This article gives some info on costs/energy savings...http://www.energysavingtrust.org.uk/Insulation/Floor-insulation I think the 'condensation' that KK is referring to might be what is known as interstitial condensation, and the 'membrane' a breathable type membrane, common in roof construction to prevent this happening, rather than a damp proof membrane (DPM) which is a separate issue. This article gives info on both...http://www.greenbuildingstore.co.uk/page--insulating-suspended-timber-floors.html KK, what's the reason why you're limited to 40mm depth of insulation? Personally I think it would be a waste of time putting a mineral wool type insulation down at such a shallow depth, a rigid foam board would give you much better thermal insulation properties @ 40mm. Be careful not to 'lag' anything electrical that could overheat. As an aside...I'm all for stopping noticeable cold draughts, but old Victorian buildings weren't designed to be hermetically sealed boxes, they need to 'breathe', and natural background ventilation in rooms is actually good for them, especially when the weather is too inclement to open windows. If you block up a fireplace you have to put in an air vent to prevent condensation forming inside the chimney leading to damp. If you have an internal kitchen/bathroom (i.e. windowless), Regs ask for a 10mm ventilation gap under the door to act as an air inlet. It's good for the building to have these 'changes of air'...