Design It Like You'll Have to Map It

Most of the temperature problems I find during a mapping study were built into the room years earlier. Mapping finds them. Design could have prevented a lot of them.

The skylight above the top racking. The loading door that opens straight onto the storage floor. The product parked in a warm corridor while someone finds a pallet jack. None of those are mapping failures. They're design decisions that nobody connected to temperature at the time.

WHO's model guidance for storing time- and temperature-sensitive products (TRS 961, 2011, Annex 9) and its technical supplements cover the building side in some depth: where to put the site, how big it needs to be, how to design and procure it, and how to secure it. So here's a walk through a storage facility from the outside in, with the parts that matter most for temperature control.

The cross-sections below come from our thermal mapping page. Hover over any element to see why it matters.

Start with the site

WHO's first supplement treats choosing a warehouse site as a critical strategic decision, and its requirement is simple. Sites should be located to minimise risks from natural hazards such as floods, landslides and extreme weather, and placed so that the population they serve can be reached efficiently using existing transport infrastructure (Supplement 1, section 1.1).

The model guidance adds a practical detail that's easy to overlook: vehicle access has to accommodate the largest vehicles visiting the site, emergency vehicles included (Annex 9, clause 2.1.2). A site that forces a refrigerated truck to wait on the street is a temperature risk before a single pallet comes through the door.

Size it before you design it

This is the step I most often see skipped. WHO calls correct estimation of the net volume of product the critically important first step in designing or procuring a warehouse, and without it, it's impossible to know how big the building or its cold storage needs to be (Supplement 3, section 2).

That estimate has to account for more than today's stock: how often you're resupplied, the safety stock you hold, how often you deliver onwards, seasonal swings in demand, and seasonal disruptions to resupply (Supplement 3, section 1.1).

There's a temperature reason for getting this right, not just a commercial one. An undersized store gets overloaded, and overloading changes airflow. WHO lists modifications that increase loading or affect air circulation as a trigger for requalifying a temperature-controlled store (Annex 9, clause 4.7). A room that was the right size on paper can end up permanently out of its qualified state simply because it filled up.

Work with the climate, not against it

The model guidance asks for storage buildings that are purpose-designed or well adapted for these products, designed to suit the prevailing climate with as much passive heating, cooling and ventilation as possible, built to minimise energy use, finished in robust, easy-to-clean materials, and built to minimise hiding places for pests (Annex 9, clause 3.1).

Temperature-controlled warehouses are energy-hungry, and WHO recommends adopting a recognised environmental audit system, such as BREEAM, LEED or Pearl, right at the start of design (Supplement 2, section 2.2.1). In high-bay warehouses it also notes the need for equipment to reduce temperature stratification (Supplement 8). Look at the warehouse in Figure 1 with that in mind. The roof-mounted air handling unit, the ducting and the skylight all shape the temperature profile of the racking below them.

Doors, bays and the space in between

Loading and receiving bays should be protected from direct sunlight, rain, wind, dust, and extremes of heat, cold and solar radiation (Annex 9, clause 3.3.1). Goods assembly areas should preferably sit close to the temperature-controlled store (clause 3.4.1).

The areas where product waits during arrival, order assembly and dispatch get their own clause. They should be kept within the temperature range for the goods being handled, protected from direct sunlight and the weather, adequately ventilated, and monitored while product is being handled there (clause 3.5). WHO notes that active control of those areas may not be needed if product stays inside temperature-controlled packaging the whole time.

Supplement 2 puts the packing principle plainly: products should be packed at or close to their labelled temperature. That's why the packing table in both cross-sections carries its own data logger. It's often the least controlled space that product passes through.

Inside the cold room

The model guidance sets out what a cold room or freezer room should be capable of, and the first item is the one that catches people out. It must hold its temperature range across the full annual ambient range at that location, not just on an average day (Annex 9, clause 4.3). The same clause asks for auto-defrost with minimal effect on temperature, low-temperature protection in cold climates, connection to a UPS, calibrated continuous monitoring, alarms, and lockable doors that can still be opened from the inside.

Several of the elements in the cold room cross-section tie back to these requirements. Take the refrigeration unit itself. A self-contained monobloc unit is easy to install, but it discharges the condenser's waste heat into the general warehouse space, which is the same space holding your ambient stock. That's why WHO suggests that, particularly in hot climates, a split system is the better arrangement, with the evaporator inside the room and the condenser outside the building (Supplement 2). The cold room in Figure 1 has its condenser outside the room for the same reason. The supplement also expects every cold room to have 100% standby refrigeration capacity in case a unit fails. The insulated panels, the door seal and the floor drain are all potential routes for heat to get in. And the cleanliness requirements are design questions too: floors should be fully accessible, goods shouldn't be stored directly on the floor, and frost and ice shouldn't be allowed to build up (Annex 9, clause 4.8).

Control sensors and monitoring probes do different jobs

This is where design and mapping meet, and it's a distinction that gets blurred more often than it should.

The control sensor is the one the refrigeration system responds to. In my view it should represent the room average, at a position defined by a qualified installation technician, so the cooling equipment runs the way it was designed to. Place it at an extreme instead and the unit ends up chasing one corner of the room. A control sensor sitting in the warmest spot keeps driving the rest of the space colder, and product near the evaporator can end up too cold as a result.

WHO's model guidance takes a different view here, suggesting control sensors be positioned at the hot and cold spots found by temperature mapping (Annex 9, clause 4.5.1). It does say, sensibly, that control sensors should be independent of the monitoring system.

Monitoring probes are a different matter. They belong at the hot and cold spots identified by temperature mapping, representing the warm and cold extremes of the space within its defined acceptance criteria. That's broadly where the guidance lands too. WHO places monitoring sensors where the greatest temperature variability is expected within the qualified storage volume, positioned so they're minimally affected by transient events such as a door opening (clause 4.5.2). SAHPRA's wholesaling guideline asks for continuous monitoring sensors and alarms at the points representing the temperature extremes (SAHPGL-INSP-03, section 3.10.5.7.5).

For cold rooms, WHO also asks for a record at least six times an hour at each monitoring position, a system that keeps running through a power failure, and a manual check twice a day, seven days a week, public holidays included (clause 4.5.2).

The practical sequence follows from that. The control sensor is set by the installer as part of the design, but the monitoring positions can't be set properly until the room has been mapped. That's why the cold room in Figure 1 has loggers at the top, middle and bottom of the shelving, and why the door and the evaporator are both on the mapping plan.

Design for the layout you'll have in five years

WHO is realistic about this: the internal layout of a warehouse is certain to change over the life of the building, as new product lines and new warehouse technologies arrive, so designing for long-term flexibility is critical (Supplement 2, section 2.2.2).

The practical consequence is worth building into your change control from the start. Every layout change that increases loading or affects air circulation is a reason to requalify (Annex 9, clause 4.7). A flexible building is a good thing. A flexible building with no plan for re-mapping after it flexes is a future deviation.

Security and fire belong in the brief

Supplement 4 covers security and fire protection, and it's much cheaper to design in than to retrofit. Its requirements include preventing unauthorised access, dedicated and securely locked areas for controlled or hazardous products, fire detection and firefighting equipment approved by the local fire authority, documented procedures, regular fire drills and no smoking anywhere on the premises (Supplement 4, section 1.1). Perimeter protection, a gatehouse and CCTV where resources allow are the first layer (section 2.1).

Questions for your next design brief

• Do you know the net storage volume you need, including seasonal peaks, before anyone draws a floor plan?
• Will the cold room hold its range across the full annual ambient range at that location?
• Where does product wait between the truck and the cold room, and is that space protected and monitored?
• Who decides where the control and monitoring sensors go, and on what evidence?
• Is there time in the programme, and budget in the contract, to map the space before handover?

That last one matters more than it looks. WHO's mapping supplement says new temperature-controlled storage areas should be mapped before they're commissioned and handed over by the installer (Supplement 8, section 1.1). If mapping isn't in the contract, it tends to happen after the stock has already moved in.

If you're planning a new store or a refit, our thermal mapping and qualification pages cover how we approach both, and No Action Required walks through when a changed facility needs requalifying.