houses and houses with brick veneer cladding often did not have building paper). The wall underlay prevents the insulation coming into contact with the back face of the cladding and stops water leakage through the cladding wetting the insulation.

Injected (or blown-in) insulation must not be installed into any drained and ventilated cavity (especially brick veneer) – it will restrict the cavity drainage and drying.

Installation requirements

To inject or blow in insulation, holes must be drilled into each section of wall framing cavity separated by studs and dwangs.

While installing these insulation materials from the outside is less disruptive to the building occupier, it creates a higher risk to the weathertightness of the cladding and damages the wall underlay. E2/AS1 requires underlay (where present) to be undamaged, so any tears need to be repaired and holes taped.

Installing insulation from the inside has the advantage of not damaging the exterior cladding or the wall underlay, but redecoration may be needed. 

With both application methods, it is not possible to visually determine the extent or density of the insulation injected into each section of wall cavity. Insufficiently dense insulation or gaps around the framing will reduce the thermal performance of the insulated wall. The density of EPS (expanded polystyrene) is fixed by the bead size and will not change. Urea formaldehyde foam shrinks as it cures, which will reduce the thermal performance. 

Since the above article was published further work has been carried out by BRANZ and others on the options for retrofitting of blown-in or injected insulation to existing walls. 

One important change relates to fitting insulation from the inside (removing linings) to walls that have no wall underlay. BRANZ research has shown that installing wall underlay hard against an existing cladding can cause moisture transfer and result in moisture accumulation within the insulation. As a result, BRANZ now advises that a 20 mm gap must be left between the retrofitted underlay or rigid insulation material and the back of the cladding. This is to allow for some drainage and drying and prevent direct contact. 

Some of the retrofit industry’s justification for drill-and-fill from the outside has been based on comparison with the moisture management ability of retrofitted underlay installed directly against the back of wall claddings. This is not an appropriate comparison as previous water leaks are much less likely to be detected because wall cavities are not able to be inspected before installation. 

Testing of loose-fill insulation by BRANZ and UK laboratories have found (so called) water-repellent fibres can transfer moisture without needing to absorb (and then wick) water from the back of the cladding. 

If the drill-and-fill installation method is proposed, it should carried out from the inside and should only be done where a wall underlay is present and the wall cavities are free of any moisture issues. This ensures that the moisture management performance of the underlay and the wall cladding is maintained. 

For more details on retrofitting blown-in insulation, see BRANZ Study Reports SR233 and SR234, available free on the BRANZ website. 

Remember that the installation of insulation to existing external walls requires a building consent unless a specific exemption is applied for and granted by the territorial authority. To reiterate the Build article above, blown-in or injected insulation must not be installed into any drained and ventilated cavity (especially brick veneer). 

From BRANZ Guideline, August 2016

Published in building.