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On our build we've done as others have and laid our first floor joists on an airtight membrane/ VCL which is turned up and lapped over the joist ends before tucking neatly into the first floor airtight layer. The underside will do the same at ground floor level. My question is about condensation forming towards the joist ends where they protrude towards the edge of the insulation in both upper and lower walls. Does the airtight membrane stop all air movement and therefore eliminate the risk or will warmer air still find it's way in somehow and then condense when it finds a cooler surface? I know in the US a lot is made of insulating rim boards correctly but this seems to be in situations without an airtight barrier. This must apply to a lot of TF builds or any construction type where joists carry across the walls at the 1st floor junction and I'm really wondering if the airtight membrane is a complete solution or not? Thanks
