Posted by: @iancalderbank

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there are no other heat sources

Don't forget people are heat sources. If you had a number of house guests, you had a number of mobile radiators/convectors... Estimates vary as to how hot we are (sic) but a commonly quoted figure is we are rated at 100W so if you had ten people in the house that's the same as a 1kW heater.

I agree, empirical evidence is always better than theoretical predictions. I also agree with @kev-m, even manufacturers' engineering data can be optimistic. The way to spot whether defrost cycles have been factored in is to plot the output against ambient temp, and look for a dip in output around 5 to 0 degrees ambient. If there is one, that is probably the defrost cycle effect showing up, meaning their effects have been included. Here's a quick plot for my 14kW (ha ha very funny) Midea heat pump at 50 degree LWT. It's not perfect, because of course with weather comp you don't have a constant LWT, but you can see a dip around 5 degrees either side of zero ambient, meaning defrost effects probably have been included: 

Here's another way of looking at the data, from a chart I did a while back. It's a bit harder to get your head round, as it plots output against LWT for various ambients, but the clue to defrosts is in the gap between the drab olive 5 degree line and the mid blue zero line. That gap is the effect of defrost cycles kicking in:

Posted by: @jswhite

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@derek-m 
I've been trying to think of a solution to this. Apparently the inner wall and outer stone is not suitable for cavity insulation. I was wondering if I could line the inner wall (and ceiling) with kingspan or equivalent straight onto the current plaster and then finish off with something a little more attractive. Half the room is already well insulated but the other half might benefit. We would lose width on a 3.3metre wide room but could probably live with that if it made a difference.

I'm not an expert when it comes to insulation, so I would suggest that you talk to a suitable company. Though more expensive, external wall insulation would probably be better, since the walls then form part of the thermal mass.

@jswhite you can buy insulated plaster with vapour barrier built in. However its best to install a vapour barrier if you want to insulate a wall like yours - it'll prevent condensation and will allow you to insulate the room on the inside. 

FASTBUILDSUPPLIES.CO.UK "https://www.fastbuildsupplies.co.uk/knowledge-hub/does-project-need-vapour-barrier/"

Vapour Barrier Membrane | Moisture Barriers - Fast Build Supplies

We offer a wide range of vapour barriers at the best prices. Next day delivery available.

Somehow I managed to screw up the x-axis in the second chart in the post above - here is the chart with the correct x-axis labels (can't edit the previous post):

Posted by: @jswhite

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I was wondering if I could line the inner wall (and ceiling) with kingspan or equivalent straight onto the current plaster and then finish off with something a little more attractive

I hate to be that person, but please do consider the vapour-openness of your walls. Sealing in the moisture with kingspan or a vapour barrier or even gypsum plaster could lead to issues down the line. There are insulating lime plasters (made more insulating with cork, glass bead or hemp), but are intrusive, time-consuming and expensive to install.

Posted by: @chickenbig

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Posted by: @jswhite

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I was wondering if I could line the inner wall (and ceiling) with kingspan or equivalent straight onto the current plaster and then finish off with something a little more attractive

I hate to be that person, but please do consider the vapour-openness of your walls. Sealing in the moisture with kingspan or a vapour barrier or even gypsum plaster could lead to issues down the line. There are insulating lime plasters (made more insulating with cork, glass bead or hemp), but are intrusive, time-consuming and expensive to install.

 

If problems were to occur in the future, could MVHR help to resolve them?

Posted by: @derek-m

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If problems were to occur in the future, could MVHR help to resolve them?

For the moisture inside the house? I suppose so.

GREENBUILDINGSTORE.CO.UK "https://www.greenbuildingstore.co.uk/information-hub/internal-wall-insulation/"

  seems relevant!

I'm annoyed I didn't install MVHR when I had the chance. Retrofitting would be expensive and annoying.