44 INSULATION & ENERGY EFFICIENCY Putting breathability in its place By Phil Heath, Technical Manager, Kingspan Insulation Limited
WITH THE URGENT NEED TO CUT CARBON EMISSIONS AND REDUCE OUR RELIANCE ON FOSSIL FUEL SUPPLIES, THE USE OF INSULATION TO IMPROVE BUILDING ENERGY EFFICIENCY IS A CRUCIAL CONSIDERATION. A HOST OF DIFFERENT INSULATION MATERIALS ARE AVAILABLE, AND THERE ARE A NUMBER OF ISSUES AND CONSIDERATIONS THAT ARE CLAIMED TO BE CRITICAL TO PERFORMANCE. NOW, A RECENT STUDY HAS PROVIDED EVIDENCE THAT ONE OF THESE, BREATHABILITY, HAS HAD ITS IMPORTANCE BLOWN GREATLY OUT OF PROPORTION.
B
reathability refers to vapour diffusion through a building envelope. It has been suggested that if constructions
do not breathe then moisture will become trapped inside houses, potentially causing surface condensation, mould growth, exacerbated dust mite populations and associated health problems. The related claim is that construction products with high breathability, including some insulation materials, can play a key role in managing moisture levels. In fact, the evidence does not support this view.
Air-borne moisture can be transported in and out of buildings in two ways. One way is breathability and the other is the process of bulk air-exchange, which is air that escapes intentionally through designed ventilation and also unintentionally through air leakage. The key is to consider the significance each part plays in managing moisture levels.
In housing here in the UK, breathability is rendered irrelevant during the summer because there is not generally a sufficient difference in internal and external vapour pressure for it to occur. The colder months are different and there is usually a suitable gradient in pressure, but this does not mean that it can be considered a major factor.
A new study by Cambridge Architectural Research Ltd. examined the matter by modelling different insulated wall specifications with varying degrees of breathability. When the minimum recommended bulk air-exchange rate for healthy occupation of dwellings of 0.5 air changes an hour was assumed, bulk-air exchange was the overriding factor, even in a house with “breathable” walls insulated with “breathable” mineral fibre insulation.
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In this case bulk air-exchange was responsible for 95% of all moisture transfer from the inside of the building to the outside. It is well worth noting that an air change rate of 0.5 is highly optimistic, as the effects of occupancy such as the opening of doors and windows makes it very likely that it will be exceeded in all but the most carefully controlled scenarios.
These results show that vapour diffusion does not make a significant contribution to the rate of vapour transport in housing, even in breathable constructions. In fact bulk air- exchange can be at least 19 times more important than breathability.
On a related note, it has been claimed that if an insulation material with low vapour diffusion characteristics is used as external wall insulation, moisture evaporating from a newly clad wall could become trapped between the inner face of the insulation layer and the building structure, which could result in chronic damp and mould problems. Although it is true that all masonry will contain a certain amount of water, the likely scenario is that it will gradually dry out by evaporation and vapour diffusion until it has reached a
stable equilibrium with the spaces surrounding it. Although there will be less vapour diffusion externally with a more vapour resistive insulation material, sufficient diffusion should happen internally for the moisture to be ventilated away.
This means chronic damp and mould are unlikely to happen unless the moisture content of the internal air is extraordinarily high, perhaps because of extremely low bulk air-exchange rates and high water vapour production preventing the wall from drying out.
However,
as has already been noted, such is the dominance of bulk air-exchange over vapour diffusion, that in those circumstances the wall is likely to be chronically damp regardless of the vapour resistance of the insulation material.
The Cambridge Architectural Research study shows that the reason breathability is not conventionally considered a factor in buildings is that the evidence points overwhelmingly against it having any significant effect. This view is supported by the requirements of the Building Regulations and the contents of documents such as BR262 "Thermal Insulation Avoiding Risks". Instead, creating a healthy building is down to good thermal design and ensuring adequate bulk air- exchange through controllable ventilation.
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