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That's a good point and something that I thought about yesterday. If I go for a mix then I can gain some decrement delay/noise insulation benefits.

I think I will do now. I focused of Nyrock as it comes in 150mm, so two layers would nice fill the 300mm rafters.

But does it in the real world. Every job i see they seem to put the flashing in after. If dpc is on brick below is it actually going in underneath

Never heard of them. Are they limited to smaller systems? Why do you have 2 units?

Me too. I don't like grey wood. I've done this loads of times. We used Sika stain on treated timber. That's 2 layers resisting wasps on the outside, and still 1 on the inner faces. The treatment had to be one of the 'not green or dark brown" ones: your BM will get it in if not their standard.. To keep it looking original we used the Sika 'light oak' colour which you can barely see when applying it. But the timber stays that colour, maybe becoming yellower, and is very long lasting. (Dark colours fade in sun over 10 years). Perhaps the next shade towards brownish is better for rural effect. We didn't try other brands. NB paint overlapping boards before fitting, to allow for movement. I've had wasp nests in the attic and hate to think where their harvest is from. They will also chew pir to form a cavity but that ends up as dust.

You'll get more understanding on BH, not necessarily all agreeing, than from the industry who just do what they do, and repeat what their suppliers have told them. The greatest ignorance and resistance I have found is re air to air. The plumbers, which is what they are, don't understand it in my experience, or simply their supplier doesn't do it. Screeders only screed. Neither understands structure or building geometry. It's a big decision and you might (should) redesign your building methods to suit.

That must be Kelvin weeny Hookes.

And if it doesn't quite? It should work well.

So keep looking. It's truet that the pir market is very competitive. But so should be bats. I got a better price fir PIR from the local than from online, incl from "seconds". And it meant I could order for no waste and get more if necessary. The trouble with pir is that your roof will not be standard cc. 600cc +/- 2? Deduct 45mm width +/- 1. Then allow for twists in the timber. Builders end up cutting small and putting squirty foam in. A pragmatic solution for 150mm fill is 50mm pir, with some gaps, then 100mm mineral wool bats, which push in tight.

Agree. Our Territorial emissions have fallen by alot more than our footprint (roughly 50% and 25% fall iirc) Ah yes, I would argue we have been successful at the generation end of decarbonisation - mainly building wind though solar and nuclear have been less successful. But the other, demand end, has been much more lackluster. Both in the deployment of heat pumps, definitely the upgrading of housing, transitioning to BEVs is only now starting to pick up. And for industry there has maybe been less success. Our high energy prices are largely driven by high oil and gas prices since 2022, though policy has very much made the spark gap wider. Ironically those same high energy prices are also driving some of the decarbonisation! On the whole I think we agree, though I think you are maybe a little to harsh on us given our pretty impressive decarbonisation of electricity generation but I agree we could do better at the other side of the equation.

Have you priced up Frametherm 32?

@Novice what is you normal heat system - ASHP?

When inside is hotter than outside. That is the only time.

Bypass mode, as it's name suggests, bypasses the heat exchanger. In a hot summer, that will result in HOT outside air entering via the mvhr with no cooling effect. Leave it in normal mode. Then outside air that is hotter than inside, will be cooled by the cooler extracted air. I have struggled to find any time when it is actually advantageous to use bypass mode.

We have been living in our certified Passivhaus since it was completed in August 2017. It's a relatively large house (approx. 400 m²) and overall has proved extremely comfortable and energy efficient. However, in recent summers we have increasingly experienced overheating in the main living areas. Despite using night purging and running the MVHR in summer/bypass mode, there are periods when we struggle to bring the internal temperature back down. We are therefore considering installing a couple of air-to-air heat pump/split AC units – probably one serving the main living area and another on the top floor. My main concern is maintaining the integrity of the Passivhaus airtight layer when installing them. The wall construction is shown in the attached drawing. The airtight layer is 12 mm SmartPly/OSB on the inside of the 300 mm twin-stud timber frame, with a 38 mm internal service cavity before the plasterboard. Obviously the installation will require refrigerant pipework, condensate drain and probably cabling to pass from inside to outside. I don't want an installer simply drilling through the wall and filling around the services with expanding foam. Has anyone installed split AC/air-to-air heat pumps retrospectively in a Passivhaus or similarly airtight timber-frame building? In particular, I'd be interested to know: How did you detail the penetration through the OSB airtight layer? Did you use proprietary airtight service grommets/collars or tapes around the pipe bundle? Is it better to install a larger airtight sleeve through the wall and pass all the services through that? How was the sleeve/penetration dealt with at the breather membrane and external cement board? How did you deal with the condensate drain without creating an air path? Would you pressure-test the house again afterwards to confirm the airtightness hasn't been compromised? I'd be very interested to hear from anyone who has done something similar, particularly on an existing Passivhaus.

My bad, I meant bats - currently looking at Nyrock 32 and have a decent quote from SIG that is almost £2K cheaper than a cellulose quote I've had! So the pir I mentioned is 1.5K cheaper than the bats, and they in turn are almost £2k cheaper than cellulose (waiting on other quotes).

I just want a second opinion on the below as the implications are significant and I'm struggling to understand the proposed wind posts locations and also the excessive amount of ladder reinforcement at 225mm centres. The internal block leaf is 7N medium density.

Yeah, I have a bad feeling about that. The information available is scarce, with some mention of them being made for 600mm centres - but then pictures show then boards as being 600mm wide! I'll see if anyone local has them...but even then, I wont know if seconds will have something different.

Agree, but that's not what I was responding to. We have been somewhat talking past each other. I responded to you saying the UK is a good example of decarbonisation and that by extracting more oil/gas we are risking that. My response is that while we have achieved a significant reduction in our emissions, a significant proportion of that is because we decided (intentionally or indirectly by actions) to just stop doing a lot of high carbon things. We aren't a good example of transitioning existing industry to low carbon, industries in this country are shutting down because they can't operate in our business environment*. Other countries have done a much better job of that. Talking about Burberry and Mulberry is a distraction. They are relatively small components of our economy that are not generally replicable. * Mainly it's electricity/gas prices (which we could have addressed in a different way) but it's also a general complication of rules over the last few decades (and Brexit) that adds friction. When other countries have adopted holistic carbon reduction schemes that encourage and support companies through the transition with active government support (and in doing so have achieved some great results) and we don't, we really can't claim to be a good example.

We’ve gone fancoils upstairs and UFH downstairs. Dual zone setup on Ashp so we can run the fancoils with proper chilled water in hot temperatures. Hoping we will be in in next few months and will report back in summer if it works!!

Decarbonising doesn't mean stopping - merely removing (where possible - see pt 2). There are plenty of industries which burn gas and sometimes oinnthat can switch. Paper making, brewing, wood work and countless others use steam. Often generated by a gas boiler. Those processes can be replaced by electric heating - they can even be replaced by heat pumps in some situations. Secondly, there will always be some processes that just require gas (or oil or coal) normally because they are being used as much as a chemical feedstock as for heat (or even entirely as a feedstock). These processes will have to remain as fossil fuel consumers - it is net zero not absolute zero. Aviation is a classic example. I cannot see long haul aviation get away from fossil fuels for quite some time. We would need to have a significant surplus of electricity to begin to start making sufficient quantities of synthetic fuels (I don't think bio fuels from current farm practices will cut it). But my point was that industry changes all the time due to different pressures. Energy prices are part of the mix but probably don't affect a bespoke sports car maker as much as skilled labour and parts availability. I don't think Burberry and Mulberry are as bothered by the price of electricity as they are the availability if leather and skilled leather workers (and probably most importantly the whims of international fashion)

This is correct, thanks Rick. The heating and hot water work well when the ASHP is running. The issue is that the ASHP randomly cuts out and flips into immersion mode. Given that immersion mode never gets above 20 degrees, it's effectively useless at that point. It kicks back in after I restart the system. The installer annoyingly put the main controller in the garage next to the tank - always fun being out in my dressing gown first thing in the morning So my Ecodan set up should (I think) have the water on permanently. It's set to heat to 50°. It then has a 10° drop off before kicking back in, and presumably going back up to 50. That seems correct because when it's working, the water is rarely below 40° according to the MelCloud (Mitsubishi) app. I'm not sure how to manually check the diverter valve but I'm pretty sure that's fine too as the heating works well when the ASHP is running. Hmm, so very frustrating.

Then for the same u value use a decent high density mineral wool batts.

Not required if you use a combined/directional inlet outlet. One of our two MVHR units is in the middle of the house, the two ducts effectively go in void above built-in wardrobes to the outside wall. Our manifolds are in the same void. But it only feeds two bedrooms and extracts from two bathrooms.

Thanks.