FAQ - Frequently Asked Questions About Insulated Raft Slab Foundations

What is an insulated raft slab foundation?

An insulated raft slab foundation consists of a reinforced concrete slab which sits within an insulating material such as XPS foam. Importantly, the insulation is also placed around the edge of the slab and acts as the formwork when pouring the concrete. The foundation effectively transfers structural loads to the ground whilst also improving the overall thermal performance of the building.

What are the benefits of an insulated raft slab foundation?

The benefits of an Insulated Raft or Slab foundation are numerous.  They include;

1. Creates a complete thermal envelope which is uninterrupted by thermal bridges.

2. Simpler and faster method of construction than traditional footings.

3. Typically more cost effective than traditional footings.

4. Structural loads are spread over a larger area making it ideal for weaker or expansive soils.

What are the costs of an insulated raft slab foundation?

The costs of an Insulated Raft or Slab foundation are usually less than those of a traditional footing with a rising wall and suspended floor.  We carried out a cost exercise of actual payments made by a major housebuilder in the Midlands for their foundations across a large volume site and found that GreenRaft pricing came in at 22.6% less for that particular project.

If you would like a free and instant quote, please use our U-value and Cost Calculator or send us your drawings to info@greenraft.co.uk

How do I calculate the u-value of an insulated raft slab foundation?

This is not as simple as calculating the U-value of a wall or a roof.  The complication is caused by the fact that most heat is lost around the perimeter of the structure and much less in the centre.  Therefore, the calculation method is very dependent upon the relationship between the perimeter of the building compared to the area of the building.  The soil type for the site also makes each site unique, as well as the thickness of the insulation used and it’s respective Lambda value.

If you want to calculate the U-value (and cost) for a particular design, then please use our U-value and Cost Calculator, which has been created using the methodology laid down in British Standards, or alternatively send us your drawings to info@greenraft.co.uk

Can insulated raft slab foundations improve the sustainability of a building?

Yes. Insulated raft slab foundations are a great way to improve the sustainability of a project for the following reasons;

1. Superior thermal performance drastically reduces operational energy demand for heating and cooling.

2. A raft foundation can often use less concrete than traditional foundations and therefore has the potential to lower the embodied carbon within the foundations. We are also working towards using alkaline activated cementious materials instead of concrete which have a much lower environmental impact.

3. The simpler construction process requires less heavy plant on site for excavations etc and so using an insulated raft foundation can reduce carbon emissions associated with the construction process too. 

How does an insulated raft foundation compare with a traditional strip footing foundation?

1. Typically more cost effective and is quicker and simpler to construct.

2. Creates a consistent and unbroken thermal envelope where traditional strip footings would make this significantly more difficult.

3. Achieves much higher levels of airtightness because the slab does not allow the passage of air. Only the edges need to be sealed to achieve an airtight floor.

4. Loads are spread over a larger footprint and can simplify internal building adaptations later on.

5. Requires much less excavation than traditional strip footings.

Can a self-builder manage to construct an insulated raft foundation?

Yes. We believe that this is not a difficult thing to do, for anyone with basic carpentry skills and a little common sense. 

The key to a well built Insulated Raft is in preparing the base.  A well compacted sub layer of stone is typically required followed by a sand blinding layer. Levelling the sand blinding layer is the most important step in the build.  If the sand is level then everything else will be simple from then onwards.  However,  it is recmmended that an experienced concrete team is brought in to pour, compact and level the concrete. Plenty of pro-builders get in such a team to do this part and to get the perfect finish on top.

Our first Self-Build project in the UK using GreenRaft had eight corners on the structure.  The self-builder who had never built a house before got it so that seven corners were millimetre perfect to level and one corner was only 2mm out.

Why would an upstand be required around the edge of the raft?

An upstand or rising wall is often required for timber frame buildings to raise the timber at least 150mm above ground level, this is part of the building regulations. 

The GreenRaft system enables the seamless integration of an upstand wall and an insulated raft slab foundation. It also maintains the continuous thermal envelope with minimal thermal breaks. 

The construction of the upstand and foundation is done simultaneously and therefore rolls multiple processes into one. This reduces the time it takes to construct, the number of trades on site and overall cost of this particular aspect of the build.

The GreenRaft system also enables superior accuracy and precision due to the unique installation process. This makes it perfect for SIPs construction or similar where site accuracy is of utmost importance to ensure that prefabricated components fit correctly.