Friday, September 21, 2012

Floor screeding the extension

21 September 2012: [Extension] On a day that feels more like late October than mid September, Adam and Ken (Harrison) re-appeared for a few hours to lay the floor screed. Although I have done screening before, the thought of hand mixing 300 litres of screed without a mixer was too off putting. There is to be no heating in the extension, but it has negligible thermal loss, and the floor will be important for providing a heavy thermal mass that provides stability.

Ken is ready to start. The extension space has been immaculately prepared with a strong pencil line to indicate the final finished floor level, and a line of foam that indicates precisely the structural screen level. There are absolutely no thermal bridges, anywhere!Ken's made a start with the first bucket fulls.
The line that runs along the previous wall is first covered, there is a thin layer of insulation over the old bricks to make sure there is no thermal bridge.The first batch had too high a water content. Adam is outside doing the mixing and is asked to reduce the water content significantly.
Ken is keen not to be 'painted into a corner' and we speculate for a while whether he could jump out of that position if he did. Fortunately, he is aware and starts at the other end…. leaving the centre to the last. This now has the correct water content, there is no loose water floating to the top, and the unlevelled screed looks very sandy - this is correct. 
Adam and Ken discuss how many more buckets are required to finish the job.The mixing zone is at the front of the house. We have ordered some thin 'Thermalboard' that will be used to face the brick wall around the lintel to reduce thermal bridging.
A long bar of wood or metal is ideal for levelling the screed, and in this case, Ken uses a long spirit level which ensures precisely that the screed will be perfectly level.Nearly finished, Ken applies the final touch, vibrating free water to the top, and filleting the edge down where it meets the existing tiling. There has to be an expansion joint because the existing floor is actively heated and can have a high delta-T, whereas the new floor is merely a 115mm thermal mass flywheel which has to get its heat from the air in the room.

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