Why Is It Important to Avoid Air Gaps In Insulation Layers? – Can I Just Ask? | Ep.13

Continuity of insulation is one of the most important — and most frequently compromised — principles in both retrofit and new build. A common scenario on site is a 170mm joist or rafter cavity filled with 160mm of insulation, with a board fixed over the top and the remaining 10mm gap simply ignored. That gap, small as it seems, creates a pathway for air movement through the structure — and on a cold, windy day, precisely when the insulation is supposed to be working hardest, convective loops within that void can bypass the insulation almost entirely. The rule of thumb is straightforward: flexible batts between timbers should always be cut slightly oversized so they friction-fit with no voids, and all insulation layers should be kept in direct contact with one another throughout.

The airtightness layer deserves the same attention. A membrane installed with even a small gap between it and the insulation allows air to circulate in that space, carrying heat with it and undermining the performance of both layers. U-value calculations assume perfect contact between all elements — as soon as gaps are introduced, those assumptions break down and the as-built performance can fall well short of what was designed.

For capillary-active insulations like wood fibre, direct contact with the wall surface is especially critical. The moisture-buffering properties that make wood fibre such a valuable internal wall insulation material depend on physical contact — moisture moves through capillary transfer, not just vapour diffusion, and an air gap severs that connection entirely. Worse, a void between an insulation board and a cold wall creates exactly the warm, humid, still conditions in which mould thrives. Ventilating that gap is sometimes proposed as a solution, but it introduces wind washing and cold air movement on the back face of the insulation, which creates its own set of problems. The conclusion is simple: don’t do gaps.