IGP

Blown EPS beads for me.

I would try to keep it as simple as possible, and without knowing the layout etc, if I was in your position I would personally; Go as big as you can without being daft on loft/roof insulation. It's cheap and simple. Fill the cavities where you have them with blown EPS beads. If you're going back to brick/stone anyway - perhaps the vapour open insulating plaster (diathonite?) would be sensible over slapping PIR over it and introducing interstitial condensation risk. Have MVHR - I would prioritize 24/7 air quality and humidty control over an occasional log burning from an non-room sealed log burner. Air-tightness generally is key as I'm sure you're aware. Foam and tape everywhere! Go ASHP, if you're having UFH anyway make sure they are 100mm centres for the lowest flow temperature possible and continuous heat. Don't worry too much about losing too much heat to the ground - earth is insulating (otherwise GSHP's with loops wouldn't work), and the priority should be designing the heating system to get the lowest flow temperature as possible get the efficiency gains here rather than spending *loads* on fancy insulation like VIPs (unless you have some *crazy* thermal bridges). Get a really good ASHP that runs v quiet, I'm going for a Viessmann Vitocal 151-A which seems to be super quiet, v efficient and also has an integral DHW cylinder. This may or may not fit the bill, but there are other options out there for sure. That's what i would do, as you can go as far as you like with insulation but it it becomes diminishing returns quickly and having an efficient ASHP setup can more than make up for imperfect insulation levels.

As other state earlier, don’t go PIR in cavity, all too easy to get wrong. Example thread. Full fill all the way be that mineral wool batts or blown EPS beads.

You could just PIR in this zone being careful to foam and tape to stop draughts coming up between the PIR and joists? The PIR is a better insulator than fibrous insulation, so you can go thinner for the same performance - keeping that air gap for those joists. Then just carry on as you were for the rest of the floor, that’s what I would do with the info you’ve provided so far.

100% this. Example of PIR done badly is here

Wow, that’s er not good in the politest sense… I wonder if you could drill through the wall and through the PIR, and using a really long toggle to allow you pull the PIR to the inner leaf, working from the bottom up. Otherwise, given that the PIR is loose inside the cavity, I don’t suppose there is an option of removing it somehow, then once done, fill with blown EPS beads?

It didn’t make a difference to the air temperature as that set to a steady 20c, but the floor itself was like 12c prior to insulation and now is about 19c post. Can happily walk on the floor barefoot and not be cold at all. We’ve got about 180mm of mineral wool for the insulation layer. Definitely put a vapour membrane on the top and tape to wall, stops moisture from the room air condensing inside the insulation layer. Do it once, do it right.

I did the Ecological Building systems approach in my 30’s semi detached, and it’s made the world of difference. No draughts anywhere, and a warm floor in winter. Highly recommend.

I’d love to have gone Enerphit just for the warm fuzzy feeling of being in a highly insulated house. However, I just couldn’t make the sums work, and ended up focusing on airtightness, loft insulation, CWI, suspended floor insulation, MVHR. No major insulation work like IWI / EWI though. It’s a 1930’s semi detached. Then, I’ve done my heat loss, upgraded all the rads to run at 38c flow at -3c OAT and getting my energy use down further through high SCOP of my heat pump I’ll be installing later this year. So, I’ll be going from 18,000kWh+ gas/year pre-renovation to 10,000kWh now and then to around 2500kWh electricity (assuming all goes well with the HP as planned) for heating and DHW. But horses for courses and people do it differently 🙂

Don’t zone if at all possible. Adjust the spacing of the UFH loops to match the intended design temperature for particular rooms while running a single low flow temperature.

Yeah the design is like something your mate takes home at the end of night of a few jars, after he hasn’t had anything in a while. I’m going Viessmann Vitocal 15X-A, partly performance, partly just looks better than that monster.

I did something similar but used foil backed PIR, foamed and taped. Then put a vapour check membrane over that and taped that to the wall, floor and windows. So far so good, but I have MVHR now so that keeps the humidity low during the winter.

I'm not so sure on electricity being a one-way bet, unless all you're thinking about its price cap/fixed tariffs. Taking Octopus agile as the most extreme example of the smart tariffs, it's clear that the average price is in the 15-18 p/kWh range if you can avoid lots of consumption between 4-7pm. Given that a HP would run low and slow all day, the vast majority would be at these averages (you could also do a setback during the 4-7pm peak). There are numerous other smart tariffs, each with their own pros and cons but they usually mean there are bargains to be had if you want to take them.

Which is why I’ve come to the conclusion that we probably should be only recommending 300mm loft insulation, CWI (if have cavities wide enough) and upgrading to DG from SG for *most* properties, unless you happen to be doing more work anyway and can insulate more while there. New builds should be much higher standards. These are the highest impact per £ invested. And then HPs at <45c design flow temp as that is around at the point where SCOP for most HPs beats the spark gap and saves money for people (and be therefore can be seen as better, not just equal to gas). But that’s just my rules of thumb.

I have the same, I installed between the MVHR unit and the supply and extract distribution boxes. You want to attenuate the fan noise of the unit to the house, not the the outside to the unit - unless i'm massively mistaken! Hope that helps