Fitting insulation that is airtight and watertight

Two kinds of tightness often confused

Watertightness concerns liquid, airtightness concerns the movement of gas through the surface. A product can stop one without stopping the other, and it is the second that weighs most heavily on a heating bill.

Why air costs more than conduction

A trickle of air crossing a ceiling carries heat away far faster than it travels through the material itself. A perfectly insulated surface pierced by air paths behaves like a mediocre one.

Overlapping the widths

Two widths laid edge to edge leave an open line. They overlap by a few centimetres and that junction is closed with a suitable tape, which turns a succession of strips into a continuous membrane.

The perimeter, the point most often forgotten

The edge of the ceiling, the corners and the penetrations are where air passes. Perfect fitting in the middle and careless work at the edges gives an average result, because performance always follows the weakest point.

Penetrations

A duct, a cable or a conduit each create an opening. Each is dealt with individually at the moment it is made, never afterwards, because a closed plenum is not reopened to correct a detail.

How to check the work

By hand, before closing up, feeling for draughts along the edges on a windy day. It is a rough method but it finds most of the defects a visual check lets through.

Airtightness is developed on the dedicated page, and watertightness on the corresponding page.