Technical brief - membranes for airtight roof designs

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There are compelling energy saving reasons for the trend towards airtight un-ventilated roof designs. An average house loses about 25% of its heat via the loft space and roof. If a roof is made airtight, heat loss reduces to about 15% making a total energy saving of over 5%. According to tests carried out by the Building Research Establishment (BRE), total energy saving could even be as much as 9.5%.

However, new regulations also focus on un-ventilated roof designs as a means to achieve new EU targets for reduced CO2 emissions. Building Regulations – Conservation of Fuel and Power Part L state that the air tightness of a building, known as ‘air permeability’ or leakage rate, should not exceed 10 m3 per hour per square metre of the building envelope.

All new buildings over 1000 sqm gross floor area have required a mandatory test for air permeability. Domestic buildings and smaller commercial properties are incorporated in to the same legislation and all new and refurbished roofs in the UK will also have to be built to the new standards. Un-ventilated roof designs therefore make a substantial contribution towards reducing the building’s leakage rate and towards saving energy.

Design implications
Inevitably there are design implications with airtight un-ventilated roofs. Air leakage can occur through gaps, holes or cracks in the fabric of the building envelope. These gaps are not necessarily visible to the naked eye and degree of leakage invariably affects the building’s air permeability performance. Therefore it is particularly important in un-ventilated roof systems that all penetrations into the roof space are properly sealed. This is necessary to minimise moisture-laden warm air from entering the roof via the building’s cavities and any openings in the ceiling. Whilst this requires careful detailing and workmanship, these additional measures have the benefit of providing up to a 25% reduction in heat loss by air leakage through the ceiling and from the loft space to the outside of the building.

Selecting the right breather membrane
An average household produces about 20 kg of water vapour per day and unless this volume is removed there is a substantial risk of condensation and mould growth. Traditional roof constructions normally have eaves ventilation provided in the roof to remove any moisture build-up. An airtight roof design does not provide the means to disperse moisture using ventilation. Therefore the ability to transmit water vapour through the breather membrane is particularly important in selecting a suitable breather membrane. By design, breather membranes allow water vapour to dissipate through the breather membrane whilst preventing liquid water from penetrating inside and soaking the insulation.

With a Water Vapour Resistance of only 0.13 MNsg-1, Powerlon UltraPerm is classified as a Low Resistance (LR) Breather Membrane. This makes it extremely breathable and results is a Water Vapour Permeability of between 1,500 - 1,700 gm-2 24hr-1 . In practical terms, this means that Powerlon UltraPerm releases over 1 kg of water vapour per square metre per day, comfortably coping with an average household generating 20 kg a day. It can also withstand a 3-metre water column which makes it more than adequate to keep out rain. It is particularly important to note that using an appropriate membrane by itself will not achieve air-tightness. The membrane is a component within a system and air-tightness can only be achieved by ensuring that gaps, holes or cracks in the roof are properly sealed.

Other energy saving membranes
One of the main benefits of using a breather membrane is that it allows moisture to escape though it. This maintains the integrity of the insulation by helping it to stay dry. A vapour control layer (VCL) is often used to prevent air leakage. The VCL can be fitted on the warm side of the insulation to effectively block any moisture migrating into the insulation material from inside of the building. VCLs should be installed with care, ensuring that all joins are taped and that any protrusions through the membrane are also sealed. Powerlon VCLs are chemically inert and do not support mildew or other fungal or organic growth.

However the best performing VCLs have an encapsulated layer of aluminium and are generally known as Vapour Barriers. These are so effective in blocking water vapour that they are used in high humidity environments such as swimming pools, laundries and bakeries. Since the Powerlon Vapour Barriers incorporate a layer of Aluminium, they are also widely used in loft conversions. The Aluminium layer provides energy-saving thermal reflectivity and contributes significantly to reflecting heat back into the loft.

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