Building a Sun Room extension - expert opinion required

Doozergirl

Giving this some thought, I wonder if you can speak to your BCO and make a trade off for the lack of floor insulation with having extra in the walls.  

Giving an example, if one builds a heavily glazed extension, the glass has a higher u value than a wall so one has to get a specialist to produce a SAP calculation to prove that the building is thermally efficient enough and the u value overall suitably low.  

I suspect if you ask very nicely that they may allow you to do something like use solid insulation in the walls instead of rock wool, as a trade off for a lack of depth in the floor.  

After all, most heat is lost (in order) from roof, walls and then floor, so if you upgrade one of the others, you'll still have the overall result, albeit a floor that isn't warm to walk on if it has a solid surface covering like tile.  

stuart45

A trade off might work out a bit cheaper considering the prices of polymer screeds and Optim R insulation.
Floor only loses about 10%.

Doozergirl

The OP could offer to produce a SAP calculation to prove overall compliance.  I wouldn't offer it up immediately in the hope of saving a bit of money, but I don't see why it shouldn't be allowed if it works with heavily glazed extensions.  

frogga

Thank you both, I had no idea that there could be any negotiating? I will be interested to hear what the builder says. We were going to ask a different Building Regs company for the next bit but I now think it would be better to keep with the same one - they agreed the work so far so they can be part of the solution

GDB2222

Section62 said:

stuart45 said:

Reinforced slabs are normally used in poor ground or deep fill etc and usually sit on the inner skin. They do the same job as a block and beam system. 

^This.

The significance of mesh being omitted will depend on the type of soil the structure is being built on, and the care taken in preparing the formation and sub-base.

The way most domestic work is done, the function of the mesh has more to do with crack control rather than providing massive structural strength.

The over site was done 5 years ago, and I assume it hasn’t cracked, as the OP did not mention that. Is that enough for the OP not to worry? 

Bendy_House

The current situation, Frogga, is that you have a cast concrete slab and how much room above this to get to finished 'floor' level?

Ie, if you allow for 22mm T&G chipboard floor to be floated on top of the rigid insulation (and for a small room like yours, 18mm will probably be fine), how much space does this leave for the actual insulation (and a thin sand blinding layer)?

frogga

Yes no cracks at all in the concrete thanks GDB

Hi Bendy, yes that is the problem. I have 70mm from the top of the concrete to the bottom of the door frame of the door leading into the sun room. so there is only 70mm left for the insulation and the screed and the tiles or whatever like floorboards yes. Maybe it would be better to do insulation and then floorboards rather than screed and tiles? Is that allowed?

Thank you for your interest Bendy x

GDB2222

Floorboards will be warmer underfoot. I think that wood is nicer than tiles, but obviously that’s just a personal preference.

You may be best off with engineered wood, as traditional floorboards require some sort of joists/bearers to nail them to. I don’t know whether you could put say 50mm of insulation with 20mm engineered wood directly on top, without any sort of screed?

Doozergirl

GDB2222 said:

Floorboards will be warmer underfoot. I think that wood is nicer than tiles, but obviously that’s just a personal preference.

You may be best off with engineered wood, as traditional floorboards require some sort of joists/bearers to nail them to. I don’t know whether you could put say 50mm of insulation with 20mm engineered wood directly on top, without any sort of screed?

Noooo! 

We've gone from worrying that building regs aren't being met to throwing them out of the window. 

You can't put floorboards directly onto insulation, solid or not.   In no way is it stable enough.   It isn't a base for anything and needs either a decent screed or joists.  

We had a thread on here by someone whose builder had done exactly that with boards then tiles and it was absolutely terrible with everything ruined.  One of the worst cowboy acts I've seen on this board.  

frogga

OK Doozergirl, I'm listening dont worry. I will talk to the builder and make sure we do as best we can. Very very grateful for your comments xxxx

I really want tiles rather than wood as we have dogs, and sometimes puppies have accidents, and sometimes old boys do too and wee sits on tiles but seeps into wood. 

I have heard you, we need to insulate and screed, but we may be able to do it thiner than the plans