Tim2021

I did a little basic maths to give me some confidence....If I just calculate at the heat loss into the floor and ignore the power heating the house for a moment. The extension floor is 25m^2, if I wanted it to be at 35 degrees C in January (when the ground below is 5 degrees) the power required will be (35-5) x 25 x 0.19 = 143W If I had 150mm PIR insulation the U-value would drop to 0.11 W/m^2K and the power required would be 83W So the additional burden on the heat pump is 60W or 1% of it's 6kW capability, in the worst case winter. So I can see why the BCO is cool about it. 60W is not nothing, but do I really dig the floor back out for a 1% improvement in the GSHP? I could scrape 25mm off, add a layer of polystyrene and reduce the excess loss to only 30W.

Thanks for the assistance. BCO has just emailed and confirmed target 0.2 and that >70mm will do it. (I wish I had checked here earlier and added insulation but I really don't want to dig it all out again.) Heating is by GSHP / Solar PV so little carbon impact long term. I will put another polythene layer below the insulation as advised on this thread. Tim

I found it in Part L I think, I will email my inspectors (Stroma) and ask. They've had the plans for months and haven't questioned it.

Thanks for the input. The online calculator says that 75mm will give a u-value of 0.19 (target is <0.2).... what u-value would you suggest for this please? Is my target wrong?

Getting ready to pour the floor slab here..... Plan is: 100mm polystyrene in the cavity, cut flush with the outer wall. 75mm PIR inside, flush with the top of the inner wall. (Inner wall is 7N aircrete block for thermal bridging) Radon barrier above the PIR, out over the polystyrene, extending right through to exterior. A142 mesh out to the slab edge above the inner wall on 30mm mesh-men, UFH pipe tie-wrapped to it. Does that all sound OK? Do I also need another polythene layer between the 75mm PIR and the compacted sand/gravel beneath? Thanks, Tim

Thanks for the reply. BC just told me that if I use (at least) one course of thermalite blocks supporting the slab and bring the insulation down below the slab they will be happy.:-) Sorted ?

I've poured foundation strips and am just about to start building the wall on my cottage extension. The plan was to have 100mm dense block - 100mm full-fill cavity - 100mm dense block construction. Inside would be a 100mm floorslab containing UFH heating pipe over 75mm insulation, and edge insulation around the slab. Radon barrier below and up the sides of the slab. BC have just told me that, in this high Radon area, I must use a 100mm slab with rebar, supported on the inner wall, radon barrier below, continuous, out to the cavity. I understand the construction of that but it will make an obvious thermal bridge. I think I need to minimise this? Can I / should I use lightweight blocks on the innerwall supporting the slab to reduce the thermal bridge? Can I still use dense blocks above? (for cost and more thermal mass). I can't easily put insulation above the slab and UFH in a screed on top of that as it ends up too far out of the ground. Any other ideas for this construction? Thanks. Tim

Thanks for the comments, I guess it confirms what I thought and there are no easy solutions. I'm going to carry on assuming I can put the small ditch along the back. Worst case, I'll have to dig up the track and re-lay it a bit lower if the inspector insists.

Thanks very much for the reply. It is my farm track, I put it in myself about 15 years ago. I had never been inside the old cottage at the time and had no idea of the awkward steps down inside it. In theory I can do anything with it... I could even dig it out deeper but would prefer not to obviously...it's C35 with rebar and a hard bed below. I'm also trying not to reduce the size of the extension of course, so am over-constrained in every way! Can you suggest what a "watertight slab and wall construction" would look like please? How thick? I'm imagining a cavity wall with the outer being concrete/rebar strip maybe 150mm thick?, 100mm cavity, inner wall 100mm block? DPC between..something like that? Would BC want a whole load of structural calculations?? ? At the lower end of the track, (coming out of the picture towards you) the level continues dropping to be 100mm below the internal floor level within a few metres. Tim

Hi, I am hoping that someone can suggest construction details for the rebuild of this lean-to extension on the slightly sloped site. The concrete farm track shown below, is slightly angled between 400mm and 1200mm from the rear wall of the finished project on the plans. Unfortunately, at the highest end, the farm track is 200mm higher than my internal floor level. I am demolishing and rebuilding the existing lean-to structure (currently the walls are mud/granite, very damp, no foundations and nearly 700mm thick). So I need a wall design as thin as possible, 200mm higher outside than inside, suitably damp coursed and insulated. My planning approval is to rebuild the lean-to as timber-framed, but there can surely be a concrete/brick dolly wall under that. I have made an initial proposal to BC but didn't include this detail and they haven't done a first inspection yet. I want to have it clear in my mind when they visit. Thanks, Tim

The Georgian one I'm hoping to copy has no pillars (apart from at a small gate). It's an aesthetic choice, a 200mm thick wall just looks too "heavy" for the garden it would be in.

? 700mm thick walls all round too.

The brick wall would all be inside the cottage garden, no public anywhere near.

My understanding is that I can claim the VAT back for a "new building" on everything described in the "principle planning application"? I believe it would be considered a "new build" for VAT purposes as I propose to take the existing structure down to ground level and re-build it with 0.18W/m2K walls instead of the solid granite (c. 2.3W/m2K). It's tiny with just 5 x 7.2m footprint The idea is to include on the main planning appn. a substantial wall extending away from the house (which definitely does need PP/BC). The brick wall to enclose the vegetable area, (a small version of the classic Victorian kitchen garden) then being a "U" made of 3 x 10m lengths. I agree with you, it's only worth involving the aggro of BC if I can get the VAT back on the wall materials (20%)/contract bricklayer. Any advice gratefully received.

Very helpful link. It says 450mm max for single skin but I guess that's a "guideline" rather than law and I can still use the structural engineer approach.. Thanks for your help I know exactly what to do now ?