First things first.
Measure your daily energy usage for the next couple of months. If you can work out your water usage as well, even better.
Then it is relatively easy to work out what you really need.

As for Solar Thermal (ST), even self fitting and maintaining is probably going to be more costly that a MCS installed Photovoltaic (PV) system. Self installing PV may work out even cheaper.
It does depend on your roof area and angles though. PVGIS and the Energy Saving Trust websites are the places to play about with setups and see what you can get from them.

Most of us are wary of pellet boilers, with or without RHI. There is a boom in them at at moment, but if the RHI is reduced too much or even removed, expect pellet prices to increase. They are already at about the price of natural gas (does the IoM have gas?). Also they are anything but environmentally friendly or carbon neutral.

Have you looked into Heat Pumps (HP), either air source (ASHP) or ground source (GSHP). With ASHP the main thing to remember is that they frost up, so even if they do claim to have a decent Coefficient of Performance (CoP), they have an even better one at a lower output temperature.
Oversize is the rule with ASPHs.

Anything with the word 'foil' in it makes many of us nervous. There is a whole, and very long thread, on here about Multifoil Insulation (MF, sometimes refereed to as the M Word). It is still rumbling on many years after it was started.
Realistically, 6mm is not going to give you much in the way of insulation.
Thermal Conductivity is measured in W/(m.K).
So if you take the best that Wikipedia has to offer, which is Perlite in a partial vacuum, it has a value of:
0.00137 W/(m.K)
So with a bit of algebra, a 6 mm layer of that will allow 0.23 W of energy to pass though it for every degree difference.
If the floor temp is 15°C and the plates are 35°C, then that will be 4.56 W/m^2.
50 m^2 of that will use nearly 3 kWh/half day in just losses.

So can you insulate more underneath it?

Also look at your airtightness, the more air that leaks out in an uncontrolled way, the more you have to heat to replace it.

(It is late and I may have got the sums wrong, others will come and check them I am sure).

New windows and external doors are going in.

I'll get everything as air tight as possible. Seal every gap I can possibly find?

As a heating engineer keen to use his skills, you won't like this, but ...

The goal of real eco-building is to arrange the fabric - walls, roof, windows etc - to do all the work of keeping warm in winter/cool in summer - with the aim of doing away with all the fascinating machinery - boilers, rads, heat pumps, biomass etc. Solar panels (wet and/or PV) are something else, because the fabric can't take care of hot tap water (DHW) or electricity generation.

PH or near-PH-standard insulation, airtightness and 24/7 mechanical heat recovery ventilation (MHRV) cuts spaceheating demand to a tenth or less, so that calculated solar gain, body heat, cooking and lighting can provide all the spaceheating load, for all but a very few extreme days of the year. Your existing crappy space heating system, even if it's an ancient floorstanding iron boiler, can provide that - a tiny top-up just a few days of the year - as well as contributing to DHW when the sun don't shine.

So save your money on machinery, don't upgrade your heating, spend it on insulation, airtightness and MHRV. Of course you can replace ugly old rads with fewer, much smaller ones.

Cavity insulation is better than nothing, but nowhere near - 200-250mm of black EPS external wall insulation (EWI), incl the system's thin-coat render, all over your walls.

Roof insulation can be at loft floor level, but much better to take the tiles off, skin with OSB glued and screwed airtight, more EWI over that, uniting with the wall EWI; replace breather felt, cross-battens, tiling battens and tiles a bit higher up, incl increasing eave overhang if nec to cover the top of the wall EWI's thickness.

You end up with an unbroken tea-cosy of insulation (and, to the roof, air tightness) dropped right over the fabric's exterior - highly effective for the money and providing opportunity to improve the building's look and 'styling' while you're at it. The existing masonry becomes entirely part of the interior thermal environment.

New made-to-measure European triple glazed windows are incredibly cheap, £350/m2 or less, fully finished, from e.g. Russell Timbertech, Munster Joinery and a few others. They are mounted outboard, not touching existing masonry etc, within the EWI zone, giving you lovely deep window cills.

Don't rip up your floors - dig an external trench right round the house, down to found bottom (but abs no lower) and run the wall EWI (unrendered) right down to there. Put a perforated pipe as french drain in the trench bottom and backfill with Lightweight Expanded Clay Aggregate (LECA) topped with topsoil. That way, your whole structure is guaranteed bone-dry from the ground, and you've created an insulation 'coffer dam' enclosing the block of subsoil (and any crawl space) under the house. It creates a long path, through the subsoil, for any heat loss down, curving outward and back up to the external surrounding cold ground surface, which is the major heat loss from any floor (straight down much less so). Even subsoil becomes an adequate insulator, given that long path length (insulation 'thickness').

So, Fabric First, join the campaign to make your machinery unnecessary (except solar panels).

Yes, and there other details to mention.

Didn't we agree that just the 'nylon' render mesh reinforcement (which someone said is in fact glass fibre) wrapped over the u/ground EPS (held in place by dabs of base coat) would deter the varmints?

Posted By: fostertom
Don't rip up your floors - dig an external trench right round the house, down to found bottom (but abs no lower) and run the wall EWI (unrendered) right down to there. Put a perforated pipe as french drain in the trench bottom and backfill with Lightweight Expanded Clay Aggregate (LECA) topped with topsoil. That way, your whole structure is guaranteed bone-dry from the ground, and you've created an insulation 'coffer dam' enclosing the block of subsoil (and any crawl space) under the house. It creates a long path, through the subsoil, for any heat loss down, curving outward and back up to the external surrounding cold ground surface, which is the major heat loss from any floor (straight down much less so). Even subsoil becomes an adequate insulator, given that long path length (insulation 'thickness').

It would be as well to know what the subsoil - or rock? - is for 1st 2 or 3 metres down and how wet it is down there to get at least some idea that it will actually provide a sufficient improvement in insulation to warrant the effort. I've an idea it rains a fair bit where you are .

So handy this forum. Thanks for helping me.

How are the Windows fixed into EWI? Or more relying on the reveals holding them tight?

Quoting FT :-"Mike's caution is I think about whether the 'coffer dam' insulation idea will work very well, or merely be a significant improvement, depending on whether u/ground water movement will take some, or a lot, of the stored heat away"

Er well ... it's also about whether it might be a waste of effort for an insignificant improvement. The IOM house could easily be on a metre of soil over moving groundwater or runoff. For a 100m sq slab I calculate the average u-value, if the soil is dry enough to have a conductivity of 1.0 (in the customary units!), would be 1.23, and adding a vertical metre of insulation 375mm thick would reduce that to 1.10.

If that is the situation down below or something like, and it was me doing the digging I'd be looking hard at relaying the floors with insulation under.

Someone on the rock must hire out mini diggers suitable for getting through a doorway if you fancy taking the slab out.. I know my brother did it with his place down south ..

Gas prices on the island are high indeed, something to do with a scandal a few years back involving disappearing funds/directors but I don't recall the details.

Very much doubt they have MCS anything, RHI etc.. It's like a version of England from decades ago. One look at the permitted development regulations for example, and you just know that the house of keys is a group of farmers banging out laws by writing a few notes up based on what the uk has done recently :)

Don't know how crazy you'll go on insulation in the end but I can't imagine that it's cheap.. Either wait til bnq have a sale on(uk sets their prices) or look into whether it's economical to get secondsandco.co.uk to ship to Heysham and have graylaw pick it up.

If the outside ground level is say 300-400 mm about the footing along part of the house, is it worth EWI the plinth from the Dpc downwards? Ok it has wind and rain at the wall but will it lose as much heat as say the back of our house where the ground level is only 100mm down from Dpc.

Cheers

Remember that the thermal bridge at the bottom of the wall is also reduced with downstand perimeter.