A working vocabulary.

ASHRAE classes

Class AA

21 °C ±2 / 50% RH ±5, with proofed daily fluctuations of ±2 °C and ±5% RH and no seasonal RH adjustment. The strictest class. See Protocol I.

Class As

Same daily fluctuations as AA but allowing seasonal RH adjustment of up to ±10%. The most-deployed class in our practice; energy-favorable; preservation-equivalent for most collections.

Class Ad

Same daily fluctuations as AA but allowing seasonal T adjustment (typically 5 °C cooler in winter). Less common; energy-favorable in cold climates.

Class B

±5 °C, ±10% RH daily, with seasonal RH adjustment of ±10%. The class we use for historic retrofits when the building cannot hold A.

Class C

RH controlled (typically 25–60%) but T uncontrolled. Suitable for archival paper in dry climates.

Class D

RH below 75% only. Equivalent to a dry attic. Acceptable for some metal collections; unacceptable for most paper.

IPI environmental ratings

NA factor

Natural Aging factor: a multiplier on the room-temperature aging baseline (NA = 1 at 21 °C/50% RH). NA = 12 means twelve times slower aging. Reported by the IPI Dew Point Calculator.

MD risk

Mechanical Decay risk. Driven principally by RH stability rather than RH absolute value. Categorical (low, medium, high).

MG risk

Mold Growth risk. Threshold around 60% RH and dwell time. Categorical.

MC risk

Metal Corrosion risk. Driven by RH absolute value. Categorical.

Dew point (DP)

The temperature to which a parcel of air must be cooled to reach 100% RH and condense out water. Measured in °C. The single most important number in conservation HVAC after RH.

Mechanical & controls

AHU

Air-handling unit. The box that conditions air (cooling, reheat, humidification, filtration) and delivers it to the rooms.

BMS

Building Management System. The control software that runs the mechanical. We deploy Niagara.

Cascade

A two-stage refrigeration scheme used for sub-zero cool storage. The first stage cools the second stage, which cools the room.

Chiller

A water-cooled or air-cooled refrigeration unit that produces chilled water for the AHU’s cooling coil. We typically deploy Trane CenTraVac or Daikin Magnitude magnetic-bearing units.

DOAS

Dedicated Outside-Air System. Conditions outside air separately from recirculated air, allowing finer humidity control on the make-up.

ECM blower

Electronically Commutated Motor blower. Variable-speed, energy-efficient. Standard on our designs.

ERV

Energy Recovery Ventilator. A heat-and-moisture exchanger between exhaust air and incoming outside air. Recovers ~78% of the latent and sensible energy that would otherwise be exhausted.

Iso-steam humidifier

A humidifier producing isothermal steam (no heating effect on the room). Cleaner than atomized humidification.

Maglev chiller

Magnetic-bearing centrifugal chiller. No mechanical bearings, no oil, no friction. Higher partial-load COP. Read more at GSA.

Plenum

An air-distribution chamber where pressure is equalized before air is delivered to ducts. Half of the practice’s name.

Reheat coil

A coil that warms the cooled air after dehumidification, to deliver it to the room at the right temperature without re-humidifying.

VRF

Variable Refrigerant Flow. A multi-zone heat-pump architecture, useful in residential conservation work for its discretion.

Sensors & telemetry

Burn-in

The 14-day period in which a new sensor is held at controlled conditions against a reference probe to verify its calibration before deployment.

Calibration drift

Slow change in a sensor’s reported value over time, due to component aging. Modern Conserv nodes drift < 0.3 °C over 5 years.

LoRaWAN

Long Range Wide-Area Network — a low-power, long-range radio protocol for IoT, operating on unlicensed sub-GHz ISM bands. Penetrates masonry buildings far better than Wi-Fi.

Reference probe

A NIST-traceable T/RH probe used to calibrate working sensors. Ours are kept in the sensor lab on the second floor of the shop.

Ribbon

A long-form plot of T and RH over time, typically days to years. The single most-read artifact in our practice.

Setpoint

The T and RH value the BMS is targeting. Distinct from the actual T and RH the room reaches.

Building physics

Air change rate (ACH)

How many times the air in a room is replaced per hour. Measured at 50 Pa pressure differential for envelope tightness.

Envelope

The thermal/moisture boundary of a building or room. Walls, windows, roof, floor, doors. The envelope is the system; the chiller is what is in the room because the envelope is not yet good enough.

Hygrothermal

Pertaining to combined heat and moisture transport. WUFI is the modeling tool.

Vapor barrier

A continuous membrane that prevents water vapor from passing. Placed on the warm side of the insulation in heating-dominated climates.

Vestibule

A buffered transition space between the controlled environment and the uncontrolled environment. Conditioned to an intermediate setpoint to prevent moist outside air from entering the vault.

Conservation

Acclimatization

The collection’s slow adjustment to a new environment. See equilibration.

Acetate base

Cellulose-acetate film base, vulnerable to vinegar-syndrome hydrolysis. Cool-storage candidate.

Cellulose nitrate

The first plastic film base, used 1889–1951, fire-hazardous, decomposes via autocatalytic chemistry. Requires sub-zero vented storage. See mixed-media storage.

Conserve O Gram

The NPS’s short technical-leaflet series on collection care. Free, online, indispensable.

Equilibration

The process by which a collection comes into balance with a new environmental setpoint. Typically 2–18 months. Gate M5 in our method.

Foxing

Reddish-brown discoloration on paper, often associated with humidity. A symptom of unstable storage.

Glass-transition zone

The thermal range over which a polymer transitions from glassy to rubbery behavior. For glass-plate negatives, the zone is around 11 °C and is a stress-prone region of any cooling ramp.

Mirroring (silver)

Surface migration of silver to the emulsion-air interface in silver-gelatin photographs, producing a metallic sheen. A symptom of long-term oxidation.

Proofed fluctuation

A daily T or RH fluctuation that has been demonstrated, by direct observation of the room’s behavior, to be sustainable. Distinct from a setpoint specification on a drawing.

Stop-work event

An out-of-band excursion that triggers an emergency response per a written stop-work procedure. We have observed approximately 0.4 stop-work events per year per retainer site.

Vinegar syndrome

The autocatalytic hydrolysis of cellulose-acetate film base, releasing acetic acid (vinegar smell). Slowed dramatically by cold storage. See Protocol II.

Practice-specific

Binder

The hand-bound 60–180-page document delivered to a client at final commissioning. Indexed, signed, working.

Daybook

The form in which we file site-walk notes. Borrowed from Aldo Leopold. See winter shutdown walks.

Gate

One of the six method gates: M1 walk, M2 risk register, M3 schematic, M4 build, M5 equilibration, M6 standing watch. See protocols page.

Quiet ramp

The two-week period at the end of construction during which the system runs to setpoint without the collection in the room.

Ribbon, signed

The long-term ribbon that signals Gate M5 closure: the room has held setpoint within band for one full annual cycle.

Rooms-within-rooms

The mixed-media compromise scheme: a baseline regime for the room, with smaller dedicated envelopes inside the room for material classes that need a different regime. See mixed-media storage.

Walk & ribbon

Gate M1: a two-week instrumented baseline before any design proposal. Sixteen sensors, two walks, an interview with the staff.

Missing a term?

Write studio@plenumandreed.example. The glossary grows by a few entries a quarter.