Why Your Building Needs to “Breathe”

Why do some buildings feel fresh and airy while others feel damp, stuffy, or just… uncomfortable? The answer often comes down to something invisible: the way your building breathes.

The word breathability gets thrown around a lot in the construction world, often loosely and sometimes misleadingly. Neil May’s paper clears this up right away: breathability in buildings is not about air. It is about water.

Water vapour in the air. Liquid water in the walls. Moisture in your floors, your roof, your insulation. How water moves through a building, and whether it can escape again, determines whether your building stays healthy or slowly falls apart.

May describes the three distinct ways materials handle water, and understanding the difference is crucial:

1. Vapour permeability — how easily water vapour (gas) passes through a material. Think of it as how “open” a material is to moisture moving through it.

2. Hygroscopicity — a material’s ability to absorb and release water vapour as humidity changes. Highly hygroscopic materials act like a natural sponge, soaking up excess moisture when the air is humid and can release it when the air dries. This is what stabilises indoor humidity levels.

3. Capillarity — a material’s ability to absorb and release liquid water. This is what determines how a wall handles rainwater, or how quickly a wet material dries out.

These three things can be completely different from one another — and a material can score high on one while scoring low on the others. This is where a lot of confusion (and a lot of building failures) begins.

Here is a striking statistic from the paper: an estimated 75% of building failures are caused by water. Not poor structural design, not subsidence, not bad workmanship, but water. And most of the time, the problem is not a leak or a flood. It is moisture that builds up slowly, inside the fabric of the building, with nowhere to go.

The consequences include:

Mould and rot — leading to structural decay and serious health risks for occupants

Reduced thermal performance — damp insulation can lose a significant portion of its effectiveness, meaning your heating bills are far higher than they should be

Poor indoor air quality — linked to asthma, allergies, and other respiratory conditions

Building fabric decay — timber frames rotting from the inside, pointing failing, renders cracking

The UK has one of the highest rate of asthma in the world, with millions of people suffering from it. May draws a direct line between this and the way we build our homes: poorly breathable construction traps moisture, raises humidity, and creates the exact conditions in which dust mites and moulds thrive.

This is perhaps the most interesting part of the article and can be uncomfortable reading for anyone in the construction industry. Many of the materials and methods we consider standard — plastic insulation, vapour barriers, hydrophobic renders, closed-cell foam — are contributing to the very problems they are supposed to solve.

A few telling examples from the paper:

Timber frame buildings — the standard UK approach puts a polythene vapour barrier on the inside and a relatively impermeable OSB board on the outside. In theory, if the vapour barrier is perfectly installed and stays perfectly intact for the life of the building, this works. In practice, it is punctured by electricians, plumbers, and carpenters — and once moisture gets into the timber frame, it has almost nowhere to go. The timber absorbs it, the moisture content rises, and decay begins. This is not a rare failure; it is happening at scale across the UK.

External wall insulation (EWI) with polystyrene — case studies from Germany, Switzerland and Scandinavia show that polystyrene EWI (external wall insulation) systems regularly perform 30% or more below their designed thermal performance. The main reason? The polystyrene traps moisture in the original masonry, reducing its thermal resistance — and the sun and wind, which would normally help the wall dry out, are now blocked by the insulation. Add new draught-proofed windows, and you also have conditions ripe for mould growth on the inside.

Cavity wall insulation — filling cavities entirely with vapour-impermeable insulation can trap construction moisture for years, reduce thermal performance, and cause damp and mould on internal walls. Many cavity walls in the UK never achieve anything close to their designed thermal performance.

With all the problems identified, there are also solutions put forward. The materials that work best with water — that allow buildings to manage moisture — are not space-age inventions. They are largely the materials that have been used for centuries.

Unfired clay has exceptional hygroscopic properties, absorbing moisture rapidly when humidity rises and releasing it slowly — acting as a natural humidity regulator. Clay plasters in kitchens and bathrooms can eliminate surface condensation on tiles that would otherwise require constant ventilation.

Timber (used correctly) is highly hygroscopic, though the rate of absorption varies significantly depending on whether the grain is exposed end-on or along its length.

Natural fibre insulations — woodfibre, hemp, flax, sheepswool — are vapour-permeable, hygroscopic, and capillary-open. They absorb moisture when conditions are damp and release it when conditions dry. Crucially, research by the Fraunhofer Institute has shown that the latent heat stored as these materials absorb moisture is released as they dry, compensating for any temporary reduction in thermal performance. One UK study found that flax insulation performed around 10% better overall than mineral wool of the same designed resistance.

Lime plasters and renders — when properly specified — allow walls to manage moisture movement far more effectively than cement or synthetic alternatives.

Woodfibre insulation boards used as external wall insulation on Victorian brick buildings can actively draw moisture out of damp masonry and release it to the outside — the opposite of what polystyrene does.

May concludes his paper with four design principles. I think every client — whether commissioning a new build or a renovation — should be asking their architect or builder about these:

1. Compatibility of materials — do all the layers of a wall, roof or floor work together? Do they have similar vapour permeability, hygroscopic qualities, and capillary behaviour? Incompatible materials create dangerous interfaces where moisture gets trapped. The sealing of vapour barriers is a classic example of a critical detail that only becomes necessary because of an incompatible design choice in the first place.

2. Make the structure do the work — what is the capacity for the structure to manage water in the building? A well-designed breathable building manages moisture, maintains thermal performance, and supports good indoor air quality through its materials, not through mechanical systems that need maintenance and can fail. Mechanical ventilation is not a substitute for good material design — it is a last resort.

3. Safety nets — is it designed for a perfect theoretical scenario or actual real and imperfect use? Breathable construction is forgiving of the inevitable imperfections of real-world building work and real-world occupation. A timber frame designed around vapour-open, hygroscopic materials can tolerate a punctured membrane, wet timber on site, or a bathroom that gets steamy. A conventional vapour barrier design often fails. Design for robustness, not theoretical perfection.

4. Whole house design — is there a complete strategy in place for how moisture can travel through the building? Moisture does not respect the boundaries between systems. The choice of insulation affects indoor air quality. The choice of internal plaster affects interstitial condensation. The heating system affects how effectively hygroscopic materials can do their job. Everything should be understood as a whole.

If you are planning a renovation or new build, the key questions to raise with your design team are:

Are the materials in our wall, roof and floor build-ups compatible with each other from a moisture perspective?

Do we have hygroscopic materials on or near the internal surfaces, where they can genuinely buffer indoor humidity?

Are we relying on a perfect vapour barrier — or does our design have built-in safety margins?

If we are insulating an older solid-wall building, are we using materials that will allow the wall to continue managing moisture, or are we trapping it?

The good news is that, as May argues, building well is not necessarily more expensive than building badly. The materials that perform best — natural fibres, clay, lime, timber — are often simpler to specify and apply than the synthetic systems that require perfect installation to work at all. And the long-term cost of getting it wrong — mould remediation, structural repairs, health consequences, premature replacement of failed systems — is very high indeed.