Internal Insulation in Historic Buildings

It is not dangerous, it is demanding, and the difference matters. External insulation forgives many execution errors because it leaves the masonry on the warm, dry side: even if something is done badly, the consequences are rarely serious. Internal insulation has no such margin, because it moves the wall to the cold side and reduces its ability to dry. That said, the cases where it causes damage almost always come down to three concrete, avoidable errors. The first is having done it on a wall that already had an unresolved moisture problem, typically ingress through the facade or rising damp: in that case the work does not cause the problem, it aggravates and hides it. The second is discontinuity: cavities behind the insulation and a broken vapour check, which concentrate all the condensation at points that cannot be inspected. The third is failing to assess the beam ends. If those three points are addressed, the intervention is technically sound, and it is often the only one possible on a listed building.

This is the most serious question in the whole intervention, and also the one most often skipped, because it means opening up a floor before quoting. The problem is that the beam end is embedded in the masonry, that is, in the part the insulation makes colder, and timber in a cold, damp zone is exposed to biological decay. There are three workable routes. The first is to interrupt the insulation locally around the bearing, accepting a limited thermal bridge in exchange for a beam end that stays warmer: it is the simplest solution and often the most sensible. The second is to ventilate the bearing, letting air circulate around the end so any moisture can leave. The third, where the timber is already compromised, is to intervene structurally by replacing or sistering the end. What is not acceptable is the fourth route, which is not looking: the beam end is invisible, the decay is slow, and by the time it shows the floor has already lost its bearing.

A humidity-variable check, in almost all historic buildings, and it is worth understanding why the sealed barrier looks like the safer choice and is not. The barrier does one thing very well: it stops vapour entering the wall. In winter that is exactly what is needed. The problem is that it does so in summer too, when the flow reverses and the wall, warmed by the sun, would tend to dry by giving up moisture inwards. With a sealed barrier that route is closed, and since the facade of a historic building is rarely perfectly watertight, moisture entering from outside accumulates with no way out. The humidity-variable check resolves the contradiction by changing permeability with relative humidity: in winter, with dry indoor air, it behaves almost like a barrier; in summer, with high humidity, it opens and allows drying. In both cases, though, performance depends entirely on continuity of installation: a perfect check with three open joints is worth less than a modest but continuous solution.

Little is lost, and that is why aerogel exists in this market: with thin materials the loss of area is a few centimetres per wall, and in small rooms it is often the only acceptable solution. The gain, though, should be described honestly, because it differs from that of external insulation. On heating consumption the benefit is real but smaller, because thermal bridges remain that external work would have solved, typically at floors and cross walls. On perceived comfort, however, the effect is often more evident than the calculation suggests, for two reasons: the inner surface of the wall becomes much warmer, and the cold-wall sensation felt sitting near an external wall disappears; and the room reaches the desired temperature far more quickly, which counts enormously in intermittently used spaces. In summer, by contrast, the balance is worse than before, because the masonry's inertia is lost: someone living in a historic mountain house barely notices, someone in a city notices a great deal.