Question 1

What is thermal bridging, and why is one corner of my room always damp?

Accepted Answer

A **thermal bridge** — sometimes called a cold bridge — is a path through the building fabric that conducts heat out **faster than the surrounding structure**. The surface on the inside of that path stays colder, and cold surfaces attract condensation.

**Why it produces damp and mould in specific places.** Warm indoor air holds moisture. When it touches a surface below the **dew point**, that moisture condenses onto it. A thermal bridge creates a localised cold patch, so condensation forms **there and nowhere else** — which is why the problem appears as a defined patch in a corner, along a line, or in a band, rather than across a whole wall.

Mould then grows because it needs surface moisture, and it will grow at surface humidity well below visible wetness — which is why mould can appear before you ever see condensation.

**Where bridges typically occur:**

**External corners**, where more external surface loses heat relative to the internal surface, and where air circulation inside is poorest.

**Window and door reveals, lintels and sills**, where insulation is interrupted by the opening.

**Wall–floor and wall–ceiling junctions**, and the edges of a floor slab.

**Structural elements passing through the envelope** — a steel beam, a concrete balcony, a joist end built into a solid wall.

**Wall ties and fixings** in cavity walls, and **repeating bridges** through timber or metal framing.

**Gaps and compression in insulation**, which is a workmanship issue and extremely common.

**Why it has become more noticeable.** As buildings get better insulated and better sealed overall, the remaining bridges stand out more sharply, and reduced ventilation means indoor humidity is higher. Retrofitting insulation to part of a wall can **create** a bridge where none was apparent before.

**What helps:** improving ventilation to lower indoor humidity; raising the surface temperature with insulated plasterboard or insulated reveals; leaving furniture away from cold external walls so air can move; and treating existing mould while addressing the cause, since cleaning alone guarantees its return.

Question 2

How do I tell rising damp from condensation and penetrating damp?

Accepted Answer

The three have completely different causes and completely different fixes, and misdiagnosis is expensive — most damp diagnosed and treated as rising damp is actually one of the other two.

Question 3

What is a vapour barrier and which side does it go on?

Accepted Answer

A vapour control layer (VCL), often called a vapour barrier, is a membrane restricting water vapour from passing through a building element into the insulation and structure behind it. Why it exists.

Question 4

What causes condensation on the inside of my windows?

Accepted Answer

Condensation forms when moist air meets a surface cold enough to bring it below its dew point. Windows are usually the coldest surface in a room, so they show it first. The moisture is not coming

Question 5

Why is my wooden floor cupping or gapping?

Accepted Answer

Because wood moves with moisture, and both symptoms are the same mechanism producing opposite results. The underlying principle. Wood absorbs and releases moisture until it reaches equilibrium with

Question 6

Why does my house make cracking noises at night?

Accepted Answer

Almost always thermal movement — building materials expanding and contracting as temperature changes, with the sounds concentrated in the evening because that is when the temperature is falling

Question 7

What is a cavity tray, and why does it matter?

Accepted Answer

A barrier built into a cavity wall to collect water that has crossed the outer leaf and direct it back outside — and missing or badly installed cavity trays are among the most common causes of

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