I have two offers for EWI on the table:
- One company offers to do a thorough job at reducing thermal bridges: going up to the rafters with the EWI which means redoing the soffit, insulating the 50mm wall bit around the window perimeter with 15mm thick PIR, insulating below the DPC.
- Another company is reluctant to do any of the above (it does not fit their standard programme). Their offer is however 7k cheaper.
I would appreciate some help with understanding how much "doing a proper job" at eliminating thermal bridges would really benefit the house insulation overall...
Opinions?

Thermal bridges may not loose a lot of heat in quantity but, if you leave them uninsulated while insulating elsewhere, they can represent a significant percentage of the heat loss. But the number of years to recover the 7K additional investment could be a lot, though many people wouldn't hesitate to spend a lot on double glazing that may have an equally long payback period. In fact the double glazing I've just put in would have a stupidly long payback period if I were to calculate it...

It's also possible that eliminating the thermal bridges may be enough to boost your score in a future more sensible EPC rating, which could add value to the house.

Aside from finance, if you have issues of condensation on the window reveals, it's possible that strip of insulation could be enough to stop it appearing.

More generally, I'd just be wary of a company that skips the trickier parts. Their reluctance to go further could be a recognition that they or their workforce aren't trained in why/how to do it, or could be a symptom of corner-cutting.

Either follow Jonti's advice (a good idea anyway), or look for a 3rd quote so you have 2 comparable ones to choose between.

''More generally, I'd just be wary of a company that skips the trickier parts.''

Yes, this is so depressingly common!!

I would say make your choice about the roof end (have a look - if you can do so without having to pay for it - at what the PAS 2035 standard says) but look very carefully at what the cheaper contractor intends to do at the eaves. If you have insufficient oversail it is 99.5% certain they will be proposing to use an 'industry standard capping detail' - a piece of bent alu usually 'gunged' in silicone, with a 'downstand', not an 'upstand'. Make your own assessment of whether this will provide a 25-year plus weathertight seal.

At the top and the bottom (roof and dpc) look at levels. I have seen countless jobs where the EWI has stopped *below* the bedroom ceiling level, and I have *never* seen anyone do the sensible thing and cloak the thermal bridge internally with a 'downstand' of IWI (except me ;-)).

If, perchance, there is room to use an 'upstand' alu capping detail things are not so gloomy. You still have to see whether the EWI covers past the ceiling height of the top-most room, but if you have room (enough brickwork above the top of the EWI) to use an upstand you can do a proper flashing detail.

Equally I have seen injected dpcs where the injection is just above internal floor level. Along comes EWI contractor, gives it 'space' (leaves 150mm above the injection line before fitting his base rail) and you have 175 - 200mm un-insulated, and worse, it may be hiding behind (and ultimately rotting, via condensation) tall skirtings.

And above all, think whether the finished article passes the Nick Parsons Cardinal rule: 'The end result must not look stupid'!

Actually doing things right is priceless!

If you (and the cheaper contractor) want examples of poor detailing just put the search term 'Disastrous Preston' into a search engine.