Why Is It Important To Include Airtightness With Insulation? – Can I Just Ask? – Ep.11

Insulation and airtightness are two distinct but inseparable parts of a well-performing building fabric, and a common misconception is that one can substitute for the other. Airtightness in this context means fabric airtightness — eliminating the uncontrolled infiltration of air through gaps and junctions in the building envelope. It is entirely separate from ventilation: a well-ventilated building can and should also be airtight, with fresh air delivered in a controlled way rather than leaking in and out wherever the fabric allows.

The reason the two must work together comes down to the physics of heat loss. Insulation addresses conductive heat loss — the transfer of heat through solid materials. Airtightness addresses convective heat loss — the movement of warm air through and around those materials. Without a proper airtightness layer, warm internal air bypasses the insulation through gaps in the fabric, carrying heat with it and rendering even a well-insulated wall far less effective than its U-value would suggest. With internal wall insulation specifically, this becomes a moisture issue as well as a thermal one: warm, humid room-side air tracking through the fabric and reaching the cold wall behind the insulation will deposit moisture there, creating exactly the conditions that lead to long-term damage.

This is why vapour control and airtightness layers are included as standard in internal wall insulation specifications — not as an optional extra, but as an integral part of making the insulation work as intended. Once both are in place together, a further benefit follows: with air movement through the fabric controlled, maintaining good indoor air quality through a deliberate ventilation strategy becomes significantly more straightforward.