Carol W

I've seen others mention doing that but I'm not sure how much of the house the void extends to ....I need to take a good look. And I'm guessing the quantities involved would make it really expensive

Just clay soil unfortunately Just clay soil unfortunately so maybe we could use a block under each pedestal to spread the push force ?

I think that block and beam floors need up to a 225mm void beneath them (we're on clay so I'm being cautious). We have about double that. How about (by taking out a few blocks and wriggling under) I use plinth/pedestal patio stands to hold Celotex up against the underside of the block and beam? It would take a lot of wriggling and effort to make sure each board was butted up to the next but would I end up with a warmer floor?

Yep...trying to sort the problem out myself because no builder can know it all. He's a nice guy and all the people who work for him on our job are also lovely. He'll do whatever I ask him to do but I'd rather bring him along on the journey of discovery. Not sure he'll get deep into the heat calculations but he'll listen to what I want.

Thanks for the link. I'll give it a go at the weekend. Never enough hours in the day !

Thanks. Do you not think there would be any issue with ply/OSB ? It's a kitchen/lounge/diner and I've had some people tell me to avoid wood under the tiles. I suspect they are thinking of moisture getting through

I have absolutely no building knowledge ! We have a new extension which will be open plan with two old rooms which formed part of our 1994 build home. My original plan was to have radiators in the old section and UFH in the new. The plumber suggested we use Wunda rapid response overfloor heating so we could have the same throughout the big room but I really didn't want the floor height to change. So, the builder decided it wouldn't be too much work to dig up the screed in the two old rooms. It turned out there was only 40mm of screed before block and beam. No insulation. I've looked at replacing the blocks but I think you then need a structural screed on top and I'm not sure we have the depth for that. Their plan was to put slurry on the block and beam (apparently the original builders should have had this) and then a layer of ply, then the Wunda trays, then self levelling, then the porcelain tiles. The screed in the new extension was applied (over deep insulation) to the same level as the block and beam in the old so that both parts of the room could have these same layers. Then I started investigating and realised that in the old section of the room there would be nothing much stopping the heat travelling down to the block and beam and I'd be wasting money heating the ground! I really want to work out how much that would cost me in money terms so I can show the builder. How can i do that ? So I'm going to tell the builder I want to go back to the original idea of radiators in the old part and ufh in the new. How should he build up the layers to do the best job in each section (one with and one without ufh) given that we only have 40mm (including a 10mm tile) to play with and the Wunda trays are 20mm. I have been told that ply is a bad idea but the plumber says the trays must have a really solid base to sit on or be screwed onto. Help! My husband really isn't into any of this and although I'm learning a lot, I know my knowledge is limited.

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.

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?

Hi All. We moved into our home nearly 30 years ago when it was brand new and I have spent all that time thinking that it was built the wrong way round to catch the sun. Now I've designed a big extension to capture lots of that light. Everyone thinks we're mad, extending after the kids have moved out and it's just the two of us, but we want to spend the rest of our lives here so I want to make the most of it. Looking forward to getting other peoples opinions on issues we are facing.