Internal insulation for block cavity walls

[Roger440]

Thanks for the explanation.

 

I see 2 issues preventing me here. One, i wouldnt have the first clue about how to manage or deal with the electronic/IT side of all that.. But probably more importantly, if it subsequently showed a problem, that would mean major rework to deal with it.

 

Thats not something i can do. If i do something it has to be right first time. 

 

Paying for WUFI is yet more money not spent on actual upgrades. Its not about justfying it, you still have to have it. If it just ended at that report it wouldnt be so bad. But it doesnt.

 

As i suspected, and you essentially confirm using a ubakus model isnt without risk.

 

Which leaves doing nothing as the only "risk free" option. Which, im not keen on as the extension is really quite poor thermally, not helped by fronting into the prevailing wind. The old stone part of the house is far better in that regard.

 

As regard joist ends, that something ive yet to establish what has been done. If its through to the cavity, that probably the end of cavity fill ideas.

[Mike]

12 hours ago, Roger440 said:

i wouldnt have the first clue about how to manage or deal with the electronic/IT side of all that.. But probably more importantly, if it subsequently showed a problem, that would mean major rework to deal with it.

Yes, if you're not into electronics and don't know someone who is, then monitoring is out - but if it did show a problem then it would be better to resolve it than to wait for structural damage to set in. However I'm not suggesting using it as a way of pushing the boundaries, only for curiosity or reassurance. I'm doing it for both reasons - WUFI suggests that my ceiling will be OK but, as it's a highly unusual situation and I still have a few reservations with WUFI, I want to validate it.

 

12 hours ago, Roger440 said:

Which leaves doing nothing as the only "risk free" option. Which, im not keen on as the extension is really quite poor thermally

Then option 1 on my list is probably going to suit you best, if you still want to consider IWI. That is, go to the technical department of Pro Clima or the like and ask if they'll do an evaluation for you.

 

12 hours ago, Roger440 said:

not helped by fronting into the prevailing wind

If you add your proposed rain screen (and it's not airtight), the chances are it will add to the drying capacity of the wall and be in your favour. Although if you can add a rain screen, maybe you can add external wall insulation, which is going to be preferable.

 

12 hours ago, Roger440 said:

As regard joist ends, that something ive yet to establish what has been done. If its through to the cavity, that probably the end of cavity fill ideas

There are many properties with cavity fill and joists embedded in the wall without problems so, if you forget the IWI and just fill the cavity, then you should be fine. However make sure that the rooms are adequately ventilated and you make the wall airtight, particularly at the joist-wall junction. At ground floor level, add plenty of ventilation to the sub-floor void (if that isn't already the case) and insulate the floor well - there are other threads on here about both.

 

Edited January 29 by Mike

[Roger440]

21 hours ago, Mike said:

Yes, if you're not into electronics and don't know someone who is, then monitoring is out - but if it did show a problem then it would be better to resolve it than to wait for structural damage to set in. However I'm not suggesting using it as a way of pushing the boundaries, only for curiosity or reassurance. I'm doing it for both reasons - WUFI suggests that my ceiling will be OK but, as it's a highly unusual situation and I still have a few reservations with WUFI, I want to validate it.

 

Then option 1 on my list is probably going to suit you best, if you still want to consider IWI. That is, go to the technical department of Pro Clima or the like and ask if they'll do an evaluation for you.

 

If you add your proposed rain screen (and it's not airtight), the chances are it will add to the drying capacity of the wall and be in your favour. Although if you can add a rain screen, maybe you can add external wall insulation, which is going to be preferable.

 

There are many properties with cavity fill and joists embedded in the wall without problems so, if you forget the IWI and just fill the cavity, then you should be fine. However make sure that the rooms are adequately ventilated and you make the wall airtight, particularly at the joist-wall junction. At ground floor level, add plenty of ventilation to the sub-floor void (if that isn't already the case) and insulate the floor well - there are other threads on here about both.

 

Thanks for all the info.

 

As always, hard to know what to do. The cavity AND IWI gives a wall that meets current standard, which i hadnt thought was realistically achievable. Im more keen on heavily insulating the extension and do rather less in the old part which is 2 ft thick stone walls, so lots of unknowns and variables.

 

Good news is, floor is concrete, albeit uninsulated (as far as i know, laid in 19080.) And yes the rainscreen will be ventilated. All pretty conventional.

[Julestools]

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. 😊

 

[Redbeard]

I know someone who tried to use NHL for the same 'economy' reasons. I think the main problem was using it as the 'adhesive coat' behind the boards. As I recall it simply did not stick at all - not even the 'light tack' afforded by RK70. I don't think he ever tried it on the boards, since he had to buy RK70 for the 'adhesive' coat anyway. Particularly if you are doing it yourself and have not done a lot of plastering I'd recommend RK70. It is so 'forgiving'. Slow, in colder weather, but relatively easy for non-plasterers to pick up.

[Julestools]

On 31/03/2026 at 09:18, Redbeard said:

I know someone who tried to use NHL for the same 'economy' reasons. I think the main problem was using it as the 'adhesive coat' behind the boards. As I recall it simply did not stick at all - not even the 'light tack' afforded by RK70. I don't think he ever tried it on the boards, since he had to buy RK70 for the 'adhesive' coat anyway. Particularly if you are doing it yourself and have not done a lot of plastering I'd recommend RK70. It is so 'forgiving'. Slow, in colder weather, but relatively easy for non-plasterers to pick up.

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😊

[Redbeard]

Hi @Julestools. I don't know what mix was used. That was before my involvement. I am not a lime expert but I think I share your idea that something as rich as 1:2 or even approaching 1:1 might be required. I don't know if having it that rich has any downsides. (Come to think of it I have used lime putty when I want a really creamy mix. Could you do that?).

 

Out of nosiness I note that you are proposing mechanical fix only. I take it this is probably onto existing gypsum plaster. Others (including specialist merchants) disagree, but having gypsum in  a 'sandwich' always makes me a bit uncomfortable. I usually strip that off and do a lime parge coat, but then maybe I like making work for myself!!

[Julestools]

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? 

[Roger440]

8 hours ago, Julestools said:

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? 

 

The way i see it is there is a risk. Big, small, hard to say. Removing it means that risk is removed. 

 

Given the problems and rework required "if" its a problem, personally, i wouldnt take the risk.