Visual inspection is a mixture of subjective judgment based on information gathered from our five senses, and our gut feel and weight of experience. In other words our judgment is limited. Objective measurement tools also have their limitations. The tools available include moisture meters, simulations, thermal imaging cameras, blower door testing, materials and biological analysis.

What are the limitations?

Thermal imaging can only show temperature not moisture, and moisture meters don't consider temperature. Biological analysis tells us what is there not how it got there. Blower door testing shows overall leakage rates related either to the internal air volume of the building, or the surface area of the building. It doesn't measure mould spores, wet or dry, decay, structural integrity, material composition or age.

Blower door testing does give an objective number, a 'rate of leakage' shown in number of air changes per hour. However what you do with that number becomes subjective.

• It does give an indicator of thermal performance (subjective) because "A house that bleeds a lot needs a lot," but how much is too much?
• It does give some indication of wetting and drying capacity of the structure, though we would argue the reason for wet structure could easily be due to lack of airtightness (subjective).
• It can give an indication of indoor air quality (other measurements also required).
• It will be an indicator of flanking sound pathways when considering acoustics.
• It does show particular leakage pathways for example around windows — regardless of single or double glazing.

So what use is airtightness and its measurement?

Firstly, the principle of draught stopping is simple, draughty buildings are uncomfortable to exist in. Draught stopping is the same as airtightness, just at a different point on the continuum because "the more controlled [comfortable] the environment, the more liveable [fit to inhabit] it becomes."

Secondly, draughty buildings cost more to run, and are by default less durable.

Blower door testing aims to identify weak points in the building envelope of an existing building, or in the airtightness layer during construction and assist in repairing defects in order to prevent structural damage due to vapour convection.

So why wouldn't a test for airtightness be a supporting measure of building performance?

I would suggest it is an important consideration, especially for newer houses.

An objective measure of existing buildings against a recognised standard is another tool to better understand and predict building performance.