Thin insulation between beams

Beams you do not want to hide

In an old house, exposed beams are part of the value of the property. Insulating between them rather than below them keeps that reading, but imposes a continuity that fitting under the ceiling would not have required.

The weak point is the joints

Insulation fitted between joists has as many joints as there are bays. Each junction against the timber is a place where air can pass, and that is where real performance is decided, not in the middle of the panel.

Why a flexible product helps

It is cut with a knife, it follows the irregularities of old timber and it is fixed by stapling then taping. An eighteenth century beam is never straight, and a rigid product would leave gaps against it.

The thickness available between joists

It is often limited, particularly in old floors. A thin product allows the air gap needed in front of the reflector to be kept, which insulation filling the whole space does not.

The air gap, again

A reflector pressed against timber no longer reflects anything useful. There must be a void in front of it, closed at its edges. That rule is the same as on a solid ceiling, and it is harder to hold between beams.

What stays visible at the end

The timber. That is the whole point of the method and its main constraint, since the work is judged on what cannot be seen but will be felt all winter.

The fitting is set out on the page about fitting thin reflective insulation, and airtightness on the airtightness page.