Specifying insulation for inverted roofs
Contents |
[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.
[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
- Domestic roofs.
- Flat roof.
- Heat transfer.
- Inverted roof.
- Inverted roof defect - case study.
- Limiting fabric parameters.
- Roof coverings.
- Roof insulation.
- Roofing defects.
- Solid wall insulation.
- Thermal bridge.
- Thermal insulation for buildings.
- Types of roof.
- U-value.
--Jablite
Featured articles and news
The general election and the construction industry
As PM, Rishi Sunak announces July 4 date for an election.
Tackling the lack of knowledge or confidence in heat pumps
The CICV Green Home Festival part of the Edinburgh Fringe.
Home improvements swapped for green energy upgrades
As applications down since and desire to reduce bills up.
Eco apprenticeships continue help grow green workforce
A year after being recognised at the King's coronation.
The Use of Natural Insulation Materials in Retrofit
New paper from STBA with support from SDF.
National Retrofit Hub in partnership with InstallerSHOW.
Permitted development rights for agricultural buildings
The changes coming into effect as of May 21, 2024.
Biodiversity and the built environment digital series
The nature of building, with CIOB and CWP.
A call for prevention and sensitive re-use.
Risks to the long-term effectiveness of the BNG scheme
National Audit Office publish review of Biodiversity Net Gain.
The CIAT principal designer register
Providing assurance and verification of the capability and competence of registered ATs.
Building Safety recommendations and Northern Ireland
The NI roadmap to improving safety in high rise residential.
Specifying rendered external wall insulation for fire safety
How to interrogate the evidence provided to the specifier.
The benefits of writing articles for your organisation
How to create a profile for your organisation and publish for free.
Scottish Government action to reach net-zero targets
Retrofit expert group highlight critical actions needed.
A forward thinking, inclusive global community of members.
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:
It is important for readers to understand when each option should be used as they are both valid solutions
-AB