The UK's Second Staircase Proposal And Why You Can't Ignore Stack Effect 

Effective building design has many elements, from the choice of building materials and the shape of the structure to the placement of windows. The design of a building is made of many aesthetic choices – but just as many serve crucial functional and safety purposes.

For instance, the location of doors, chimneys, windows, HVAC systems, and any unsealed openings, among others, can significantly affect air flow throughout a building. This has particularly significant implications on a phenomenon known as stack effect, which can have a significant impact on not only the comfort of building occupants, but their safety as well.

A law recently introduced in the UK, known as the Second Staircase regulation, can impact stack effect, and buildings designers and developers need to pay attention.

What is the Second Staircase Regulation UK?

The UK government has announced new rules in London to build on recently introduced fire safety measures. This staircase legislation, which took effect in February 2023, has mandated that new residential buildings over 30 meters tall must have a second staircase – providing another avenue of exit in an emergency for both residents trying to exit the building safely and first responders going in and out of the building.

Who Does the Staircase Regulation Impact?

The Second Staircase regulation affects building developers of new and newly proposed buildings that reach at least 30 meters in height. Building designs that previously did not have a second staircase included must now adapt the design in order to include one as per the legislation requirements.

How Will the New Regulation Impact Developers?

This can mean reducing the size of units in a residential building, adding additional square footage, changing the location of lifts, and otherwise adjusting the floorplans. Budgetary effects, such as increases due to the cost of labor and materials, for designs that must be updated must also be considered.

Another major effect of the second staircase law that building developers must account for is the influence on building performance – specifically, elements such as heating and cooling systems, as well as air flow, air quality, and ventilation. A key feature in the effective delivery of quality building performance is managing stack effect within the building.  

What is Stack Effect?

Stack effect, which can also be referred to as the chimney effect or stack ventilation, is often observed as the vertical flow of air through a building. It is driven by the internal-external temperature difference and building height. It is often at its strongest on cold winter or hot summer days and in tall buildings. 

Stack effect can be useful in some instances, such as when it contributes to effective natural ventilation in a building. However, poorly managed stack effect can introduce many issues, including whistling doors, doors that slam, energy loss, odor migration, lift issues, as well as hot spots at the tops of buildings. These issues can become especially pronounced in tall buildings and modern buildings where improvements to tightly seal building envelopes put additional focus on areas with operable components, such as vents, windows, and doors. 

How Extra Staircases Can Increase Stack Effect

Stairwells, and the staircases they house, increase the ease of air flow throughout a building, subsequently increasing routes for stack effect and the potential for issues to occur. Thus, it is crucial to consider their placement and design closely. Without accounting for stack effect, a second staircase could complicate the air flow path through the building, increasing the risk for stack effect issues, as well as influencing the migration of odors and pollutants through the building. Evidence indicates there could be potential issues surrounding ventilation, both normal day-to-day ventilation and ventilation during abnormal situations, as well.

How to Manage Stack Effect with a Second Staircase

Managing stack effect with a second staircase can be a multi-stage process – a key component of which is the review of stack. Modelling to represent varying flow paths, such as how air flows through stairwells, is a powerful tool during this review to develop a holistic understanding of stack effect in a building. Results from modelling can better inform the most appropriate strategies for reducing stack effect. Common strategies include:

However, not every building requires every strategy – and the combination of those needed can also vary. This is precisely why testing is vital in determining the most appropriate management strategy, particularly with the new second staircase regulation in the UK.

What Comes Next?

Now that this legislation has come into force, stack effect review and modelling is even more important. Second staircases are another variable to consider in building design, after all, though their benefits, including those related to safety in emergency situations, are undeniable.

Early stack effect studies in the design of a new residential building can offer high level design guidance about potential risks and accommodate necessary changes without significant setbacks down the line. Detailed review and modelling studies in the later stages, which are strongly recommended for buildings 25 meters tall or taller and recommended for shorter buildings with certain design elements, offer the opportunity to test and refine design elements and mitigation strategies that can relieve stack effect and benefit the building.