Breathable building & airtightness explained

There are a number of terms concerned with construction that are often misunderstood or misused, would you be comfortable explaining ‘breathability’ to someone else? before reading on try to create a definition in your mind you would be happy to go public with.  There is no intention to deceive it’s just that building terms are often widely used without anyone asking for a full explanation, maybe people don’t ask due to shyness or maybe because the word itself appears self explanatory, or so you might think.

When the term breathability is referred to with reference to building materials it is not concerned with air, but with water in both liquid and gas form. Breathability tends to be used to describe the passage of water vapour and there are a variety of related terms that are used to quantify breathability.

Why is this important?

"Except for structural errors, about 90 percent of all building construction problems are associated with water in some way."[1]

The list of the harmful effects of water in building materials and structural elements is indeed a long one. In many instances the water by itself is not harmful, and only when combined with other phenomena does it cause rapid deterioration. On the other hand, the other phenomena involved will not cause deterioration in the absence of water. It follows then that if water can be controlled a building can be made more durable and the maintenance and repair costs reduced. If this could be achieved only by the use of very expensive materials and construction it could be argued that it is better to let the building deteriorate and to replace the damaged portion from time to time. In fact durable construction can be achieved with relatively inexpensive materials and designs, provided that the designer understands the behaviour of water in its various forms and applies the necessary controls to prevent it from accumulating in harmful quantities.

What is a breathable Material?


If you are confused we don’t blame you. The term breathable is used loosely and variously across the industry and it appears it can mean different things to different people.

In summary, to avoid confusion, for a material to be breathable it must have the following characteristics:

  • Vapour Permeable – it has the ability to allow vapour pass through
  • Hygroscopic – it has the ability to absorb moisture
  • Capillarity Active – it has the ability to release absorbed water through its pores or capillaries (microporous) or by molecular transfer (monolithic)

As we mentioned above if any of the materials used in the system don’t display ALL of the above properties they cannot be called “breathable” in the true sense of the term in relation to a building element.

Why is this important? Because (as we have seen above) water affects everything in building from the health or decay of building fabric, through to the thermal performance and to the health of occupants. Particularly as we try to increase the air-tightness, thermal performance and indoor air quality of our buildings, breathability has become a critical issue, affecting all areas both of new build and renovation.

One group of building materials does have an advantage when designing for breathability that is the ability of natural and hygroscopic materials to absorb and release water whilst remaining dry which reduces the risk of interstitial condensation and ultimate fabric failure. Natural fibres constantly adjust humidity levels away from extremes of damp and dryness helping maintain air moisture at comfortable levels, reducing the risk of both surface condensation and the negative health risks from moulds, mites and viruses. Of course, fabric breathability is not an alternative to a good ventilation strategy, but should be considered as part of a robust and healthy building design.

To achieve increased levels of airtightness the use of vapour tight systems are the most common solution used on the market today, this can be achieved by the use of ‘wet trades’ plaster typically deployed onto block and brick or by the use of membranes on timber structures, roofs or walls.

Intello-Application-273x150 Achieving airtightness through the use of internally applied membranes, in this case Intello Plus

It is essential that a building element needs to be either non breathing – a barrier or breathing, it cannot be both. The area most commonly encountered where this becomes a more complicated issue is refurbishment and extensions. Certainly this is critical if retrofitting insulation where non breathing components such as gypsum plaster, vinyl paints and cement render need to be removed. This is because a “non breathable” wall cannot cope naturally with the movement of water. This approach requires the elements to be assessed in order to meet the 5 critical factors which must be present to ensure that the building element is designed and delivered as breathable, they are:

  • Continuous vapour control layer – plaster or membrane
  • Continuous insulation layer – no thermal breaks
  • Thermal bridging due to multiple fixing
  • Thermal looping within an unventilated cavity
  • Adequate treatment of thermal bridging – reveals, returns & floors

The key factor is that the wall must be vapour tight which simply means that the system you choose must stop water vapour transferring from the warm side to the cold side of the insulation. Failure to address this key issue will lead to the water vapour condensing on the cold wall and turning to liquid water.

Over a period of time this will lead to the reduction in the performance of the insulation material but in addition the formation of mould and degradation of the fabric of the building will occur.

PIR badly fitted Poor fitting insulation allows air leakage

The second issue is that the insulation must be continuous and have no breaks. Studies have shown by the Fraunhofer Institute that, a 1mm gap in the insulation layer can lead in a drop in performance by a factor of 4.8. Therefore a system, designed to perform at 0.3 U Value, will in effect perform at a U value of 1.45.

If there is a cavity or air gap behind the thermal insulation system installed then this must be “un-ventilated” to keep this air warm and still. Failure to achieve this reduces the effectiveness of this air gap which is used to enhance calculated U values on the wall.

Thermal looping and cold bridges Thermal looping within a cavity in a wall

Failure to adequately deal with cold bridging at critical junctions will increase the heat loss at this point after the insulation has been installed which can lead to mould growth in these areas. A recent report published by the BRE states that heat loss at un-insulated junctions can be as high as 50% in certain cases. The effect, of not dealing with Cold Bridging contributes a large part of the “Performance Gap” the acknowledged difference between ‘design and actual’ performance of a building.

In short a breathable building element can deal with the formation of water due to vapour condensing and move it in the direction of least resistance, either to the outside or inside face of the wall. Breathable elements allow a degree of buffering and this moderating effect ensures a more pleasant and healthy environment plus it is a more forgiving and fail-safe structure.

Further reading.

Click the image blow to read a report titled 'Breathability: The key to Building Performance' This was written a few years back but the principles and observations it makes remain unchanged from our view today and the conclusions and recommendations are still relevant to the way we construct now.










[1] ASTM E 241-77, Recommended Practices for Increasing Durability of Building Constructions Against Water-Induced Damage. Philadelphia, PA: American Society for Testing and Materials, 1977.