Airtightness in Buildings

About this course.

Building airtightness is the ability of the building fabric — the external walls, roof, floors and the junctions between them — to resist the uncontrolled flow of air between inside and outside. The short answer to why it matters is that air which leaks through unplanned gaps and cracks carries heat and moisture with it, so a leaky building is harder and more expensive to keep comfortable, more prone to draughts and uneven temperatures, and more vulnerable to hidden condensation and mould. Airtightness is not about sealing people into a box; it is about making air movement deliberate and controlled rather than random and wasteful.

It helps to separate three ideas that are often muddled. Insulation slows the conduction of heat through solid materials; airtightness stops air itself moving through gaps; and ventilation is the intentional, controlled exchange of air for health and comfort. A wall can be thickly insulated and still leak air badly, and when it does, the insulation cannot perform as intended because air is bypassing it. Airtightness and insulation are partners: insulation reduces heat flow through the fabric, and airtightness stops warm or cool conditioned air simply escaping and being replaced by unconditioned outside air. Neither substitutes for the other.