I have taken up my office chipboard floor in order to replace it with floorboards. I'd also like to insulate the floor. The chipboard was nailed to 125mm deep joists which were resting on a poured concrete base with DPC below. The office was built in the 1970s and I don't think there will be any insulation under the concrete. There are no air vents in the cavity walls. I was planning on using nails to support 100mm Kingspan between the joists, leaving a 25mm air gap below. Is this the best idea or are there better/cheaper alternatives?

The timbers have lain on the concrete floor for almost 40 years and there is no evidence of rot. I assume the damp proof membrane under the concrete has prevented moisture being able to reach the joists. I want to stop heat escaping through the floor. Are you saying that if I now insulate between the joists I will increase the chances of condensation? Why is that?

I remember labouring for my builder brother when he converted our barn a few years back - if memory serves he dug out the earth floors, put down compressed type 1, with sand on top, laid damp proof sheeting on that, then a concrete pour, then battened the floor with kingspan between the joists and floorboards over that. Everything was passed by building control.

At present the chipboard is offering little thermal resistance - so all the fabric below is gradually absorbing your internal heat energy . ( You want to put a stop to that of course) . Any condensation that periodically arises can disperse across all that open void space .

If you introduce insulation now - the concrete will be thermally separated from the internal space . It will get cold
Condensation will tend to arise more frequently and in greater amounts . The insulation being vapor resistant will leave only the timbers to cope .

I would ask Kingspan for a project specific condensation risk analysis - sent to you in pfd format - before relying on the detail link above

Not sure about Kingspan - but this product is BBA certified - see section 3 - its gives pretty detailed advice . you can float a timber floor onto it - no battens ( except at doorways )

http://www.vencel.co.uk/assets/bba/1796_web.pdf

I'm planing a similar thing in one of our rooms.

existing situation is a earth sub floor. I am intending a layer of hardcore, then a layer of geotex or similar. Sand blinding then dpm. Then insulation. On top of the insulation a layer of ply with battens which will support the floorboards.
I intend that the water and electricity pipes will run between the battens and then I'll stick in rockwool type stuff.