Class AA gallery design & commissioning.

What Class AA is

The ASHRAE classification system for cultural-heritage HVAC, codified in Handbook — HVAC Applications Chapter 24 and elaborated in Stefan Michalski’s 2007 Getty paper, distinguishes six classes — AA, As, Ad, B, C, and D — by the size of the band and the rate of change a room is held to. Class AA is the strictest: a setpoint of 21 °C ±2 and 50% RH ±5, with proofed daily fluctuations no greater than ±2 °C and ±5% RH, and (this is the part that defines AA against As) no seasonal RH adjustment. The room is held within a 10% RH band twelve months a year.

The collection categories that demand AA are well known: oil paintings on canvas with significant cleavage history, works on paper of museum-quality photography, photographic prints with vulnerable emulsions, panel paintings, and any object that has been quietly drifting toward instability inside a previously laxer regime and now needs to be locked down. The classes are not a hierarchy of virtue; AA is not always the right answer. But where it is the right answer, it is the only answer.

Who is asking for Class AA

In our practice, the institutions that have asked for Class AA in the last five years have been: a regional art museum installing a long-term loan of a Caillebotte; a university library accepting a 19th-century French print collection on terms that named ASHRAE AA in the loan agreement; a small institution renovating its single climate-controlled gallery before bringing in a touring photography exhibition; and one private collector with a Squirrel Hill rowhouse and four Audubon double-elephant plates that her insurance carrier conditioned at AA. The mix of motivations is typical: a loan agreement names the class, an insurance policy names the class, a peer institution holds AA and the board feels it should also hold AA. Sometimes all three.

Class AA is rarely the right answer for an entire institution. It is most often the right answer for a single gallery or a single cluster of galleries, with the rest of the building held to As, B, or C. We have run AA-in-a-larger-building projects more often than full-building AA.

The mechanical parts list

A representative parts list for a 4,500 ft² Class-AA gallery suite, three rooms, single zone, single AHU. Specifications are typical, not universal.

The components are mostly off-the-shelf. Class AA is not bought at the chiller. It is bought at the envelope, at the controls, and at the equilibration. The chiller is what is in the room because the envelope is not yet good enough; the goal of every AA project we run is to leave the room able to hold AA on the smallest possible chiller.

Commissioning & equilibration

The mechanical build is typically four to seven months. The commissioning is typically four to eleven months on top of that. The two are separate phases — we do not consider the system commissioned until the room has held AA, within band, for a full annual cycle, and we will not return the collection to the room until that cycle is observable. The collection-return date is therefore decided by the ribbon, not by the construction schedule.

Equilibration is the slow part. The collection has been living, until now, in whatever envelope the institution had previously installed; the new room is now considerably tighter; bringing the collection from old to new at no more than 1 °C and 2% RH per week typically takes between two and twelve months. The longest equilibration we have run was 18 months — for the cordage warehouse case (case 031) where the building had no functional thermal envelope and the collection had been at 24 °C/68% RH for fifty years.

What it costs

A typical Class-AA gallery project at the 4,500 ft² scale prices between $620,000 and $1.4M total construction, with engineering and commissioning at 9–14% of that on top. The variance is driven by the envelope you are starting from. A new-build addition designed for AA from day one prices toward the bottom of that band; an early-20th-century masonry building with limestone walls and steel windows prices toward the top, sometimes well past it. We will tell you, before Gate M2, where in that band your building is likely to land.

The 24/7 standing-watch retainer (Gate M6) on an AA system runs $32,000–$64,000 per year, depending on site count and after-hours response window.

If not AA, then what?

For most institutions, most of the time, Class As is the right answer. As permits seasonal RH adjustment of up to 10% (a softer wintertime band, a softer summertime band) without compromising the daily fluctuations — and the energy savings are large, often 18–26% on annual HVAC operating cost. We have moved more clients from A to As than from A to AA. Petra has written about this trade-off at length in Dew point as a protocol and in her 2024 AIC talk; we encourage you to read both before naming a class on a loan agreement.

For collections of color photography, motion-picture film, or acetate-base material, Class AA is also not the right answer; cold storage at 5 °C / 30% RH is. AA is for the room you walk through; cold storage is for the room you keep things in.

Begin a Class AA project

Most institutions that take this protocol have already named the class in a loan agreement, an insurance policy, or a board resolution. If yours has, write us. Begin a project. If yours has not but is considering it, the right next step is a 30-minute call with Petra to talk about whether AA is the right answer at all.