Insulating Foundation Product Information

We believe that an insulating foundation, whether raft or footing supports a structure, the choice of materials is paramount. For this reason, we only use Extruded Polystyrene or Cellular Glass Insulation in our insulated raft system.

XPS Insulation

Extruded Polystyrene (or XPS) is one of the few insulation materials that is virtually waterproof due to its closed cell structure. It also has very high compressive strength and consistency of structural quality superior to other insulants. This makes it the ideal material for supporting large structural loads in potentially wet conditions.

A board of extruded polystyrene used in most GreenRaft foundations

Cellular Glass Insulation

Cellular or Foamed Glass Insulation is also a high strength and waterproof insulation.  It is unique in the world of insulation as it is the only rigid insulation that is both fireproof and waterproof. It is made primarily from recycled glass but the high cost of production keeps this as a very expensive insulation option.

A board of cellular glass insulation

Both XPS and the Cellular (Foamed) Glass Insulation are available options in the GreenRaft product range but only XPS is kept in stock for immediate delivery. Foamed Glass is a special order.

We supply what is needed, when it is needed to efficiently build your perfect foundation.

Bespoke Solutions

The detail needed at the edge of the insulated raft foundation is dependent upon the target U-value, the structure above, the ground levels and the chosen wall cladding.

We work with you to create the exact foundation detail which you require.

We offer a bespoke supply service in terms of :

  1. Insulation Type – XPS or Foamed Glass
  2. Depth of Insulation to meet a target U-value
  3. Height and thickness of the closer block cut to size and shape on our CNC hot-wire cutting machine
  4. Splitting the insulation between above and below the concrete (Ideal when a screed is required)
  5. Creating a particular height of upstand – ideal for timber and SIPS construction
edge of Insulated raft foundation with upstand for timber/SIPS structures and with our thermal skirt
Raft Upstand for SIPS with thermal skirt
Placing concrete in an insulated raft foundation using digger bucket

Raft Foundation Costs

Against other Foundation Types

We recently undertook a pricing comparison exercise using information from a major housebuilder.  The price per square metre they were actually paying for a trench footing with a suspended beam and block floor was £23 per square metre more than the built price for our insulated raft system.

The faster build speed and the reduction in build team numbers are also factors to be considered when comparing systems, as well as a potential opportunity to reduce heavy plant on site during the groundworks phase

Raft Foundation Costs

Against our Competition

The GreenRaft system differs from the competition in that our simple components are used to build accurately on site whilst our competitors pre-manufacture bespoke components for each project. In theory, bespoke prefabricated components make for a better quality end product but in reality this is often not the case.

Without the expense of off-site pre-manufacturing we give the builder the freedom to build accurately and quickly using just a handful of basic components.  

After conducting a price comparison against our competitors, we have found that the GreenRaft system is around 20% less expensive. Try our online cost calculator to see for yourself!

An Alternative to Concrete

The Environmental Argument on Concrete

Traditional Portland Cement is known for being extremely high in embodied carbon. Foundations in particular can often account for a very large proportion of a buildings total embodied carbon.

As part of our long term intention we have formed strong ties with leading experts and companies working in the field of Alkaline Activated Cementious Materials (AACM) and Geopolymers.  This is the cementitious material that replaces Portland Cement in the concrete mix and can lower the embodied carbon by up to 80%.

Though we are keen to promote this environmentally-friendly material, some important players in the construction warranty industry will not countenance its use until there is a BS EN document published. Until such a time, ordinary RC35 concrete will be the norm.

In the meantime, any forward-thinking designers who would like to go down this route and are not reliant on a mortgage for their build should get in touch to discuss. We would love to hear from you.

Ground Preparation

The Sub-Base

Excavation and ground preparation is minimal with the GreenRaft system. It has been designed to reduce the processes which are typically required to construct foundations.

The XPS or Foamed Glass ground boards are laid on top of a well compacted sub-base of crushed stone with a sand blinding layer on top. The key to a successful build is getting the sub-base and the blinding as level as possible. From there it is a simple process of placing the insulation.

Full Drawing

Request our drawing details either as pdf or dwg files by contacting us by telephone or email.


If you want to know more about how the GreenRaft system can help you achieve the highest levels of thermal performance, please give us a call.


Click here or contact us directly to dicuss quotes and more.