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

Mind the Gap

Insulation doesn’t just work because it’s there, it works because it’s continuous. Gaps, even tiny ones, can dramatically reduce thermal performance, increase moisture risk, and undermine your airtightness strategy. In both retrofit and new build contexts, overlooking the detailing of insulation continuity can mean the difference between high performance and high bills.

If you’d prefer to watch the video, just click play below to watch Chris and Andy from our technical team talk through this question in Can I Just Ask? – Ep.13 👇

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The Invisible Weak Spot: Air Gaps

Whether you’re specifying for a timber frame, retrofitting a Victorian terrace, or building a passive house from scratch, one principle holds true: insulation must be continuous to be effective. Gaps in insulation, whether from undersized batts, poor fitting, or design oversights, create thermal bridges and allow air movement through the structure. That air movement, especially in windy conditions, can carry heat away just when you need it most.

It’s not uncommon on site to hear things like: “We’ve got a 170mm deep rafter, so we’ll just fit 160mm of insulation and it’ll be fine.” It won’t be. That 10mm gap might seem negligible, but even small voids can allow significant convective air movement. This “wind washing” effect rapidly degrades thermal performance, turning carefully calculated U-values into wishful thinking.

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Overstuff It – Within Reason

When using flexible insulation between timber studs or joists, it’s usually better to slightly oversize the insulation so it fits snugly. A tight friction fit helps eliminate voids and ensures good contact with surrounding materials. This is particularly important when detailing layers for airtightness.

The air control layer, typically on the warm side of the insulation, should be pressed tightly against the insulation itself. Any separation here can form a secondary cavity where air can circulate, bypassing the thermal layer and leaking energy. Think of it as your insulation wearing a windbreaker: the closer the fit, the better it works.

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Internal Wall Insulation: Contact is Key

When using capillary-active insulation materials like wood fibre internally, the stakes are even higher. These materials rely on direct contact with the masonry to function properly. Gaps between the insulation and the wall don’t just reduce thermal performance – they also compromise moisture management.

Wood fibre insulation, for instance, is designed to absorb, store, and redistribute moisture through capillary action. Introduce a gap, and that capillary pathway is broken. Instead of buffering humidity and protecting the wall, the cavity becomes a breeding ground for mould, especially if it’s warm and unventilated.

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Designed vs Delivered: Closing the Performance Gap

In modelling and U-value calculations, we assume perfect contact between all layers – insulation, airtightness membranes, substrates. Any deviation from that ideal introduces a performance gap. Gaps don’t just bypass thermal protection; they also complicate moisture control, lead to interstitial condensation, and make it harder to hit energy targets.

The solution? Don’t do gaps. Detail insulation carefully, install it snugly, and ensure good contact between all layers. The payoff is real-world performance that matches your design intent, and a building that’s warmer, healthier, and more resilient.