Water Vapor Permeability

Water vapor permeability describes how easily water vapor moves through a building material or layer. It decides whether a wall can dry out, and it becomes critical when an old solid wall is renovated. Designers express it with the μ factor, the diffusion resistance factor of a material, and with the sd value, which turns that factor into a figure for a whole layer.

What does water vapor permeability mean in practice?

Water vapor moves from the side with higher vapor pressure towards the side with lower pressure, which in a heated building in winter usually means from inside to outside. Permeability is how quickly a given material lets that movement through. Highly permeable materials are vapor-open, and those that barely let vapor through are vapor-tight.

Permeability is not airtightness. Air carrying vapor leaks through joints and gaps, a separate and usually larger problem handled by the airtight layer. Diffusion through the material is slow, and permeability answers whether that flow can leave the wall before it causes harm. Harm happens when vapor condenses inside a cold layer, as described under interstitial condensation.

What does the μ factor tell you about a material?

The μ factor is a dimensionless number that compares a material with still air. It shows how many times more resistant the material is to vapor diffusion than air of the same thickness. Still air is the reference, so its μ is 1.

The μ factor belongs to the material, not the layer, so thickness does not change it. It is the figure you look up in a product data sheet. In Slovak technical documents it appears as faktor difúzneho odporu, and the same concept is covered in the entry on the vapor diffusion resistance factor.

How is the sd value calculated from μ and thickness?

The sd value, the equivalent air-layer thickness, is the μ factor multiplied by the thickness of the layer in metres: sd equals μ times thickness. The result is the thickness of still air that would resist vapor in the same way as that layer. It turns a material property into a figure you can add up across a whole build-up.

Thickness enters the calculation, so the same material can matter more or less depending on the layer. A thin membrane with a high μ can be the tightest layer in a wall, while a thick open board can stay open overall. Designers therefore sum the sd values of all layers. Treat the result as a planning figure, only as reliable as its μ value.

Layer	Relative openness	Role in a renovated wall
Mineral wool board	Very open	Insulation that dries in both directions
Lime plaster	Very open	Finish that lets the wall dry outwards
Clay plaster	Very open	Finish that also buffers humidity
Wood fibre board	Open	Open insulation that buffers humidity
Cement render	Tight	Outer finish that can trap moisture in masonry
Expanded polystyrene (EPS)	Tight	Insulation that blocks outward drying
Vapor-tight paint	Tight	Interior film that blocks drying inwards

Which direction should a wall build-up dry?

A wall build-up should become more vapor-open towards the outside. Each layer should be at least as permeable as the layer inside it, so vapor that gets in can move outwards and leave. A tight layer on the outside, with more open layers inside, traps moisture because its only exit is closed.

This is the logic of vapor-open construction. The inside is controlled first, with an airtight layer and, where the climate requires it, a vapor retarder. The outside stays open so the wall can dry. What counts is the total outward permeability of the build-up, not any single layer. Unusual moisture loads need a specific check.

Why is an old solid brick or stone wall a renovation trap?

Old solid brick and stone walls are thick, mineral and usually vapor-open. Many carry moisture from decades of rain, and they release it mainly outwards, so that outward path must stay open.

The trap appears when a renovation seals that path. A cement render applied to fix cracks, an expanded polystyrene external insulation system plastered over the masonry, or a vapor-tight paint on the inside can each look like a sensible repair. Together they stop the wall from drying, so moisture builds up behind the new finish and mould or salt damage can follow, often only after several seasons.

Before sealing the outside of an old wall, ask where the moisture will go.

Which materials should you choose for a renovation instead?

For solid masonry, choose vapor-open materials: mineral wool or wood fibre insulation, with lime or clay plaster as finishes. Lime plaster, known in Slovak as vápenná omietka, is vapor-open and suits the moisture old walls carry. Mineral wool, minerálna vlna, must be protected from rain during the works. Wood fibre insulation, drevovláknitá izolácia, combines openness with good moisture buffering.

Renovation choice on a solid masonry wall	Effect on drying	Verdict
Cement render as the only outer finish	Tight, blocks outward drying	Risky on damp masonry
EPS external insulation with a cement-based finish	Tight, blocks outward drying	Needs a moisture assessment first
Mineral wool with a lime plaster finish	Very open, lets the wall dry	Usually suitable, protect from rain
Wood fibre board with a lime or clay finish	Open, lets the wall dry and buffer humidity	Usually suitable, with careful detailing

How does water vapor permeability differ from a vapor barrier?

A vapor barrier is designed to be almost impenetrable to vapor, usually placed on the warm side of the insulation. Permeability is the broader property every material has, from open to tight. A vapor barrier is the extreme tight case.

The purpose differs too. A vapor barrier stops vapor reaching a cold part of the construction. A permeability figure is used to design the whole build-up so that drying in both directions still works. A vapor retarder, or parobrzda, sits between the two: it lets some vapor through and slows the rest.

Is a vapor-open wall enough without an airtight layer?

No. Vapor-open materials solve a diffusion problem. Air leakage moves far more moisture, carried through joints and gaps into a cold layer where it condenses. An airtight layer on the warm side stops that flow, and permeability cannot do the same job. A wall needs both, checked separately.

A breathable wall cannot take on any moisture load. Vapor-open construction lets a limited amount of moisture leave, and it works only if the materials are reasonably dry when installed and rain is kept away from the insulation.

When does a renovation need a hygrothermal calculation?

A simple rule of thumb can serve for a dry, well-understood wall. An old, damp or unknown wall deserves a hygrothermal simulation, which models temperature and moisture through each layer across a full year and shows whether the build-up dries or accumulates water.