Most efficient insulation with UFH?

[Philxyz]

We are rebuilding an old cottage and have only 120-130mm available for insulation underneath the UFH (probably in 55-65mm of liquid screed). What type of insulation, what suitable product, will give the most insulation?

 

Thanks!

[Nickfromwales]

Topic moved. 

[JohnMo]

Could use a phenolic insulation boards like Kingspan Kooltherm K103 Floorboard.  About 10 to 15% better than normal PIR, but quite a bit more expensive. Or normal PIR boards.

 

Put it down in two layers with staggered joints, all a tight fit. Fill all gaps with expanding foam and tape the top layer. Thin polythene sheet, staple the UFH pipe to insulation through poly sheet and then screed.

[Nickfromwales]

If you can sacrifice some more headroom, go to 150mm of PIR as the better you invest in the Insulaton now, the cheaper this will be to run forever. 
Go regular PIR for the 100mm base layer, and 50mm phenolic for the top layer ( if possible ). 

[Marvin]

Most PIR for floors has the same sort of thermal resistance so your really only discussing thickness. I would go for as thick as you can stomach!

 

As @Nickfromwales says save you every year!

[Iceverge]

Get digging I say, only one chance to get this right. 

 

300mm of EPS or 200mm of PIR is a good starting figure. 

[Nickfromwales]

7 minutes ago, Iceverge said:

Get digging I say, only one chance to get this right. 

 

300mm of EPS or 200mm of PIR is a good starting figure. 

Amen, reverend . Would be the best money spent.

 

UFH gets done so badly, go often, it's genuinely hard to believe. 

[SteamyTea]

9 hours ago, Iceverge said:

Get digging I say, only one chance to get this right. 

 

300mm of EPS or 200mm of PIR is a good starting figure. 

Yes.

Insulation is not very glamourous, but it works.

And never trust a builder to get it right. 

[Dave Jones]

10 hours ago, Iceverge said:

Get digging I say, only one chance to get this right. 

 

300mm of EPS or 200mm of PIR is a good starting figure. 

 

yep do this. 300mm eps is good start point.

[Philxyz]

OK. Sorry for the delayed response, and thanks very much for the pretty unanimous and clear need to go deeper.

 

Looks like we can get down to 180mm, will try to go deeper but wont know til on site. Will do the 2 staggered layers, foam, tape and polythene. Will look into the price for a top layer of phenolic.

 

Thanks everyone!