Preparing the ground floor

Installing the radon barrier/damp-proof membrane

Now that the roof is mostly complete, it’s time for us to move on to inside the house. Over the past few days, we’ve been preparing for a special date – the day the liquid screed is poured to provide our solid concrete floor which we’ll then finish right at the end.

The step before the first step was to make sure our block and beam floor was clean and flat enough for the next steps. We used a combination of an axe to chip away at any raised bits, and our trusty site vacuum cleaner James to clear away debris and sawdust.

Once that was done, we moved onto the first step – the radon barrier and damp-proof membrane. Since we’re in Cornwall and radon gas in the ground is a recognised issue here, all new houses need to have a radon membrane installed on the ground floor. This stops the gas rising from the ground and into the house. A damp-proof membrane is also needed to stop damp from rising through the floor. We found a two-in-one membrane that handles both and installed that – that’s the yellow sheeting in the photo. All the seams need to be overlapped, taped with double-sided tape and then sealed with duct tape. The membrane is taped to the damp-proof course running around all the walls (in black), so that any gas underneath the membrane can escape out of the building.

Structural grout underneath a steel beam

Of course, nothing is ever simple, so before we went ahead to the next step, we needed to make sure the steel beams were secured. They come attached to the floor with some bolts, but other than that and maybe some plastic packers, there’s a gap between the bottom of the beam and the floor. To make sure the beams are well supported, our carpenter fitted some simple formwork around them, and we filled it up with a structural grout, which looks a lot like sloppy concrete. It dries pretty quickly, however, and provides a strong support base escasing the base of the beam.

Rigid insulation and a black membrane ready for the next step

Once that was all done, we were ready for the next step – insulation! We have 150mm of rigid insulation boards across the ground floor to keep the heat in and provide a base for our underfloor heating.

These boards come in large sizes but can be cut easily with a handsaw and are relatively light.

In the middle of the photo you can see a conduit with a cable sticking out, and this is also visible in the first photo. This one is for a floor socket and so we needed those to be in place first. The bottom of the insulation board was then cut out to slot nicely over the conduit, and any gaps filled with expanding foam. The boards are staggered and nicely butted up to each other to provide a continuous base.

After this comes the last of our layers – the black membrane. This is very similar to the radon barrier/DPM, but it provides a sealed surface for the liquid screed to be poured into so it doesn’t leak and escape – therefore it’s very important that any holes are patched up. We laid it with overlaps just like the radon barrier and taped the seams and edges with duct tape, making sure there were no gaps. Finally, we laid a strip of edge foam around the perimeter, stapled to the timber frame. This foam provides space for the screed floor to expand and contract as it heats and cools.

We worked pretty hard into the night to make sure it was all ready for the next step – our underfloor heating, which I’ll write about in my next post.

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