Specifying insulation for inverted roofs

[edit] Introduction

EPS (expanded polystyrene) and XPS (extruded polystyrene) insulation are specified in three different grades - 200, 300 and 500.

The load imposed on a roof by plant for air handling units is typically 1 tonne. This will be supported by 6 spacer pads at 150 mm diameter giving the load exerted onto the insulation as 73 kN/m². Only exceptional loads require over 90 kN/m².

A habit of over specification of inverted roof insulation has meant that until recently the minimum grade used was 300.

When an exceptionally heavy load is required, a roof can be designed using 300 grade for the specific load area and 200 grade for the rest of the roof. This approach offers a number of benefits; it is cost effective, lighter and helps to achieve BREEAM as 200 grade is A+ rated in the BRE Green Guide to Specification.

EPS has other strengths compared to XPS when tested for permanent deformation under cyclic loading:

XPS showed a permanent deformation of 25%.
XPS loses elasticity at around 45% compression.

EPS remains elastic up to 65% compression.

EPS recovers back to 98% of its original thickness when compressed up to 50%.

[edit] Drainage

Drainage, and how to achieve compliance to BS 6229 and BS EN 12056-3:2000, is probably the most contentious issue about an inverted roof design.

A minimum fall of 1:80 is best practice, but the problems of using a screed layer mean that many roofs are constructed with a zero fall.

Tapered insulation bespoke-designed to the roof, removes the requirement of a screed layer to create the fall for drainage. The insulation boards can be laid straight onto a concrete deck and add just 2% the weight of a screed layer.

A major benefit of a bespoke tapered insulation compared to the screed solution is that there is no drying time for the insulation, which can add up to 40 days to a roof installation.

[edit] Thermal performance

The specified thermal performance of the inverted roof is achieved through the roof design and the thickness of the insulation used. U-values as low as 0.10 can be achieved.

[edit] 200 and 300 Grade

For most inverted roofs with typical pedestrian traffic, a 200 grade EPS insulation board for inverted roofs is a cost-effective product that will provide the required compression performance.

300 grade insulation should be specified when a roof has exceptionally heavy loads. Another option is to design a combination roof that gives performance and cost benefits.

[edit] Tapered

Jablite launched the first tapered insulation for inverted roofs. This enables a roof to be installed according to best practice to achieve the required fall without the addition of a screed layer.

[edit] 1mm increments

Insulation boards for inverted roof are available in 1 mm increments. In some cases this can mean up to 19 mm less material on a large inverted roof giving a substantial saving in cost and load weight. This option eases roof design constraints especially around door thresholds onto balconies and roof terraces and on inverted roofs with public access.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

--Jablite

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Comments

Are the loads given in the introduction for AHU correct 73-90 kN/m² seem so large that I wonder whether a decimal point might be missing? (I.e 7.3-9.0 Knus/m²).

-bo

Thank you for your comment. The loads given are correct as this is specific to our high density product for inverted roofs application. I hope this answers your query. Many thanks,

Jablite Team

This page is written as if EPS is the best solution for all applications (from a supplier of EPS), without listing any benefits of XPS. From my understanding, XPS also has benefits such as:

Less likely to be damaged by freeze-thaw weathering (water cannot permeate between the beads). Also means EPS has reduced performance when wet?
Higher compressive strength for comparable densities
Less likely to have condensation forming as the water is not retained below the insulation

It is important for readers to understand when each option should be used as they are both valid solutions

-AB