SteamyTea

At those prices (shows it is worth hunting around), and if my spreadsheet is correct, EPS comes out at £30 (0.35m thick) and PIR at £26 (0.25m thick per meter squared. Those are for a U-Value of 0.1, which in reality, is pretty good. If there was a 20°C temp difference (quite possible with UFH) and you have 100m2 floor area, then: 20°C x 100m2 x 24h = 4.8 kWh/day. That would be for a very cold day.

Excess can also be easily and cheaply stored as domestic hot water

Have you run the system though PVGIS, I think they have a storage calculator now as well, but not sure on that.

I must be bored. Here is a chart showing price comparison between insulation types. I have only got the prices from 1 site, and have used median k-Values. To get better results, do your own research. There is little difference between XPS and Phenolic at the 0.1 W.m-2.K-1 level, about £10.m-2. There is a 0.1m thickness difference though, which may or may not be important depending on how you excavate and dispose of the dirt. (as usual, there may be errors as I am tired)

That gets me to @Pocsterland, or about a third of my bi-monthly trips up country. I find filling my mind with thermodynamics problems passes the time.

Is this New Year confession time. I drilled though a wall to install a water pipe for a shower, long drill bit. Went through the wall fine, and then into the 300lt cylinder that was the other side. Measure twice, drill once.

I just knocked it up from assumed k-values. It is a 1D model, so edges, which can have greater losses, would need to be added on. I think that is caused by the charting software, see below for a different curve algorithm (changed to a B-Spline from a Cubic Hermite Spline). This is where it gets difficult. It depends, primarily, on energy prices and installation prices. UFH would cause greater losses than just heating the air in the building, the building form factor would make a difference as well. This is why individual buildings need to have heat loss calculations and not have the heating system sized by a rules of thumb i.e. just floor area or number of windows (I have heard a plumber assess the heat loss that way). Losses though the slab are uneven as they can be affected by the room temperature above, the ground type below and if there is any ground water movement nearby. So probably safer to pop a bit extra in, rather than too little.

Use this simple cut out guide to work out your ideal insulation thickness for your desired U-Value.

No idea. Would be OK for windows and doors, but not very good for slab insulation.

You are still comparing overall thickness though (I know you had a (expletive deleted) up are limited on thickness). What you need to do is pick a U-Value (i.e. 1.2 W/m².K) and compare the prices to achieve that, not compare prices to fill the gap up. Not if you had made that decision in 2015, 6 years before our energy prices doubled.

Have you work out the price by U-Value, rather than thickness. Regarding unfaced sheet. Unfaced are used a lot in the composite plastic industries because it is easier to adhere to. For any sheet insulation there will be a certain amount of compression when the initial pour of concrete is added over the top. There has been some discussion in the past about whether it should be polystyrene first then PIR, or the other way around. I can't remember what the outcome was, but seemed to remembered it was counterintuitive. The insulation manufactures should have that information. There will also be a long-term creep as well. PUs can keep shrinking for years. They also absorb moisture (as do most materials) that can affect the k-Value quite significantly. I think polystyrene is a bit better in this respect (though it does depend on density. It really comes down to what you want from your floor insulation. I am with @JohnMo in this respect. Get the very best performance you can financially justify, you can put it right later.

If you read the work of J Richard Gott, he states that the longer than something has been about, the longer it will last. AC lighting has been around a long time compared to modern DC lighting. So you have to consider the replacement costs, the potential difficulties of getting like for like replacement, separate wiring circuits etc etc. You can buy, from Poundland, LED builds that plug into an ordinary bayonet or Edison Screw fitting for a pound. Why complicate things, it is a light build.

Bullshit Jobs

Several years ago, a neighbour of mine was working for a large, London based legal company. She had it written into her contract that she could use the companies IT facilities to do her Amazon trading. One of her duties was to post many legal documents, she used to slip in her own packages with no one noticing.

Isn't OFGEM meant to put a cap on it on predicted future prices (they are not really predicted as they are forward contracts as far as I know).