Air tight membranes have been used for some time to control vapour however misconceptions in the industry still exist that such membranes can only be used internally meaning a lot of time-consuming detailing. A breathable alternative could provide the answer, offering the performance but avoiding the hassle for contractors.
Although achieving air tightness in constructions seems like a relatively new concept, it has been around for a couple of decades at least, and recent updates to Part L of the Building Regulations have meant an increased focus on air tightness to improve energy efficiency. It can now be said that there is a traditional approach to achieving air tightness while minimising condensation within the building, and that is to install an air tight vapour control layer (VCL) to the interior (warm) side of the wall and roof, behind the plasterboard and insulation.
While this is a perfectly effective method of minimising most air coming into the building and preventing the vapour within a structure meeting the cold insulation layer and condensing,
the problem is for contractors that such internally applied membranes require a lot of detailing and sealing around the plethora of fittings in a typical building interior to ensure air tightness. These include junction boxes, rafters and light fittings and heating and drainage services ducts as well as overlaps. In order to achieve Scandinavian levels of air tightness a similar plethora of special accessories are required, from air tight gaskets and junction boxes to rafter shoes, tapes and sealants in order to finish off your construction adequately to pass the air tests.
Because they are installed on the inside of the building, standard vapour and air tight membranes also carry the risk of being damaged by follow-on trades. Such a time-consuming VCL approach although established in the industry can be prohibitive for many contractors, however there is an alternative.
A 'reathable' i.e. vapour permeable air-tight membrane from the A. Proctor Group, Wraptite-SA, can offer a solution which avoids all of the detailing necessary with internally applied membranes. This is because it allows vapour to escape, and therefore is uniquely able to be applied externally as it will not lead to condensation due to vapour meeting the cold layer of a structure. This makes it an easy-fit as well as an effective air tightness solution, further enhanced by the self-adhering design, which has been developed to adhere to any substrate.
The main objection to using external air tightness membranes is one of convention - the industry tends to think that an air tight layer should be located internally. Installing them externally is certainly a new concept which the UK has yet to fully adjust its thinking to, according to the A. Proctor Group's Technical Director, Iain Fairnington: "People tend to think of the air tight layer also being vapour tight, and obviously you wouldn't put a vapour tight product on the outside of the building, as this would lead to condensation as your vapour control should always be on the warm side of the building."
Wraptite SA - breathable air tightness. Wraptite-SA comprises a triple layer of microporous polyethylene film covered with a vapour permeable acrylic adhesive, meaning the whole structure
is vapour permeable as well as air tight; much like a Gore-Tex jacket it will keep the heat in but allow the structure to breathe. In short, it makes it possible to install an air tight product on the exterior of the building when this was not previously possible. Recently certified by the BBA, Wraptite-SA makes a significant contribution to a building's thermal performance by preventing lateral air movement, achieving air tightness of below 1 m3/m2/hr in some installations, meaning it is in Passivhaus territory. Wraptite recently achieved an air tightness score of 0.87 m3/m2/hr on a Code for Sustainable Homes Level 5 project for Kingspan Timber Solutions in Cambridge.
The product also however helps maintain a healthy living environment and structure thanks to its vapour permeability rating of Sd 0.039. This also contributes to the important task of preventing a building overheating which has been a feature of increasing airtightness to meet increasingly demanding Building Regulations.
The self-adhering membrane fully bonds to virtually any substrate, requiring no mechanical attachment, seals or tapes to suppress air leakage around junctions or penetrations. The product is ideally suited for rainscreen applications, which only have fixings and possibly air vents to interrupt what is otherwise a continuous sheet of membrane, minimising potential air leaks. As well as full wall applications, Wraptite can also be used on pitched roofs or for window flashings or other details. Wraptite-SA membrane is accompanied by a tear-resistant Wraptite Tape which offers the same vapour permeability but enables convenient sealing to be achieved around penetrations and at overlaps, making detailing quick and easy.
Wraptite installations offer valuable wind-proof and water-resistant characteristics, as the product will adhere across the whole of a substrate meaning no air pockets which can be lifted by wind, offering additional useful benefits for contractors on site. It requires no primer, and due to its breathability will allow damp sheathing to dry. Its very practical features have seen the product used successfully on a wide range of projects, from commercial office buildings to domestic developments with rigorous energy efficiency demands.
Another major benefit in the current context of space-constrained development footprints is that due to its thinness it provides an opportunity to provide a high degree of efficiency while maximising living space within a building. The A. Proctor Group has identified in a study that it would take an additional 230 mm of Rigid PU insulation to make the equivalent Designed Emission Rate savings of well installed Wraptite.
Iain Fairnington concludes: "The arrival of Wraptite-SA means that excellent airtightness doesn't have to be difficult to achieve. Using an external air barrier greatly reduces the amount of time spent on difficult detailing, and with less to go wrong, there's also less chance of encountering expensive remedial works."
