Julestools

Interesting. What was your proposed IWI build up?

What indeed? I look forward to reaching a well-informed decision. Thanks for all your input so far👍

Making a good decision has become more complicated than I thought, that's for sure! It looks like whether I use 50, 75 or 100mm mineral wool (or indeed hemp or wood fibre), my current estimated U-value of around 0.5 would end up around 0.3 or even less. This is below what is considered safe for a solid wall. However, if I am led by the more optimistic things I've read and been told, I could happily go ahead with my IWI..so long as I take care of room ventilation, airtightness of walls etc. So, although I've been learning a lot, I'm no closer to deciding my iwi design. If only there had been a unanimous 'it'll be fine'. I know a builder or 2 who would have just slapped some celotex on the walls and be done with it! I think, Mike, I will go with option 3, do some more reading and let you know if gets me any closer to the answer I want to hear! Thanks for your replys👍

Ah ok, that's good. We had plenty ventilation, fans going and woodburner chugging away👍

Hi, and thanks! I had read something about the risk of trapped moisture (between the old DPM below the slab and my new DPM above) potentially causing issues with mould, but decided I was over thinking it and as my leveling stuff passed the old polythene sheet taped to the floor test, I've gone ahead and done the floating floor.. almost finished it today, a productive was to spend a rainy Sunday😊

Hi Nick, yes I've levelled the original screeded slab. Since I posted the above, I decided the RonaScreed FastDry stuff had done it's job and went ahead with the floating floor, so there's no going back now! The original slab seemed bone dry, though my gut feeling was to not trust the old thin DPM below. I used a mix as dry as I could get away with for levelling the worst low areas and SBR to bond it. Technically, adding the RonaScreed, it should have been dry enough in 4 days to take 'moisture sensitive floor coverings' so all should be ok. I was just curious about what others thought of the old 'plastic sheet taped to the floor' test. Thanks for the reply👍

Thank you Gus, for a great explanation of the whole insulation, ventilation condensation thing! (I think I'm going to have had enough of these words by the time I finish!) Perhaps I am over thinking.. and here is a good place to do it. Trying to be pragmatic, I'd say that if I moderately insulate with mineral wool (75mm?), ensure good airtightness and ventilation, then it is unlikely that the dewpoint will move to the inside or cause problems given that the inner leaf is already warmer due to the cavity insulation, would you agree? Thanks again.

Yes, I think I am a bit "hybrid" (not hydrid)! 😁

I think I'd use 45x70 with (cork) spacers if using 100mm mineral wool; 45x45 with continuous cork spacers if I opt for a (theoretically safer!) 75mm wool. Feels like I'm splitting hairs now.. It's hard to make a decision!

Brilliant, thanks. I'll look into ventilation options👍

Thanks Mike. The Historic Scotland study is very interesting and offers hope for trouble free IWI for my walls. I'm assuming that I am at less risk of condensation, having an already insulated cavity and therefore a warmer inner leaf? Nor sure if I want to go down the road of modelling the whole building with WUFI. It seems the current U-values of my walls are likely to be around 0.5, so perhaps 75mm mineral wool would be an adequate upgrade. I will look into hemp, and there's also cork boards (no plasterboard needed)... I'm just no so keen on the cost! Thanks again

Hi Gus, no I have a solid concrete floor so no ground floor joists to worry about!

Ah, thanks, ok got it. The cavity is already insulated.. They had some kind of fluff pumped in back in the 90's. I'm looking to improve the walls with internal wall insulation.

ETC, I'm not sure what you mean!??

Mike, I'm thinking of using 100mm mineral wool between studs .. Do you think that is too much, or OK due to the lower risk of condensation due to the walls being cavity, with the cavity already insulated? Also, I had the idea if using 45x45 treated studs/battens with 45mm spacers of some sort every 600mm or so, where the studs are screwed to the wall, giving me a 45mm space behind the studs to fill with insulation, then the regular 100mm batts fitted between the studs. I guess it's a kind of 'warm batten' design, cheaper on timber, but solid enough to fix plasterboard to. I have seen continuous insulating spacer strips used, only 10 or 20mm thick behind the studs ..but I'm wondering if my idea is a daft one, as I've not seen it done quite like that before!