A vent makes an enclosure more waterproof
Breathing membranes, and the pressure differences that force water in.
A deliberately fitted vent can make a sealed enclosure more waterproof, not less, because a completely sealed box heated by the sun or by its own internal electronics expands the air trapped inside it, and once that box cools again the same air contracts, pulling outside air, and any moisture riding along with it, back in through whatever tiny imperfection the seal has, a cycle that a correctly designed vent interrupts by letting air pass freely while still refusing to let liquid water through.
The mechanism behind pressure cycling
A sealed enclosure is not actually sealed against pressure the way it is sealed against liquid water, since the air trapped inside still responds to temperature the way air always does, expanding when warmed and contracting when cooled, and a fully sealed box left in the sun and then left to cool overnight goes through exactly that expansion and contraction cycle daily, pushing air out through the smallest available gap while hot and pulling air back in through that same gap while cooling. That pulled-in air is never perfectly clean or perfectly dry, and over many cycles it can carry in enough moisture, or enough dust fine enough to exploit the gap the previous article described, to slowly compromise an enclosure that was, by every static measure, correctly sealed the day it was built. This is a genuinely different failure mode from the ones the earlier articles in this set covered, since it has nothing to do with the seal's squeeze, its groove, or a stray trapped particle, it is a consequence of the enclosure simply existing in a world where temperature changes, a condition no amount of care applied to the seal's own geometry can design away on its own. A seal perfectly squeezed, correctly oriented for pressure assistance, and machined into a flawless groove is still fighting this same daily cycle regardless of how well every other detail of its design has been executed, since none of those earlier fixes address the underlying cause, only how gracefully the seal happens to tolerate the repeated strain the cycle keeps putting on it.
The thermos-flask comparison
A vacuum flask kept tightly sealed holds its temperature well for a single use, but a flask that is never opened to release the pressure that builds as its contents cool can become surprisingly hard to open later, the trapped air inside having contracted against a seal doing exactly what it was designed to do, resisting exactly the kind of pressure difference this article describes. An enclosure with no vent at all is fighting a version of that same battle constantly and invisibly, every warm afternoon and every cool evening cycling pressure across its seal in a way a single well-sealed flask only experiences once, and a seal asked to resist that cycling day after day for months or years is being asked to do a fundamentally harder job than the same seal asked only to resist a single, static pressure difference. A flask released once, its lid cracked open just enough to let the trapped pressure equalise before being sealed shut again, goes right back to holding its temperature well without any lasting harm to the seal around its lid, which is essentially what a vent does automatically and continuously for an enclosure, releasing the same kind of pressure buildup before it ever has the chance to work against the seal at all.
One figure worth keeping in mind
A vent fitted with a breathable membrane, a material with pores small enough to block liquid water's surface tension from forcing a droplet through, yet large enough to let individual water vapour molecules and air pass relatively freely, equalises internal and external pressure continuously rather than letting it build up and force its way past the main seal in a sudden pulse, which removes the driving force behind the whole cycle this article has been describing rather than merely making the enclosure marginally more resistant to it. The size gap between a liquid water droplet, held together by its own surface tension, and an individual water vapour molecule travelling alone is large enough that a membrane's pores can sit comfortably in the narrow window that blocks one while passing the other, the same physical principle behind waterproof, breathable outdoor clothing fabric applied to a rigid enclosure instead of to cloth.
Why this matters in practice
Because the vent handles pressure equalisation and the main seal handles liquid exclusion, the two can each be designed and specified for the specific job they are actually doing, rather than asking one single seal to somehow resist both continuous pressure cycling and liquid ingress at once, a combination that pushes a seal toward compromises that would not be necessary if pressure were allowed to equalise safely through a dedicated, purpose-built path instead. An enclosure without a vent has to be sealed more aggressively than one with a properly specified vent to achieve the same practical reliability, which means a vent, correctly understood, is not a hole weakening an otherwise sealed design, it is a feature that makes the rest of the seal's job genuinely easier.
What this does not explain
A vent only helps if its membrane itself remains intact and unobstructed, since a torn membrane defeats the whole purpose by opening a direct path for liquid water, and a membrane clogged with dust, oil, or debris stops equalising pressure properly and can leave the enclosure worse off than having no vent at all, back to fighting the full pressure-cycling problem this article describes while also carrying a membrane that no longer does its job. A vented enclosure therefore trades one maintenance requirement, periodically checking the membrane's condition, for the pressure-cycling problem it solves, and that trade is worthwhile only where the vent can actually be inspected and replaced as part of a realistic maintenance routine, which is not every application this kind of enclosure ends up used in. An enclosure destined for a genuinely inaccessible location for its entire service life may reasonably be sealed without a vent at all, accepting the pressure-cycling problem this article describes as a smaller risk than a membrane nobody will ever check.