Why poor insulation installation is often mistaken for product failure

All insulation types, whether they be fibrous, expanded foam or other technologies can deliver long-term performance when correctly specified, fitted, supported, protected from moisture, and installed to NZS 4246. IAONZ promotes installation quality, training, and evidence-based practice across all types. We discuss fibrous insulation in this article. There is a long-standing perception in some parts of the building sector that some fibrous insulation products can collapse or lose loft over time and therefore lose thermal performance.

It is an understandable concern, especially when older insulation is seen during renovation, demolition, or upgrade work.  In many cases, what is described as “slumping” is not evidence of product failure. It is more often the visible result of poor installation, lack of support, moisture damage, compression, disturbance, or older building practices that pre-date modern insulation standards and training.

This distinction matters.

Fibrous insulation, whether glass mineral wool, rock mineral wool, polyester, sheeps-wool-based, or other fibre types, relies on correct specification and correct installation. When the right product is installed properly, remains dry, is not compressed or disturbed, and is appropriately supported, it will provide long-term thermal performance for the building.

International context

Internationally, insulation industry bodies and energy-efficiency organisations place strong emphasis on installation quality, product suitability, and quality assurance.

In Australia, industry organisations have focused strongly on installer accreditation, safe installation, product compliance, and independent quality processes. In North America, fibreglass/glasswool and mineral wool industry bodies have directly addressed claims about settling and long-term performance. 

IAONZ’s position is consistent with that international direction: insulation performance is not determined by product alone. It depends on correct specification, correct installation, correct support, protection from moisture and damage, and appropriate quality assurance.

Walls: poor fit is often mistaken for slump

In wall applications, a common issue is not that insulation has slumped over time, but that it was not properly fitted in the first place.

NZS 4246 provides clear guidance for insulation installation. Wall insulation needs to fit firmly within the framing cavity, typically by being cut slightly oversize, around 5 to 10 mm, to create a friction fit. This helps ensure the insulation fills the space properly and remains in contact with the surrounding framing and linings.

If insulation is cut too small, loosely pushed into place, folded, compressed, or left unsupported, it may later appear to have slumped or collapsed. That can happen with any fibrous insulation type. The underlying issue is installation quality.

Another issue arises where the insulation thickness does not match the wall cavity depth. For example, if 90 mm insulation is installed in a 140 mm wall cavity, the insulation needs to be restrained so it remains in the correct position, generally against the internal lining side. Without restraint, the insulation may sit in the wrong position within the cavity, reducing effective performance and creating the appearance of movement or slump.

Again, this is an installation and support issue, not a material failure.

Ceilings: insulation needs to be properly maintained after installation

Ceiling insulation also needs to be considered in context.

Once correctly installed, ceiling insulation should not be walked on, compressed, displaced by storage, disturbed by other trades, or allowed to get wet. If insulation becomes wet, the primary issue may be a roof leak, plumbing leak, condensation problem, or other building defect. Wet insulation will lose performance and should be replaced, but the cause needs to be addressed at the building level.

Older ceiling insulation can also look thin when viewed many years later. That does not automatically mean it has lost loft. Many older homes were insulated to much lower R-value requirements than those applying today. The insulation installed at the time may have been significantly thinner than modern ceiling insulation. When viewed beside current products, older insulation can easily appear to have reduced in thickness, even when it may simply reflect the lower specification of the time.

Renovations often reveal old installation practice

Many claims of “slumped” insulation arise when walls are opened during renovation or demolition. Once the internal lining is removed, poorly fitted insulation becomes obvious.

This needs historical context. NZS 4246 was first introduced in 2006. Before that, New Zealand had less consistent guidance for insulation installation, fewer trained professional installers, and less focus on installation quality. Insulation was often installed by people without specialist training, including general labourers, apprentices, or other trades.

If insulation was simply pushed into a wall cavity without being cut correctly, without full edge contact, without restraint where needed, and without a quality check, it is not surprising that the result looks poor years later.

That should not be confused with a failure of a particular product.

The real lesson: installation quality matters

Insulation performance depends on both product quality and installation quality.

A compliant insulation product that is badly installed will not deliver its intended performance. Gaps, folds, compression, voids, misalignment, poor support, moisture damage, and post-installation disturbance can all reduce the effective R-value of the building element.

The better conclusion is not that “fibrous insulation slumps”. The better conclusion is that poor installation has been around for a long time, and renovations continue to expose it.

New Zealand now has a much stronger focus on thermal performance, energy efficiency, healthy homes, and building quality. NZS 4246 provides the technical basis for good insulation installation, and IAONZ training helps installers apply that guidance in practical site conditions.

Correctly specified, correctly installed, properly supported, dry, and undamaged insulation provides long-term benefits for the building and its occupants.

The value of good insulation is not just in the product. It is in the product, the specification, the installation, and the quality assurance working together.

That is what delivers performance for the life of the building.