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.
- 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
- 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
Licensing construction; looking back to look forward
Voluntary to required contractors (licensing) schemes.
A contractor discusses the Building Safety Act
A brief to the point look at changes that have occurred.
CIOB Construction Manager of the Year award
Shortlist set to go head-to-head for prestigious industry title.
HSE simplified advice for installers of stone worktops
After company fined for repeatedly failing to protect workers.
Co-located with 10th year of UK Construction Week.
How orchards can influence planning and development.
Time for knapping, no time for napping
Decorative split stone square patterns in facades.
A practical guide to the use of flint in design and architecture.
Designing for neurodiversity: driving change for the better
Accessible inclusive design translated into reality.
RIBA detailed response to Grenfell Inquiry Phase 2 report
Briefing notes following its initial 4 September response.
Approved Document B: Fire Safety from March
Current and future changes with historical documentation.
A New Year, a new look for BSRIA
As phase 1 of the BSRIA Living Laboratory is completed.
A must-attend event for the architecture industry.
Caroline Gumble to step down as CIOB CEO in 2025
After transformative tenure take on a leadership role within the engineering sector.
RIDDOR and the provisional statistics for 2023 / 2024
Work related deaths; over 50 percent from construction and 50 percent recorded as fall from height.
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