I'm about to put in a rough concrete floor down in my 6x3m outbuilding. I want to put some insulation in first, and have a selection of leftover boards which are varying in thickness between 40mm-60mm. 

Is it a big no-no to use slightly different sizes? Or can i still use them, as the finished level of concrete will still be the same? 

I'm not overly bothered with the finish, it's just an outbuilding for storage mainly, but just thought would be prudent to stick some insulation down whilst I have the chance... 

No harm at all, just make sure they are all flat and don't rock about.

just another quick question linked to this. A couple of the boards are non foil backed. does this matter? 

cheers. 

Put some polythene over the top, as concrete reacts with the aluminium 

ok, finally going to move ahead with this now the weather is nice. 

one final question if you please:

 

I think I saw somewhere that you shouldn't pour the concrete right up against the walls, in order to allow the building to 'breathe'. I now can't seem to find any reference to this, and although it still seems logical, am starting to wonder if I made it up in my head! 

 

if it isn't supposed to directly against the walls, then what forms the perimeter - wooden form work/shuttering? If so, does that remain in place or is it somehow removed? 

 

Many thanks again good peeople. 

Edited May 21, 20233 yr by TANK

use a dpm under the lot.

Are you planning some internal insulation in the walls? If so, have an insulation upstand from the under-floor insulation to just above finished floor level, so that your internal insulation can connect tightly to it. I think I'd do an upstand in any case.

2 hours ago, TANK said:

ok, finally going to move ahead with this now the weather is nice. 

one final question if you please:

 

I think I saw somewhere that you shouldn't pour the concrete right up against the walls, in order to allow the building to 'breathe'. 

 

 

A perimeter upstand of say 25mm PIR will reduce the conductive heat loss from the slab -> walls -> ground. 

 

I'd you're not bothered about this you can just pour to the walls. 

 

 

 

 

 

 

Ah right. Yes that will work. 

Thank you.