Museum Display Case Airtightness: Is 0.1 Air Changes per Day — or per Hour

The Same "0.1," Two Different Time Units—a 24× Difference

Anyone specifying display cases has seen this: Supplier A says their case achieves 0.1; Supplier B says 0.5 is just as good and half the price. Ask one follow-up question: "0.1 per day, or per hour?" "—and the conversation often stalls. In many cases, the person quoting the number hasn't separated the two either.

This is the most common trap in museum display case airtightness specs: without a unit, the number means nothing. 0.1 per day versus 0.1 per hour is a 24× difference. The first is the threshold for a conservation-grade case; the second is not a figure display cases reach at all.

Air exchange rate is not folklore. It is a hard specification that can be measured, graded, and written into acceptance terms. This article covers four things: what it measures, where 0.1 sits on the scale, how such a number is actually built into a case, and how to ask and verify when you buy.

What Air Exchange Rate Actually Measures

How much of the case air turns over each day?

The air exchange rate (AER, in d⁻¹, i.e., "per day") measures what proportion of the air inside the case exchanges with outside air each day.

The physics is straightforward. Door gaps, panel joints, lock holes and cable entries are never perfectly sealed; outside air seeps in, inside air seeps out. The slower the leakage, the more stable the internal temperature and humidity, and the less sulphur dioxide, ozone, dust and acidic volatiles get in. So AER is not really a "ventilation metric"—it is a quantified result of how well the enclosure is sealed.

How it is measured: tracer gas

The industry uses the tracer gas method: release a tracer gas inside the case, record how its concentration decays over time, and back-calculate the exchange rate. The Canadian Conservation Institute (CCI) technical bulletins reference carbon dioxide (CO₂) and nitrous oxide (N₂O) as tracers and note that a leakage rate measured with tracer gas must be converted to an equivalent water-vapour leakage rate based on material permeability.

Because it measures the whole case, any corner cut in glass, doors, gaskets or joints shows up honestly in the result. That is why it is more trustworthy than a materials list.

0.1 per Day Is the Threshold for a Conservation-Grade Case

The international benchmark: below 0.1 per day

In the display case selection guidance published by Museums Galleries Scotland, the first criterion for a conservation grade case is: sealed from airflow, with an air exchange rate of less than 0.1 per day. The same guidance also requires chemically stable materials, avoidance of wood and wood composites such as MDF, a stable microclimate, options for controlling internal pollutants, no internal heat sources, UV filtering, and security hardware compliant with indemnity standards.

In other words, 0.1 is not a vendor's selling point. It is the international dividing line between a conservation-grade case and everything else.

CCI's three-step scale: 1 / 0.3 / 0.1

To judge where a number sits, use the scale set out by the Canadian Conservation Institute:

1 per day — a typical, moderately sealed display case. CCI notes this is a commonly used default reference value;

0.3 per day — achievable fairly easily by a well-designed glass-and-metal case; well sealed;

≤0.1 per day — very well sealed, i.e. conservation grade.

Note that this is a graded scale, not a universal recommendation. CCI's position is that lower values give a more stable microclimate, but cost and process requirements rise accordingly. Which step you choose depends on what goes inside.

From 1 to 0.05: what each step down actually costs

As a manufacturer, most of the spread in our quotations sits on this curve. Laying out how each level is achieved makes clear where the money goes.

Around 1 per day is the natural level of conventional sealing — a proper double-gasket door and correctly sealed joints get you there. Suitable for general gallery cases holding low-sensitivity objects.

0.1–0.2 per day enters conservation-grade territory: doors move from point contact to continuous perimeter sealing, gasket material and compression must be specified rather than assumed, joint tolerances tighten, and the finished case is leak-tested before dispatch.

Below 0.1 per day adds the third-party test gate. A single failed test means stripping down, re-gasketing and retesting. The price a supplier quotes at this level includes yield and rework, not just materials.

0.05 per day approaches the practical limit of volume manufacturing. To be clear: this figure appears mainly in the technical requirements of individual high-sensitivity projects or in manufacturers' own spec sheets — it is not a unified international standard. Adopting it usually means engineering a specific case separately, often with a microclimate humidity module.

Our standing advice: set the target by object sensitivity, and don't pay for performance you don't need. Move budget from "0.05 everywhere" to "below 0.1 on the critical cases, below 0.5 on the rest" — overall protection is usually better.

Why Museums Are So Strict About This Number

One step looser, a hundred times the pollutant concentration

Most people assume airtightness is only about humidity. The real reason is subtler: a sealed case concentrates whatever its materials off-gas.

Industry research on board-material emissions gives a widely cited order-of-magnitude comparison: in a case at 0.1 air changes per day, volatile organic compound (VOC) concentrations can build to roughly 120 times those of an ordinary indoor environment at 12 air changes per hour. That figure comes from specific experimental conditions and should not be extrapolated to every case type — but the trend is unambiguous: the better the seal, the more concentration can accumulate.

What that means in practice: if a case uses non-compliant board, adhesive or coating, emission levels that are imperceptible in an ordinary room become concentrated enough inside a tight case to corrode silver, fog lead, and embrittle paper and silk. The tighter the case, the sharper this double-edged sword — the more airtight the enclosure, the less room there is for compromise on materials. That is why conservation-grade material requirements (acid-free, low-VOC, Oddy testing) are always bound to airtightness.

Humidity control assumes the case can hold what you put in it

Whether passive or active, humidity control carries an implicit precondition: the case must hold its atmosphere. In a leaky case, moisture buffered by silica gel is continuously diluted by outside air, and a humidifier keeps topping up while the case keeps leaking — good equipment doing futile work.

CCI's technical bulletin on silica gel is explicit: the enclosure's leakage rate is one of the key parameters determining how much sorbent you need, and the better the seal, the less sorbent and the longer it lasts. Hence the old rule: fix the envelope first, then talk about equipment.

For more information, read our guide to museum vitrine climate control and the factors affecting humidity stability.

How We Build a Case That Holds 0.1 per Day

Air exchange rate is not dialled in through settings; it is built from structure and process. A case that reliably holds below 0.1 per day usually earns it in three places.

Door sealing is the main battleground

Most leakage happens at door gaps, not across the glass. Continuous perimeter sealing outperforms point-contact catches by a wide margin — the latter compresses at a few points, the former closes along the entire frame. The gasket itself matters: EPDM or silicone, typically Shore A 40–50, with 15–25% compression. Both figures are engineering practice rather than mandatory standard — though the 15–25% range is consistent across manufacturers' specifications — and the logic is unforgiving: too little compression leaks, too much takes a permanent set and stops sealing within six months.

Joint craftsmanship sets the floor

In engineering practice, visible joint width should be held to ≤0.3 mm, continuous and even, with no triangular gaps. Glass-to-frame junctions, cable-entry sealing and lock-cylinder penetration are each potential leak paths. This is won by assembly precision, not by upgrading materials.

Acceptance criteria force the process

Air exchange rate is a whole-case outcome; no single component can compensate for another. Our internal rule is therefore simple: the sealing structure must be verified on a complete case, not asserted with a materials list. It is also the most useful question you can ask any supplier.

How to Ask, Specify and Verify When Buying

First, ask for the unit

When a supplier quotes "0.1", the first question is: d⁻¹ or h⁻¹? Measured on a complete case, or extrapolated from materials? Writing "0.1 per day" where "0.1 per hour" was meant hands the supplier an impossible task; conversely, dressing up "1 per day" as "near conservation grade" is wordplay.

Second, match the number to object sensitivity

A defensible approach sets targets by sensitivity rather than applying one number museum-wide:

• Low-sensitivity objects (metals, ceramics, glass): around 1 per day is usually adequate;
• General conservation needs: 0.1–0.5 per day;
• High-sensitivity objects (paper, lacquered wood, corroding iron): below 0.1 per day;
• Exceptionally sensitive objects or projects with special requirements: targets in the 0.05 range.

Writing targets into the specification against the object list, level by level, is far more effective than a blanket "highly sealed".

Third, demand the report

A verbal "highly sealed" does not count. Acceptance should rest on a third-party test report using a recognised method such as tracer gas, stating test conditions, ambient temperature and humidity the tracer used, and the concentration decay curve. Test the sample case once and spot-test on delivery — keep evidence at both ends.The most common mistake in one line: ask for the unit, then the scale, then the report.