Cold Storage Construction: What Decides the Building Envelope

Thicker panels do not make a better cold store

When a cold store underperforms, the first instinct is to blame the insulation and specify thicker panels next time. In practice the panels are rarely where it went wrong. A cold store fails at the places where the insulated box is interrupted: panel joints, the wall-to-floor junction, doors, steel passing through the envelope and every pipe and cable that enters.

At those points warm, humid outside air meets cold surfaces. Moisture condenses, soaks into insulation and, in frozen rooms, turns to ice. Wet insulation insulates poorly, so the refrigeration runs longer, which makes the surfaces colder, which draws in more moisture. For the business the cost is a rising power bill, ice on floors and doors that becomes a safety hazard, damaged stock, and in the worst case a frozen-store floor that heaves and has to be broken out.

The envelope is decided by what the room holds. Cold stores are usually described in two broad categories, and the difference changes more than the panel thickness.

The categories are descriptive. The room temperature, panel thickness and protection measures for a project are set by the refrigeration and envelope designer.

The vapour barrier belongs on the warm side

Moisture moves from warm, humid air towards cold. In a cold store that means from outside, inwards. In most Indian climates the outside air carries a lot of moisture for much of the year, so the drive into a cold store envelope is strong and constant.

The rule is to stop moisture on the warm side of the insulation, before it reaches the cold zone where it would condense. With metal-faced sandwich panels the outer skin is the barrier, which makes the joints the critical detail: every panel joint, corner and penetration needs sealing on the warm face, and the seal has to be continuous with the barrier under the floor. Sealing only the inside face, which is easier to reach, traps moisture inside the panel joint. Interstitial condensation explains the mechanism, and the same dew point logic behind office window condensation and mould applies here at a much larger temperature difference.

Panel selection still matters. PIR insulated panels give high thermal performance per millimetre, and the panel system as a product is covered by EN 14509. IS 661 is the Indian code of practice for the thermal insulation of cold storage. Neither replaces sealing the junctions.

The floor: insulation and frost heave

The floor is the part of a cold store that is hardest to fix later, because it sits beneath the racking, the stock and the traffic.

Floor insulation has to carry rack leg loads and forklift wheels without crushing, so it is typically a high-compressive-strength board such as extruded polystyrene, chosen from the manufacturer's data against the loads the structural engineer calculates. A vapour barrier sits below it, and a reinforced wearing slab above it spreads the loads.

Frost heave is the slower risk. Under a frozen room, cold works down through the insulation over months and years. If the ground below falls below freezing, water in the soil freezes and expands, lifting and cracking the slab. Frozen stores are therefore typically built with underfloor heating (electric or fluid-based) or a ventilated underfloor that lets ambient air through pipes or a void beneath the insulation. Which method suits depends on the site, the ground and the room temperature, and is the cold store engineer's decision. The wearing surface on top is a separate choice; PU concrete flooring is commonly used where washdown and thermal shock are part of the duty.

Doors, air curtains and the loading face

Every time a door opens, warm humid air rolls in at the top and cold air spills out at the bottom. In a busy store the door is often the largest single source of heat and moisture, larger than the conduction through all the panels.

The design tools are well known: fast-acting doors that are open only while traffic passes, insulated sliding or hinged doors for less frequent use, strip curtains and air curtains to reduce exchange while a door stands open, and an anteroom or chilled dock between the outside and the coldest rooms. Frozen-room doors usually have heated frames so seals do not freeze to the frame. Where the store loads trucks directly, the dock seal and leveller become part of the envelope too.

Frozen rooms also need pressure relief. When a room is pulled down to temperature the air inside contracts, and when a door opens and closes the pressure swings. Without relief valves, panels can be pushed or pulled out of shape and joints opened.

Thermal bridges and the refrigeration interface

A thermal bridge is any path that lets cold travel through the insulation. In cold stores the common ones are steel columns or beams passing through the envelope, panel hangers from the roof structure, the base of the wall where it meets the floor, and fixings through both skins. The cold end of the bridge chills the warm end, and moisture condenses or ice forms on it. Keeping the structure entirely outside or entirely inside the insulated box avoids many of them.

The refrigeration interface adds more penetrations: refrigerant pipes, drain lines, power and control cables, and the supports for evaporators hung from the ceiling. Each needs a sealed sleeve detail agreed between the panel installer and the refrigeration contractor before panels are cut, not after.

Fire is an envelope decision too

Insulated panels with foam cores are the usual cold store envelope because they combine insulation, vapour control and a cleanable face in one product. The cores are combustible, and fires in cold stores have shown how a fire can travel through panel cores where it is hard to see and hard to reach. Cold stores are also often unoccupied for long periods and full of packaging, which adds to the fire load.

That makes the panel core a fire decision as well as a thermal one. The core's reaction-to-fire class to EN 13501-1 should be named in the specification, and what the building requires, including any compartmentation and whether non-combustible cores are needed at particular lines, is decided by the fire consultant under NBC 2016, Part 4. Insurers frequently take their own view, so it is worth asking them before the panels are ordered.

Common mistakes

• Specifying thicker panels while leaving joints sealed on the inside face only.
• No frost protection under a frozen room, because the floor was designed as a normal warehouse slab.
• Floor insulation chosen for thermal value alone, without checking it against rack and forklift loads.
• Refrigeration penetrations cut on site without a sealed sleeve detail.
• Steel structure passing through the envelope without a thermal break.
• Panel fire class left unstated, then raised by the insurer late.
• No pressure relief on frozen rooms.

What to ask your designer and contractor

• What room temperatures and loads were used for panel thickness?
• Where is the vapour barrier, and how is it continuous at joints and the floor?
• How is the floor protected against frost heave, and who designed it?
• Which doors, curtains and anterooms are proposed for the traffic expected?
• What is the panel core's reaction-to-fire class?
• How are refrigeration penetrations sealed, and who is responsible for them?

Standards referenced

Thermal insulation of cold storage under IS 661; metal-faced insulating panels under EN 14509; reaction-to-fire classification under EN 13501-1; fire and life safety under NBC 2016, Part 4. Room temperatures, panel thickness, floor protection, refrigeration design and fire strategy for a specific cold store must be established by the project's refrigeration engineer, structural engineer and fire consultant; nothing on this page replaces that design.