Iceverge

Just use the thickest layer you can. 100mm PIR is normally good value. 200mm might cost the same as 150mm if you can fit it.

The best windows with the best install are typically still worse than the worst new build wall by about a factor of 4. Depends on your window frames. I would leave say at least 10mm of the frame exposed. Beyond that fit as much insulation as you can.

As soon as I posted it I regretted the level of smut I had sunk to. Clearly you've rubbed off on me.

Largely an airtighess and ventilation issue. Full fill fluffy every time for me in this regard. EWI boards should be foamed at the perimeter and at all joining faces for install too.

PIR + plasterboard or insulated plasterboard to the head jambs and sill.

More details needed. Is it for a shower or a foundation or flat roof or something else?

This stuff. It won't do huge holes, and may need a second hit for a deep one but it finishes well and seems to last for ages in the bucket.

I would say it's very unlikely that you'll be anywhere near the foundations at that level. It'll be more digging out but it's worth it I would say. Consider something like seconds&co for a deal on PIR boards. I would work on 200mm as a good figure as 100mm boards typically have a good price to depth ratio. There's no rocket science to airtighess. Basically just seal up all the gaps.

What type of windows are they? Inwards opening or ouward openers?

Welcome welcome. Sounds like a good plan. I would the only thing I might focus on more is Airtightness and more insulation under the floor. 200mm PIR or 300mm EPS would b a good level.

That's over double the price in Ireland.

4 years. I haven't inspected the inside of the pipes mind you.

Airtight paint. I love to see it!! Use something capable of taking a screw. Our builder just put in plaster board and it struggles with curtain rails.

Didn't bother here. Haven't noticed any issues.

Pir in cavity wall. Nope. Use mineral wool batts instead

Great idea @ProDave. Personally I would be aiming for level or close to level with the new beam if I could without hurting the old structure too much.

We have MVHR in the house a PROAIR 600pli unit. Filters are not cheap at about €22/pair but the unit is fine. I change them every 6 months. I did dismantle it to give the insides a clean after about 2 years. I think I may do the same and change some bearings this year after 4 years operation. I wouldn't say it would be unreasonable to allocate £100/year in maintaining, servicing and running the unit if you were outsourcing the job. It recovers many times more than that in energy for us but more importantly it gives healthy and comfortable internal air. The dDCV I have out in the office is a plug in, pick your setting and completely forget. No heat recovery but then it was about £60 from eBay. You can hear it if you are beside it. I would compare the noise to a PC fan on tickover. I wouldn't put it beside a bed but it would be inaudible in an adjoining room.

Perhaps ask for a quote for both for interest.

It's the sealing that I'd be concerned about. Old fashioned sliding doors rely on brush seals which are completely useless. You can get lift and slide or a straightforward French door which are all compression seals and perform much better.

Another example of Buildhub being a decade ahead of Industry.

Those CV2GIP fans are excellent. Thakd for the tip @JohnMo Thinking about the categorisation I would split Mechanical extract ventilation (MEV) into 3 categories. 1. Intermittent extract fans such as those annoying noisy bathroom fans we all hate. 2. CEV continuously extract ventilaton that run at a constant level all the time. 3. DCV Demand Controlled Ventilation that adjust the a airflow according to demand based on CO2 or humidity. Then add a small "d" to indicate a decentralised version. I would say the Greenwood CV2GIP is a dDCV. Would you agree/disagree?

Welcome welcome. Nice to have someone from down under and to learn from your particular situation. A friend of mine built a hemp Crete house recently but from blocks in Ireland. Lime render 75mm hempcrete blocks 170mm hempcrete batts in a stick built timber frame. 75mm hempcrete blocks Lime render. I don't know if the blocks are available on Oz but it was less labour intensive than shuttering on site hempcrete. For the TF have you considered stick built on site? I like the hammer band they use in Sweden rather than a double top plate. Maybe it would solve your lintel issue. See 9mins20secs into the vid. https://youtu.be/4nRz0D0bNmg?si=qOs-IhmdRxzuLNad

Good ones that won't be draughty will cost a fortune. Most of them are rubbish at air sealing. I would get french windows.witb fixed sidelights. They'd be fraction of the cost and seal much better.

Those flexible silencers aren't great. I would use a rigid metal one in preference. Another option if there is no solid wall for the MVHR unit is just to sit it on the ground floor on rubber feet or a matt. Mechanic ventilation with or without heat recovery is always worth it regardless of airtightness levels. True it won't do much on a leaky house on a windy day but it will serve a valuable function on days with no wind in keeping the interior healthy. dDCV is cheap and easy to install if you can live with out HR.

Sounds enormous, too big I would say. I know of a couple of circumstances where people built a "garage". Say 5.5m x 8m externally with a converted attic above. 2 rooms upstairs and 2 downstairs and a w/c shower too. 70m2 total. Specs at a guess are 150mm floor insulation, 150mm EPS beads in the walls, Second hand uPVC Double glazing and 200mm mineral wool between the rafters. All within planning but not built to current building regs. However it was wired and engineered safely. In any case better than almost all rental properties. Given how hard it is to find housing here and the need to get kids started in local schools etc it was perfectly understandable in my mind. When the main house was finished then they moved in there, used the garage as storage and used upstairs as an office.