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Now managed to have a play around with a calculator and figured out that if we fill the rafters fully with soft wood fibre wool, and then add PIR below, we will meet regs for heat loss and manage a decrement delay of 9.15h, although the solid wood rafters probably won't keep the heat out as much as the soft wood fibre wool in the voids in between, so not sure how accurate that 9.15h measurement is. Assuming it is more or less accurate, is that enough to be comfortable for the type of bedroom I describe above, or should I add an additional layer of wood fibre below the PIR to try and hit 11h or even 12h decrement delay? This would compromise head height, but it might be worth it. Was hoping @SteamyTea might share his thoughts on this as I think he's done quite a bit of research and experimentation on this.

Anybody aware of any other free decrement delay calculators?

I have what i think is known as a mansard roof over our first floor extension. It is flat at the top and then has a pitched perimeter along the three sides (on the fourth side there is no roof as that is where it adjoins our house). We had planned on a warm roof, but it's looking like we don't have space due to some design changes that we need to make. The rafters are 175mm. Area of room, which is a rectangle is about 4.3m long by 3.75m wide. Half of that floor space (half of the 16.125 m2) is in the extension and the other half is in the first floor of the original house, with a second floor above it. So although the majority of the external walls are brand new cavity walls, there is a 2.1m bit of wall that is just solid wall with 40mm of K5 insulation on the outside. The flat part of the roof has a large skylight 900mm by 1800mm which has solar control double glazing SN70/35 (g value 35%, U value 1.0). The only other window in the room (about 2.2m by 1.4m) is made from the same glass (g value 35%, U value 1.0) We were hoping to use wood fibre to increase decrement delay of the roof so that overheating is minimised. My architect wasn't entirely sure this was necessary as although the outlook of the room is west facing, until about 11am, possibly later, the majority of the roof will be shaded by the large loft conversion on the floor above and the neighbouring house. But with afternoon sun, that might still make it uncomfortable in summer. But with switching to a cold roof, with insulation between the rafters, I only have 175mm of space for insulation. My architect is checking to see if 100mm of wood fibre plus 75mm of PIR will meet regs. If not, he says we just build up the inside with additional PIR or insulated plasterboard. But I know regs only care about keeping the heat in, not keeping the heat out. Anyone know how much wood fibre, when used with PIR, is needed to have a decent impact on preventing (or delaying, to use the decrement delay terminology) heat from coming in from this type of situation? I know I should do a proper model, but don't know where to start with that. Alternatively, is there a wood fibre product that is rigid enough to be used on top of the rafters and double up as a structural board for the roof covering? I don't have a lot of space, but could maybe squeeze something in.

@PeterW you are a genius, thank you. Not sure why that size isn't available on the sites I was looking at.

We have removed the pebbledash from a 1930s semi and had planned on installing 40mm of external rigid board insulation onto the brickwork which we have exposed, and then rendering over that. Kingspan K5 is a phenolic core which is faced on both sides with glass tissue (more details here). Problem is it comes in 20mm and 50mm, but not 40mm. 40mm is already making things a bit tight in terms of keeping the original features of the house, but we can just about make it work. 50mm would be too much though. Can anybody recommend something similar to K5 that comes in a 40mm thickness? I don't think PIR is an option because that tends to be foil faced making rendering over it not really an option. We could use two lots of 20mm thick Kingspan and just fasten the fixings of the outer layer through the inner layer and then onto the underlying wall, but it is effectively twice as much labour, so I rather not do that.

I'm not keen on ASHP because of the noise and unsightlyness of them and the requirement in our area to get planning permission for them. A solution to most of these problems is to go with a super expensive unit like an Oschner, but even then they require a lot of space for the internal compressor which we didn't really have unless we severely compromised our utility room. Next house. The Solar Thermal idea however might work quite well for us. How much do you think that would cost? We have quite high hot water requirements (our UVC is going to be either 300L or 400L in size).

Thanks @SteamyTea, here are some drawings. The first drawing shows part of the roof plan. The dimensions are about 8.2m by 3.15m; I'd forgotten we had to make the dormer slightly smaller - for reasons I won't go into to). The ridge line is centred with the chimney stack (shown as a rectangle with 8 circles in it). The second drawing shows a cross section. You can see the pitch of the small piece of pitched roof between the flat roof and the ridge line is 45 degrees. But the diagonal length of that bit of roof (i.e. the hypotenuse on the cross section) is only about a metre, so I'm hoping there won't be much shading from that. Obviously the chimney stack will cause a bit of shade, but there isn't much we can do about that as we use it and it is also jointly owned by our semi detached neighbours.

@delta9 I really think you should: 1) find a good mortgage broker with experience of these transactions; and 2) speak to the specialist lenders that focus on the sector of the market. Both should give you useful info for free.

It’sa flat roof, ie the lid of a dormer. Hasn’t been built yet, so possibly time to reinforce it if necessary.

No, I was hoping to get those moving mounts that track the path of the sun as the back of the house faces west and the front of the house east. Not sure if they are expensive though.

Incidentally, I bought a vitra concealed cistern/frame and matt black flush plate for the very reason that matt black will work with our colour scheme (all our windows are matt black framed) and vitra seemed very good value. I didn’t have the confidence in Vitra to buy this brand for all the toilets though. Have gone with them just for one toilet that won’t be used as much as the others. @tanneja to answer your question, @Temp used his skills to find me some more affordable brass effect plates in the Grohe range from this website: https://insani24.de/ I ordered two a couple of weeks ago. The website is a bit weird because after ordering I didn’t get a confirmation email, but when I log on to my account the order is there. It was meant to dispatch a couple of days ago, but for some rain it hasn’t, so I actually emailed them today to check what’s going on (your message reminded me - thanks!). It looks all legit, so I’m sure it’s fine, I’m hoping it’s just a couple of weeks till I get them. I still haven’t ordered the Grohe cisterns/frames yet. Thinking of going with the new model that comes with a flow regulator, though I’ve never been an early adopter of new technologies, so seems a bit risky. Just appealed to it by the fact that you can tweak the flush flow rate to work perfectly with any rimless toilet - I’ve probably just fallen for the marketing!

Yes, well this is why I don’t like the idea of a thumb turn! Having said that, our letterbox is a through the wall one with a lockable security flap.

The reason I’m against a thumb turn is that our door is like this. As you can see, it is mostly glass. So although it is PAS24, and the glass will therefore be laminated, my gut is telling me that a burglar looking through the glass could see that behind the steel lockbox is a thumb turn, and that might inspire him to try to break the glass to get in. I’m not sure he (or, let’s be egalitarian, she) would actually get in, but as good as PAS24 is, it is not perfect. i can design the door to have a wider lockbox, so more of the thumb turn is obscured by the steel, or I can just lose the thumb turn and ensure there are spare keys in a place that everybody can find easily in a fire. Thoughts?

So how does one go about doing this? I will have scaffolding up in a couple of months when the loft is converted so good opportunity to do it, and hoping to put PV panels on my flat roof (above the dormer conversion) which is about 8.5m by 5m. Got a couple of quotes from companies that also install and it was just crazy prices.

Good news: the BCO has backed down and admitted that “on reflection” there is no requirement for a thumb turn.

What multiplier did they give you?

You need to find a cheaper broker! Mine charges £260 per transaction, and only after you have an actual mortgage offer. Pm me if you want the details.

So what’s the solution when I have gone to the expense of going with a private BCO and he is being a complete twit?!

I’m wondering whether these actually work well in practice. Chipboard doesn’t sound like it has the right properties to deflect heat upwards, or is the idea that you install spreader plates onto the pregrooved chipboard.

Our house is a 1930s semi. A few decades after it was built, a timber porch was added (with clay tile roof). Although that timber porch had a lockable front door, the more secure original front door of the house remained, so one had to open the porch door and then the front door to get in. As part of our renovation/extension project we are knocking down the old timber porch, building a cavity wall porch, and doing away with having two lockable doors. Instead we will just have one door within what will form the new external wall of the house/new porch. There will be no lockable door where the original front door of the house was. The new front door has multi-point locks, all glazing is laminated and it is PAS24 certified, so it definitely applies with Approved Document Q (even though it doesn't need to, as this is not a new build or a dwelling formed by a change of use). My question is, as a matter of fire safety under Approved Document B, do the building regs require me to ensure it can be opened from the inside without a key. My BCO seems to think that it does, but when I challenged him on the wording of Approved Document B (which I've pasted below) he was lost for words and said he "had to go and check". Not sure what he had to go and check. Is there further guidance on how to interpret the approved documents which are not contained within the approved documents themselves? The relevant wording, so far as I can see, is the following: I therefore cannot see that there is any requirement to have a thumbturn on an external door. In particular, 2.10(c) makes clear that the lock on an escape window may be fitted "with or without removable keys". There is therefore no logical reason as to why the same wouldn't be true of an external door. Now it's possible he will refer to provision 2.10(b) and say that if the front door is locked and one doesn't have a key one cannot get out. But it would be bizarre to interpret it that way in the absence of a clear provision that creates a positive requirement to have a thumbturn. Another query I have is whether any of this applies at all. What if I was doing a renovation that was generally caught by building regs but chose not to replace my perfectly good front door. Would i then suddenly have to upgrade my lock to one that had a thumbturn just to comply with building regs?

There is no chance in hell that they will simply outlaw all stoves regardless of when they were installed. The industry would die overnight and there would be many claims for judicial review on the basis of a legitimate expectation that law Is not enacted with retrospectives effect. We are going for an extremely efficient drive that complied with the 2022 EU requirements.

I am in London yes. My understanding is that when the legislation changes it will be prospective, not retrospective. Any idea when the new law is due to come into force?

@Omi you are lucky to have found an installer that will do this. Common sense suggests it should work, but I not that the installation manual for the Burley does not provide for your suggested solution as an option: https://burley.co.uk/wp-content/uploads/2019/02/9103-9112-Burley-Wood-Burner-Installation-Instructions-JANUARY-2019.pdf (pages 9-10).

That’sa good idea. Only issue is he is only there early mornings and towards the end of the day, and not every day. I could buy his men lunch, but there isn’t really any tension between me and them.