Cavity and Loft Insulation

Two different jobs share one logic and one risk. Insulating a loft floor means stopping heat loss where it is greatest — heat rises — with work that does not disturb the occupied rooms, needs no scaffold and takes a few days. Blowing insulation into an existing wall cavity means filling a void that is already there, without touching either facade or interior: on the cavity-wall houses of the sixties and seventies it is often the only feasible way to insulate. The shared risk is that the result cannot be verified by eye. A badly insulated loft shows up later, in the bills and in condensation; a badly filled cavity does not show up at all, because the wall is closed and the holes are made good. This is why thermography is not a flourish: it is the only way to know whether the work paid for was actually done, and it must be run when there is a temperature difference and before final payment. Before blowing, a survey is needed, and it is not a bureaucratic formality. It must be established that the cavity really exists and is continuous, that it is not already partly filled with building debris, and above all that it is not wet: blowing a hygroscopic material into a wall that takes water means creating a permanent sponge inside the wall. In many buildings, moreover, the cavity is interrupted by lintels and ring beams that create separate chambers: miss them and half the wall stays empty. In the loft a different rule decides everything: above the insulation you need ventilation, below it you need air-tightness. Warm humid air rising from the flat through an unsealed hatch or a leaky downlight reaches the cold insulation and condenses; if the loft does not ventilate, that moisture never leaves and wets the roof structure. The two go together: ventilating without sealing is not enough, sealing without ventilating is worse.

It is the formula to remember, and it is worth understanding what happens when only half of it is applied. Insulating the loft floor makes everything above the insulation cold: the roof structure, formerly lukewarm, now sits at outside temperature. If warm humid air keeps rising from the flat — through the hatch, through downlight openings, through pipe penetrations — that air meets cold surfaces and leaves its moisture there. Sealing without ventilating produces the same effect by another route: even the little moisture that gets in anyway has no way out, and accumulates. Ventilating without sealing means cooling the surfaces further while still pushing vapour upwards: ventilation removes moisture, but if the vapour flow is continuous it is not enough. The two operations are neither alternatives nor ranked in importance: they are two halves of a single measure, and insulation laid without both is the premise of a roof structure that rots in silence.

There are cases where the best work is the work not done, and recognising them is the designer's responsibility before the contractor's. The first case is the damp wall: if the cavity is wet from rising damp, from facade penetration or from a flashing defect, filling it means putting a hygroscopic material into a permanently damp environment. The insulation saturates, loses most of its performance, and the water that used to evaporate in the ventilated cavity is now trapped: decay accelerates instead of stopping. The second case is the cavity that has a function: in some facade constructions the air space is part of the drainage system for water passing through the outer leaf, and filling it brings that water into direct contact with the inner leaf. The third case is insufficient width: below a few centimetres the benefit is marginal and does not justify the risk. In all three, the right answer is to tackle the cause of the damp first and reconsider afterwards.