It's easy to say that suggests you need some insulation between your garage and the room above it. What would probably be more sensible is to prove it. I'd say it's worth you getting hold of a thermal camera (hiring is fairly reasonable, I believe) and see where your cold spots are inside and out. We actually bought one so we could lend it out to friends and family too, but when we did it provided some surprising results.

Things we thought would be a problem were in fact fine and one thing we didn't even think of (our front door) turned out to be abysmal. Sorting out what we found before getting heat pump people in to look around and quote turned out to be a REALLY good investment.

@majordennisbloodnok 

Thanks for the reply.

I do have a thermal camera so will get some images with it.

The room has 1 external wall which now has beaded CWI installed. The window in that room has been replaced with a new aluminum framed double glazed unit which has been installed with compriband/foam and membrane.

2 internal walls have rockwool in, more so to reduce noise transfer. The UFH sits on 100mm PIR.

The remaining internal wall is a block wall dividing us and our neighbour (semi detached property).

And finally above the room is 300mm loft insulation.

Always worth insulating the floor from the garage underside if there's reasonable access. If you store much in there, including cars, I would fire proof the floor as well. Inert mineral wool or non-flammable insulation combined with 13mm dry lining + skim or 2x13mm staggered joints would work. Cost is minimal for the thermal and safety benefit it will bring.

Ground floor heating distribution can be compensated for with larger or additional rads, not quite so easy if already UFH. If you have UFH with an oil boiler you'll probably be heating water up way beyond the temperature you need under the floor, and then blending it back down with mixing valves or a manifold. That's thermodynamic lunacy and very wasteful. Older boilers often have a minimum leaving water temperature and can't be set to a suitably low temperature to avoid blending with cooler water for UFH. An ASHP would heat the water to the flow temperature needed automatically compensated for outdoor air temperature.

The 135mm crs could be problematical for a lower UFH supply temperature, but an experienced designer would advise on that. With a reasonably insulated home, say EPC C, you don't want to be designing an ASHP for any higher than 45degC maximum flow temperature. Ideally nearer to 40degC with UFH. Ditch the zone controls, ditch all the blending valves, upgrade the rads where needed, insulate whatever you can, fit an ASHP on open circuit control properly designed and installed, and you'll not regret it. 

@allyfish 

hi and thanks for the reply.

So the floor in that problematic room has 100mm PIR underneath already. I can get access and will add as much as possible extra.

Ground floor is being renovated starting next month and plan here is to have the existing slab taken up, and dug down enough to install 200mm PIR, then UFH stapled to that (100mm centres), then 100mm concrete. Finished floor here will be tiles.

At present we only have heating on the first floor (UFH) and the oil combi works for that via a buffer. This is just to reduce the short cycling of an on/off oil combi. And yes at present the heating water is taken from that and blended down.
Aim is with an ASHP installed, we'll get rid of the blending valve and have the temp set by the ASHP via WC.
Also at present it's zoned but with the ASHP I'm hoping this is gone and it's just run as a 1 loop over both floors.

I know the house didn't have an EPC of C when we purchased in 2019 but it hasn't taken into account any of the new work.

Posted by: @vala

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I know the house didn't have an EPC of C when we purchased in 2019

Great stuff - sound like to you a very good plan. EPCs can be notoriously inaccurate. It was only until quite recently the installation of a heat pump could actually make an EPC score lower due to the absurd way EPC SAP software used to consider ASHPs as direct electric heating with 100% efficiency. They finally fixed that, not before time.

Posted by: @vala

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...

I know the house didn't have an EPC of C when we purchased in 2019 but it hasn't taken into account any of the new work.

...

Just to say that we bought our house with an EPC banding of D. It now has a banding of B and is within spitting distance of an A.

As @allyfish has said, EPCs are notoriously unreliable, being largely at the whim of the assessor who is trying to shoehorn what they've discovered into box-ticking forms. Nonetheless, if you ignore the rating itself and look at the areas being assessed it's not a bad checklist of stuff to consider.

@majordennisbloodnok We received a rating of C (72) with potential to be a  B (83) if fitted with solar water heating and 2.5 kWp. of PV panels. We have more than done this with 8.1 kWp. of PV and of course, we have fitted an ASHP, battery and so forth. I think that by that means of assessment, we are now an A. They did not even allow for under-ground floor insulation at all. Regards, a smug Toodles.

So just to update this thread, we have a HG coming over next week to complete a heat loss survey. I don't know/doubt if they'll spill the beans on how to get the house heating up to the target temp, in all rooms, with it all being on 1 loop. What puzzles me here is I'd assume they'll use the Vaillant controls such as the sense comfort, however this surely will have a thermostat in it and will shut off the heating once this hits target temp. And in which case, how can I be sure that the problematic room on the first floor is also up to temp?

Does anyone have any ideas?