Vapour barrier

Air is not only oxygen and nitrogen — it always contains a small amount of water vapour, an invisible gaseous water. How much depends on the temperature.

Warm air can hold far more vapour than cold air. That is the key to the whole subject. The figures from AMU-Syd's compendium illustrate it clearly [Grundlæggende isolering s.18]:

• At 30 °C air can contain at most 30 g of water per m³
• At 20 °C: about 17 g/m³
• At 7 °C: about 7 g/m³
• At 0 °C: about 4.5 g/m³

When warm, humid air meets a cold surface, the air layer close to the surface cools down. The cold air layer can no longer hold all the water vapour, and the surplus condenses — densifies — into liquid water. It is exactly the same physics as:

• The droplets on the outside of a cold can of cola. Take it out of the fridge at 5 °C into a room at 22 °C and 60 % humidity: the room's dew point is 13.9 °C, and the can's 5 °C is far below it. Result: visible water droplets.
• The bathroom mirror after a shower. The mirror's glass surface is marginally colder than the vapour-saturated air, and vapour settles on it as a film.
• The face mask in winter — warm breath vapour meets the cold inner side of a winter jacket and turns into moisture in the clothing.

In technical insulation the problem is the same — just on a much larger scale and with long-lasting consequences. A cold pipe at 7 °C in a server room or a commercial kitchen will have a surface temperature below the room dew point. Vapour in the room air will migrate towards the pipe, and every single droplet that condenses does not disappear — it accumulates in the insulation, eats away the λ-value, corrodes the pipe and eventually drips down onto the ceiling below.

The vapour barrier is the physical barrier that stops vapour migration before it reaches the cold surface.

Water vapour is a gas, and like all gases it moves from high pressure towards low pressure. The vapour pressure in a body of air depends on two things:

1. How much vapour the air actually contains (absolute humidity)
2. How much vapour the air can hold at its temperature (saturation pressure)

Warm air has a higher saturation pressure than cold air. Even if two bodies of air have the same relative humidity (RH %), the warmer air has a higher vapour pressure.

That gives two scenarios in technical insulation:

Hot pipes (above room temperature — district heating, hot domestic water, steam): Inside the pipe there is typically no significantly higher vapour pressure than outside — the medium flow is water or steam, and the pipe's metal wall is practically vapour-tight. Even if the pipe's surface were moist, vapour would diffuse from inside OUT towards the cooler room air. This is harmless, because the vapour moves away from the pipe — it never reaches a cold surface where it can condense. That is why no vapour barrier is typically required on hot pipes — only protection against energy loss [isover-hvac-haandbog s.11].

Cold pipes (below room temperature — chilled water, refrigerant, condensate): The pipe's surface is colder than its surroundings, so the air immediately outside the pipe is cooled and has a low vapour pressure. The room's warmer air has a higher vapour pressure. Vapour diffuses from OUTSIDE IN towards the pipe. On its way the vapour passes through the insulation, where the temperature falls gradually towards the pipe. Somewhere inside the material the vapour reaches its dew point and condenses inside the insulation. First as invisible moisture, then as droplets, then as running water, then as corrosion on the pipe.

On a cold pipe the vapour must be stopped before it reaches the cold surface. That is the whole purpose of the vapour barrier.

The rule is simple and decisive: the vapour barrier always sits on the warm side.

This is not intuitive for a beginner. On a building construction it means: the vapour barrier sits innermost, against the warm living space, not outermost against the outdoor climate [bolius.dk efterisolering-fugt-og-dampspaerre].

On a pipe installation the logic flips, and can seem counter-intuitive:

• Hot pipes: The warm side is innermost against the pipe. The pipe's own metal wall is typically vapour barrier enough — no extra layer is required.
• Cold pipes: The warm side is outermost against the room. The vapour barrier must sit right at the very outside of the insulation — as alu foil, alu facing or vapour-tight outer cladding.

If you come from the building trades and think "vapour barrier = plastic film innermost", you can get it wrong on cold pipes. Here the logic is reversed — and it is one of the most frequent sources of condensation damage: the vapour barrier is placed wrongly or left out altogether.

Rule of thumb: Draw a thermometer diagram through the insulation. Find where the temperature is at the room dew point (typically 10–15 °C). The vapour barrier must lie on the warm side of this point — otherwise the moisture risks condensing on the inside between vapour barrier and pipe.

A material's vapour tightness is described with two parallel concepts:

• μ-value (my-value): A dimensionless number. "How many times worse does the material conduct water vapour than still air?" Air has μ = 1, glass wool has μ ≈ 1 (open), PE film has μ ≈ 100,000, elastomeric foam μ ≥ 10,000, Foamglas μ = ∞.
• sd-value (equivalent air column, m): μ × material thickness in metres. "How thick an air column would be needed to give the same vapour resistance?" A PE film of 0.15 mm with μ = 100,000 has sd = 100,000 · 0.00015 m = 15 m. Realistic figures are often quoted higher (50–80 m) [danskfacadepuds.dk].

In Danish construction you also encounter the Z-value (GPa·s·m²/kg) — an alternative measure of vapour resistance used in older standards. A vapour barrier with Z ≥ 50 GPa·s·m²/kg is normally sufficient in building constructions [BYG-ERFA ordbog Z-værdi]. For technical insulation, μ and sd are used almost always.

What do the figures mean in practice?

• sd < 0.5 m = diffusion-open. The material is practically permeable. Ordinary paint, plasterboard, mineral wool.
• sd 0.5 – 50 m = semi vapour-retarding. Certain special boards, PE film in thinner versions, PUR foam.
• sd 50 – 1,500 m = strong vapour retarder. PE film 0.15 mm, PVC cladding, elastomeric foam.
• sd ≥ 1,500 m = effective vapour barrier. Aluminium foil ≥ 50 µm, alu-faced glass wool, Foamglas. Practically vapour-tight.

For condensation insulation on cold pipes, sd ≥ 1,500 m is the target — typically achieved with aluminium facing and full taping of all joints.