FFU ceiling layout: coverage, class and grids

By ZY Cleanroom · Updated 2026-11-01 · Sources at the end of the page

Start from airflow, not coverage

LBNL defines room air velocity as total supply airflow divided by floor area. It defines air change rate as total supply airflow divided by room volume. It adds that no simple rule links a class to one velocity or one air change rate.

These published values frame the target.

Example calculation (assumed values)

Example only, with assumed values. Room: 6 m × 4 m, 3 m ceiling. Floor area is 24 m² and volume is 72 m³. Assume a unit delivers 800 m³/h at its working point. Use your own room size and datasheet value.

ISO 7 at 30 air changes needs 72 × 30 = 2,160 m³/h, or 3 units. ISO 5 at 0.45 m/s needs 0.45 × 24 = 10.8 m³/s, or 38,880 m³/h. That is 540 air changes and 49 units.

The table shows this example ISO 5 case at three coverages. Face velocity equals 10.8 m³/s divided by filter area.

Ceiling and grid details

USP <797> requires inlaid ceiling panels to be caulked to the support frame, and light fixtures to be flush and sealed. Annex 1 asks for ceilings sealed against contamination from the space above.

Returns in a USP <797> suite must be low on the wall unless a smoke study shows no stagnant air. Repeat the smoke study when equipment moves or the HVAC changes.

Test the finished ceiling

ISO 14644-3:2019 covers installed filter leakage (Annex B.7), airflow velocity and uniformity (B.2), airflow direction and visualisation (B.3) and recovery (B.4).

FDA's 2004 guidance scans the filter face 1 to 2 inches away. A single reading of 0.01% of the upstream challenge signals a significant leak. Annex 1 requalifies Grade A and B every 6 months at most, and Grade C and D every 12.

Put these in the room data sheet

State the class and occupancy state. State the velocity target and where it is measured. State the coverage assumption and the face velocity limit per unit. State how many units may fail before the room fails its class, and give access for aerosol injection and scanning.

Is an FFU ceiling the right system?

LBNL compared recirculation systems in the ISO 5 rooms it reviewed. Fan-tower systems were generally far more efficient than distributed air handlers and FFUs. Efficiency varied widely from room to room.

Efficiency is one factor. Cleanliness class, process risk and maintenance access also count. Compare the options on airflow delivered per watt of fan power, using measured data, before you fix the layout.

Who approves the room

Your regulator judges the facility, not the contractor. The contractor builds the ceiling to your data sheet and supplies test records. To have a room list reviewed, send it to ZY Cleanroom, a cleanroom design-and-build contractor with its own panel factory in Tianjin, China.

Related guides

• Cleanroom pressure differentials: cascades, values, monitoring
• Cleanroom HVAC design: AHU, recirculation and FFU options
• HEPA filter integrity and leak testing
• Medical device cleanrooms: writing ISO 13485 design inputs
• Turnkey cleanroom contractor
• Room list builder

Sources

• Tengfang Xu, Lawrence Berkeley National Laboratory, Considerations for efficient airflow design in cleanrooms (peer reviewed, 29 July 2004; checked 31 October 2026)
• FDA, Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing, Current Good Manufacturing Practice (September 2004, nonbinding; checked 31 October 2026)
• EudraLex Volume 4, Annex 1: Manufacture of Sterile Medicinal Products (published 25 August 2022, applied from 25 August 2023; checked 31 October 2026)
• Washington State Department of Health, Pharmacy Self-Inspection Worksheet, 2026 USP 797 Sterile Compounding Addendum, DOH 690-296 (January 2026), which reproduces USP <797> text by section; checked 31 October 2026
• ISO 14644-3:2019, Cleanrooms and associated controlled environments, Part 3: Test methods (published August 2019; contents list checked 31 October 2026)