A_L

No recommendations but here are some. https://renergise.ie/shop/energy-saving-products/ventilation/ https://www.partel.co.uk/products/lunos-ventilation-with-heat-recovery https://www.ambientika.eu/en/ambientika-advanced/4893273177373 https://cvcsystems.co.uk/our-products/brink-air-70/

@ProDave , @SteamyTea I had a Philips microwave that had a slowly rotating metal 'fan' below a microwave transparent floor which distributed the microwaves instead of a turntable

@RobLe,unless there is a heat source in the garage the insulation will do very little. Poor air tightness in the garage and uninsulated walls will strip the heat away. Is there EWI on the house/garage common wall? If there is there will be even less reason to insulate the roof as there will be less heat entering the garage

Bifacial (two sided) panels might be worth investigating, even a North-South fence? https://www.solarpowerworldonline.com/2018/04/what-are-bifacial-solar-modules/ just search for 'bifacial panels' for manufacturers

Yes, plus your view on energy price inflation ?

This will reduce heatloss by about 2-2.5kWh/m2/yr, so the question is, how much will it (cumulatively) cost to put 2-2.5kWh of heat into the heated space?

If you mean the bit of the gable which will become an internal wall then breathability is irrelevant. Insulation on the sandstone wall along the line of the new extension's perimeter would increase the path length for heat using the sandstone wall as a thermal bridge and increase the surface temperature of the sandstone wall in the region of the extension's perimeter, reducing the risk of surface condensation in cold weather

As a one storey building originally built the walls would represent 20-25% of the total heat loss, the ceiling would have been around 30% but even 100mm of insulation would reduce this by 85%. The windows would have been 15% max so new would reduce this by 50-75%. 75mm of PIR/PUR insulation on walls would reduce their heat loss by over 90% Assuming no interstitial condensation in the structure then 'breathability' is unimportant

The U--value of a wall with 100mm of EPS EWI but a clear cavity is about 0.21, with a cavity filled with silver EPS beads is about 0.16. So not an insignificant improvement, particularly if you can get it at a subsidised rate. The EWI will handle the rain /wind and a clear cavity unnecessary. There is also the possibility that the cavity is ventilated and you would lose some of the insulating effect of the EWI

The SAP software uses data from the Products Characteristics Database (PCDB) https://www.ncm-pcdb.org.uk/sap/searchpod.jsp?id=17

The energy use of cookers and appliances is not included in SAP Here is a decent overview, https://ggbec.co.uk/sap-an-overview/

@Cpd, no air gap increases the heat loss by conduction A 25mm air gap without reflective surfaces has a thermal resistance of 0.18m2.K/W, an air gap with a low emissivity surface 0.44m2.K/W. 25mm of PIR has a thermal resistance of 1.09m2.K/W and 25mm of EPS about 0.7m2.K/W. pages 19/20 of attached PDF BR443-October-2019_consult.pdf

@Eric, provide floor dimensions (or exposed perimeter), and floor area and I can calculate U values.

If you must have 0.15 then take a look at Walltite, and injectable polyurethane foam from BASF. A filled 150mm cavity will be around 0.14 but it will not be cheap.

@WGL , try the Energy Saving Trust, they maintain lists of suppliers in your area. You might even qualify for a subsidy (0800 444 202). If you are fitting EWI as well then there is no reason not to use blown fibre (e.g. supafil from Knauf)