Insulated plasterboard

Is it an option to externally wrap the house? That will sort any heat loss air tightness issues.

Had it done myself mate. Know all about it.

yes the more insulation internally, vs whats in the cavity will move the due point, if moved enough you could wet the internal leaf and end up with mold growth behind the new board etc, this needs to be carefully considered before any decision is made

From memory it's one third. Say 60 mm in cavity so 20 mm max. internal. However you would be better advised to get it calculated properly. Even an email to the tech dept of the insulation manufacturers would get you an exact answer.

Looking at getting a Heat Loss Survey Done Similar to this video I would imagine https://www.bsria.com/uk/compliance/airtightness/ - The cost will be close to €800 (Seal up Ventilation/Add FAN to door (Simulate Cold/Windy Day) /Take Images) they simply provide Infra Photos and Advice! Would investing in a Thermal Imaging Camera that you can use over a longer period be a better DIY option? I would imagine that on a cold day like today Air Leakage would be easy to see with such a device ?

You could go that route but tbh in the absence of training and experience you likely will end up with a poor result (then again as I have been doing these surveys for nigh on 2 decades I would say that wouldn't I!). A TI camera is just a tool in a tool box, no different to a hammer or saw in a builders tool box but in the hands of an untrained/ inexperienced amateur ...

Any pointers on best practice to insulate north facing eaves ?

Prevent Cold Bridging ? What is the best approach ?

Thanks for engaging MicktheMan, Not doing EWI just yet, Just looking to get the attic Insulatated correctly via DIY. Currently, the Attic Insulation does not meet the Cavity Wall Insulation, While using the diagram attached as a reference my Attic Insulation stops approx where the green Arrow is , This detail (found on web) indicates that there should be continuous insulation linking the wall and attic insulation. Is that correct?

I am not sure of the detail of my Bungalow (built 2005/2006) as there seems to be some kind of cap/closer and I can feel rigid insulation coming up to meet the plasterboard but not sure about the rest

1st Picture is Soffit/Brick detail. I don't know anymore as I did not build the house.

I can feel rigid polyiso-type insulation meeting the ceiling plasterboard as indicated by the red box. Is this insulation likely to be the Cavity Insulation ?

Would I be doing the right thing if I join the attic insulation to the cavity wall insulation while leaving a 50 mm air gap?

What's your wall makeup from inside to outside?

Got the IR Camera , Cold Spots visible - Removed the existing insulation and replaced with 300MM , Massive Improvement. Thinking of Drylining External Walls with 2 Layers of overlapped 25 PIR then plasterboard then add a decentralised heat recovery unit in both bedrooms replacing the 6" Holes in the Wall. https://www.vent-axia.com/heatsave - What do you think ?

So are you saying that 62.5mm insulated plasterboard is not a vapour barrier and there is a risk of interstitial condensation ?

Based on your posts EWI 600mm below finished floor level is the only show in town , And even if someone asks about a blocking a vent you manage to shoehorn in that they require an Air Tightness Survey ! People are looking for actual advice and solutions. I have 2 problem North Facing Bedrooms , I don't want a whole house solution.

Really? Talk about being ungrateful. Maybe you should consider employing an energy consultant privately and pay for the advice.