Flat roofs are a common feature of many buildings here in Britain. When insulating these constructions, most are familiar with a warm roof approach (where insulation is fitted above the deck). Inverted roof approaches offer an alternative which may help to extend the life of both the waterproofing system and the roof deck.
In this article, we’ll look at how to insulate inverted roofs, including:
- what U-values inverted roofs should achieve
- which insulation products are suitable for inverted roofs
- best practice tips on how to install inverted roof insulation.
What is an inverted roof?
As its name suggests, in an inverted roof application, a typical warm roof construction is reversed so that the insulation layer sits above the waterproofing system. The insulation is typically then covered with a range of ballasts including gravel, paving slabs or a green roof to prevent wind uplift.
What are the benefits of an inverted roof?
The key benefit of an inverted roof construction is that the insulation layer helps to protect the waterproofing system against the elements including:
- extreme temperature changes which can cause the waterproofing to expand and contract
- sun exposure which can lead to UV degradation
- impact damage during storms which can lead to tears or punctures in the layer.
Which insulation is best for inverted roofs?
Inverted roof insulation needs to be able to:
- handle significant compressive forces from the ballast
- have minimal water absorption
- withstand freeze / thaw cycling.
There are a handful of materials which meet all of these factors.
EPS and XPS
Historically, inverted roofs have been insulated with either:
- Expanded Polystyrene (EPS)or
- Extruded Polystyrene (XPS) such as Kingspan GreenGuard
Our Kingspan GreenGuard range is available in grades up to 700 kPa and is resistant to both water absorption and the effects of freeze-thaw cycling.
The trade-off is both EPS and XPS have fairly high thermal conductivities. In the case of Kingspan GreenGuard, these start at 0.034 W/mK. This means that significant thicknesses of insulation may be required to achieve the ambitious U-values in the updated Building Regulations and Standards.
Encapsulated VIPs
Encapsulated vacuum insulation panel (VIP) systems such as OPTIM-R E can offer a much slimmer solution for these applications. Our OPTIM-R E panels are covered in a protective spray coating to provide a more robust finish which also makes them easier to handle than other Vacuum Insulation Panel products. The panels can achieve a declared thermal conductivity of as low as 0.008 W/mK, and a design thermal conductivity as low as 0.009 W/mK (the design value is used for U-value calculations in inverted roofs).
In our OPTIM-R E Inverted Roofing System, the encapsulated VIPs are supplied in combination with Kingspan GreenGuard boards of the same thickness. These boards can be cut as infill strips to fit around the perimeter of the roof, fill awkward spaces between the VIPs and to enable penetrations through the insulation layer, allowing for elements such as rooflights or ventilator kerbs.
What U-value should an inverted roof achieve?
This will depend on the building type, its location and whether it is a new build or refurbishment. The tables below cover the requirements for inverted roofs on new buildings under the guidance documents which support the Building Regulations and Standards in England, Scotland and Wales.
Each country provides area-weighted limiting U-values, which the total roof area must meet on average. In all cases, no area of roof should achieve a worse U-value than the threshold value shown below.
An area-weighted Notional Dwelling/Building specification U-value is also provided. This specification is used by energy assessors and is applied to a notional building of the same dimensions as the actual building. From this, they can generate the performance targets for the actual building. Whilst you can diverge from the U-values, they often form the starting point for building specification.
England
| Notional Building (Wm2K) | Limiting U-value (Wm2K) | Threshold U-value (Wm2K) |
Domestic | 0.11 | 0.16 | 0.35 |
Non-domestic | 0.15 (side-lit and unlit activities) | 0.18 |
Wales
| Notional Building (Wm2K) | Limiting U-value (Wm2K) | Threshold U-value (Wm2K) |
Domestic | 0.11 | 0.13 | 0.35 |
Non-domestic | 0.18 | 0.20 |
Scotland
| Notional Building (Wm2K) | Limiting U-value (Wm2K) | Threshold U-value (Wm2K) |
Domestic | 0.09 | 0.12 | 0.35 |
Non-domestic | 0.11 | 0.16 |
Learn more about the energy targets in the Building Regulations and Standards
It is important to be aware that U-value calculations for inverted roofs need to be corrected to take into account the local climate and the average rate of rainfall. This means that the calculation should typically be carried out based on the project’s specific postcode. However, some ‘worst case rainfall’ constructions are available on our U-value calculator which can be used.
What to consider before installing inverted roof insulation
There are a number of design considerations which need to be taken into account before fitting inverted roof insulation – these include:
- Assessing the resistance of the ballast, insulation and waterproofing system against wind uplift and flotation in accordance with BS EN 1991-1-4: 2005 + A1: 2010 (Eurocode 1. Actions on structures - General actions - Wind Actions).
- Ensuring a sufficient fall is provided. BS 6229: 2018 (Flat roofs with continuously supported flexible waterproof coverings. Code of practice) recommends uniform gradients of not less than 1 in 80. However, because of building settlement, it is advisable to design in even greater falls.
- Providing sufficient drainage outlets in accordance with BS EN 12056-3: 2000 (Gravity drainage systems inside buildings. Roof drainage, layout and calculation). The rainwater outlets should be double entry type, to allow rainwater to be drained from the roof surface at both the membrane level and the upper surface level.
- Considering whether to lay a water-infiltration membrane, such as Aquazone, above the insulation. This can dramatically minimise heat loss by reducing the amount of rainwater that flows between and beneath the insulation boards.
Specific best practice when fitting the OPTIM-R E Inverted Roofing System or Kingspan GreenGuard is available in the ‘Sitework’ section of the product literature.
