Great post @grahamh 

We're about to start renovations at home and looking to install an ASHP along with Solar PV, battery and MVHR. We will also be insulating our house as we live in a 1930s detached house that feels like it's similar to the churches you refer to! On insulation, we were planning to insulate externally to avoid reducing the room sizes, and it means we should also insulate the floor voids between floors that we wouldn't be able to do if we were doing internal insulation. Do you have a view (or evidence) of whether external insulation is superior or inferior to internal?

In the days when I grew up it was expected that most science 'knowledge' should be conveyed to pupils/students by the use of hands-on practical experimentation.
Oh - happy days! 😀 

One task was to evaluate the best hand-washing strategy using soap to remove bacteria.

We started by brain-storming the likely variables

• the length of time taken
• the temperature of the water
• the quantity of water into which the bacteria were to be washed

etc

Experimentation was done, hand-swabs taken and cultures grown.

Alas, there was very little variation between any combination of these variables.

Finally the professor suggested that we'd missed out a particular variable...
... the number of times that a washing procedure was undertaken.

And that transpired to be the most important factor of all.

Irrespective of all the other variables, it was the number of times that the hands were washed which greatly outweighed anything else.

Why?

Suppose any hand-washing cycle was around 90% effective....
thus 10% of the bacteria were still present.

Repeating that cycle would then remove 90% of the bacteria which had remained, resulting in just 1% left on the hands.

Repeat the hand washing a third time, and we have only 0.1% of bacteria remaining.

Watch a surgical team preparing for an operation and you'll see them putting this into practice.

The same is true for the task of keeping heat within the home.

Heat is the movement of particles.
The atoms and sub-atomic particles oscillate or vibrate.
Cool everything down to Absolute Zero (-273°C) and all movement stops.

The three categories of heat have different properties:

Conduction is the movement of atoms within a material.
In a copper pipe, an atom receives a 'nudge' and passes this on by bouncing against the adjacent copper atoms.

Convection is the movement of hot air. The atoms are further apart than those in cold air, which means warm air is less dense.
The lighter air rises until it hits the ceiling and then moves across it, losing energy to the plasterboard.

Radiation is energy being transferred as photons.
Unlike the visible light spectrum, you can't see this heat with your eyes.
It's in the Far Infra-Red (FIR) region, with wavelengths of 3–100 μm.

The best approach to home insulation is to use several layers through which the heat energy must travel before reaching the outside air.

Having 'light' aerated concrete blocks for the inner-leaf of the wall is a good start, but that's just one layer.

If you have a 1930s house (as I do), I then fill the cavity to prevent the movement of air.
That's the second layer of defence.

But both of those measures are more effective at reducing heat transfer by conduction and convection.
What about radiation?

The holes within the aerated block and cavity insulation are enormously greater in size than photons, which will pass through unimpeded.

The PIR (Celotex) insulation board which Graham mentioned at the start of this topic also incorporate a thin layer of aluminium foil.
That adds a defence against heat radiation.

In my own renovation project I've been identifying cold walls and adding PIR rigid board panels, held in place by a wooden framework.

The (scrap-) wood battens are deeper than the (2nd hand) insulation boards.

They don't get pushed back against the plastered wall behind them because the rear aluminium foil layer would then be in contact.
I don't want to allow heat to be conducted from the foil to the plaster.
So the few millimeters of gap prevents that.

The timber battens are then covered in (heavy) cementitious boards.
Fibre-cement boards are made by STS and Fermacell.

These boards have high thermal-mass which acts as a warm layer.
The wall surface retains heat which is being radiated from the underfloor heating.

Once again, there is a 3mm gap between the back of the fibre-cement board and the PIR insulation board.

Posted by: @nickmorgan

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Do you have a view (or evidence) of whether external insulation is superior or inferior to internal?

And you can probably now guess my answer.

There is negligible difference between these approaches.

Each one is merely adding one more layer of defence.

What you're actually looking for is a multi-level solution.

External wall insulation (EWI) is generally thought to be better than Internal wall insulation (IWI) because it keeeps the thermal mass of the walls warmer and it reduces the potential problems of condensation within the wall structure. My walls have but a single layer of insulation; it is a certified passivhaus.

Posted by: @nickmorgan

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We're about to start renovations at home and looking to install an ASHP along with Solar PV, battery and MVHR.

Nick - I have installed Solar PV, storage battery and MVHR, all using a DIY approach in my 1937 detached house.

If you raise any of these issues in fresh topics, please tag me.

As a self-builder who is also involved with future energy strategy, I don't get to read all topics here on the Forum.
But I would want to reply to you with photos, diagrams and lists of potential suppliers if you raised these subjects.

Equally, please post relevant photos of your house as it currently exists. 🙂 

@transparent thanks for the note. I'm new to this site so not sure if direct messaging is allowed or facilitated but it would be great to pick your brains on what you've done.

Yes, DM is available, although the line-formatting is naff. 😥 
Simply click on my name in blue.

You'll arrive at the info page for me.
Click on the envelope at the right hand side to open the message editing window.

However, we do prefer that discussions are open on the forum.

The idea is that:

• we can pool ideas across a number of forum members
• others who arrive in future with a similar issue can read what we've already discussed

Some of our input will depend on your own situation.

Be prepared to share with us your approximate location.
We can then respond in ways which reflect

• your electricity Distribution Network Operator (DNO)
• your Local Planning Authority (LPA)
• available local energy resources

@transparent understood, thanks very much. I'll create a new forum accordingly