Hi, we have a new extension and an old room in our 1995 house, forming one big room. We wanted to have underfloor heating but didn't want to create a change of level with the other rooms downstairs. On that basis we though we should use the foil coated polystyrene boards with grooves for the pipes as they can be quite shallow. The builder dug out the 40mm screed in the old room and found uninsulated block and beam below. He has now insulated the extension and laid screed so that it is level with the old block and beam. He decided it was best not to do different UFH methods in each section  (ie: deeper pipes in the new section and shallow foil coated boards in the old) but to keep them the same. 

I need to check with him but I think the boards will be about 20mm so we have about another 20mm above the block and beam (and screed) to add any insulation. 

What would people recommend to use between the block and beam and the ufh boards? Should we remove the blocks from the block and beam and replace them with new blocks with better thermal properties? Is there any way of insulating under the blocks without messing up the cavity underneath? 

You want about 150mm insualtion for UFH to be effective and economical.  How much and what type went in to the extension?

 

For the block and beam, you can get EPS blocks in place of the concrete blocks, but may not fit your existing beams. And you'd still want more PIR above this anyway. 

 

I'm not sure on the option of completely removing the beam floor and replacing with a solid slab, a structural engineer would be needed.

Loads of insulation went into the extension....to current regs. I guess we just need to do the best with what we have in the old room. I'm interested in replacing the blocks but wouldn't want to replace with a solid slab as the void also goes under other rooms...it would be massive.

 

Is be sticking to rads in the existing part, tbh. You'll not get enough insualtion in. 

Yep, rads in the old part and ufh in the new, provided that there is 150mm + of PIR down there. 
 

We ended up with 200mm.