Wednesday, April 21, 2010


How to prevent water damaging buildings and health

The term ‘breathability’ is becoming ever more widespread and, although a useful concept, it’s often misunderstood. It sounds as if it is about air but in the context of building performance it’s about water. Breathability describes how a structure reacts to water as a liquid or vapour, primarily through:

* Vapour permeability (how materials allow water vapour to pass through)
* Hygroscopicity (how materials absorb/release water vapour)
* Capillarity (the transmission of liquid water)

Breathability should be central to the design and renovation processes. It affects the health of the building and the occupants, especially as we try to improve the thermal performance of our homes. It also effects the environment, as the materials that provide the best breathability are often the natural, more traditional ones, although only recently have we begun to understand why.More info at SOLAR SERDAR.

An estimated 75% of building failures are due to water - either rainwater penetration, interstitial condensation or inner surface condensation. This starts with the outside of the building where vapour closed finishes (such as hard cement renders or high resin paints) can cause considerable damage by trapping moisture. It’s possible, often desirable, to have a capillary closed external finish but it should usually be vapour open. Research into modern timber frame constructions shows that drying is more effective through vapour open materials than through “vented” cavities, which often have no air movement.

Moisture effects performance. If the moisture in mineral wool increases by 1 or 2% its thermal resistance halves. Water molecules effectively form a cold ‘bridge’ and fill up the insulating air voids. Insulation gets wet in a number of ways and with non-breathing materials it takes a significant period to dry out, resulting in longterm loss of thermal resistance and allowing moulds the opportunity to develop. Completely water impermeable materials are not effective either, as the moisture content of walls can increase or remain damp, lowering thermal resistance.

It is Irish practice to use non-breathable materials and vapour closed designs in standard timber frame constructions. While this works in theory, if the timber or insulation gets wet while building or if vapour barriers are ‘punctured’ by alterations, water will be trapped, the timber can start to decay and moulds can develop. In breathable structures the moisture is drawn harmlessly outwards. As German regulations make clear, no timber treatment is necessary in timber frames with breathable construction. Only vapour closed constructions require preservative chemicals. Similarly in roofs, rot can result with vapour closed constructions or vapour resistant insulation. If the roof gets wet in construction or the vapour barrier is breached, as it usually is, the timber will collect water as it is the only vapour permeable and hygroscopic material present (it may retain it for a significant time).

The standard methods of internal wall insulation (battening out plus a vapour barrier behind plasterboard) can also lead to problems. Joists and floor junctions are in the most danger as moisture can collect here, so rising damp and even structural failure can occur. If a vapour permeable hygroscopic woodfibre board is used, these problems are usually avoided.

Breathable paints and plasters can be used internally to halt surface condensation as they absorb and disperse water droplets. This is especially helpful in renovated buildings, where installing ventilation is difficult.

Consequently, a knowledge and use of breathable materials will ensure that buildings have a good thermal performance, and are safe from moulds and rot.

In improving energy efficiency, we strive to make buildings more airtight. This affects indoor air quality, which affects human health. Large amounts of trapped moisture have been directly linked to allergic reactions (particularly asthma) and other autoimmune diseases, as both moulds and dust mites thrive in high humidity conditions. We should pay particular attention to this in the UK and Ireland, which has the largest incidence of asthma worldwide. Low humidities also have an adverse effect, as certain bacteria can flourish and mucous membranes become dried and vulnerable to dust and toxins. A healthy environment has a relative humidity of between 40 and 60%. This is also the most comfortable.

Current solutions use mechanical ventilation to maintain humidity. Many people feel that this is not enough and that we need the design and materials of the building to control moisture as well. This would stop us relying on ‘external’ systems, which need maintenance, repair and have a limited life. Achieving a healthy, robust and breathable building is not difficult. It involves simple safe designs and fully breathable materials which are already in common use across much of the continent and in historic buildings world-wide. Natural materials and systems have a proven track record and are also best from an embodied energy and resource view point.

For the health of both our buildings and ourselves, a proper understanding of breathability is essential. Only in this way can we improve energy efficiency, reduce building failure, and attack the root causes of many health problems. And, as an added bonus, it’s good for the environment too!


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