A Shropshire home owner undertaking a large renovation and retrofit project got in touch to ask what his options are for insulating his solid floor. He had planned to dig out the concrete floor base and replace with a new base, insulation and under-floor heating. However having tested part of the area and found it’s 8 inches thick they discovered it would take considerable work, and shaking of an old house with dodgy lath and plaster ceilings, to remove.
He thought it might be possible to put insulation on the existing slab – if it was thin. So asked us for recommendations for underfloor insulation products that wouldn’t increase the height much and prevent the need for all the concrete to be dug out and replaced.
Carole Ryan-Ridout BSc.MA MRICS MCIfA IHBC
A good option to prevent heat loss from solid floors is carpets and rugs, preferably all wool and absolutely with a hessian backing as all such floor coverings in traditional building construction on solid floor must be allowed to breathe, wicking away any moisture – do not use glue or rubber-backed carpets. This does lead to a higher relative humidity within the room, so ventilation is necessary. Make full use of trickle vents in windows.
Wool carpets have to be well treated against wool moth. Wool carpets are of course expensive, and any carpet will resist heat loss downwards. Jute, sisal, seagrass, pampas and coir are all good options for breathing floors as they are natural materials. They will resist heat loss but allow the moisture in the floor to escape. This is necessary because most older buildings do not have a damp proof membrane under the tiles or flags.
Wooden boarding is another option to consider. Wood is a natural insulator as it contains a structure that resembles drinking straws, the walls of which are made of cellulose. Cellulose is naturally permeable and hygroscopic, thus wicking away any moisture from the solid floor. The wooden boards need to have a moisture content of around 12% when first laid.
It should be remembered that ceramic tiles and to some extent stone flags immediately abutting a heat source such as an Aga or Rayburn cooker, or wood burning stove will start to heat up and act like a storage radiator, retaining the heat until long after the heat source has cooled.