Passive slab completed – is this the first in the UK?

There are certainly other low energy houses in the UK with passive slabs but they are usually built by specialist companies with materials and often manpower from Germany, Sweden and Ireland.  The Colemans Hill slab was built by two non-specialists – with a little help from friends at the week-end.  It’s not rocket science but it is time consuming, especially for a house with 22 corners, and involves some creative and innovative thinking.

“The Passive Slab is an Insulated Foundation System that is PassiveHaus approved to deliver the lowest U-value available on the market (as low as 0.08W/m.K) and eliminates the critical wall to floor cold bridge. Expanded Polystyrene is used to wrap the foundation of the building ensuring there is no thermal break between the wall and the foundation. The structural strength of the Passive Slab results from the combination of Expanded Polystyrene (EPS 300), concrete and steel“.

Perimeter walls built up to DPC level 

Coarse sand is added over the compacted stone

Sand is built up to about 30mm to allow for compaction

and then compacted using a whacker plate . . .

. . . before levelling ready for the polystyrene

ESP300 L sections are first laid around the perimeter

Corner showing half lap joint

A further 250mm of EPS300 is added to the

base section and 500mm pieces of EPS300 placed on top

1200X1200 mm EPS100 are added to fill layer 1

followed by more EPS100 to complete layer 2

A DPC membrane was then laid over layer 2.

No radon barrier was required in this location.

Steel mesh was then laid on layer 3 . . .

. . . and a strip supported on clay "chairs" added
around the perimeter to form a ring beam

Areas supporting load bearing walls have no layer 3
and additional steel mesh is added . . .

. . . these areas and the ring beam will therefore contain
200mm of concrete whereas the main slab is only 100mm thick

50mm wide timber strips were used to keep the EPS away from the brick wall and retain a 50mm cavity and small wood blocks were used to keep the strips of steel centered in the ring beam and other load bearing areas. 

Whilst EPS can be cut with a saw it produces a lot of polystyrene granules so we built a simple hot wire cutter which although a little slower, produced a cleaner cut and no mess.

After wiring and spacing all the steelwork the slab was finally ready for the concrete pour.  Regrettably it did not prove possible to use the ultra low carbon cement as CEMEN was still working on the new production facility and the bagging plant was not operational.

Concrete filling the 400mm perimeter "ring beam"

The completed passive slab


From English Brothers Managing Director - Jay Hubbard


"Tom tells me your base is perfect - a first!
I applaud the work of you and your site team as that is pretty much unheard of.
Well, I've never known it in seven years, put it that way".