A vent that works on the difference, inside against outside

The actuator is a printed bimorph: PA-CF, which takes up two to three per cent water and swells roughly half to one per cent linearly, printed as a laminate against PETG, which does not swell. The difference makes the strip curl; 0.8 mm total thickness over 90 mm gives six to ten millimetres of tip travel between 45 and 85 % RH, amplified through a printed 1:6 lever driving the louvre. What I think is the point is the pair: one strip sits in the interior air, a second faces outboard and works against it, so the vent responds to the difference rather than to interior humidity alone. It opens when it is damper inside than out and shuts when wet outside air would push moisture in. A printed over-centre detent gives hysteresis so it does not chatter around the crossover. Housing in ASA-CF, sensing strips behind a rain baffle, a 120 mm louvre that goes into a cover panel or an existing mushroom vent. No power, no battery, nothing to see to for six months.

Calibration is a sealed box, a salt solution for a fixed humidity and a cheap hygrometer.

Then the objections, which are real. Mechanical humidity-driven vents are not new — Aereco and Velux have sold them into buildings for forty years, sensing with polyamide bands. What I cannot find is the opposed, differential version, which is why this is published openly: then it stays free to use. And PA-CF responds in hours, not minutes. For a six-month lay-up that is probably right, but it also means the advantage over a single sensing strip behind a rain baffle has to be proved on the bench before anyone pays for it. This is an idea. Nothing is built and nothing is measured.