Question 1

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.** Warm indoor air holds far more moisture than cold air. That vapour migrates through walls, floors and roofs toward the colder outside. If it reaches a point cold enough for it to condense — inside the insulation or against a cold surface within the construction — you get **interstitial condensation**: hidden damp, degraded insulation performance, rot in timber and corrosion in fixings, all invisible until serious.

**The rule for which side.** The vapour control layer goes on the **warm side** of the insulation — the inside face in a UK climate. This stops moisture entering the construction in the first place.

The corresponding principle for the outside is that the construction should get **more vapour-open toward the outside**, so any moisture that does get in can escape rather than being trapped. The shorthand is **"vapour tight inside, vapour open outside"**.

**Getting it backwards is worse than omitting it**, because a barrier on the cold side traps moisture inside the construction with no route out.

**Where it matters most:** internal wall insulation, warm-roof loft conversions, timber frame construction, and insulated floors.

**Vapour barrier is not the same as a breather membrane.** A **breather membrane** goes on the cold side, keeps liquid water out while allowing vapour to escape — the opposite specification, and the two are routinely confused on site.

**Airtightness matters as much as the membrane.** Most moisture reaches a construction by **air movement through gaps** rather than by diffusion through materials. A perfect membrane with unsealed penetrations at sockets, pipes and junctions does little. Tape joints and seal penetrations.

**Ventilation is the other half.** Restricting vapour into the structure means it stays in the room, so extraction and background ventilation must handle it.

**Get a specific design** for anything complex — this is where retrofits fail.

Question 2

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 3

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

Question 4

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

Question 5

What are trickle vents and do I need them?

Accepted Answer

Trickle vents are small, controllable slots in a window frame or head that provide continuous background ventilation without opening the window. Why they exist. Modern homes are sealed far more

Question 6

Why do old houses need to breathe?

Accepted Answer

Because they were built without a damp-proof course or a cavity, and are designed to absorb moisture and release it again. Sealing them with modern materials traps that moisture inside the fabric,

Question 7

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.

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