Insulation options

[lookseehear]

I often see 300mm EPS specified under a wet UFH system, rather than a thinner layer of PIR. Is this for cost reasons or for structural reasons or both? I've also seen 100m EPS + 100mm PIR. What would be the reasons for mixing and matching?

[Thorfun]

ultimately it comes down to cost (or maybe SE specifications if a specific load bearing is required). i would suggest to start with a target U-value and then work out the most cost effective way of achieving that and then ensure that your structural engineer is ok with what you want.

[lookseehear]

We're trying to avoid excavating more than we have to due to site limitations (it's an extension rather than new build) so thinner build-up would be better in our case, so I assume we'd be OK to go with just PIR.

[JohnMo]

I would (did) 200mm PIR, poly sheet 16mm pipes stapled to PIR and though poly sheet and then screed.

[Jenki]

I did 300mm EPS, but had a lot of depth to occupy, so worked well.  Either is a good solution especially if you beat the Min U value requirements.  Multiple layers with staggered joints is the way to go with the lowest U value you can achieve.

[saveasteading]

Nothing to do with structures. If 150mm does not compress then 300 won't either.

There are diminishing returns on thickness, especially if ufh is not on all day.

Also depends on the floor area ( and distance to the cold outdoors).

If I had a 300mm space to fill then I might do 150 eps below 150 pir. A compromise.

Edited October 1 by saveasteading