XPS in cold-store floors: insulation, water vapour and frost protection

Extruded polystyrene boards come in a range of thicknesses. Select the thickness in relation to the rest of the envelope and the cost of refrigeration in operation. An underspecified floor can allow substantial heat gain even where the walls and ceiling are well insulated.

In a cold store the vapour drive can be the reverse of that in a heated room in winter. Diffusion runs from higher to lower partial vapour pressure, usually towards the cold room. Check the direction using temperature and humidity on both sides. Detail the vapour control layer and its sealed joints. Laying XPS alone does not produce a continuous vapour barrier.

Include the diffusion resistance of the boards in the calculation, together with joints, service penetrations and the wall connection. The build-up determines the position of the vapour barrier. A detail taken from a dwelling or additional XPS thickness cannot resolve a break in that layer.

The slab, racks, stock and vehicles all load the XPS. Design the load-spreading slab for the actual points of force application. Obtain creep data as well as compressive stress at ten per cent deformation to assess sustained loading.

Replacing floor insulation later can take a chamber out of service. At design stage, allow for any planned increase in stock, determine acceptable slab movements and check the tolerances specified for the handling equipment. Use those limits to select a grade that meets both thermal and mechanical requirements.

Prolonged sub-zero operation can cool the soil beneath the floor to freezing point. Depending on site conditions, a designed frost-protection system such as heating below the insulation may be required. The presence of XPS reduces heat gain into the room but does not establish that the ground will remain frost-free.

Assess the insulation, structure and soil together. Record soil type, temperatures, operating duration and access for monitoring the protection system. A frost-protection heater controls conditions beneath the insulation while the storage chamber above continues to operate below zero.

Thermal insulation, seals and floor finishes meet at the perimeter. Detail the junction so that the XPS meets the adjacent insulation. Gaps increase local heat gain and can create unfavourable moisture conditions, particularly at columns, thresholds and door frames.

XPS can be cut closely around a penetration, but the penetration still needs its own seal. Provide protective layers suitable for the specified use and cleaning regime. The exposed board is unsuitable as a working surface or hygienic lining.

Ravago, URSA and Austrotherm are among the suppliers of XPS. Check the proposed grade against temperature, load and moisture exposure. Declared performance after freeze-thaw cycling may also matter. A generic product description for cold stores is insufficient for procurement.

For a thick insulation layer, agree the number of board layers and joint pattern in advance. The available stock sizes alone should not determine the build-up. Record the selected configuration and how the insulation will be protected during concreting and assembly of the cold room.

Before covering the XPS, inspect its bearing, board joints and the continuity of water and vapour control layers. Photograph penetrations and perimeter junctions. Base the cooling start-up sequence on the installed materials and plant design. Concealed work must already have been inspected at that point.

Suitable XPS helps reduce continuous heat gain and gives the refrigeration system a predictable floor load. Coordinate the slab, vapour barrier, insulation and any ground protection before ordering boards or scheduling the first delivery of goods.