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eskay-
Free! I should have said it was the one @hoefla posted, so thanks to her for that.
https://woodfibreinsulation.co.uk/courses/I did the retrofit one, which was especially relevant for me as I'm planning on using wood fibre batts for external and internal wall insulation, with hemp flax combi jute in between the floors because it has a higher heat capacity and better soundproofing qualities.
With it being free my expectations weren't high but it was actually really useful.
Fox
I actually disagree slightly with the orthodoxy that it's not worth insulating between floors.
This is coming from someone who's spent months going down eco insulation wormholes (and I've just finished an online course on it too).
There are a few other advantages to insulating between floors. At the risk of stating the obvious, it helps you zone the rooms by keeping some warm while not heating others, which is more efficient. If you spend most of your time downstairs in your living room/kitchen then you won't be paying to heat upstairs rooms which may not need it in winter.
Current thinking on heating and insulation has fortunately developed past the 'heat your house as one big box' approach to focus the heat where it's really needed (see things like Tado TRVs, energy firms giving out electric blankets), so IMO it only seems logical to insulate between floors and rooms to extend this thinking and only pay for heat where you need it.
It can also help with overheating problems in summer. Our bedroom is at the top of a three story terrace and it gets way too hot. Some of that is heat which is rising up through the house - through the fabric of the building but also up the stairs which are currently open to the living room - so we'll have a lovely cool living room but an unbearably hot bedroom. Adding in insulation between the floors with a high specific heat capacity will slow that heat transfer by storing up the heat and slowly releasing it over time, buffering the overheating effect.
But hoefla is right of course and the main reason to fit it is acoustic. This is another big reason we're doing it and I wouldn't underestimate how much good acoustic insulation can benefit the final feel of a room or building. Objectively it enhances privacy, improves comfort, boosts productivity and there's some evidence it improves your health too, but more subjectively people who've had a lot of insulation fitted tend to perceive rooms as being more cosy and 'softer'. Which is probably not surprising if like us you currently have 175mm joists and nothing in there but dust and a load of old shit - it's like living on top of a drum 🥁
They who shall be obeyed do agree with me because intra-floor insulation is now required in new builds under part E of the regs. Personally with our renovation I'm bringing things up to current regs standards where I can, so another reason it's a no brainer for me.
Worth adding that if you're putting it into an older building it needs to be breathable and ideally hygroscopic to help maintain the health of the building and prevent sagging - just in case kid in said room knocks a drink over in the future...