Rick734

Cheers @Iceverge another super helpful post. That AVCL is exactly what I was looking at earlier so would make sense to go with that. Random q about the service cavity, I had assumed it would be empty so you could feed through pipes/wires at later dates as needed. I guess the assumption is that we are organised enough to not need to do that, if we are filling it with insulation?!

Thanks @Iceverge, super helpful stuff here. Take your 11mm OSB. It has an Sd value of 2.5m which is quite vapour open. A house with an internal RH of 60% at 20deg will exert a vapour pressure towards the outside on a winter's day. Assume it's 5deg and 90% outside. Over an hour about 0.18g will diffuse through 1m² into your wall. What are your thoughts on sealing around edges of the OSB and just using that as the vapour barrier?

@Iceverge can give us chapter and verse on this but my understanding is that a vapour barrier completely blocks moisture, which means that if any does get behind (e.g. leaks, perforations etc) it makes it impossible for it to escape again, potentially leading to interstitial condensation (between surfaces inside the wall) and therefore mould etc. Whereas a vapour control layer slows down the rate of moisture transfer, but is still technically slightly moisture permeable so trapped moisture can escape. Shoot me down if I'm wrong... If anyone's interested I have a link to my conversation with ChatGPT...seems to have a strong opinion on this subject: https://chatgpt.com/share/e/694b98bc-d6a8-8000-a4b4-5b08ee8d0ea9

@Redbeard this is super helpful as the Pro Clima is one of the options I was looking at. Any comments on performance (or I guess there's probably not much to tell)? Did you end up doing the OSB at all or leave it off altogether? Just wondering now if it's best to put the membrane inside or outside of my OSB...

Thanks Mr Punter - just tossing the coin on whether to do a 25mm service void or not at the moment...due to the layout of the room it may be the case that I do it in some areas but not others. Any recommendation? The construction is a 145mm C47 timber, 120mm PIR with 9/11mm OSB on the outside plus renderboard and Fassa render system.

Thanks @JohnMo - any particular vapour barrier? Are the smart/intelligent ones e.g. Pro Clima Intello / Isover Vario KM Duplex worth the money? Re timing - yes I don't want to bodge it, but I do see people on here looking at getting the ultimate performance from a project as it is their forever home and they are prepared to put significant effort in to achieve that. As long as it isn't going to cause damage or serious building issues in the long term, I'm not super worried about getting the highest possible specs out of it. I guess I'm looking for more of a balance of performance of practicality, speed and performance, rather than just performance, if that makes sense?

Further addition to the post...decided to have a bit of a chat with ChatGPT to get his views on the subject. The interesting bit was that when I mentioned I would be lining the internal face of the external walls with 18mm OSB (for fixing purposes), he/she/they/it decided that OSB can replace the whole vapour barrier/air tightness thing and is fine by itself...?! Sounds great, but I am cynical unless someone can weigh in on this please?

Afternoon gents, Been racking my brains on this one a bit and thought to turn to the helpful people on here. Who probably could run the country a lot better than the current shower. Anyway... Stick-built TF extension on the side of my house, over the existing garage walls. Everything up, roof slated, external walls currently being rendered when weather permits. Now working on the inside on the first floor and my understanding is a VCL around the walls is a good idea on TF to prevent moisture moving into the timber frame from the inside - fine. I have myself some silver reflective VCL from Tradestore ready to go - but a couple of questions I've been thinking about: 1) How to seal the edges of it, where it meets the existing house - how much do I need to worry on this front. Can I just lap it over onto the render, run a bead of silicone down & tape over? 2) At floor level, is it just a case of taping it down onto the chipboard? Or is there some technical way to seal it? My brain says just seal it down and be done with it, but at the same time there is the ground floor below, so surely moisture could rise through the ceiling structure and get up into the wall structure...or is that just overthinking it. I should add, I am more about speed and getting the job done at this point, rather than perfecting it. Definitely not a forever home and I'm in a hurry so not trying to spend time achieving microscopic increases in building performance...if that makes sense - quickest solutions that are acceptable, ideally! Over to you! Image below of one of the vertical junctions where the TF meets the masonry...

Perfect thanks @Gone West, that's the image I had in my head! Musta seen it before somewhere.

Thanks @Temp I had been wondering about STs fibre cement boards, because of my wall-hung (that was what I meant earlier) toilet which can obviously put a terrific amount of force onto the backer board so cannot have any flex...I will be doing a low profile shower tray as well, just can't really see that much point in full wet rooms.

Awesome, thanks @Nickfromwales 👍👍

Just jumping on the back of this thread if that's OK. Got a bathroom coming up which I'm just doing studwork for and then will need to go through the process above. Any particular kits/brands/methods recommended for tanking please? And to go off topic very slightly, I seem to recall from some other threads on here that cement fibre boards doubled up or with a layer of ply behind, are the go-to where you have a back-to-wall toilet...?

Good evening gents, many thanks for your advice - afraid I haven't had a chance to go through it properly yet today but just wanted to acknowledge your assistance and links/details provided - this is what BH is about! 😎 Will go through and follow-up hopefully tomorrow if I have time.

Thanks my fellow Rick Makes sense. I guess I am somewhere in between accepting the logic of the low and slow approach, while also liking the ability to warm up the house quickly if desired! Regarding insulation/air-tightness, I have been: renewing some of the 200-300mm loft insulation, replacing all windows & doors with new double glazing, adding expanding foam every gap I can find 😅, dry-lining walls with DPM and 50mm PIR where I am converting single skin areas into living space. Any other suggestions you would add? Thanks!

Thanks JohnMo super helpful, so just so I've understood your point about the DHW requirement correctly...my rad kW requirement x 24 then /22 should give me the DHW element of the boiler size? So as below...would you say that's correct?