SBMS

@Dave Jones when you say calculating heat loss ‘through’ - isn’t both sides of your marmox insulated? CWI one side and the floor PIR the other? Or is this not how it works

We’ve got floors that are built into blockwork in current house. They bounce a bit on a couple of long spans but don’t squeak. anyone else any experience with hangers? I wonder if we should specify 400mm spacing if we go with hangers..

Did you seal around the hangers for airtightness?

Interested to hear people's thoughts here - how did you fit your intermediary floor joists? A very quick comparison: Built into blockwork: quick and easy for brickies, no squeeky floors, difficult to airtight (Tony Tray needed) Joist Hangers: better for airtightness (but detailing still needed to account for mortar drying, timber shrinking), pre-engineered by posi joist designer, have heard they move and can be squeaky Ring Beam: ideal for airtightness, brickies and just construct the walls, more work to construct, structural engineer needed to specify bolts and sizes etc, again have heard they move and can be squeaky Thoughts?

Have you quantified the difference in heat loss from the increased thermal bridging? I calculated that a perimeter row of 215mm thermoblock would be circa £2k additional cost. I wonder what the energy saving would be, but I'm not sure payback justifies it? No hard data for that though, just a gut feeling...

Thanks, yes it's not been updated yet following SE's drawings. DPC is in wrong place too!

Thanks @JohnMo got it now. Out of interest how would one typically ‘stop’ the EPS beads 225mm above the concrete cavity fill? A cavity tray? FYI this is the architect detailing showing the DPC/cavity tray in red dotted line - does this catch the EPS beads?

Got you. I think the thing I couldn’t make sense of is why I’ve read in a few places on here that members have taken their insulation to the bottom of foundations. In my head in only makes sense to take it to the same depth as the slab insulation (dotted red line below) Am I correct on that? So in summary, I just need to change the two courses of inner block immediately below DPC (marked blue below) to aircrete blocks and that’s it?

At the risk of sounding daft (I don’t mind!).. if the cavity below ground is filled with insulation, how is this thermal bridging any different to above ground - ie the same makeup with a block wall and then 200mm of insulation? Is it because it’s a solid mass below ground on the inside of the wall as opposed to mostly empty space (ie a room) above the DPC? Or is the primary reason for using aircrete blocks simply to negate the thermal bridging of the bottom most block sitting on the foundations? I hope my naivety will be useful for others!

Is there any reason why aircrete blocks would be needed if we brought the cavity insulation (EPS beads) right down to the bottom of the cavity? Would that negate any thermal bridging? I don’t know whether a Cavity tray at DPC level would still be needed.

Did you do similar detailing re DPC as suggested? Were you on PIR for cavity insulation or beads?

@Dunc that’s really useful thank you

So, unlike @JohnMo’s suggestion you’d advocate some marmox instead of aerated blocks? I wonder what the payback would be based on additional cost of marmox.. we’ve got 50m perimeter.

With that DPC - how would we get the beads below the DPC?

Thanks. Out of interest if this is as effective as marmox thermablock - why would anyone use them instead of the above detailing?

Thanks. Architect has specified PIR upstand around screed (not on engineer drawings above). Is that what you mean? Just to clarify - your two courses of aerated blocks - are they the ones directly below FFL (adjacent to the slab, insulation and screed?) Where do you mean regarding the DPC movement?

Would that not be based on the joist depth?

We currently have the following 'standard' detail for foundation arrangement: We are using EPS beads for cavity insulation. Have seen a number of different schemes to reduce thermal bridging. Enhancement one: the 'denby dale detail' where the bottom four courses of inner block are switched out to aerated concrete. Enhancement two: putting a course of marmox thermablock adjacent to the screed to minimise bridging (about £1400 additional cost for that). Enhancement three: instead of the concrete cavity fill before ground, we could bring the beads right down below ground level. Which of the above would recommend incorporating into the above detail with regards to a cost vs benefit analysis?

Dave - really useful, have you got any more photos (especially at corners) of the installation?

Currently specifying the posi joist to wall junction. My plan is whichever detail we use, we'll still paint the junction internally with passive purple. As I understand it, the tony tray method is 'tried and tested' and clearly works - but there can be issues with on site installation (tearing when joists are lifted on etc). Is there any reason why a joist seal such as https://www.manthorpebp.co.uk/air-leakage/joist-seals-wide-range-sizes-and-permutations/joist-seals isn't an easier, less risk prone solution?

I don’t think that would be clearly justifiable or necessary. The principle when granting planning permission is that the new dwelling that is approved is assessed for approval in the context of retaining its PD rights. The removal or those rights via a condition should be justifiable in the context of the application and site. For example, a contentious site that only just passed in green belt would typically have PD rights removed so as to afford the LPA the ability to control future development of the site. It should be site specific and necessary and justifiable. Our LPA no longer adds this condition by default (it used to) following a number of successful applications to remove the condition post approval by a fairly industrious local planning consultant… each time he asserted the test of necessity and justifiable to the site and the LPA didn’t even question it. I think an earlier post regarding appeal inspectors dim view to removal of these rights is valid.

Yes I don’t quite understand it myself - but makes sense it’s an exponential simulation. We are in the north west (Lancashire) so…wet! Our site area for positively drained area is 0.3ha - does that make a difference? Yours is roughly half; could that account for the 2x difference??

Is it vaulted if you’ve got a ceiling effectively 1.4m from the ridge? If youve got a steel there anyway could you not get rid of the collar?

@Alan Ambrose, our ground floor sqm footprint is 206m2 (garage + ground floor). My basic maths: my 206m2 / your 470m2 = 43%. So of your 24.7m3 storage would be around 43% at 10.6m3. Then there's the discharge rate - mine is 2.6l/s whereas yours is 1.9l/s. my 2.6 / your 1.9m2 = about 136%. So that 10.6m3 is about 7.8m3. I'm not sure why the figures I've tried to normalise above are still double when removing the 'permeable' area, and adjusting for the lower discharge rate.. However I suspect that the simulation isn't linear (i.e. half the sqm doesn't yield half the required attenuation) and is possibly more like an exponential curve... someone smarter than me can answer that? What I would say is counting your permeable areas as impermeable seems up for debate and might drastically reduce your storage requirements. Secondly - how did they calculate your discharge rate? Our site is currently brownfield and they adjusted it 'up' as if it was greenfield (so there is a betterment), but still theres a 30% difference.. maybe one to investigate? And finally, the great irony in all this, is that currently, every single bit of our site - where the house is going, the driveway and our garden - totally 1500sqm - is currently a thick concrete slab (brownfield, former piggery and stores) with some rudimentary drainage discharging to the local beck about 160m away. The site never floods, and neither does the beck, so anything we do - probably even without attenuation - might be entirely unnecessary??

Is it enough? He’s done the storm calculations and says it is - that’s what we are paying him for, so got to assume that his climate adjusted 1 In 100 year storm figures are correct? One thing I have noticed is some engineers count the driveways (even if permeable) into modelling/attenuation figures and some don’t - did yours (assuming your driveway areas are permeable?)