UFH difference in insulation height between kitchen and hall

[mattwaddy]

anhydrite screeds expand at a rate of 0.012mm per meter, per degree. In essence this means if your house is 10 metres wide, and your screed is laid and dries at 15 degrees C, when you run it at 27 degrees C, the floor will expand by 0.7mm on each side of the room.

One of three possible things could happen when the floor expands

* The walls and PIR are so strong, the screed will bow up in the middle by 73.5mm
* The screed and the PIR are so strong, the walls will be shoved outwards along the foundations by 0.7mm
* The PIR, which won't be hard up against the wall or the screed, will compress by 0.7mm

If you're in any doubt as to which of these will occur, just press on the surface of a PIR board with your fingers next time you have one to hand

Yeah that was my thought too, i.e. surely it will compress. He did mention something about if it does compress that it won't return as it has no flexibility unlike eps which will, and so that's why they recommended also not using pir, but hey ho. I'll see how much rework will be to fit pir, but I don't think we will have time now as we're already behind schedule to move in. Is the 25mm eps going to be so bad it's worth it? I'm torn

 

[Mr Chibs]

Just checking... Is that figure correct? screed will bow up by 73.5mm

* The walls and PIR are so strong, the screed will bow up in the middle by 73.5mm

 

[robinbanks]

as it has no flexibility

a) this isn't true but..
b) so what if it was?

Even if the PIR was so perfectly flat against the wall (it won't be) and the screed, such that there wasn't even a single millimetre of movement, and the expanding screed really did compress it, let's say by a massive 2mm, and then you turned the floor off and the screed shrank and pulled away, you have a 2mm gap of air..

..what is the negative consequence?

Is the 25mm eps going to be so bad it's worth it?

Depends what your sense of worth is
The extra heat loss is only going to amount to a few watts, and paying to replace it might reach a hefty 15 quid a year.. But these slight slips in attention to detail tend to occur all over a build, and they all add up.

I made the same mistake next to a bay window, and there is very little insulation between the screed and the wall (even worse than your current) and it still irks me to this day that when I stand on that bit in bare feet the floor is noticeably colder. I know it only costs me a few quid, but I was striving for getting the job right, and the screed guys turned up early and poured before I could get there to fit that bit. At the time I had bigger things to worry about, so I left it..

 

[Mr Chibs]

Funnily enough, saw a document this morning, from the council... Building Regulations Guidance Booklet (June 2022).

"It is now a requirement to provide a 25mm perimeter upstand of PIR insulation as standard, with the exception of floating floors".

 

[mattwaddy]

Funnily enough, saw a document this morning, from the council... Building Regulations Guidance Booklet (June 2022).

"It is now a requirement to provide a 25mm perimeter upstand of PIR insulation as standard, with the exception of floating floors".

Ok thanks, I wish building control were more helpful! Ok so if I swap out the 25mm eps for 25mm pir and also try and use an expansion strip. Would the strip go behind or infront of the upstand? I'll end up with dpm lapped up the wall, upstand + expansion, then a slip 500 gauge layer over the insulation and upstand etc to allow w flow screed to be laid. I could do with getting it right to avoid more rework

 

[robinbanks]

Honestly, it won't matter about the expansion strip (it won't hurt but if you're pushed for time, don't kill yourself over it)

I was new building, a timber frame setup with an internal VCL. I don't know if you have the same but my VCL was TyVek AirGuard. I was advised not to have the alu layer in contact with the screed, and to pour the screed then glue the air guard to it. It turned out much simpler and more effective to bring the airguard down the wall, over the slip membrane and then turn it back up the wall 6 inches, so that the screed ended up sticking to the back (non alu) side.

Worthwhile trick if you're installing a VCL