The houses currently have solid concrete floors with no insulation underneath them. As a part of the retrofit we are excavating these original solid floors, and replacing them with a combination of insulation laid under a new concrete floor. Putting the insulation underneath allows the mass of the concrete to store heat and then give it back up as the air temperature in the house cools, thereby helping to keep the house warm.
Effectively the floor is then acting as a storage heater. The heat absorbed into the floor is from a combination of passive solar gain and incidental gains from occupants, cooking and appliance use. Basically whenever the air temperature in the house exceeds the temperature of the floor slab, the floor will absorb that excess heat. The insulation under the floor slab stops the heat escaping to the ground below, storing it to be returned to the house as the air temperature subsequently cools.
The existing solid walls will act as a heat store in a similar way once the new external wall is built up with the insulated cavity outside the existing wall.
The video below shows one of the houses with the old floor excavated and Nick summarises our plans for the new floor.