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 because 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 and attaching to the wall or roof.
It also 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 in order 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, which is actually inside the airtight layer.
The electrician at our Hennerwood project told us that his first fix wiring time was at least twice as quick on our frame as on a previous frame with a membrane formed airtight layer on the inside – this is because his phase on work on ours didn’t interact with the external airtight layer.
That’s a significant saving to consider!
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).