Iceverge

Welcome, a mate is building with hemp/timber frame at the moment. Materials are about 5 times € vs dense blocks and EPS beads. It is much nicer to work with however and environmentally more sound.

@Patrick used closely spaced boarding instead of ply which I prefer TBH as any moisture under the Zinc has somewhere to dry to.

No sorry, thermalite Vs standard 7N blocks. Rockwool (other brands available) or EPS beads is practically the best solution here I would say. If they had wanted better performance then I would have widened the cavity. Given they're extending to a solid walled house going much better with insulation on a small extension won't make much difference due to all the thermal bridging interfacing with the old walls. They could put some insulated plasterboard on the internal walls including the "internal" wall that used to be the external one. All this pales into nothingness without a solid airtighess strategy mind you. Another option to materially improve things at this stage is decent triple glazing Vs double.

The difference in swapping to dense blocks is about 15mm of rockwool. The wider the cavity the less % difference the blocks it make.

No that'll be fine. Almost all the heat loss is stopped by the rockwool anyway.

Blown beads. Leave the cavity run as low as you can. Right to the foundation if you can get your SE to agree. Don't cross the cavity with the DPM. Have a separate DPC on the outer leaf. This will give some allowance for mortar droppings not to bridge the cavity above DPM as they'll just fall to the abyss harmlessly. Then fill the cavity with blown beads. If you have a line of aerated blocks in line with the floor insulation you'll get performance matching an insulated raft foundation.

Or don't bother with foam if you can manage it.

It could be done for the living areas where there's plenty of circulation with doors open etc. However for the bedroom's without ensuites they'll be a ventilation dead end when the doors are closed and would suffer high CO2 levels at night. Well planned the ducting is quick easy and cheap to install.

Kooltherm £££££ Another steak dinner for the Architect and a week in the Rivera for the Kingspan salesman.

If there is a continuous layer of mineral wool as per the cavity at the outside of the steel it'll be fine.

If bothered you can counter batten your service cavity internally at 400cc. You tube has shown 1200cc rafter spacing so I wouldn't worry about it falling down.

Mineral wool batts or blown beads. 250mm cavity here with blown beads. Have you seen the denby dale videos from green building store? Also @tonyshouse did a blog.

A big gas boiler and a thermal store would work I would say.

Interesting I hadn't seen that. I had it pictured as a variable intake duct.

I wouldn't bother.

Have you done the calculations on stainless steels ties? Over a wide cavity, the difference is tiny in heat loss but dramatic in money saved. We used these guys ship but I don't know if they ship to the UK. http://www.vartryengineering.com/products/extra-long-wall-tie-large-cavities.html

@JtG Have you measured your DHW use? Heating a large UVC with a cheap time of use tariff has the cheapest lifetime cost for low volume users with my sums. Heat pump DHW is only really cheaper over a lifetime for higher volume users. On another point, an ESHP (all in one) DHW heater can be set to extract heat from the house or the outside. I've not been able to find any accurate data on the COP of said tanks Vs inlet air temperature so I'm undecided which I better. However at full chat they will tend to over ventilate a house. A simple operable butterfly valve or two in a duct could make this switchable season to season and fine tune it for best ventilation/COP.

I have most faith in EPS. It is 98% air so suffers almost no thermal drift unlike other insulants. When clearing out some old abandoned sheds I found some EPS that had been outside, wet and tangled in briars and scrub for at least 30 years. It was in perfect condition. In a thermally and UV stable setting I think it would last forever. In fact it has crossed my mind as an environmental concern.

That would work too. I'm just exceptionally cheap.

This is what I mean. The notch will sit outside a 100 x 50 soleplate. Without the notch you'll not be able to "squeeze" the OSB sheathing into the sole plate. It'll just bend the OSB. The notch will create a 50mm gap outside the sole plate to stuff some mineral wool to break that thermal bridge. Despite what @saveasteadingsays I think It's important to keep the soleplate warm. Without it there's a risk of it getting cold, especially in contact with the slab and then you get condensation and rot. Not an issue with a permanently heated house but an intermittently heated garden room it'll save you in the long run. Line up all your studs on a bench and clamp. Snap a line and set your saw to 50mm and cut them "end on" along Line A . Set your circular saw to max depth/100mm and cut along line B. You'll probably need to do some tidy up with a hand saw but it'll be quick. Have a look at the detail above. I would use all 50mm sheets of EPS. Use 150mm under the main part of the floor and then thin that down to 100mm under the thickened edge of the slab. Also use 50mm for the insulation upstand. Lay the DPC as with plenty of spare over the edges. Lay your at the centreline (50mm) of the concrete on chairs. Tie the foundation bolts to the mesh in the correct place with some wire or cable ties. Pour your concrete. Level with a stick and float by hand. When it is set take your soleplates and use the hammer method to find the right hole location. Drill out holes and ensure it's correct/square. Notch your studs and build the wall as normal including OSB sheathing. Fold DPM over Upstand. Apply A/T tape for air sealing. Similarly tape all joints in OSB externally. Very effective and simple. Tape outside of doors and windows similarly. Apply breather membrane, batten and clad.

Have a look at this one I did earlier. You may need to notch the studs to fit over the wall plate but you'll have a continuous line of insulation from the upstand to the wall. Use at least 150mm screws for the bottom layer of sheathing so that they go all the way through the studs into the wall plate. You'll have a 100mm bearing on the concrete which will be fine.

Why not just wet plaster it. Solves a multitude of issues.

The vertical joint in brickwork/blockwork.

Beware water ingress via this route. At the very least the OSB sheathing should be flush with the insulation so that the breather membrane can lap over the insulation. Preferably outside it in my opinion. Hence my suggestion earlier of a 100*50 sole plate with 150*50 studs on top. The 50mm overhang will allow you to put insulation outside the sole plate too breaking the thermal bridge of the wall plate. For the doors, fit them that that they rest partially on the concrete and partially on the insulation upstand. This will sort your thermal bridge here. Are you committed to bi-folds? They are draughty. Would a french door with a fixed panel suffice? Cheaper and better IMO.

Block house here. 0.31ACH. Airtight paint to wall floor junction. Airtight paint to all chases. OSB window boxes taped to windows and airtight painted to walls. Wet plaster to all walls. Airtight membrane below cold attic continuously ran over all internal walls. Returned to wall and sealed with airtight sealant. Service cavity for ceiling. Airtight attic hatch. Our Tony Tray ripped under the hollowcore plank so I cut it away and sealed later with foam and airtight paint. DIY blower door was the secret though. It really showed up any faults. Things I would change. Pour the ground floor slab over the internal foundation walls continuously and continue the walls from slab again rather than introducing lots of separate areas for leakage. Use the green building store detail for plywood window boxes rather than fitting OSB ones post window install. Ust the green building store detail for a timber ledgerplate for the first floor. Pretape the windows before install. Tape wasn't that dear. About €15 per 25m. Much tidier than foam or sealant.