Solid walls have no cavity, so their inside face gets very cold in winter and condensation forms on it, leading to black mould. Better ventilation and steady heating help, and where the mould keeps returning, a thin internal thermal lining such as Isotherm can warm the wall surface so condensation is far less likely to form.
A lot of houses round here, older terraces and stone cottages especially, have solid walls. There’s no cavity, just brick or stone right through. In winter the inside face of an external solid wall can get very cold, and when warm, moist air from everyday living touches it, the moisture condenses. Give it a few weeks and you’ve got black mould in the corners, round the window reveals and behind the furniture.
Painting over it or wiping it down doesn’t last, because while the wall stays cold the mould keeps coming back. You’ve got two levers to pull: reduce the moisture in the air and raise the temperature of the wall surface.
Start with the basics, which cost little or nothing: use extractor fans, keep trickle vents open, heat the house steadily rather than in short bursts, keep furniture off cold walls and dry washing outside or in a vented dryer. Consider continuous mechanical ventilation, such as a PIV unit, if the house is generally humid.
If that isn’t enough, which is common in small rooms with two cold external walls, look at warming the wall itself. Full internal wall insulation with insulated boards is one option, but it takes up room space and on solid walls it needs careful design, because the wrong system can trap moisture within the wall.
A thin thermal lining like Isotherm (ISO-THERM) is a good middle ground. It’s a thin insulating layer bedded in its own adhesive and skimmed over, so you lose very little room space. It lifts the surface temperature of the wall, so it is far less likely to drop to the dew point where condensation forms. We take it into the window reveals too, because they’re often the coldest spot in the room.
Two important points. First, a thermal lining is for condensation on cold walls. If the wall is wet from rain getting in, a leaking pipe or bridging at the base, you must fix that first, or you’re just covering up a problem. Second, it’s not a substitute for ventilation. You still need to get the moisture out of the house.
Get the diagnosis right, deal with any water getting in, improve ventilation, and then warm the wall. In that order.
When to call a specialist
If mould keeps coming back on cold external walls even though you're heating and ventilating properly, have the walls assessed to confirm it's condensation before choosing a lining. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
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Frequently asked questions
Does Isotherm stop mould?
It warms the wall surface so condensation, and therefore mould, is much less likely, but ventilation and fixing any water ingress are still needed.
Will thin internal insulation make my room smaller?
Very little; a thin lining with a skim finish adds only a small amount to the wall thickness.
Can I put insulation on a damp solid wall?
Not until the cause of the damp is fixed; insulating over a wet wall can hide and worsen the problem.