An AVCL should always be installed over areas of high humidity regardless of whether the system is ventilated or not. This applies to bathrooms and kitchens, and to swimming pools and saunas in particular.
Low humidity areas, such as offices, do not require an AVCL but can benefit from better airtightness and vapour control if one is installed. This can also help improve energy efficiency.
They also help to achieve airtightness and reduce convective heat loss from the building, providing the joints are sealed. Yes, to allow vapour to escape to the outside atmosphere. Yes, but the breathability of the membrane will be less effective. Yes, for 4 months, providing the membrane is secured sufficiently to prevent wind damage. While this is happening, the vapour resistance can be as low as 0. When all joints are taped, it forms a barrier against convective heat loss, and provides limited vapour control.
When all joints are taped, it forms a barrier against convective heat loss and provides limited vapour control. Put simply, an air and vapour control layer AVCL helps to keep warm air inside the building and prevents condensation build-up with its airtight but vapour-open structure — keeping air in but allowing water vapour to pass through the membrane. Breather membranes are positioned to the outside of the insulation acting as a weather barrier while still allowing moisture particles to escape from the inside.
How to install a roof breather membrane Lay the breather membrane. Next, lay the breather membrane parallel with the roof eaves — normally, when installing breather membrane the printed side should face outwards.
Ensure draping occurs. Lay the rest of the breather membrane. Breathable sarking membranes negate the need for ventilation directly above the insulation layer. They do this because they have a low vapour resistance, which allows water vapour to escape through the breathable membrane to the outside of the building. Breather membranes are installed to the outer side of the insulation — for example, either over or under the counter-battens on a pitched roof — and allow water vapour to escape from inside a building without the need for ventilation.
They also repel any water, most commonly rain, that tries to enter the building. What is the best breathable membrane? Cromar Vent 3 Classic - Breathable Felt. Klober Permo-Forte Breathable Felt. Klober Permo Air Breathable Felt. Redland Spirtech 2s Breathable Felt Membrane. BS clause 8. Breather membranes are water-resistant but vapour permeable which means that they can protect your building from damp related issues while releasing any surplus moisture vapour into the air. They are also able to prevent penetration by external environmental factors including snow, wind and rain and they keep out contaminants such as dust and dirt.
One common problem with homes, especially new builds, is condensation. This can be caused by a number of factors, and a poorly fitted and installed breather membrane can be a contributing factor. Breather membranes are installed to the outer side of the insulation and each roll should contain detailed fitting instructions. You may also need to check the ventilation requirements of the membrane will be met before you begin the installation.
Some breathable membranes are also approved as being air open as well as vapour permeable. These underlays are the most breathable on the market and are particularly suitable where high levels of condensation are to be expected such as the drying out period of new build properties , or for more complex roofs where traditional ventilation options may be problematic.
Next, lay the breather membrane parallel with the roof eaves — normally, when installing breather membrane the printed side should face outwards. Make sure the bottom of the membrane is overlapping the eaves support tray or UV resistant membrane by at least mm, but not by so much that it will still be exposed when the roof tiles are laid. The breathable roof membrane should drape slightly between the timber rafters or counter battens, meaning there is a dip of around mm.
When the tiles or slates are fitted they will provide a more secure fixing. Continue laying the lengths of breather membrane, working up the roof. Each manufacturer should state the amount of overlap required between each length of the breather membrane. If the overlap does not coincide with a timber batten, an additional batten can be installed to ensure the overlap remains, and to prevent the tiles from touching the membrane.
These can be added across the top of the underlay, but the drape of the underlay must still exist to allow the rainwater to pass down the surface of the membrane.
If the roof includes ridge ventilation, the breather membrane must be cut on each side of the ridge to ensure the path of ventilation is still clear. If there is no ridge ventilation, the breather membrane should go over the ridge, and overlap each side of the ridgeline by the required overlap. On roof valleys, ensure the breather membrane is extended by a minimum of mm on each side, measured from the centre of the valley.
At abutments, the underlay should be turned up by at least mm — this will create a secondary water-resistant barrier. With wet verges, also known as verges that use mortar bedding as fixings , the breather roof membrane needs to lap onto the masonry by at least 50mm. When used with dry fix verges , the membrane needs to be laid so that it extends past the face of the gable.
For windows , soil pipes and other penetrations cut the membrane and turn it so that it folds up against the penetration. If working with circular penetrations, cut the membrane with an Asterix shape to create flaps to fold upwards. If needed, tape the cut out sections to the penetration to prevent them from folding back down.
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