HOW TO SPECIFY EXTRUDED POLYSTYRENE (XPS) INSULATION FOR A CAR PARK DECK

If a flat roof forms part of a building’s thermal envelope as well as providing car parking then it is necessary to specify an insulated car park deck. This means choosing a thermal insulation product capable of meeting the energy efficiency requirements of national building regulations and bearing the loads imposed by vehicular traffic.

Any building element should be designed and specified to meet an individual project’s requirements. As such, there are different ways to approach the build-up of an insulated car park deck. In this post, we’ll offer some general advice on approaches that can be taken, and look at typical structural loads associated with car park decks.

What is a typical build-up of an insulated car park deck?

Illustrations of typical car park deck build-ups tend to feature warm flat roof constructions. While this doesn’t automatically rule out inverted roof solutions, it is hard to see an advantage in specifying a separate waterproofing layer on the structural deck, and surface layer suitable for vehicle traffic above the insulation.

From a perspective of choosing extruded polystyrene (XPS) insulation, either a warm or inverted roof solution is acceptable. The build-up might be guided more by the requirements of any system supplier you choose to work with, and their warranties/guarantees. Taking this into consideration, an example warm roof car park deck specification could look like the following.

• Paving grade asphalt.
• Mastic asphalt waterproofing.
• Sheathing felt.
• Reinforced screed, laid to falls.
• Building paper.
• XPS insulation.
• Vapour control layer.
• Concrete deck.

The requirement for a vapour control layer may depend on the humidity levels inside the building, and appropriate project-specific condensation risk analysis should be carried out.

What waterproofing should be used for an insulated car park deck?

The example car park deck build-up described above features mastic asphalt waterproofing. Mastic asphalt is a thinner and lighter alternative to hot-rolled asphalt and asphalt concrete.

Sheathing felt isolates the mastic asphalt from movement in the structure, and offers surface friction against it contracting in cold weather. Because of mastic asphalt’s extremely high vapour resistance, the sheathing layer also provides a route for moisture vapour to disperse through the construction without blistering the asphalt. This explains why a vapour control layer is sometimes shown not to be required, as long as the building use is not a high humidity application.

Liquid-applied waterproofing is an equally viable choice. The speed and relative ease of installation, comprising fewer layers, are good reasons why it might be a preferred choice. XPS insulation can be specified in conjunction with both mastic asphalt and liquid-applied waterproofing.

Depending on the waterproofing chosen, more information on specifying the surface layers of the car park deck may be sought from the manufacturer of the mastic asphalt waterproofing or the Mastic Asphalt Council (MAC), or the manufacturer of the liquid waterproofing or the Liquid Roofing and Waterproofing Association (LRWA). Possible other sources of information include the local authority’s highways department, or the British Parking Association (BPA).

What loads act on car park deck insulation?

While it’s tempting to think of structural loadings only in terms of the vertical loads imposed by vehicle weights, car parks are also subject to other loads.

When we consulted with a structural engineer on this subject, they told us about horizontal braking forces of a car. They explained a situation where a grating was specified at the end of a parking ramp. The grating was designed to withstand the weight of the car from above (compression) but wasn’t suitable to resist the force imposed by the car braking.

Whether active loads such as these are accounted for should form part of the discussion you have with your project’s structural engineer about the suitability of the car park deck specification.

“The dead weight of vehicles parked on the car park will apply some vertical load to the roof build-up, including the insulation layer,” said Rob Firman, Technical & Specification Manager at Polyfoam XPS. “However, they are not permanent dead loads in the conventional sense. The ‘active’ nature of the live loads imposed on the car park deck means compressive creep is less of an issue for the insulation’s performance than might normally be the case.”

Read more about different types of loading and compressive creep.

What Eurocode loading categories apply to car parking?

BS EN 1991-1 Action on structures is otherwise known as Eurocode 1. Part 1 deals with General actions. Densities, self-weight, imposed loads for buildings and features tables for a variety of loading categories, giving designers information about likely maximum loads.

Of most interest to us in this blog post is table NA.6 with ‘imposed loads on garages and vehicle traffic areas. It features two categories: category F for gross vehicle weights less than or equal to 30 kN (about 3 tonnes), and category G for gross vehicle weights greater than 30 kN and less than or equal to 160 kN (about 16 tonnes).

Both categories are accompanied by the following two quoted loading figures.

• Prescribed occupancy load per square metre (q_k), given in kN/m² – this can be summarised as distributed loads.
• A separately applied roving point load (Q_k), given in kN – this can be summarised as concentrated loads.

For categories F and G, q_k is given as 2.5 and 5.0 respectively. This equates to approximately quarter of a tonne and half a tonne of force load per square metre respectively.

Where Q_k is concerned, only category F is given a figure: 10.0 kN, or around one tonne of load. If category G applies then the Eurocode says Q_k is to be determined for the specific use.

If you would like to understand more about them, we have a separate blog post that looks at units of measurement for load and compressive strength.

What Polyfoam XPS insulation product is right for car park decks?

Polyfoam XPS offers two grades of insulation for roof applications: Extra and Super. The Extra grade has a declared compressive strength of 300 kPa, while the Super grade has a declared compressive strength of 500 kPa. Those declarations equate to around 30 tonnes of force load and 50 tonnes of force load, per square metre, respectively.

Following the lead of the BBA, who issue third-party certification regarding the fitness for purpose of construction products, Polyfoam XPS applies a safety factor of 2.5 to declared results to derive a suggested maximum load.

For Extra grade product, our suggested maximum load is 120 kPa (approx. 12 tonnes of force load per square metre), and for Super grade product, our suggested maximum load is 200 kPa (approx. 20 tonnes of force load per square metre).

Even these suggested maximum figures for both grades are clearly well in excess of the maximum loads given in Eurocode 1. However, two key points should be kept in mind.

1. Compressive strength declarations and suggested maximum loads only take into account vertical compression loads, and not active loads like horizontal braking forces. Structural engineers should therefore advice on the best all-round grade for dealing with all load types.
2. Polyfoam XPS’s safety factor may not be the only safety factor applied to structural calculations. Structural engineers might employ multiple safety factors at different stages of their calculations, which is something we discuss further in our blog post ‘A practical guide to the compressive strength of insulation’.

About new Super grade insulation from Polyfoam XPS

Super grade insulation, with its declared compressive strength of 500 kPa, remains the go-to choice for high-load applications like car park decks. Like all products in Polyfoam XPS’s range, it is lightweight, strong, moisture resistant, and provides thermal performance and strength for the lifetime of the building.

Being stronger than Extra grade, the Super grade insulation provides a particularly flat, strong, robust, and dimensionally stable base for the car park deck overlay. It can withstand the loads imposed by parked and moving vehicles, subject to final sign-off by the project’s structural engineer.

And now, as of July 2024, Polyfoam XPS is manufacturing a brand-new version of its Super grade product on its new production line. Read our news post announcing the launch of the new line and the new Super grade, with its improved thermal performance. You can also contact us for advice around the benefits of Super grade insulation for your car park deck specification.