Insulating the floor and installing under floor heating would be the biggest challenge, we have been told, in doing the eco makeover. It is certainly the first challenge.
We were hoping that the surface screed on the concrete floor would be up to 50mm thick. A bit of hacking with a cold chisel shows that it is asphalt concrete and just 20mm thick. Drilling a hole through the slab has confirmed that there is 100mm of concrete above 100mm of compacted hardcore.
Removing the screed layer and then putting in 75m of insulation and a screed, containing the heating pipes of 65mm would raise the floor level by 120mm. Door opening would have to be raised and losing five inches off the heights of the rooms would do nothing for their proportions.
Breaking and removing 5 cubic metres of concrete sounds like a very expensive and risky option. Much more digging would be needed to make room for insulation, the new concrete slab and the screed. Can we afford that and do we want to take the risk of collateral damage? But it would be nice to reach a passive house standard.
Searching through internet forums brings conflicting experience. Some say they have dug-out floors, but don’t say how much it costs. Others say it is too expensive, but again don’t quote figures.
The height increase on the existing slab can be reduced by fitting the heating pipes into the top of the insulation and using a heat spreader. But that means there is no thermal mass in the floor and the heating has to run at a higher temperature which reduces efficiency.
Every solution seems to have disadvantages. Mad thoughts flash through my mind: might not the existing slab provide a big heat store if we just laid the heating screed directly on it. Then solar gain, through the big south-facing windows, could compensating for the lack of insulation?
Dreams, dreams of an easy solution. We are just going to have to cost the alternatives and decide what to do.
We can’t be the first people do face this dilemma. It would be good to mown what they did.