Archer

Thanks for the suggestions, this is really useful. I've abandoned the idea of laminating something myself... Working out the number of panels that I'd need and it's a lot of work with uncertain results. The company linked above looks pretty much exactly what I'm after with the XPS and 4mm ply facings. The "Tekwarm" loft boards are similar but with PIR and a single 8mm OSB facing.

Ok, if it's of use to anyone else - there does still seem to be a breathable, perforated EPS insulation board available, which could be another option for me. Baumit Open Therm https://baumit.co.uk/products/external-wall-insulation/opensystem-breathable-ewi/baumit-opentherm

Yup, very drafty loft for sure. Between the rafters are already filled with mineral wool. We will be boarding 50% of the width of the loft and half of the length, so much more than a few feet. The risk I was imagining is warm, moist air from below rising and getting trapped on the underside of the insulation and possibly rotting the rafters. I don't know if this is imagined but there a few horror stories floating around online and surveyors recommending that foam type insulation is removed from lofts... I think problems are probably unlikely and rare and that I'm overthinking, but still. You're right though, PIR would be twice as effective thermally for the same depth of insulation. They used to sell a perforated "vapour open" foam board, but I can't find it online any more

It's an older 70's house so not a new build. Loft hatch is very limited in size. I can get all the timber and cork cut to size though

Again, really limited understanding of the structural theory... But I thought that the core material mostly acts like a web to keep the two face materials apart. Hence being able to use quite weak stuff like EPS. Cork insulation is quite strong in compression, but not in tension.

Merry Christmas everyone. I know this is a bonkers idea before I've even typed it out from my sherry addled brain. Anyway, let rip, I can take it. Thinking goes something like this... 1. Want to board a section of my loft for easier access and light storage; 2. Head height is very limited (about 1.4m at the apex; 3. Rafters are about 8cm deep at 50cm spacing (I think, need to remeasure) 4. Worried about adding too much weight; also want to make sure that any insulation and boarding is vapour permeable 5. Want the easiest DIY option possible - loft is not a nice place to work Now obviously loft legs with mineral wool underneath and 18mm OSB is the obvious choice. However, I've been idly thinking that finding a way to screw an insulated panel directly to the rafters would be more straightforward than faffing about with the loft legs, balanced between rafters... There are PIR options for loft floors with foam insulation laminated to a single sheet of 18mm OSB. The weight of these is 12-15kg/m2 so I might only be able to safely store another 5kg doing this... Also, I'm not sure how rational it is but it feels risky to encase a section of the rafters in impermeable foam in case moisture vapour from below rises and gets trapped. (Any thoughts on this risk?) So... I've been wondering whether, structurally speaking, you could DIY a breathable "Loft SIP" by laminating 2x face sheets of either 3mm or 6mm hardboard either side of a 80mm cork insulation core using D4 adhesive or something similar. 3.2mm hardboard is obviously incredibly flimsy by itself, but am I right that in a sandwich panel, the thickness of the face material is less important, because it's acting like a flange (ie. in a metal SIP for eg.). The panels would only need to support the weight of a person across a pretty short span... Still, don't want people dropping though the ceiling! Is there any straightforward way to calculate this or is it just flat out a terrible idea? I was thinking to mock up a single cork SIP panel with some offcuts and test it but happy to get some science before going further! Cheers

I'm still trying to get my head around trying to size the inverter. Our ASHP (12.5kw) says it's power output is 4kw or 19amps. I'm assuming that would be the max if it's working flat out for a period? And then we'd want to allow for an electric oven/ induction hob (3kw?!) and maybe some spare capacity for adhoc things... But an 8-10kw inverter seems much bigger than almost everyone has so I'm assuming that I'm misunderstanding and most appliances only draw the peak for a very short time, is that right? I'm getting some expert advice now, but it seems to be that 5-8kw inverter is probably the sweet spot for our usage. Does that sound about right?

Good luck with it all however you decide to progress. Our house is also 70's and we ended up keeping the ground floor steel ducts and adding a second zone in with new ducting and vents from the ceiling in the first floor. Obviously with heat pumps the supply temperature is much lower than gas warm air so you need to run the system for longer essentially to achieve the same input of heat. Most of the time this is beneficial because you can get the same level of comfort at a lower set air temp when the system is running. Ours feels a tiny bit underpowered when it's really cold. It still gets up to temperature but it takes hours to do so. Only seem to notice this at freezing and below, I think because of the heat pump defrost cycles and generally greater heat loss ( our house isn't that well insulated) I'm still not entirely sure how we got it wrong. The A2A heat pump itself is oversized so should be fine. My suspicion is that the 2 indoor air handlers are too small because the upstairs duct work is custom made so unlikely to be the problem. Either way, it's a bit annoying but otherwise the retrofit has worked fine. Much better than the 70's original gas system we replaced.

This is all really helpful stuff, thank you. Just to add that I think the restriction on VAT to solar installs has been lifted - at least according to this. All battery installations are now exempt (is this right?) https://contact-solar.co.uk/blog/uk-government-scraps-vat-on-battery-storage/

I'm normally sceptical but Fogstar seem to be well rated for budget batteries. Uk customer service, long trading history, lots of previous customers etc. Not as dodgy as it seems. But also riskier than getting a big brand through a well known installer

Definitely getting warmer I'd say. By the time you've paid for an installer and with real life / non Nirvana usage, it would likely be a 5-6 year payback. But that's decent given it has an 8 year warranty and should last 10+ Feels like a good deal, but still brings me back to trying to work out how to get a decent installer who would fit one and know what they are doing to get me started on the energy management side.

That's the one! Well a chap on Money Saving Expert reckons he's got an "all in" price of £4.5k and that was based on buying the Fogstar for more (£2500). Feels quite daunting to find an installer though unless anyone has suggestions https://forums.moneysavingexpert.com/discussion/6457277/fogstar-home-batteries-domestic/p5

Also, another thing I don't understand - from reading other posts it seems there are efficiency losses between the grid, your battery and your home. Is it easy to quantify what these losses are? I've seen people use 10% to 25% which is quite a big range

No, this is what I needed to understand and is really useful. All of your risk points are totally valid. There is also the question about whether the technology now gets superceded quickly by something better/ cheaper and also whether increasing adoption flattens out the difference between peak & off peak power demand. You can get interest free finance and 10/12 year warranties which reduces some of your risks above but obviously not completely. I'm not sure if I'm allowed to link products in forum rules, but there is a 15.5kw Fogstar home battery with great reviews that is currently retailing for £2000. Changes the payback to 4-5 years but (allowing for the inverter and capable electrician to install). Does anyone have any experience with DIY installation - it's definitely beyond me but I'm not sure how to go about finding someone who will be comfortable installing this sort of kit...??

Putting the environmental arguments to one side for a moment, I could use some advice on how strong the business case is for home batteries in our circumstances. Our house is not well insulated by Buildhub standard - it's very much average for the UK stock with around 70mm mineral wool in walls and floor and a better insulated attic. We have a heat pump running heating and hot water but no solar PV (partly because of shading and partly cost). I estimate on our current tariff we're spending around £1300 a year on electricity at 23p/kwh and perhaps around an average of total 16kwh per day (if my calculations and understanding are sound... always a danger). Installing a 13.5kw home battery seems to cost around £7.3k with an advertised saving of £900 per annum which is maybe a bit optimistic but feels broadly in the right ballpark (if we can get most of use through the battery at a time of use tariff around 8p/kwh) So perhaps a 9 year payback on the investment? Not spectacular but pretty solid and with the added benefit of some protection in the event of a powercut. Advice appreciated - are my logic and sums above ok... what are the gotchas here?! Compatibility with high demand heat pumps, risk of the future Time of Use Tariffs being less generous, sizing the battery for winter peak demand or summer use...? Thoughts appreciated