INSULATION Project1_Layout 1 07/05/IEliminate thermal bridging with insulated membranes Thomas Wiedmer, UK and Ireland technical director at Actis Insulation, explains why you should always use a vapour control layer or breather membrane on newbuilds or refurbishment projects t’s possible to insulate a building without using a vapour control layer or a breather membrane – there is no legislation stating one has to be used. But what are the dangers of ignoring this step? After all, why should we spend money unnecessarily? in this article, I’m going to explain why it’s a good idea to use a vapour control layer when embarking on a newbuild or refurb, and what to look for when choosing a breather membrane. “While there is nothing in the building regulations stating that a vapour control layer or a breather membrane must be used, they are recommended because they massively reduce the risk of interstitial condensation, they guarantee air tightness, and can protect the building from precipitation and wind driven rain if installed correctly. Some vapour control layers, such as Actis HControl Hybrid, are dual purpose. While it is CE-marked as a vapour control layer and has a high vapour resistance, it also acts as an insulation, which means the thickness of the main insulation can be reduced to achieve the same required U-value. It is used on the warm side of any insulation material, behind the internal finish in roofs, walls and ceilings. A breather membrane has a low vapour resistance. Again, like the vapour control layer, some, such as Actis Boost’R Hybrid also act as insulation. This particular product is CE-marked as a breather membrane but, like HControl Hybrid, is dual purpose. Reflective and watertight, yet vapour permeable, it’s used on the cold side of roofs and walls. Because water vapour molecules are smaller than those in water droplets, it lets moisture out, but doesn’t let water in. It also helps improve the building’s air tightness as the membrane not only keeps damp out, but it aso boosts thermal resistance, keeping heat in. Thermal modelling trials carried out by certification body BM TRADA have shown that Actis insulated membranes have a dramatic impact on counteracting thermal bridging, and act as excellent thermal blankets. Both membranes, along with their sister insulation product Hybris, are NHBC-approved and have achieved LABC and LABSS registered details, which means that they can be accepted by LABC and LABSS building control surveyors in all local authorities across 056 FEBRUARY 2018 RCIMAG.COM England, Scotland and Wales, when used according to the certification. www.insulation-actis.com Breather Membranes fact file: AKA Vapour Permeable Underlay (VPU) or breather felt Purpose of breather membranes • To allow moisture laden air to exit the building fabric • Reduce risk of interstitial condensation • Prevent moisture entering the building fabric. Location of breather membranes • In roofs, above rafters • On the external face of timber frame walls • Almost always on the cold side • Associated with a ventilated air cavity to disperse vapour laden air. Technical information • A breather membrane must have a vapour resistance of less than 0.6 MNs / g in walls and, as a Low Resistance underlay 0.25 MNs/g on a roof (BS 5250) • Joints should be lapped and might need taping depending on manufacturer’s requirements • Reflective VPMs are available, offering additional thermal resistance when combined with air gap. Vapour Control Layer fact file: AKA Vapour check or vapour barrier Purpose of Vapour Control Layers • To prevent moisture laden air entering into the fabric of the building • Reduce risk of condensation • Prevent build-up of mould • General airtightness. Location of Vapour Control Layers • In roofs, below the rafter • In timber frame walls, across the internal face of studs • Always on the warm side. Technical information • Joints should be lapped and sealed with tape, according to manufacturer’s recommendations • Various grades available dependant on application • Reflective VCLs are available, offering additional thermal resistance when combined with air gaps. Impermeable Membranes fact file: • Non-breathable membranes usually referred to as roofing felt, sarking felt of type 1F felt • Requires 50mm ventilated air gap on the warm side of the felt (above any insulation) • Commonly replaced with breather membrane.
RCI Feb 2018
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