ADLIan

I’m aware of that. My reply was more in connection with why use a 100kg/m3 slab when there is no need to. Assuming pricing is linked to density there are probably more cost effective solutions.

For acoustic insulation denser does not equal better - at normal, commercial densities, they all do the same job. Bldg Regs need 100mm thickness at 10kg/m3 as minimum, denser will be more expensive but acoustics will be the same (the ceiling, floor finish and effect of joists has a bigger impact).

I wouldn't use 032 mineral wool between timber studs. Will give little to U-value but at much more cost, 035 product will be OK.

The U-value table from Black Mountain is meaningless. U-values apply to the whole build up, including all individual elements, of the floor, wall or roof. Additionally with timber frame it does not account for 15% thermal bridging of the insulation - the Kingspan figure does. Looking at insulation in isolation the hemp has a thermal conductivity of 0.039 W/mK compared to 0.018 (?) W/mK for Kingspan. The simple ratio of these 2 values shows you need twice the thickness of hemp to equate to the Kingspan product. Mineral wool is normally the product of choice here for ease of use and robustness of install, has good 'eco' credential and not oil based.

Fire!! EPS will soften/melt, plaster will delaminate leaving eps exposed to add to the fire load. Mechanically fixed metal mesh as carrier may be an answer.

Do not render on insulation on timber frame. Render should be on carrier board with vent cavity behind. See Bldg Regs and any good document on TF construction (see STA or TRADA guidelines). Wood fibre manufacturers guidance should not be relied on. Some did have BBA certs covering direct render but these all appear to have been withdrawn.

Have you a photo showing the front elevation of this bump out rather than the side. If only a porch and the main external door is between the house and this area then it’s probably outside the thermal envelope so no insulation required. However it looks to be too long for that and may well include part of a ground floor room ceiling in which it should be insulated with 300mm (min) of quilt insulation. Looks like they did try to prevent the thermal bridge across the steels by packing with insulation - not sure how successful that would be! Run more insulation vertically against wall to minimise thermal bridge through steel. Not sure how you would do this so late in the day, perhaps have to remove roof tiles. Is house still under any warranty?

The area weighted average u-value is the way to go however if the main wall/Uval accounts for at least 90% of the total wall area just use that value

Best practice? Please read MIs, BS and BBA certs, no ply/osb above insulation. Lots of fire safety info on internet re torch on systems. Moving to single ply membranes still no need for ply/osb above insulation (it’s in the wrong place!). Normally rubber/PVC single ply membrane is mech fixed or adhered direct to insulation.

Your flat roof is a standard detail apart from the extra layer of ply above the insulation - this is not required. Kingspan, and other PUR manufacturers, have insulation that will accept torch-on membranes and it looks like the fixings are OK. Remember the flame is directed at the roll of felt to melt the bitumen layer on it, not onto the insulation. Torch-on systems seem to have lost favour due to fire risk but if appropriate safety measures are taken it is quite acceptable.

Also EPS, board or beads, is not used for acoustic insulation.

Difference with a flat roof is the waterproof membrane/layer which invariably has a massively high vapour resistance but is on the cold side of the insulation. This situation should not arise with TF walls.

@scottishjohnPlease have a look at BR 443 and the various caveats about metal (not discrete screw fixings or wall ties) penetrating an insulation layer. Kingspan composite panels normally sit above the purlins, similarly the insulation division normally show the insulation external to purlins/deck or light steel frame studs. @moldyProblem with a compressible insulation is that it's insulation value will depend upon its compressed thickness! Simple solution is to use the loft legs that are designed for this application.

The Actis claim of being equivalent to over 200mm of mineral wool was demolished many years ago. If I remember correctly it was back in 2006 when new guidance in support of the Building Regs was issued.

18 or 22mm of ply/OSB will do little to protect the top flange. Please accept the fact that metal penetrating an insulation layer really does ‘degrade’ the overall thermal thermal performance of this layer considerably. I’ve done plenty of 2D and 3D thermal modelling showing the effect. Have a look at heat losses thru steel stud walls and rain screen cladding systems

Big problem! Metal rails thru insulation is never a good idea. Top flange will be in cold loft space and will be a major linear thermal bridge (or point thermal bridge if cut into smaller lengths).

Some big pieces of metal penetrating the insulation with above option!

+1 on comments above. Not a very good roof design and the space blanket (multi-foil?) will be doing very little to add to the overall thermal performance. U-value is probably 0.25 W/m2K or even worse.

Dividing the SAP air infiltration rate (at 50 Pa) by 20 gives an approximation for ach in normal conditions. Method above, accounting for volume and exposed area, gives ach still at 50 Pa.

Must use 'Type A' wall ties (lighter with low dynamic stiffness) and minimal fixing density to limit transfer of sound between the 2 leaves. +1 on mortar droppings on ties and bottom of cavity!

Assuming under English Regs Appr Doc E has masonry cavity wall and solid wall options that ‘should’ work but need testing on completion. Alternative is to look at Robust Details (RD) and follow their design details. Unfortunately this only covers new build, not conversion, so testing probably still required unless you can convince BCO otherwise. As above detailing to avoid flanking transmission is critical as is quality of workmanship. Adding mineral wool in the cavity gives little, if any, acoustic benefit as the acoustic performance is down to the mass of the wall and 2 decoupled leaves. Mineral wool rolls or slabs, at least 18 kg/m3, are needed to prevent party cavity wall thermal bypass - a thermal requirement reducing the party wall u-value to zero (See Appr Doc L1). This may not be so critical with conversion work but check with your BCO. The RD view is that this measure must not degrade the acoustic performance of the wall hence they show both filled (optional) and unfilled cavities.

Check with manufacturer - most show short ends fully supported on joists too.

50mm insulation is unlikely to get the required U-value in a ground floor, max allowable is 0.22 W/m2K. Probably requires 80-100mm depending upon footprint. Also I do not think that screed at 12mm thickness is suitable for use directly over insulation. Specialist screed can go down to 40-50mm, sand/cement at least 65mm.

Min. density is 10 kg/m3 - see Section 5.18 of Appr Doc E (same detail in Scotland too). Manufacturers will have suitable products with this spec, see 'acoustic roll' or 'acoustic slabs'.

Conversion work comes under Appr Doc L1B (E&W) with a much more relaxed standard compared to new build. No need for SAP calculation to show compliance as simple elemental U-values are provided. (SAP is needed however to produce the EPC on completion). In basement walls and floors the U-value is dependent upon the basement depth as well as insulation type & thickness, if too difficult to insulate BCO can relax the L1B standard.