I have an extension being built on usual concrete beam & dense block over a ventilated void. The external extension area is 3 x 6m. Ventilation is via two 215x65 plastic airbricks and telescopic ducts as usual. The construction is unremarkable.

Over the beam & block is the DPM, then taped PIR board insulation, then a 2nd DPM (to prevent screed migrating during pouring), then screed, and finally the finished floor resting on the screed as usual. Perimeter insulation will be the same whatever I choose, probably 30 - 50mm PIR, so this can be ignored.


The vertical space available for insulation and screed is 220 mm. But how much of that 220 mm should be allocated to PIR insulation, and how much allocated to screed?

  • More insulation, less screed: all things being equal, absolute heat loss is smaller, but the thermal mass of the floor is also much smaller so small heat losses matter more and will be easier noticed.
  • Less insulation, more screed: all things being equal, absolute heat loss is greater, but more screed above the insulation means more thermal mass, so the slower and less any heat loss will be readily apparent.

Greater thermal mass but less insulation? Or greater insulation less thermal mass? My best guess is: given the range of annual temperatures and its use as a living space, thermal stability I guess matters more than thermal responsiveness. So more thermal mass. But too much thermal mass and there won't be enough insulation to sustain its temperature anyway.

So what is a sensible way to choose a good balance and decide the vertical depth for each of the 2 elements?

Other useful info

  • The house generally has 150 mm PIR insulation walls and roof, although some bridging is inevitable as its being made to work in an old renovated house not a new build.
  • The house will generally be occupied as a living home, with usual work patterns - more in use evening and night, a bit less during the day.
  • The location is S.E. UK, so temperatures vary from a bit below 0 C in winter, to 35+ C in recent years' July heatwaves. (Recent patterns for e.g. July 2022 have been considerable time at around 35 C, peaks almost 40 C, R.H. 60-70%, if that helps). Thermal mass to resist sharp daytime warming in summer might be useful. So might resisting cooling in winter.
  • Heating system - unfortunately unknown at this point if its underfloor or wall based. Or indeed if underfloor is viable.
  • "Heating system unknown at this point." Then cease and desist. You going through all that work and you're not putting in radiant floor heat? That would decide everything else. 2" foam under plastic pipe, with hardware cloth over it as rebar, and then 6" of concrete. And then you have a floor. Beam & block is not "unremarkable". It's a thing over another thing that would've sufficed in the first place and wouldn't leave an inaccessible gap underneath the floor for rats to move in.
    – Mazura
    Aug 11 at 2:14
  • Beam and block isnt avoidable. There are local code rules and existing historical conduits that have to be bridged, so a solid ground bearing slab was not ever an option.
    – Stilez
    Aug 11 at 6:35
  • The reason heating systems are undecided is that here in the UK, the financial incentive schemes for different forms of heating seem to change rapidly, and some types are/were/may be threatened to be forbidden in new works. During the planning and design stages it isn't clear at all what optimal or even available choices would exist, at the time of installation.
    – Stilez
    Aug 11 at 6:39


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