Building and Installation

Chimneys are one of the most persistent sources of heat loss and air leakage in traditional buildings. This episode covers how to insulate and seal a chimney properly — from lining and backfilling the flue, to airtightness at the opening and insulating the breast — without creating moisture problems or fire risks.

Show more11:07Claude responded: In timber frame construction, where you put the OSB matters more than most people realise. In timber frame construction, where you put the OSB matters more than most people realise. Placing it on the internal face of the frame gives you better moisture control by keeping the higher-resistance layer on the warm side, makes airtightness detailing far simpler, and helps absorb the natural movement of the timber before it reaches your plasterboard. If OSB has to go on the outside, you need a high-resistance internal membrane and solid airtightness to avoid condensation problems.

Suspended timber floors are a common source of heat loss in older homes, and insulating them is usually straightforward — friction-fitting a flexible batt between joists, supported by a breather membrane below and a vapour control layer above. The critical part is managing moisture in the floor void. Cross-ventilation, hit-and-miss dwarf walls, and a ground membrane are all essential. Where reliable ventilation isn't possible, replacing the suspended floor with a solid construction using something like Foamglas aggregate is often the safer long-term solution.

Insulation only works when it's continuous. Even small gaps allow air to move through the build-up, and that convective movement can strip heat away far faster than conduction alone. With flexible batts, it's better to slightly oversize so you get a snug friction fit with no voids. For internal wall insulation using capillary-active materials like wood fibre, gaps are doubly problematic — they break the contact with the masonry that the material relies on to manage moisture, turning a potential buffer into a condensation risk.

Brick creams are siloxane-based hydrophobic treatments that soak into masonry and reduce water ingestion without fully sealing the surface. They can be a useful tool, particularly in retrofit projects where adding internal insulation reduces the wall's ability to dry out. By cutting down how much rainwater gets in, they help restore some balance. That said, they're irreversible, they degrade over time, and they won't fix underlying problems like failing gutters or poor pointing. Useful in the right circumstances, but not a substitute for good detailing.

Insulation slows heat loss through materials, but it can't stop warm air escaping through gaps — that's a job for airtightness. Without it, air leakage bypasses the insulation entirely, and the warm moist air it carries can condense inside the wall build-up causing moisture damage over time. The two need to be designed together. Airtightness doesn't mean sealing a building shut — controlled ventilation still comes in, just on your terms rather than through accidental gaps in the fabric.

When you add external wood fibre insulation to a solid wall, the outer surface becomes much colder, meaning a traditional lime render stays wet for longer after rain. Silicone render solves this by shedding rainwater effectively while still allowing vapour to escape from inside. That balance of water repellency and breathability keeps the insulation drier, extends its service life, and reduces the risk of freeze-thaw damage or biological growth. It works well on other substrates too, not just wood fibre systems.

Airtightness, breathability, and vapour control are often treated as conflicting priorities but they work together when detailed correctly. Airtightness controls bulk air movement through gaps and cracks, while breathability refers to vapour diffusing through materials molecule by molecule — the two are not the same thing. A lime plaster or intelligent membrane can be airtight and vapour-open at the same time. The key is choosing the right vapour control layer for the build-up, and making sure the wall can dry in the right direction when conditions change.