Why we have an external air tight layer
While not so commonplace in the UK, installing the building's air-tight layer to the outside of the frame is a build method that has been gaining ground in Europe and the US.
The main benefit is that it delivers a more reliable result as the whole of the exterior of the building is covered with the same material, dispensing with the need to deal with complex detailing, such as fitting it around beams attaching to the wall or roof.
This means that once your build is complete, your follow-on tradesmen (electricians, plumbers, etc.) will not have any need to break into the air-tight layer to complete their work, and subsequently reseal and make good again, as they would have to on a conventional system. All necessary wiring and plumbing runs within the structure of the walls, actually inside the airtight layer.
The electrician at our Hennerwood project told us that where he would usually expect to completely wire an air-tight home with an internal layer of this size in about three months, he was able to complete his work within 4 weeks!
That's a significant saving to consider, especially when we consider time is money!
Advantages of external layer
- Significantly stronger than a traditional internal membrane based layer
- Prevents wind blowing through the insulation, which would make it less effective
- Follow-on tradesment can work more quickly and efficiently
- Avoids tricky details around floor joists / rafters (cold roofs).