Airtight Membranes and Vapour Control Layers (AVCLs)
The airtight layer is located on the inside of the building envelope. At Synergie we currently stock two Internal Control layers;
• Synergie Sd Variable – an intelligent/active vapour control layer, 100 gramme/m2
• Synergie Sd 2 - a vapour barrier with a Fixed Sd value of 2m, mass of 100gramme/m2
How does the Sd variable Airtight membrane work?
As mentioned earlier, increased insulation standards necessitate the need for careful moisture management in the structure of buildings. Our Synergie Sd Variable membrane has the ability to change its water vapour performance (Sd value), ie. the membrane becomes more diffusion open depending on the relative humidity (RH) in the area surrounding the membrane. In the colder, winter months, the micropores of the membrane stay diffusion tight, preventing vapour from the warm interior of the building diffusing into the timber roof/wall structure or insulated layer. However, in the summer drying out months, the micropores of the membrane open, allowing any vapour/moisture trapped in the insulated material to escape back into the building interior and dry out/ventilate naturally. In the long run, over a few years, this ability to act as a barrier in winter and a breathable membrane in summer, helps to protect the building from structural damage caused by excessive moisture trapped in the building fabric.
The Synergie Sd Variable membrane is ideally suited to flat roof make-ups which are not adequately ventilated or diffusion tight towards the outside. The Sd Variable membrane is also suitable in new build where excessive moisture accumulation has occurred during the initial construction phase of the building process where timber elements and/or thermal insulation has suffered an unplanned wetting.
Synergie Sd 2 Airtight Membrane & Vapour Control Layer
The Synergie Sd2 AVCL Membrane, with its optimum fixed Sd value, Sd2, is suitable for the underside of ceilings in most pitched roof applications, where breathable roofing membranes (with typical Sd values of Sd 0.02) are incorporated, as they are diffusion open to the outside. The Sd2 vapour control membrane allows for an ideal level of vapour control. The rule of thumb is that the Sd Value of the Internal membrane needs to be a minimum six times higher than the Sd Value of the External roofing membrane.
Correct use of Airtight Membranes and Vapour Control layers (AVCLs)
When considering which AVCL should be used in your specific project, the most important factors to take into consideration are as follows:
• All AVCLs can be differentiated by their Sd Value and/or weight (mass (in grammes) per m2)
• Sd Value = the measurement of the Vapour Permeability of the material/product, the rate/speed at which vapour can pass through/diffuse through the material
• Very Low Sd Values, less than Sd 1.0m indicate that the material is highly vapour permeable or “vapour open” ie. will easily allow the natural passage/diffusion of vapour through the material
• Very High Sd Values, for instance greater than Sd 100m, indicate that the material is “vapour tight”, ie. will really slow down the rate of passage/diffusion of vapour through the material.
• The cut-off point between a “Vapour Check” (a vapour retardant that controls the rate of passage of vapour) and an outright “Vapour Barrier” (not even vapour can pass through the material) is Sd 1500m
• Below Sd 1500m and the material is considered a Vapour Check
• Greater than Sd 1500m and the material is considered a Vapour Barrier …. or total vapour block.
• When looking at a substrate build-up, external vertical wall or roof structure the important objective to bear in mind is the building physics “Rule of Thumb” that the Internal AVCL must have an Sd Value a minimum of 10 times higher than the Sd value of the External AVCL.
• For instance, in typical residential construction, your typical pitched roof will have an external “breathable” roofing membrane that is very vapour open with a Vapour permeability value, Sd Value of 0.02m. At this level any vapour build-up on the underside of the material will very quickly diffuse through the material and get away very fast. If the roof structure is a cold attic space and you install the secondary, internal AVCL at bedroom ceiling level, the Sd Value of the AVCL only needs to exceed Sd 0.2m to hit the 10:1 Rule of Thumb. So use of Synergie Sd2m, on the underside of the ceiling level (below the 400mm of mineral wool insulation, as per Part L of 2013 Building Regulations) therefore comfortably exceeds the rule as you actually achieve a ratio of 100:1
• However, in either residential or commercial construction projects, where an impervous roofing membrane has been utilised on the external side, such as the use of pvc/EPDM roofing membranes in flat roof applications, very careful consideration needs to be utilised when considering which AVCL to use at the internal ceiling level, directly below the flat roof.
• The critical factor to bear in mind is to what extent this flat roof/external wall build-up is ventilated. The external wall may consist of a cement board/MgO board/OSB3/other vapour permeable material. As water vapour slowly passes/diffuses through the internal AVCL vapour check, and diffuses through the mineral wool insulation, it will naturally diffuse to the outside through the external vapour permeable material/board, provided the roof/wall structure has sufficient interstitial ventilation in place.
• However, you run the risk of condensation build-up, ultimately over a long period of time resulting in potential mould issues, if sufficient ventilation is not present.
• In this case, where a potential condensation risk could be possible, it is considered best practice to use internal AVCLs that are “Active/Intelligent AVCLs” that have Variable Sd Values, with, preferably, a wide range in the Sd Values, where the tiny, microporous holes in the functional film of the AVCL can react or “vary” (become “active”) to the relative humidity that occurs at ceiling level, so that in the winter months the AVCL will detect the presence of the relative humidity and operate at the higher Sd Value, such as Sd 20m, in order to slow down the passage/diffusion of vapour through it into the overhead roof/adjacent wall structure. Any vapour that is caught in this structure (due to inadequate ventilation) can condense and eventually lead to potential mould build-up. However, in the subsequent summer months/drying out period, the tiny microporous holes will react and open very slightly, but open enough, to allow for “back diffusion” to occur, ie. the vapour build up will diffuse back into the room below, thus alleviating any potential hazard.
• Our supply partner, Synwer GmbH, at Troisdof, just south of Cologne manufacture many different roofing membranes and also manufacture four separate internal AVCLs, as follows;
• (i) Synergie Sd2 AVCL ………………. Fixed Sd2 allowing for vapour diffusion in one direction only, and is generally utilised in 90% of AVCL applications
• (II) Synergie Sd Variable Sd 0.2m – Sd 20m AVCL ………… Variable Sd AVCL with vapour diffusion possible in two directions, ie. back diffusion can occur.
• (iii) Synergie Sd40m Reflex AVCL ………….. reflective AVCL, with relatively high fixed Sd 40m for use in timberframe, with an emissivity/heat reflective rate of 94%
• (iv) Synergie Sd 1500m AVCL …….. Total Vapour “Barrier” primarily used in very high humidity areas like saunas, swimming pool shower areas etc. where you wish to totally eliminate the passage of any vapour at all.
The airtight layer is located on the inside of the building envelope. At Synergie we currently stock two Internal Control layers;
• Synergie Sd Variable – an intelligent/active vapour control layer, 100 gramme/m2
• Synergie Sd 2 - a vapour barrier with a Fixed Sd value of 2m, mass of 100gramme/m2
How does the Sd variable Airtight membrane work?
As mentioned earlier, increased insulation standards necessitate the need for careful moisture management in the structure of buildings. Our Synergie Sd Variable membrane has the ability to change its water vapour performance (Sd value), ie. the membrane becomes more diffusion open depending on the relative humidity (RH) in the area surrounding the membrane. In the colder, winter months, the micropores of the membrane stay diffusion tight, preventing vapour from the warm interior of the building diffusing into the timber roof/wall structure or insulated layer. However, in the summer drying out months, the micropores of the membrane open, allowing any vapour/moisture trapped in the insulated material to escape back into the building interior and dry out/ventilate naturally. In the long run, over a few years, this ability to act as a barrier in winter and a breathable membrane in summer, helps to protect the building from structural damage caused by excessive moisture trapped in the building fabric.
The Synergie Sd Variable membrane is ideally suited to flat roof make-ups which are not adequately ventilated or diffusion tight towards the outside. The Sd Variable membrane is also suitable in new build where excessive moisture accumulation has occurred during the initial construction phase of the building process where timber elements and/or thermal insulation has suffered an unplanned wetting.
Synergie Sd 2 Airtight Membrane & Vapour Control Layer
The Synergie Sd2 AVCL Membrane, with its optimum fixed Sd value, Sd2, is suitable for the underside of ceilings in most pitched roof applications, where breathable roofing membranes (with typical Sd values of Sd 0.02) are incorporated, as they are diffusion open to the outside. The Sd2 vapour control membrane allows for an ideal level of vapour control. The rule of thumb is that the Sd Value of the Internal membrane needs to be a minimum six times higher than the Sd Value of the External roofing membrane.
Correct use of Airtight Membranes and Vapour Control layers (AVCLs)
When considering which AVCL should be used in your specific project, the most important factors to take into consideration are as follows:
• All AVCLs can be differentiated by their Sd Value and/or weight (mass (in grammes) per m2)
• Sd Value = the measurement of the Vapour Permeability of the material/product, the rate/speed at which vapour can pass through/diffuse through the material
• Very Low Sd Values, less than Sd 1.0m indicate that the material is highly vapour permeable or “vapour open” ie. will easily allow the natural passage/diffusion of vapour through the material
• Very High Sd Values, for instance greater than Sd 100m, indicate that the material is “vapour tight”, ie. will really slow down the rate of passage/diffusion of vapour through the material.
• The cut-off point between a “Vapour Check” (a vapour retardant that controls the rate of passage of vapour) and an outright “Vapour Barrier” (not even vapour can pass through the material) is Sd 1500m
• Below Sd 1500m and the material is considered a Vapour Check
• Greater than Sd 1500m and the material is considered a Vapour Barrier …. or total vapour block.
• When looking at a substrate build-up, external vertical wall or roof structure the important objective to bear in mind is the building physics “Rule of Thumb” that the Internal AVCL must have an Sd Value a minimum of 10 times higher than the Sd value of the External AVCL.
• For instance, in typical residential construction, your typical pitched roof will have an external “breathable” roofing membrane that is very vapour open with a Vapour permeability value, Sd Value of 0.02m. At this level any vapour build-up on the underside of the material will very quickly diffuse through the material and get away very fast. If the roof structure is a cold attic space and you install the secondary, internal AVCL at bedroom ceiling level, the Sd Value of the AVCL only needs to exceed Sd 0.2m to hit the 10:1 Rule of Thumb. So use of Synergie Sd2m, on the underside of the ceiling level (below the 400mm of mineral wool insulation, as per Part L of 2013 Building Regulations) therefore comfortably exceeds the rule as you actually achieve a ratio of 100:1
• However, in either residential or commercial construction projects, where an impervous roofing membrane has been utilised on the external side, such as the use of pvc/EPDM roofing membranes in flat roof applications, very careful consideration needs to be utilised when considering which AVCL to use at the internal ceiling level, directly below the flat roof.
• The critical factor to bear in mind is to what extent this flat roof/external wall build-up is ventilated. The external wall may consist of a cement board/MgO board/OSB3/other vapour permeable material. As water vapour slowly passes/diffuses through the internal AVCL vapour check, and diffuses through the mineral wool insulation, it will naturally diffuse to the outside through the external vapour permeable material/board, provided the roof/wall structure has sufficient interstitial ventilation in place.
• However, you run the risk of condensation build-up, ultimately over a long period of time resulting in potential mould issues, if sufficient ventilation is not present.
• In this case, where a potential condensation risk could be possible, it is considered best practice to use internal AVCLs that are “Active/Intelligent AVCLs” that have Variable Sd Values, with, preferably, a wide range in the Sd Values, where the tiny, microporous holes in the functional film of the AVCL can react or “vary” (become “active”) to the relative humidity that occurs at ceiling level, so that in the winter months the AVCL will detect the presence of the relative humidity and operate at the higher Sd Value, such as Sd 20m, in order to slow down the passage/diffusion of vapour through it into the overhead roof/adjacent wall structure. Any vapour that is caught in this structure (due to inadequate ventilation) can condense and eventually lead to potential mould build-up. However, in the subsequent summer months/drying out period, the tiny microporous holes will react and open very slightly, but open enough, to allow for “back diffusion” to occur, ie. the vapour build up will diffuse back into the room below, thus alleviating any potential hazard.
• Our supply partner, Synwer GmbH, at Troisdof, just south of Cologne manufacture many different roofing membranes and also manufacture four separate internal AVCLs, as follows;
• (i) Synergie Sd2 AVCL ………………. Fixed Sd2 allowing for vapour diffusion in one direction only, and is generally utilised in 90% of AVCL applications
• (II) Synergie Sd Variable Sd 0.2m – Sd 20m AVCL ………… Variable Sd AVCL with vapour diffusion possible in two directions, ie. back diffusion can occur.
• (iii) Synergie Sd40m Reflex AVCL ………….. reflective AVCL, with relatively high fixed Sd 40m for use in timberframe, with an emissivity/heat reflective rate of 94%
• (iv) Synergie Sd 1500m AVCL …….. Total Vapour “Barrier” primarily used in very high humidity areas like saunas, swimming pool shower areas etc. where you wish to totally eliminate the passage of any vapour at all.