Suspended Floor Insulation

1 Dec 2019
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United Kingdom
The suspended floor is now constructed and ready to be insulated. I have a query on the thickness of insulation that i need to install.

If my research is correct a minimum U Value of 0.25 is required but i have also seen 0.22 mentioned.

The Kingspan U Value calculator says 90mm PIR is required for 0.25 and 110mm for 0.22.

100mm PIR seems to be cheaper than 90mm so that would be my choice. Do i need to go any deeper?
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I'd just lob in 100mm PIR, bring it to the top of the joists and fill any gaps and tape-up the joints.
Aluminium ducting tape is cheap and works a treat with foil/foam boards.

It won't make the floor warm, but will slow down heat loss.
I would fill joists with mineral wool and 50-100mm celotex underneath if space and services allow it - avoids the cold bridging, cutting, gap filling etc. Fix the celotex with wood screws and penny washers.
Just got to be careful not to block any air vents.
Could you not fill with rock wool and a thin layer of celotex... like 25mm?

In real world terms, it's hard to quantify cold bridging through a joist of 150mm+ in depth.
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heat loss through a floor is very low, mostly it's draughts because there is no convection and almost no conduction loss.

Have you got to meet BRs? Mineral wool is better for blocking draughts because you can stuff it tightly into the space between the joists with no gaps and no effort to foam or tape.

be extra thorough round the edges, because that's where the draughts get in. do not block airbricks or ventilation route
complete waste of money. i’ll play along with 50mm . pack it in and concentrate on stopping draughts. heat rises. insulate the ceiling.
Building regs require it. BCO wants to see it before signing off.


I don't know what the P/a ratio is so that might need recalculating
PA ratio for mine is .9 i believe which spits out 140mm rockwool. I think i will juts stick with the 100mm PIR as i have a fair few boards left over from the roof.


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