For most industrial and commercial buildings, insulated core steel doorsets are often the preferred choice. Although they are not the most economical choice, they do provide a lot of capabilities that other doors do not. Steel doorsets are modular, and as a result, can be tailored to provide a combination of features, including but not limited to, security, fire resistance, and the ability to withstand high levels of wear and tear. This modularity is the reason why these doors can be seen in distribution centers, warehouses, and food production facilities among others.

Steel doorsets consist of an insulated core that is sandwiched between two exterior sheets of steel. These can either be galvanized steel or cold-rolled steel. The core can be either foam or mineral wool. Polyurethane foam is the most common choice. It has an excellent thermal performance and is available in a variety of thicknesses. However, it is also the most flammable. Where fire resistance is the primary concern, mineral wool is a much better choice since it does not contribute to fire spread in the way that foam can. Some manufacturers also produce mineral wool-steel foam hybrid doors that try to combine both thermal performance and fire resistance. However, the performance deficits in each aspect of these doors should be examined before making a specification.

For compliance under Part L of the Building Regulations, U-value is the relevant thermal performance metric. For commercial buildings, the current version of the Approved Document L puts the limiting U-value for opaque doors that form part of the building envelope at 1.4 W/m²K. Mainstream manufacturers’ doorsets that are better than this offer U-values of 0.8 to 1.2 W/m²K, meaning there is flexibility available against the limiting U-value, which is particularly important to consider when optimizing the overall energy performance of the building. The worst performing area of the doorset, and the area of the highest thermal bridging, is generally the threshold detail. A highly insulated door that is poorly framed and detailed at the door threshold will perform far worse than the doorset’s specification would lead one to believe.

Fire rating must be outlined independently from thermal performance. In the UK, fire rated doorsets are rated by integrity and insulation: FD30 and FD60 represent the most common ratings in the residential and commercial sectors and signify thirty and sixty minutes of fire resistance, respectively. If the doorset must also inhibit the passage of smoke, then the rating in the UK changes to FD30S or FD60S. For high risk areas, plant rooms, and zones where a longer resistance period is required by a fire engineer, doorsets are rated FD90 or FD120. The UK standard for the fire-resisting doorsets is BS EN 16034. Rather than relying on manufacturer claims, doorsets should be third party certified to this standard.

Incorporating security performance into insulated steel doorsets doesn’t significantly change the construct. Some UK manufacturers have doorsets certified to LPS 1175, and compliance to PAS 24, assessed for the Secured by Design standard, can be done within standard insulated steel constructions. For most commercial applications, doorsets use multi-point locking systems. However, BS EN 1125 compliant hardware is used if the doorset needs to be a fire escape. BS EN 1125 also mandates the doorset be lockable from the interior. Selecting hardware requires careful deliberation, as a doorset designed to be a security doorset that is frequently used, and a doorset designed to be a security doorset that is infrequently used, must have a different hardware configuration.

In controlled environments, thermal insulation requirements extend beyond the doorset. Cold storage doorsets particularly have heating elements built to the frame to mitigate against icing at the seal, and doorsets must withstand the pressure that occurs when a cold air doorset is opened to the warmer ambient air. There is no mistaking the difference between a standard insulated commercial doorset and a cold storage doorset. Insulating the wrong one leads to icing at seals and energy losses that are expensive and difficult to resolve.

Steel doorsets exhibit very little adjustment flexibility after fitting compared to most door types, meaning that installation tolerances matter more with steel doorsets. The structural walls/enclosure need to be built to the correct dimensions to accommodate the steel doorset frame. Validating the dimensions of the structural opening prior to frame installation is especially necessary for the steel doorsets when compared to flexible frame doorsets as the result of fitting steel doorsets into oversize structural openings will look and function much worse compared to fitting the frame into a structural opening built to the proper dimensions.