Breathability in buildings

“Breathability in buildings is not really about air. It is about water: water as a gas and water as a liquid; water inside the building, water outside the building, and water in the walls, floors and roofs themselves (though not about water in pipes!). It is not only about how water moves through structures (water vapour permeability), but also about the ability of materials to absorb and release water as vapour (hygroscopicity) and about the ability of materials to absorb and release water as liquid (capillarity). Water affects everything in building from the health or decay of building fabric, through to the thermal performance of the building and to the health of occupants. Particularly as we try to increase the airtightness, thermal performance and indoor air quality of our buildings, breathability has become a critical issue, affecting all areas both of new build and of renovation.”

— NEIL MAY, 'BREATHABILITY: THE KEY TO BUILDING PERFORMANCE

Breathability in construction is sometimes confused with air permeability, but it’s important to appreciate that these are two entirely separate things. As noted above, breathability is the result of a complex interaction between vapour permeability, hygroscopicity and capillarity. Air permeability, on the other hand, is the uncontrolled passage of air through joints and gaps – i.e. leakage. Standards of air leakage in new buildings are carefully regulated and tested, as they should be, since poor air tightness can lead to a significant drop in thermal efficiency.

Surprisingly (given its importance), breathability is largely ignored by building codes and regulations across the globe. But perhaps this is not altogether surprising, since the market for thermal insulation is dominated (at over 98% of sales) by mineral wool and foamed plastic insulation products, which have very poor hygroscopicity and capillarity. To get around this, the thermal envelopes of new buildings are often specified to include a vapour barrier, which stops any breathability whatsoever, but creates other problems.

Our system made possible by the use of Hempsil and other hygroscopic natural materials. But why is a demountable, breathable thermal envelope better than the alternatives?

Durability

“Except for structural errors, about 90 percent of all building construction problems are associated with water in some way.”

— ASTM E 241-77, RECOMMENDED PRACTICES FOR INCREASING DURABILITY OF BUILDING CONSTRUCTIONS AGAINST WATER-INDUCED DAMAGE.

Biogenic construction materials are intentionally biodegradable, but this is not the same as lacking durability or longevity. Indeed, cellulose based natural materials such as timber and hemp shiv have a service life measured in centuries, providing the moisture content of the material is maintained at less than 20%.

The problem with a vapour ‘closed’ thermal envelope, is that a vapour barrier is required on the ‘warm’ side of the wall or ceiling. While this approach is usually (but not always!) effective when the building is new, over time the membrane can easily become damaged, through alterations to the electrics and plumbing, or by simply hammering a nail in to hang a picture. Once damaged, water vapour condenses onto cold surfaces within the fabric of the wall or roof, causing loss of thermal performance, decay and mould, which is harmful to the building and its occupants.

Other water related problems arise from rain penetration. In conventional construction there are typically two ways of preventing this; either by means of capillary closed finishes (such as render and paint systems), or by allowing outward facing materials to absorb and desorb moisture in cycles. The problem with the former approach is that it is highly susceptible to defects (such as cracking over time) and poor detailing. The latter only works well if the thermal envelope has good breathability, which as noted is not easily achieved with the majority of insulation materials.

We use a demountable ventilated rain screen cladding system which protects the thermal envelope from water ingress while allowing it to breathe. This ensures that even in extreme weather conditions, the materials within the wall always remain at below 20% moisture content.

The control of moisture is central to how our system maintains stable indoor temperatures and comfortable levels of humidity. Successive layers of Hempsil and other natural fibre insulations allow water vapour to diffuse through the wall or roof build up and condense (at a microscopic level) within the pores of the material. In severe cold weather the dew point temperature of the interior air (within the thermal envelope) is always below that of the structural elements by virtue of the graduated layers of hygroscopic insulation. Moreover, the capillarity of the natural insulation acts to wick moisture away from the structure.

Air Quality

The remarkable ability of hemp to passively regulate indoor levels of humidity to between 40-60% relative humidity has been shown to inhibit the spread of viral and bacterial infections. It does this by absorbing moisture into the material during times of high relative humidity and then releasing it again when the humidity drops, which means that condensation cannot form on interior surfaces, for example during cooking, or at night from occupants breathing. Without moisture, bacteria and mould cannot form, greatly improving air quality.

By avoiding petrochemical derived building products and utilising only plant-based materials, our system removes the risk posed by a proliferation of harmful chemicals found in many modern construction products.