On almost all projects, you’ll likely need to call on some of your contingency in the early stages of the build. While you can undertake a number of different investigations to minimise your risk, especially in the ground, not all risks can be eliminated.

This is even more pertinent when working with existing buildings, especially if the entire building is going to be subject to refurbishment, retrofit and remodel.

demolishing a semi detached house

Paul’s family have moved out during the works which also include a large extension to the rear of the property

Firstly, on our home, the existing single-storey, single-glazed sunroom to the back of the house was found to be sitting on a 1m-deep slab of concrete – overkill for such a lightweight structure – which took more than three days of work with a digger and breaker to remove. It did mean, though, that once it was out we had no need to dig further to get us to our required foundation levels.

We have also removed our chimney breast up to attic level, as we didn’t use the fire. The skin was bricked up carefully but the infilling of the chimney around the flue was rather haphazard — it was made up of loosely laid blocks and bricks infilling the void, and often crossed the party wall.

Once our chimney breast was removed we could even reach our hands through to hold our neighbour’s flue through the party wall.

chimney being demolishing inside house

The chimney proved to be somewhat haphazardly built when Paul and his team began to remove it

Removing the rubble and blocking this up carefully with a decent render coat to the back of the neighbour’s party wall leaf has certainly made for a much better situation for the future.

Both of these projects required some time and a little bit of cost flexibility, but neither have impacted significantly on the cost of the build and certainly not on its performance.

The next unknown I should have perhaps foreseen required a more collaborative redesign. There is an access hatch in the corner of our dining room floor that allows us access to the underfloor void.

I had already measured the void depth at 750mm and this gave us the required depth to build up our new insulated concrete floor almost perfectly. What I didn’t check in enough detail, though, was how far the void extended at this depth. In reality, not very far.

The subfloor level changed considerably over the footprint of the house — and decreased to give us only 300mm of build-up depth in the worst case. This presented two issues. The first was that we couldn’t fit the layers of insulation required without some significant and potentially risky excavation in the confined space of the house.

The second was that to lay insulation you need a level surface otherwise you get big gaps and poor performance — especially with thicker layers.

blown glass for sub floor

The glass aggregate will act is insulation while providing a loadbearing substrate

After lots of discussion on site we chose to excavate minimally in the worst affected locations in order to get a minimum depth of approximately 350mm. We then introduced Glapor, a blown glass aggregate, to a depth varying between 50mm and 400mm. This blown glass aggregate has a decent insulation performance and provides a loadbearing and flat substrate. The aggregate isn’t cheap but is very light and easy to handle.

We then added a sand blinding (a layer of sand), laid a layer of insulation and a damp-proof membrane (DPM) before casting the concrete slab on top. This all meant that the revised subfloor cost very little more than the original scheme and still kept us within our EnerPHit target.

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