Foundations form a potentially very expensive unknown cost to a new build - with the cost dependent on the outcome of a soil survey, proximity of trees and history of the site. The typical solutions would be a strip foundation or piling (followed by beam and block floor) where necessary, both of which have an inherent cold bridge issue.
We offer an alternative, the Scandinavian foundation system which solves some problems associated with these foundation types.
The Problem
Strip foundations and piled foundations both suffer from these main problems:
- They use a large amount of concrete
- They suffer from cold bridging issues
- The thermal mass of the foundation is not within the thermal envelope
- The detailing of timber sole plate is problematic
Concrete Use
Concrete is responsible for around 7% of global CO2 emissions (c.f airlines at 4% though rising rapidly), so we should aim to minimise our use of this product in our buildings. The strip variety are a particularly inelegant solution to the foundation problem, where a trench is dug 450mm wide down to a depth that the building inspector / structural engineer deems adequate - for normal situations this would be around 1m deep, but can be upto 4m. This is then filled with concrete to transfer the load down to the trench bottom.
Sometimes these trenches are required to go to a depth of 4m if the soil conditions dictate, meaning this big hole then has to be filled back up with concrete - with the associated CO2 emissions and costs.
Thermal Issues
Cold bridging is also a problem - the concrete foundation is connected to the ground, and so will be at ground temperature which during winter will suck heat from the building to ground. To get around this, work has to be done to thermally separate the foundation from the building which is simple enough inside the building, where polystyrene is used with a screed on top, but is a difficult issue at load bearing points where polystyrene cannot be used, so these will inevitably become cold bridge points to preserve the structural stability.
For timber frame buildings it is also advantageous to have the thermal store potential of the concrete within the building's thermal envelope - this will have a stabilising effect on internal temperature, absorbing heat during the day (particularly if the floor is tiled) then releasing this heat at night.
The Solution
Our Scandinavian foundation system offers a different approach with a lower volume of concrete used, and no cold bridging present, by constructing a raft over the site with structural polystyrene underneath. This entire raft sits on the polystyrene, with steel mesh used to provide strength so that the entire building is on polystyrene, thermally separated from the ground. This method results in very little excavation work, the top layer of soil simply being removed to provide a flat surface. Since there are no extensive excavations and subsequent soil removal, further cost savings are introduced.
The main four processes involved are illustrated below:
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The system consists of clearing the top soil to 600mm below finished floor level, then covering with compacted hardcore. | On top of this expanded polystyrene blocks are laid around the perimeter, followed by polystyrene sheets inside the building. |
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| On top of the polystyrene, steel mesh is installed along with underfloor heating pipes if required. | Once everything is in place the slab can be poured which forms your finished floor screed. |
This solution has some important additional benefits over a strip system - firstly no screed is required, when the slab is poured this provides the finished floor so eliminating the screeding process. Another major advantage is thermal: since the whole foundation slab is thermally separated from the ground it falls within the thermal envelope of the home, this means it will provide substantial thermal mass contributing to a steady state internal temperature and avoiding overheating issues which can occur with poorly constructed, light weight timber frame houses
The U value for this slab is around 0.1 w/m2K
Example Project
For a recent project the soil conditions were poor, so the engineered solution was either to go for a deep strip foundation, perhaps to as much as 4m or a piled solution. The strip would have used around 60m3 of concrete, a considerable cost and source of CO2 emissions.
Our alternative was to use piles, with our slab foundation on top supported by the piles. This solution resulted in 62% less concrete used on the foundation, the screeding process is eliminated (further saving concrete / money), and the entire slab being used as thermal mass. The processes involved in this work were:
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Stage 1 - clear the site | Stage 2 - install piles | Stage 3 - level site with sand and install clay master (if required) |
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| Stage 4 - install polystyrene insulation | Stage 5 - damp proof course | Stage 6 - steel reinforcing and cages to form ring beam |
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| Stage 7 - concrete pour, power float finished for final surface ready for timber frame install |