Air-Tightness & Infiltration

Insulation slows the heat that conducts through the fabric, but heat also escapes a second way: bodily, carried out in warm air leaking through gaps in the construction and replaced by cold air that must be heated from scratch. A beautifully insulated but leaky building still feels draughty and still costs a fortune to heat. Airtightness is the discipline of controlling that leakage, and it sits alongside insulation as one of the two halves of fabric performance. This lesson covers why leakage matters, how it is measured, and the "build tight, ventilate right" principle that stops airtightness becoming a health problem.