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This looks like a very clever way to save on labour and materials and I can immediately see that laying this I can lose the 6mm rubber matting. But if you can't screw through these joist caps, is it fine to just lay a floating floor directly on top. What would that floor structure be? Wouldn't I still need 18mm ply and then some form of underfloor heating board?

To the minimum level required by building regs when you build a rear extension and a loft conversion. I'm a layman so I don't know what those minimum levels are, but the point is that the front and side of the house are not being upgraded because of a combination of lack of space and budget, whereas the whole of the ground floor is benefitting from a full width rear extension, and half of the first floor is benefitting from a half width rear extension, and the loft is being converted and insulated all over. I agree that this means the rooms with "old" uninsulated walls will lose much more heat, but due to the layout of the house there won't be many of those rooms and also, those rooms have their own heating zones and so shouldn't impact on the rooms which are benefitting from new walls/new loft insulation. I think comparing my house to a new build is not really an apt comparison in that I'm doing my best to upgrade a 90 year old house that has no cavity walls. There is a limit to what can be achieved without spending a small fortune.

Thanks for your response. To be clear, the way the house is currently insulated (or rather, not insulated) we only use the central heating 7 or 8 months a year. Although I am going to be installing new double glazing everywhere and although most of the rear of the house and the loft will be properly insulated, the front of the house and the detached side of the house won't have any insulation. Are you saying that even in the middle of winter, you don't need any additional heating in your loft? Also, wouldn't insulation between the floors (as I'm suggesting) prevent the heat from downstairs heating up the loft?

Although the rules of thermodynamics say that heat should travel upwards on its own (by conduction to the floor covering and then convection to the air above), everyone in the underfloor heating industry says that you need to insulate underneath the underfloor heating system to prevent wasting heat "leaking" downwards. I guess the purpose of the insulation underneath is to prevent the heat travelling via conduction to the subfloor and by convection to the void underneath the subfloor. So there has to be some form of insulation I think. Or am i wrong about this?

Converting a 3 bed very dilapidated 1930s semi into a 4.5 bed modern home. EVERYTHING is being gutted except the joists. Quotes are way over budget, particularly for the underfloor heating. I had specified wet underfloor heating throughout all three floors of the house (ground = 800 sqft, first = 750 sq ft and loft = 450 sqft), with the ground being imbedded into screed and the first and second floors being laid within some high performance extruded polystyrene boards made by Cellecta called XFLO. XFLO is quite expensive, about £26 a square metre, but the reason I had specified them is that they provide good insulation to make sure the heat goes up, are CNC drilled by Cellecta to meet any pipe layout requirements we specify and also feel quite firm underfoot. The other advantage of the XFLO is that you can lay the finished floor directly onto it. Underneath those XFLO boards, I had specified a 6mm dense rubber matting (which comes as a 10m by 1m rolled product, so pretty easy and quick to lay) to absorb impact sound, and underneath that 18mm tongue and groove ply which is nailed directly onto the joists (whereas the rubber and XFLO are just floating). In between the joists I was going to have mineral wool to mute airborne sound travel, and underneath the joists I was going to have the ceiling attached using resilient bars (with market name "Genie clips") to isolate the ceiling from the joists to further prevent sound transmission. The whole system is to be zoned as 9 zones, including one zone for the 3 towel radiators (which will be the only radiators in the house, one for each bathroom), although I'm going to provide Tado thermostats, one for each of the 8 UFH zones and a Tado smart valve for each of the towel radiators. One builder has quoted about £25k plus VAT for all of the above, and the other about £28k plus VAT. These quotes don't include the thermostats (I'm providing those). When I have challenged each of them on why this is so expensive (I was expecting about half the cost) they have said different things. The first builder has said that I could save £2,500 to £3,000 money by losing UFH in the loft and in most of the first floor, and instead having radiators there. The second builder doesn't agree that would save any money and is also trying to respect our reasons for having UFH. Instead he just thinks the system I've specified for the first and second floor is overkill and that I could make two changes which would save about £4k in labour: First, instead of 18mm ply plus 6mm rubber matting plus the XFLO boards (which are about 28mm thick) I should just lay a product sold by theunderfloorheating store calledProWarm ProFloor 22mm pre-routed chipboard. I've looked this up and I see the benefit. Like the XFLO boards, you attach it directly to the joists and it comes routed (though with standard not bespoke routing) and enables you to fix the finished floor directly. It costs roughly the same as the XLFO, but it saves the builder the trouble of having to lay the rubber matting and the ply, so the saving is in the cost of the matting (£6 per sq metre), the ply (about £14 a sq metre) and the labour to fit those two layers. Second, he says that in the bathrooms, I should just lay electric UFH rather than wet. He accepts it's dearer to run, but he says I can have smart timers on it to make sure I only have it on when needed and as there will be wet towel rails in there, I really won't use it that often. I think I'm willing to accept his second point and fir the electric UFH in the bathrooms, but I'm concerned about the disadvantages of the first point, which I see as follows: 1) I lose the impact sound protection provided by the 6mm rubber - is this such a big deal given I have an isolated ceiling and the mineral wool between the joists? If so, can anyone think of a way of combining a fairly cheap but effective sound impact layer with the 22mm pre-routed chipboard? It would only ever be my kids running around, so impact sound is not a major issue, but I want some protection. 2) Chipboard is not as durable or as strong as ply. Although both will feel similar at the outset of their lifespan, what will they feel like in 12 years time? I plan to be at the property for 12 years at least, but hopefully longer and I don't like springy floors. 3) no bespoke pipe channels like Cellecta offer with XFLO. But is this overkill? Maybe I don't need bespoke pipe layouts. Builder says that the 22mm pre-routed chipboard just has channels at 200mm pipe centres and that is a standard construction for first and second floors. Although most of the rear of the house is being extended, so it will have insulated cavity walls, and the loft will also be well insulated, the rest of the house has no cavity walls. Keen to hear your thoughts on the above. Thanks for your time!!!