Air Tightness & Vapour Control Membranes

Air Tightness Membranes prevent unwanted air infiltration (draughts) and heat loss. But their main task is, to protect the timber structure of your building (e.g. roof structure) against condensation, mainly caused by warm & humid room air in the winter. That´s why the most building professionals call and specify air tightness membranes as vapour control layers.

Air tightness & vapour control layers have to be installed always at the warm side of the building shell.

Examples of usage for air tightness membranes

Highest Ceiling

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Pitched Roof

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Vaulted Ceiling

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Flat Roof

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Timber Frame

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Through floor joists

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Wrapping of pre-cast hollow core slabs

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We always recommend a quailty check of the installed air tightness layers during the construction stage.

Based on our experience, this test should be done, once the air tight layers have been completed (e.g. all windows & doors taped or sealed plus air tight membranes completely installed).

The test should be treated rather as a quality control than an official preliminary air tightness test with values. It´s more important to take the time to detect existing leakages instead of gambling with current values for air permeability and air change.

Such values will be always much higher in comparison to the final air tightness test result.

air tightness membrane for straight ceilings

Mostly found in bungalows & multi storey houses, where the highest storey has been designed as full storey. In the most cases the membrane has to brought down to all the walls, to seal every room separately. Attention is needed to all perforations and penetrations through the air tightness membrane.

We always recommend the use of air tight sealant, to bond the membrane to the surrounding walls.

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air tightness membrane for pitched roofs


Typicall samples can be found in dormers and storey and a halfs. The air tightness membrane can either brought down at the knee walls or all the way down at the pitched roof towards the wall plate. Attention is needed around existing stud partitions. 

The most insulation materials (except Cellulose) have to be installed to the pitched roof area pior the installation of membrane.

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air tightness membrane for vaulted ceilings

Pretty similar to the installation in pitched roofs. The air tightness membrane has to brought down over the wall plates to the side walls and roof windows must be joint to the membrane.

Please measure and adjust the single layers of membrane to prevent an overlap close to the ridge.

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air tightness membrane for flat roof areas

Preparation and planning is the key for flat roofs. When designed as warm roof,  the vapour control layer has to be positioned right underneath the insulation (on top of the sheeting).

Cold roofs with insulation in between the joists should be filled completely with insulation material, to prevent voids and pockets and their high risk of condensation. The membrane has to be installed underneath the joists and needs to be taped and sealed to perfection.

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air tightness membrane for timber frame structure

The entire timber frame structure has to be covered with an air tightness membrane on the inside of the building envelope. Alternatively the same area can be covered with a sheeting (attention to the air tightness performance of the material is required). Every overlap and penetration through the air tightness layer has to be perfectly sealed. Professional installers and self-builders with attention to detail do achieve air change rates even lower than the recommended Passive House Standard of 0.6 ACH (air changes per hour @ 50Pa differential pressure)

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air tightness membrane through the ceiling joist level

One of the supreme details for the installation of air tightness layers. The challenging task is, to bring the air tightness layer from the ceiling level down below to the wall or roof level upstairs without discontinuity. Leaving the joist level exposed is not an option because the air will infiltrate in between the joists.

Be prepared when planning an attic conversion at a later stage.

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air tightness membrane for wrapping around ducon slabs

When using pre-cast hollow core floor slabs in conjunction with cavity wall structure, the slabs need to be wrapped with an air tight material towards the cavity. This has to be done to prevent the infiltration of air into the building. Common practice here in Ireland is the use of air tightness membranes for this purpose. Please ensure that your selected  membrane has good properties regarding tear resistance and sturdiness.

The membrane has to be layed out prior the installation of the floor slabs.

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