Julestools

While removing the gypsum would eliminate any risk, the WUFI hygrothermal simulation that was run for the house was based on the gypsum remaining, as well as other information about the house, such as exposure to wind driven rain, composition of the walls, dpc and sound external render etc. It was stressed though that a decent ventilation strategy must be in place, as it would be for any type of IWI . Given that we are renovating whilst living in the house and want to minimise upheaval, mess and also cost, I'm feeling ok, at least for now, about going ahead with the advice I've had so far.

Yes, I could try lime putty. I used a lime putty finishing plaster for inside walls on my last project (cob barn conversion), lovely and creamy it was. Putty or nhl, if it sticks well, then the embedded mesh should prevent cracks? We were told by the company that ran the WUFI analysis for us that the gypsum plaster would be ok staying, though I'm aware that others said it 'should' be removed. Hmm, I wonder what risk there is that it could cause trouble? Is mould growth more likely with gypsum or is the issue with it breaking down/dissolving in the presence of too much moisture?

Hi, thanks Redbeard. Yes, I've heard rk70 is nice to work with. I wonder what nhl mix was used that didn't stick as the adhesive coat? I imagine 1/3 with sharp sand , for example, probably wouldn't work. nhl2/soft sand 1/2 or even 1/1 may provide a creamy sticky enough mix for the wood fibre boards? I'm fine with plastering so If I don't get a categorical 'no' then the only way to find out is to get a bag of each and experiment😊

Hi, I'm back at Long last (that was aa busy couple of months!) with a update to my IWI conundrum.. We talked more to builders and suppliers of IWI, considered other materials; for a moment we even got tempted to use PIR, continued going round in circles.. Until one company we spoke to supplying wood fibre did a 'quick WUFI analysis' for us and came up with these recommended maximum thicknesses: Cavity wall as is (cavity filled), 60mm. Cavity with blown insulation removed, 80mm. We've decided to play it safe and go for 60mm wood fibre. The great thing is I can simply mechanically fix the boards to the walls, mesh and plaster, with no timber battens/studs or plasterboard. This should get the walls u value to 0.3 or so, and we're happy with that. Many thanks to @Mike and @Gus Potter amongst others for taking the time to help. The next decision is what to put on the wood fibre? Specialist lime-based plasters such as Baumit rk70 are recommended but I've heard an nhl2/sand mix will also be fine to use and stick well to the boards (along with mesh embedded in the first coat), which is a much more affordable option. Any thoughts would be much appreciated. 😊

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