A. Proctor Group's Nick Backhouse made a focused case, that an external air barrier solves the problem internal air-tightness layers keep creating, and that air-tightness, not more insulation, is often the higher-leverage move.
Nick Backhouse had one message. Everything a façade has to do with moisture, he argued, sits under BS 5250, the code of practice for managing moisture in buildings, and it means managing moisture, heat and air together, with the local weather in mind: a tall building in the middle of Manchester is going to be battered by wind-driven rain in a way a sheltered site is not.
The problem with the internal line
The conventional way to make a wall air-tight is an internal air-and-vapour control layer. The trouble, Backhouse argued, is everything that then has to pass through it, HVAC, networking, power, plumbing, structure. Every penetration is a hole in the airtight line, and in practice that internal line ends up compromised on exactly the buildings where it matters most.
Proctor's answer: an external air barrier
Proctor, a family-owned company from Blairgowrie in Scotland, and, Backhouse noted, a heavy user of certification to back its products, does it the other way round. Its Wraptite membrane is a self-adhesive air barrier applied to the cold side of the insulation, on the outside, so the airtight line runs uninterrupted by internal services. Crucially it is both air-tight and vapour-permeable, letting the wall breathe outward while still sealing it, and being self-adhered it simplifies the install. It suits both Passivhaus and non-Passivhaus work.
Why air-tightness beats more insulation
The most useful part of the talk was a comparison. Take a wall at a U-value of 0.15 and an air-leakage rate of 7 m³/m²·hr. To improve its notional carbon figure by around 6.7% by insulation alone, you would have to thicken the mineral wool from 175 mm to something like 630 mm, a wildly impractical wall. Keep the U-value at 0.15 and instead cut the air-leakage rate from 7 to 0.5 m³/m²·hr, and you get a slightly larger improvement, about 6.9%, for a fraction of the material. The point lands hard: past a certain point, chasing air-tightness is far better value than piling on insulation.
The proof: a Passivhaus leisure centre
Backhouse closed on St Sidwells Point in Exeter, the world's first Passivhaus leisure centre, and a project Proctor is plainly proud of. On completion the building achieved 0.1 air changes per hour, and it did so with no internal vapour-control layer at all: the air-tightness was managed entirely with Wraptite on the cold side. Ten minutes, he admitted, was far too short for the subject, but the case was made.
