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Vapour and Air Permeable Membranes Explained

Vapour and Air Permeable Membranes Explained

As Building Regulations are demanding ever higher thermal efficiency in both commercial and domestic sectors, today's building professional increasingly needs to consider air tightness as a way of improving the thermal performance of the building envelope, not simply adding more insulation. While considering airtightness the designer/builder must also be mindful of the implications for moisture and vapour management when improving the thermal efficiency.

Building terminology is now changing to talk about vapour permeable membranes, vapour control layers, air tight membranes and air open membranes, but what does this all mean? And how does it affect our thermal performance?

For example, no one would argue that a heavy knitted jumper will keep you warm, but up a mountain in a howling gale, the wind will blow right through it, rendering its high insulation levels ineffective. Adding an oilskin jacket will block the wind very effectively, however after walking up the mountain, it’s limitations as regards breathability will become all too apparent. A Gore-Tex jacket on the other hand, being both air tight, water resistant and vapour permeable, will maximise the effectiveness of the woolly jumper without any of the unpleasant sweatiness of the oilskin. In this example air openness would not be an advantage as it would not deflect the wind as required.

Product Vapour Air Application Link
Wraptite Vapour Permeable Air Tight External air barrier - used outside the services zone More Info
Roofshield Vapour Permeable Air Permeable High performance pitched roofing membrane More Info
Frameshield 100 Vapour Permeable Air Permeable Breathable walling membrane More Info
Procheck 300 Vapour Tight Air Tight Lightweight vapour barrier used internally More Info
Procheck 500 Vapour Tight Air Tight Standard vapour barrier used internally More Info
Profoil 861 Vapour Tight Air Tight High performance reinforced vapour barrier More Info
Reflectite Tape Vapour Permeable Air Tight Reflective Tape that creates an airtight but vapour permeable seal. More Info
Reflectatherm Plus Vapour Tight Air Tight Reflective vapour barrier that increases thermal performance More Info
Reflectashield TF 0.81 Vapour Permeable Air Permeable Reflective breather membrane that increases thermal performance More Info
Cladshield Vapour Permeable Air Permeable Breathable membrane for lightweight industrial roofing and walling More Info

The behaviour of the building envelope is no different, and BS5250 defines the following-

· Airtight layer- prevents the movement of air which may/ may not act as a Vapour Control layer

· Vapour Control Layers- a material which can limit both movement of vapour by diffusion, and air movement (sometimes called vapour checks or vapour barriers)

· Breather Membrane- defined as a membrane with a vapour resistance less than 0.6 MNs/g (sometimes this terminology is used, arguably wrongly ,in place of LR underlays

· HR underlay- an underlay with a vapour resistance greater than 0.25 MNs/g

· LR underlay- an underlay with a vapour resistance less than 0.25 MNs/g (sometimes referred to as vapour permeable or vapour open underlays and some may possess a degree of air permeability)

Microscopic photographs below show the difference in the make-up of two types of LR underlays. The one on the left is a vapour permeable/ airtight layer of which there are more than 20 in the UK with BBA certification. The membrane on the right is Roofshield which is both air and vapour permeable thus meeting the NHBC requirements for non-ventilated cold pitched roofs.

This terminology, while trying to be helpful can further confuse users. The old ethos of Build tight, ventilate right is a good start but does not tell the whole story. By “ventilate” we mean ensure the buildings internal environment is maintained at healthy levels and there is fresh air movement in a controlled manner. Air infiltration should be limited to designed inlets, not unplanned leakage.

The UK construction sector generally build with air tight layers internally which are also designed as vapour control layers, so as we move through the element from inside to out we generally try to reduce the vapour resistance. A popular example of this is in UK cold pitched roofs where we attempt to make the ceilings as air tight as possible (sometimes with the use of vapour control layers) to improve the thermal efficiency. Outside this airtight layer, above the thermal insulation, we will either ventilate the roof or use vapour permeable membranes allowing potentially harmful vapour to escape prior to condensing. The NHBC have given further guidance that even when using vapour permeable membranes, LR type, you should still provide high level ventilation, however they further state that if the vapour permeable membrane is also AIR permeable, this additional ventilation is unnecessary. This is based on evidence showing the air permeability of the underlay lowers the risk of condensation further, as in addition to vapour escaping by diffusion, a substantial air change rate is also achieved via the air permeable membrane, greater in fact than would be achieved with traditional ventilation. So in this situation it is advantageous for the membrane to be both vapour AND air permeable as opposed to air tight. A. Proctor Group Roofshield meets this requirement and is being used by many house builders throughout the UK to provide a cost effective method of reducing condensation risk whilst simplifying design. This has taken the UK industry over 15 years to embrace, proof that we need to be more adaptable and open to changing techniques and requirements. The days of “if my grandfather never used it, I don’t want to” are hopefully becoming a thing of the past.

In North America the issue of air tightness is crucial in the design of thermally efficient buildings not only to reduce heat losses but to reduce air conditioning/cooling costs. Here it is common to use an AIR TIGHT vapour permeable membrane on the walls to reduce unwanted air infiltration. The membrane reduces the movement of unwanted external air yet ensures the risk of condensation is kept to a minimum by maximising the vapour permeability. Installing the AIR tight layer externally ensures continuity is not interrupted by floor and ceiling joists, and subsequent trades damaging the air tight layer. New technologies have shown that vapour permeable air tight layers can prove cost effective externally in walls, so perhaps it is time for the UK to learn from this and install our air tightness measures externally.

The A Proctor Group, with a highly trained experienced technical team of 5, can advise building professionals of which membranes to use in order to achieve thermally efficient, healthy buildings.

New technology is constantly evolving the way we can meet the requirements of ever changing regulations and thermal insulation. By embracing this innovation the building professional will meet his clients’ demands with easier to install cost effective measures.

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