Membrane vent vs SRD AS-B: condensation test

Key takeaways — what this test demonstrates

In our test, the enclosure fitted with the vent alone condensed, the one fitted with the sticker did not. Three IP66 enclosures, ramp from 30 to 0 °C over 1 h 20: the box with the vent condensed like the control box.

The SRD AS-B sticker captures excess water vapor and holds it temporarily, which keeps the dew point from being reached. It releases the vapor when the air dries or warms up.

💡 The two devices do not perform the same function and are complementary. A membrane vent, or pressure-equalising vent, equalizes internal pressure and preserves seals over time: it is not designed to manage humidity. The sticker acts on the humidity of the internal air.

Two functions to tell apart: pressure and humidity

When specifying an IP65 or IP66 enclosure for an outdoor environment, two families of accessories routinely come up when it comes to “managing humidity”:

membrane vents, or pressure-equalising vents,
internal desiccants such as silica gel sachets or mesoporous-material stickers (SRD AS-B sticker).

These two devices are sometimes presented as equivalent. They are not: they address two different physical phenomena and are complementary. To show this experimentally, we ran a comparative test between a membrane vent and the AS-B sticker on IP66 enclosures placed in a climate chamber.

What a membrane vent does

A membrane vent is a microporous hydrophobic membrane (typically expanded PTFE). Its function: allow gases to pass freely to equalize internal pressure against external temperature variations, while blocking liquid water. This is useful to preserve enclosure seals from mechanical fatigue caused by pressure cycling over the very long term (5-10 years).

The membrane does not distinguish dry air from humid air: water vapor crosses it just as freely as air. The vent is therefore not designed to manage humidity: it manages pressure. Internal condensation calls for a different function. If outside air is more humid than inside air, vapor enters through the vent: this is the normal consequence of its function.

These accessories are sometimes presented as an “anti-condensation solution”. The test below shows what actually happens.

What an SRD AS-B sticker does

The AS-B sticker is built around a mesoporous SRD (Self-Regenerating Desiccant) material that adsorbs water vapor by capillary condensation as soon as relative humidity rises above 60% — precisely the zone preceding condensation. When the ambient air dries, the material regenerates spontaneously without energy input. Its function is different from the vent’s: it does not manage pressure, it captures excess vapor, holds it temporarily and releases it when the air dries or warms up.

For the full physics of IP65 enclosure breathing and AS-B sizing, see our dedicated article: the breathing of IP65 enclosures and the time constant τ.

Test protocol: 3 IP66 enclosures in a climate chamber

Figure 1 — Protocol: 3 IP66 enclosures (bare / AS-B / membrane vent) in foam insulation

Three hermetic IP66 enclosures (IP rating for liquid water sealing) were prepared:

Box A — bare control (reference, no device)
Box B — SRD AS-B sticker affixed to the inner wall
Box C — membrane vent

The three boxes were placed inside a foam insulation jacket allowing thermal exchanges only through the glass face — which optimizes condensation visualization. Visible mist on the glass is a direct indicator of dew point being reached inside the enclosure.

Figure 1: IP66 enclosures bare (A), with AS-B sticker (B), and with membrane vent (C), placed in foam insulation.

Initial humid environment simulation

Figure 2 — IP66 enclosures with water dishes before thermal cycling 30→0°C

Figure 3 — Mist formed on walls after dropping to 0°C: visible in all 3 boxes under initial conditions

To generate identical humid air in all three boxes prior to the final test, a water dish was placed in each (with the box C vent temporarily sealed with adhesive putty to equalize initial conditions). Boxes were first held at 30 °C for 30 min in the climate chamber to evaporate the water, then brought to 0 °C for 30 min to trigger condensation on the glass face and confirm that all three enclosures contained vapor-saturated air.

Figure 2: Initial conditions — water dishes placed in the 3 boxes. Figure 3: Mist formed in all 3 boxes after dropping to 0 °C — interior atmosphere saturation confirmed.

Final test: 30°C → 0°C ramp over 1h20

Once initial conditions were validated, the dishes were removed, the AS-B sticker was installed in box B, and the adhesive putty was removed from box C’s vent. The three boxes were warmed back to 30 °C for 30 min to evaporate residual mist, then introduced simultaneously into a chamber programmed to ramp from 30 °C down to 0 °C over 1h20. The experiment was filmed (Nikon D5100) to track mist appearance in real time.

Results: 2 out of 3 enclosures condensed

Figure 4 before — Initial state of the 3 boxes at 30°C

Figure 4 after — Final state at 0°C: mist in A (control) and C (membrane vent), no mist in B (AS-B)

Upon reaching 0 °C, the photos show unambiguously:

🔴 Box A (bare control) — heavy mist on the glass face
🔴 Box C (membrane vent) — heavy mist on the glass face, same result as the bare control
🟢 Box B (AS-B sticker) — perfectly clear glass face, no mist formed

The temperature drop created a cold point on the glass face. The dew point was crossed in boxes A and C, triggering condensation. In box B, the AS-B sticker captured water vapor as the temperature dropped, keeping internal relative humidity below the saturation threshold and preventing any droplet formation.

Figure 4: Before (left) and after (right) the 30°C → 0°C cycle. Box B (AS-B) is the only one without condensation.

Comparative summary

Criterion	Membrane vent	SRD AS-B sticker
Primary function	Equalize internal pressure	Capture excess water vapor, hold it temporarily, release it
Mechanism	Microporous gas-permeable membrane	Capillary condensation in mesopores
Blocks incoming liquid water	✅ Yes (hydrophobic membrane)	n/a (internal action)
Incoming water vapor	Lets it through, by design	n/a (internal action)
Captures internal vapor	Different function	✅ Yes, from 60% RH
Result of our test (30 → 0 °C ramp)	Mist, like the control box	No mist
Preserves seals long-term	✅ Yes (pressure equalization)	Different function (internal action only)
Energy required	None	None
Maintenance	None	None (SRD self-regenerating)
Ideal use case	Highly sealed enclosures undergoing strong thermal cycling over 5-10 years	IP65+ enclosures exposed to internal condensation risk

When to use one, the other, or both

The two devices do not compete: they address two different problems.

To protect seals from wear caused by pressure cycling (very long term, high thermal amplitude environments) → the membrane vent.
To manage internal condensation on electronics (corrosion, sensor drift, dielectric breakdown) → an internal desiccant like the SRD AS-B sticker.
For IP65/IP66 outdoor enclosures with a 10-year lifetime target and sensitive electronics, the two work together: the vent mechanically protects the seals, the sticker manages the humidity of the internal air. If the enclosure already has a vent, keep it and add the sticker. The simulator takes the vent into account when sizing the area.

Conclusion: two takeaways

A membrane vent and an SRD sticker do different jobs. The vent manages pressure, the sticker manages humidity. They are complementary: in a specification, each is specified for what it does.
In our test, the enclosure fitted with the vent alone condensed, the one fitted with the sticker did not. The box with the vent condensed like the control box: the vent managed pressure, which is its function, and the sticker captured the vapor.

To specify your humidity protection on an IP65+ enclosure, request an AS-B quote online or read our deep-dive article « The breathing of IP65 enclosures — why moisture always gets in » which details the physics of vapor diffusion and the analytical sizing of the sticker for your enclosure and climate.