Iceverge

What's wrong with both?

Meanwhile these are my problems and not your solutions. From my time on this forum, for your house I would recommend . 1. A 5kW monoblock ASHP. 2. A 300l ASHP UVC. 3. Hep20 push fit plumbing with the hot manifold preheated by convection above the cylinder. Pipes run in a radial fashion. 15mm for the baths, showers, and kitchen sink and 10mm for everything else. 3. In slab wet UFH in a single zone downstairs. 4. Electric UFH in all bathrooms. 5. Floor drains with waterless traps in every wet room for leak protection. G3 checks are simple. Just clean the filter, pressurise the expansion vessel and check the temp and pressure valves. I do my own.

The only satisfactory vented solution is a coil in tank thermal store in my opinion. I installed a 250l Maxi Pod TS to combine the oil boiler and perpetually running solid fuel Rayburn in my parents house. It works really well but needs to be kept above 65-70⁰. It would work equally well with a new build oil or gas system boiler. However...... In a low energy demand house, the high storage temperatures may cause localised overheating near the tank. Not an issue in an old farmhouse. An ASHP won't be able to provide high temp water at a reasonable COP for the store. You can compensate by having a very very big thermal store. Daikin have an off the shelf product, I can't see why it shouldn't work. We did a simple immersion in 300l unvented cylinder (UVC). It provides sufficient water for us on cheap night rate electric. It should prove very economical once I eventually install PV and a divert.

I'm not crazy about having PIR inboard of the studs. Firstly because they will be kept below the dew point for much of the year with the PIR keeping the internal heat away from them. This really limits drying for most of the year as temperature is such a large factor in evaporation. Concentrating only on the permeability of materials and ignoring the fact that a wall will readily dry inwards as well as outwards seems to be flawed logic, localised to our part of the world. Secondly because of the large daily variations temperature on a sunny day for the timbers. Look at these two diagrams and note the location of the layer of PIR. Without too much digging into it I would say that the long term stability and durability of the studs would be much improved by being inside the layer of insulation with a much more consistent temperature and humidity level. For my money something like this would be better. Dig into thermal bridging a little and maybe as build airtightness results to get a complete picture of what is realistically achievable with each method. Oh and for some reason I really want a sausage roll. Cant figure out why thats in my head.,......

Ubakus isn't perfect. It just predicts one set temperature for outside and inside. If for instance your buildup doesn't dry at -5⁰ don't fret as it's unlightly it'll be that cold for long enough to actually do any damage. On the structural side of things I think you'll need to have the OSB connected to the studs as it provides the racking strength to prevent them folding over like a house of cards. From a fire perspective I would prefer to see the OSB on the inner face of the studs or if it is external to them behind a good layer of fireproof material like rockwool or cement board.

Trusses are perfectly fine apart from the trickeries of making them airtight. Posi rafters or I joists would work fine. Weigh up the extra cost of them Vs sawn lumber however. A layer of something like woodfiber outboard with a fluffy insulation might be cheaper than an engineered product.

Sips are inflexible in a design sense and lightweight insulation in the roof is poor for heat protection and noise. They can work but you're tied into a crane and a chosen supplier. I would cut that roof and use blown cellulose.

Flexible sealant. It's dear though. You could probably get away with something like this.

Given the constraints of your circumstance you must decide where to compromise. In opting for a rigid U value target for a given width you may have missed the wood for the trees a bit. Say you get your approx 150m2 of wall to a theoretical 0.12 instead of a realistic 0.18. that's a difference of 0.06w/m2K 150m2 x dt20⁰ x 0.06w/m2K. That's about 180w in the peak heating load over say a floor area of 120m² it's 1.5w/m². At another guesstimate that's about 200kWh/annum or £32 per year. Now account for the poorer airtighess performance, material wastage during building, additional cooling load due higher decrement delay. Poorer noise and fire performance. Be very careful about putting PIR In a stud wall.

Don't put PIR between the rafters. Waste, fire, noise difficulty fitting etc. Don't do this, noise, fire, decrement delay all average Maybe, good for noise, decrement delay and fire but itchy to fit and doesn't do anything extra for airtightness. Do this. noise, airtightness, fire, decrement delay all good. Do you have any cross sectional sketches and I can have a look at a good buildup?

I have been wondering about this, Suppose you had a completely non combustible cladding like fiber cement, on fiber cement battens over rockwool. Would the cavity be mute then as there's nothing to burn in there? However as there is timber involved, ventilation behind it is vital to ensure it doesn't rot. I gather you protect this from fire by compartmentalizing the sections, normally with fire stops of some kind. However doing this obviously blocks the ventilation too. Unless the natural gaps between the uneven surfaces of the board would provide enough ventilation? What about 20mm over 20mm vertical fire treated cladding screwed onto 20mm horizontal battens at 600cc with solid blocks between inner cladding timbers to ensure no vertical paths Then vertical rockwool filled socks every say 1200mm to compartmentalise any fire to a 1200*600mm square. Then allow the natural small twisting of the cladding to provide a minimal amount of ventilation to each rectangle. Gut feeling says it would work and would also prevent fire from spreading behind the cladding. @G and J I was just thinking that's how vertical cladding is normally installed. probably an overkill and you could drop back to 1 layer of battens.

Mineral wool is mega itchy. Cellulose, not so much.

No issues making holes so long as you fill them again. A bit of mastic in the holes before the rawlplug would do nicely.

Have you had any more luck with the fire resistance necessity of the wall? I did some perusing. You'll need fire resistant cladding, which is treated wood or else fiber cement. Something like the one detailed at the bottom of the page below might be an option although I have no idea where you would find 160*60mm timbers. https://www.backtoearth.co.uk/news/fire-and-wood-fibre-insulation/ Not an engineer with any qualifications in such things but..... I would do something like. Fire treated 20mm on 20mm vertical lap larch cladding. 20 x 75mm fire treated horizontal battens over vertical 20*75mm strapping. 2*50mm rockwool rain screen slabs with joints staggered. Breather membrane 12mm cement board 11mm OSB, taped as airtight layer. 145*44mm studs with full fill cellulose 15mm fibreboard Skim Total thickness about 365mm. U value about 0.16. Maybe @Gus Potter or another knowledgeable person could suggest something or comment on my fire resistance guesswork.

I started with a bRegs basic house. Double glazing, natural ventilation, minimum insulation and 5ACH. I assigned a realistic cash value to every upgrade of the fabric. Obviously some were either there or not like the 3g and triple glazing buy others like airtighess and insulation I stepped up gradually. I then fed them into PHPP and looked at the annual heat demand reduction for each upgrade. I costed energy at the time at 10c/kWh and noted the payback of the upgrade. For example if MVHR cost €3000 but saved 1500kWh/year @ 10c/kWh = €150. €3000/€150 = 20 year payback. Anything that paid back in less than 25 years at the time got the thumbs up. It was a bit of an arbitrary target but I felt that it was at least a balanced and economic method of spending money.

How about a stick frame with OSB sheathing and a mineral wool layer before cladding? Something like rockwool frontrock external to a stick build? Sorry fire regs aren't my strong point. For a thin high performance wall, thermal bridging really does have a big effect as you can't compensate by just making everything thicker. A repeating 100mm stud will be much worse than a repeating 200mm stud for instance.

A grinder and paint makes me the welder I ain't 🤣.

Are you using blown in cellulose? If so I wouldn't panic about pin holes. In any case I wouldn't worry about nails holes with a nail already in them. We had about 600 screws through the airtighess membrane for our battened service cavity and blew 0.31 ACH

I'm not a massive fan of SIPs in general as they have issues with thermal bridging at junctions. If I was going to upgrade them I would probably opt for a layer of something like mineral wool to reduce this. How is your external skin going to be constructed? Brick/blocks or cladding hung from the SIPS.

Do the best you can is the only thing I can say. I used PHPP and my own spreadsheet to refine the insulation levels. In terms of money spent to improve energy efficiency it's a little further down the list than you might expect. From memory the best bang for your buck was 1.Airtightness 2.MVHR 3. 3g Windows 4. Roof insulation 5. Wall insulation 6. Floor insulation. The metric I used was payback years. EG an extra 100mm of attic insulation might take 30 years to payback and and extra 150mm might take 50 years. I started at bRegs and worked my way up year by year until everything was at 25 years payback. I stopped there as I was at passivhaus performance and a bit extra. At least I knew then I was spending our cash in as balanced a manner as possible. In the end I had 450mm cellulose in the attic, 250mm EPS bonded beads in the wall, 200mm EPS70 in the floor. I could have probably put 300mm everywhere but in our case it would have been dearer for the same result as the floor insulation was much dearer than the cellulose.

There is acoustic benefits to triple glazing too. Not to be overlooked.

Good air sealing is far more important than U-Value in my opinion. A neighbours new build, with MVHR and good airtighness . Excellent U values of 0.1-0.16W/m2K. However they opted for new sliding sash windows with brush seals. The house was drafty and sitting in the kitchen with the heating on in winter you would occasionally get a little shiver. Whatever you pick, make sure you have good compression seals, joined properly at the corners (not just loosely butt jointed). Preferably 3 layers of sealing and multiple locking points to ensure they squeeze tightly to all sides.

I would argue that absolute humidity tends to equalise rather than travel in any particular direction. Indoor absolute humidity tends to be higher and therefore the trend to migrate outwards. By keeping a layer of continuous insulation outboard of any organic materials like timber then it will remain above the dew point for almost all of the year and condensation cannot happen. As there's only space for 50mm of PIR in this case, below about 5 Deg there is a risk of moisture accumulating on sheathing/PIR interface. This assumes that room air comes into direct and regular contact with the sheathing AKA terrible airtighess. In reality in our neck of the woods we are very rarely exposed to prolonged cold temperatures and if you plan on doing a good airtighess job then the risk is truly minimal as ,like I've said before, almost all moisture gets into a structure via air paths and cracks and not through diffusion through materials. I'm afraid , probably quite realistically in many cases, you've assumed terrible workmanship. Flat roofs go wrong when people take a token (if even that) approach to airtighess internally, have multiple down lighters etc, and token offcuts of PIR jammed between joists as a insulation. We have a cold roof. 450mm blown cellulose at ceiling level. However the external breather membrane was fully sealed so there's no ventilation in the roof. This was only possible because of the really diligent job we did on the airtighess layer below the trusses ensuring there was no way for air currents to take moisture up there. The carpenters were all spitting with indignation when I asked them to do it, telling me our roof would rot . But 4 years on and the timbers are as dry as a bone. Hybrid roofs are also an option if you aren't an idiot installing them and will save a lot on extra long fixings the thicker insulation in a pure warm roof of the same U value.

Not necessarily, I would omit the inner OSB layer in any case as it's not needed and will just add to the thickness which you want to avoid. You may want some Plywood or noggins/blocking if you plan on hanging furniture from the wall etc.im certainly areas. This localised ply inner layer won't be airtight as there'll be many penetrations for wires etc so I wouldn't consider it much of a barrier of any kind.

I would limit any material of low vapor permeability to one layer in a buildup. This will aid "drying", either inwards or outwards. The important thing is to have a good airtightness layer somewhere. This will limit "wetting". It's a tricky concept to get your head around but so long as "drying" exceeds " wetting" you're sorted.