Hello,

I very much recognise and feel for your situation. It reflects most of my journey building my house and I'm still in it.

IWI is more difficult to detail I think precisely due to the reasons you've mentioned.

A good source I have found that includes detailing for this kind of project is the retrofit pattern book https://retrofit.support/

It's free to register and has a number of details for IWI https://retrofit.support/category/IWI/

Well worth a look if you're inclined.

I myself am rather biased towards the natural insulation systems such as woodfibre and sheeps wool. I bought all my insulation through Ty Mawr who have some brochure info on the IWI systems they provide. They're also a very helpful lot that have good experience with conservation and older building so always worth a conversation with them. Here's the link to their online brochure https://view.publitas.com/p222-14383/retrofit-insulation-systems-for-old-buildings/page/4-5

I'm not entirely sure there is ever a right answer, every system has its pros and cons and it often comes down to a balance of compromise that gets you close to what you're ideally after.

+1 on wood-fibre and the possibility of EWI to the gable. Up till recently I have used Pavatex boards and Baumit plasters. Baumit stuff is wonderful to use. I am not a plasterer but I can get an excellent finish. It is slow, but forgiving. I am usually a 'chaser of low U values', but for IWI, given the choice, I would use WF (or cork - never used it but I cannot see why it should not be just as good) and accept a U value just a bit higher (worse) than the 0.3 in Part L. Your BCO should accept it as you have a traditional property built using water-0vapour-permeable materials. General feel of the room is transformed.

Sorry, maybe I missed it, but where are you? That will influence who people are able to recommend.

Posted By: geubendecided that we are going to have to remove the skirting, moldings etc as even something as thin as 13mm SpaceTherm would end up looking a bit odd against the profiles on the trim (and the uneven existing plaster barely leaving a defined edge in places). I've managed to convince my partner that losing 40-50mm off two walls in a 5x5m room won't really be noticable (I hope I'm right).

Some quick maths puts 40mm of WF @ u-value of 0.04 roughly equivalent to 15mm of SpaceTherm aerogel but at <1/10th the price so absolutely worth looking at again.

Still leaves the problem of finding someone to do the work. I would give it a go myself but it'll take me an age and it'll probably work out less stressful to pay someone else to do it. If after seeing it done in person it still seems doable I'll give it a go in a less important room, like my office, where only I will see my mistakes.

Losing 50mm off the walls of a 5 x 5 m room will definitely not be noticeable! I have used 75mm of Celotex in my two dormer bedrooms (approx 5 x 4m each) and plan to do the same in the lounge (approx 4 x 6m).

You can definitely DIY this I can assure you, even if you could just get to the plaster-boarding stage and then have a plasterer to skim the walls for you. This is what I intend to do in my lounge, which will involve removal of skirting boards and a dado rail on the two external walls (I will leave the two internal walls alone).

The bedrooms were plaster-boarded using taper edged boards and I used ready-mixed "mud" to fill the gaps. It was an "ok" finish - plastering is definitely not one of my better skills - most of the time it looks good but when viewed in a certain light it is possible to see where the joins are, hence I would leave the lounge to a professional!

Absolutely, I don't to want to go through all the effort and expense only to get a minimal improvement, even in Building Control would allow it.

150mm feels like it'll be too much around the windows. The end result would be heavily recessed windows and I don't think they would look good (any I'm much more accommodating than my partner).

Perhaps I need to go for a mixed approach; cheaper WF where it can be thicker (marked red in the attached image) and something more expensive like SpaceTherm where the thickness will be an issue (blue).

The size of the windows means that the amount of SpaceTherm needed might not be too bad, need to do some sums.

Are they any additional things to consider when using multiple technologies together? Both WF and SpaceTherm would offer the vapour permeability wanted right?

I would not advise 150 WF internally without a dynamic interstitial condensation risk assessment. Most suppliers recommend a max of 80-100mm.
You rightly say ''Absolutely, I don't to want to go through all the effort and expense only to get a minimal improvement, even in Building Control would allow it.''

I'd be amazed if you did do 80-100mm WF (with attendant air-tightness preparation) and not notice a significant difference.

I just had an out-of-the-blue text from a client last week telling me how the 100mm WF installed (to 2 ext walls in one room) in the summer had effected a complete transformation.

60 or 80mm woodfibre IWI.
20 or 40mm for window reveals depending on width window frames and positioning of wall reveal.
Concentrate on air tightness around window/wall detail.

Run IWI straight up through the floor (can cut flooring back with plunge saw - might need to put new joist set back from wall to allow for IWI)

What is on the inside of the external walls? Wallpaper/skim/lime plaster underneath?
If going for wood fibre you want it to be breathable.

The black is almost certainly lime mortar with 'black ash' as aggregate, v. common in the Sheffield area and probably equally over the border. Don't worry about the hollow spots. They will be pulled up tight to the wall by the mechanical fixings. My normal method of fixing WF boards (as spec'd by Pavatex) is to 'butter' the back of the boards with a toothed coat of lime plaster, which when 'pulled up' by the mech fixings will tighten up the cracks. As long as the exg lime plaster has not been painted with impermeable paint (and by the sight of what's there it hasn't) it will still 'breathe' and act as an a/t parge coat.

Edit: floorboard removed, parge coat between joists, Plinth of 40mm Diffutherm, and Pavaflex between GF ceiling and F Floor on parge coat.

Posted By: geubenPhenolic foam does look to have a good price/performance ratio but it's not vapour permeable so I think that rules it out for me.

With vapour permeable insulation types presumably you'll be adding a vapour barrier on the inside. If any trace moisture builds up behind vapour impermeable insulation during hot wet weather it will go out through the external walls during more normal weather.

Bear in mind many modern paints are relatively vapour impermeable!

Posted By: Nick Parsonsact as an a/t parge coat.

It won't be airtight if it has cracks in it! Ah, I see you use a continuous layer of lime on the boards to seal the cracks. As PiH says, that won't fix any bits that have blown though.

Posted By: Nick Parsons'contract matt'

Yes, Dulux Trade Supermatt is a bargain. They actually publish an Sd number for it! We used it as an undercoat on our internal (gypsum) walls with clay paint over the top. Otherwise I can recommend clay paints but there are other types.

Posted By: Nick Parsonsmy take on it has been that even a 'baggy' bit of original lime plaster is likely only to crack around the edges of the baggy bit, and with another layer sandwiched on top, between the old plaster and the WF, I think that's a pretty good chance at air-tightness.

Indeed so, but that doesn't fix the strength as regards a fixing surface for the subsequent layers. Perhaps it's an abundance of caution but ...

geuben,

I did internal insulation to an end-of terrace flat (effectively the top two floors of an end of terrace house) 5-10 years ago and came up against all of the same doubts and uncertainties.

You might be interested to read this thread:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14973&page=1

a few comments:

- my research led me to believe that the main worry was the walls drying out sufficiently from wetting from the outside (ie rain). This worried me more than the interstitial condensation issue.

- I ended up leaving a small cavity between the insulation and the masonry. This may not have been necessary

- I put monitors into the wall to see what actually happened in the long term. The results are discussed in that thread (basically, so far, no indicationof problems)

- because I did the building work myself, I could be fastidious about airtightness and consequently the risk of vapour transmission. If someone else were doing the work, it would be different.

- I remain unclear about whether the "breathable wall" stuff makes sense. I ended up with a somewhat indecisive approach - some parts of the walls use PIR board and a polythene vapour barrier, others, EPS insulation (still unclear about whether this allows any meaningful level of vapour transmission) and no vapour barrier other than a layer of OSB with joints made meticulously airtight

- The most difficult bits were passing the insulation layer across the zones where floor joists join into the wall. Sealing the insulation layer around a bunch of wonky victorian timbers was incredibly fiddly and time consuming. I didn't have to deal with the ground floor condition in my case, because my flat is only on the upper floors

- I think that your approach of varying the thickness of the insulation according to where you can afford to lose the space makes sense. This is effectively what I did. You can perhaps see on the attached floorplan.

- don't ignore that part that windows play. In one room, I replaced the window with a good 3g one. In the other room, it's still the low quality double glazed uPVC ones I inherited with the flat. Eventually I'd like to replace these. It's noticeable that they compromise things in that room - you can feel it just walking near them on a cold day.

- part of my project involved very thoroughly insulating a roof space. Here the difference in comfort is absolutely unquestionable (we basically never need to use the heating up there). In the lower floor, where it was the walls that I insulated, the difference is not quite as stark. Those rooms are compromised to some extent by the limit of what I could do at the junctions with the flat below, and currently to some extent by the windows as mentioned above. We do need to use the radiators in these rooms sometimes. However, it's the first time I've lived in a house of this type (victorian terrace in London - I've lived in a few) where you basically never wake up in an uncomfortably cold room, or get home to a freezing house.

- it might be that my comments do nothing to help your indecision. But know that you're not alone in finding it very difficult to judge the risk/benefit calculation when it comes to IWI. I got around it by doing it myself on my own property, with an ability to know that I'd done things thoroughly, and with an awareness of the risks that I decided would be worth taking. So far, it seems to have paid off. But it was a *lot* of work. And I remain wary about recommending IWI to lay people in a professional capacity.

Lineweight - when you say: "I think that your approach of varying the thickness of the insulation according to where you can afford to lose the space makes sense. This is effectively what I did" - what about the notion that Building Regs require any additional insulation has to comply with latest building regs with regards U value? Did you consider this?

Myself, I take the view that any additional insulation has to be a good thing regardless of whether or not a particular U value is achieved. I did discuss this issue informally with a local BCO and he basically told me to get on with what I was planning. Maybe he had enough on his plate at the time!

Posted By: Jeff Bwhat about the notion that Building Regs require any additional insulation has to comply with latest building regs with regards U value?

https://www.planningportal.co.uk/info/200130/common_projects/32/insulation/5 says:

"Where a solid wall has been upgraded by the installation of insulation it must meet the minimum energy efficiency values set out in the Approved Documents.

"However, if such an upgrade is not technically or functionally feasible, the element should be upgraded to the best standard which can be achieved within a simple payback of no greater than 15 years."

Posted By: Jeff BLineweight - when you say: "I think that your approach of varying the thickness of the insulation according to where you can afford to lose the space makes sense. This is effectively what I did" - what about the notion that Building Regs require any additional insulation has to comply with latest building regs with regards U value? Did you consider this?

Myself, I take the view that any additional insulation has to be a good thing regardless of whether or not a particular U value is achieved. I did discuss this issue informally with a local BCO and he basically told me to get on with what I was planning. Maybe he had enough on his plate at the time!

In my case - the insulation I added was way beyond what building regs deems acceptable, even at its thinnest points.

But as others have pointed out, the regs allow you to upgrade to a lesser extent where you can show that it's limited by what's technically feasible, and take into account payback periods.

Even aside from that though, you can also satisfy the regs by a weighted approach, where a low U value in one area can be offset by a higher one elsewhere.