Hi,

We went for a hybrid option, the main external wall we did with wood fibreboard and lime plaster and painted, with no tiling other than a tiled skirting. Our bathroom is located on the ground floor and we chose to put a plinth later of XPS at the base of the wall, which sat directly on the concrete slab. This mitigated the otherwise risk of the wf boards sitting on a potentially damp concrete slab.

We had one partially external wall which would be regularly soaked in water from the shower and which the bath was set against. This was to be tiled, so here we used marmox XPS (siliconed all the joints) and covered them with their waterproofing tape. This is therefore a non-breathable wall.

As the floor is a concrete slab we put down marmox XPS on that and tiled over. I didn't like the idea of floating chipboard/OSB over PIR as used elsewhere in the house, in case of water damage. We have a freestanding acrylic bath that sits on the tiles and there's no issue with the weight. The XPS can take considerable loads where they are spread out.

We made sure that all external walls were pointed in lime mortar.
Made sure everything was airtight with appropriate pro-clima tapes.

Been in place for two years, the woodfibre wall gets the odd splash and it's been no problem at all..

We do get a slight issue with condensation gathering towards the lower part of a solid internal partition wall, which sits directly on the slab, so acts as a cold bridge.

Good mechanical ventilation is really important! We went for a single room mvhr with humidistat and automatic purge option. A decent extractor fan with built in humidistat would be fine.

Hope that helps, can post some photos when I get a minute

Good luck!

Chris

Thanks for the responses!

This external wall will get a fair amount of water as the bathtub and a shower will be right in front of it. So I am getting the idea that I should avoid getting the wall too wet?

Would tiling over the wood fibre and lime plaster be a reasonable option, so I sacrifice breathability but prevent the dampness?

Good tip about the marmox XPS and stormdry, the floor on this bathroom is concrete so it might create a cold bridge. We do plan to install underfloor heating though, so hopefully that will help?

I am all for breathability, but I feel a bit puzzled about bathrooms!

As Nick says, if you're tiling then there's no point in breathability of other materials. So as well as the WF it might be more expensive to use lime rather than regular plaster (I may be wrong?). Also don't trust the tiles and grout to prevent all water passing behind them, unless they're specifically done carefully enough to guarantee it, if it matters.

https://www.gov.uk/government/publications/retrofit-internal-wall-insulation-best-practice

If you read the BP guidance above (released, but apparently almost imperceptibly, by BEIS just around a year ago) you will see that they recommend that any non-breathable insulant should be used on IWI *only* where a fully ventilated air-space of at least 25mm can be 'engineered' between it and the wall. Anything else, I guess they would argue, is 'on your own head'. This is good advice, but it will complicate a lot of people's thought-processes. The example I used earlier did not have an air-gap, but it was done with an almost anal obsession with air-and vapour-tightness.

Posted By: number_thirty_threeI've done a lot of thinking and research on bathroom/kitchen finishes in the past, although we're still ages away from actually doing ours!

It sounds like you've been through a lot of the same processes we did.

A breathable natural material for finishing wet areas such as a bathroom is Tadelakt. ... It's not a massively cost-effective option but if compared to a fully tiled bathroom i believe fairly similar. A lime plasterer should be good to apply it (or you can even have a go yourself if you fancy it because the finish is quite forgiving!)

We looked at tadelakt for our downstairs wetroom, for the outside wall I mentioned elsewhere. Our lime plasterer, Arthur Philip, was quite keen on it. It is very expensive though, because it's an enormous amount of hard work.

We are planning on Marmoleum flooring for the bathrooms

I would have liked to use marmoleum in various places but was put off by the limited choice of colours. In the wetroom we went for contract hospital vinyl installed by a local commercial firm, mainly to get the roughened anti-slip surface but also to guarantee a truly waterproof finish to form a shallow dish. It has done the job. Bamboo is an excellent flooring material; very stable and scratch resistant and stands up to water much better than a wood floor. Looks good too.

Hi
I think it is worthwhile understanding what the purpose of breathable insulation is ... As far as I understand breathable insulation is for getting interstitial moisture out of walls not helping remove moisture from the air in a a bathroom. That is what ventilation is for.

There are obvious merits to insulating the walls as this will reduce/prevent condensation on the walls. I also think it important to consider wall coatings. Tiles are obviously a vapour barrier but so are almost all emulsion paints. interestingly while paints typically used in bathrooms in the UK often are very waterproof and contain ant fungicide, in Italy and other Mediterranean they use a chalky ,dusty paint to cope with condensation. I am talking here about ceiling and walls that wont be exposed to direct water rather than wall next to a bath or shower where a waterproof surface is essential.

The concept of moisture buffering surfaces is very useful here. This is the ability for some materials to absorb and hold moisture during times of peak humidity and release it slowly in drier periods - not unlike thermal mass in effect. I did an MSc at CAT a few years back and researched this very topic for my dissertation. I looked at bathrooms as they have intermittent but very high humidity levels -reaching 99% RH for short periods. Ventilation is needed to maintain healthy humidity levels but this results in heat loss and so needs to be balanced.

To test the effects of material choice I created an insulated, vapour tight chamber, with a humidity controlled ventilation fan. I measured the air movement, the humidity and temp of the outside air and the internal air. In one condition I lined the chamber with conventional foil backed plasterboard with gypsum skim and painted with standard emulsion paint. In the second condition I used conventional foil backed plasterboard coated in a lime/ hemp fibre plaster skim and a clay paint. The plaster I used was Adaptavate Breathaplasta. I also took a small sample of each and suspended it from a set of scales in the chamber so I could measure the changes in mass as the material absorbed and desorbed moisture. For each condition I humidified the chamber using a steam generator twice a day for 7 days. I found that the moisture buffering lime hemp plaster/ clay paint combination resulted in lower humidity levels in the air in the chamber and reduced the need for ventilation by about 30%. Within an hour of a steaming event the lime hemp plaster had fully dried out. I should caution that my setup was rather heath robinson and that wildly varying external temperatures and humidities ( the weather was very changeable at the time) could have confounded results so I cant be totally sure of my results but it does suggest that we can manage moisture levels very effectively by exploring moisture buffering wall coatings rather than the UK convention of super water tight surfaces.

Food for thought.

BTW the plaster skim was 4mm thick as the moisture doesnt penetrate far into the surface according to the research papers i read in preparation for my experiment. I should also add that the breathaplaster was really easy to use. I got a plasterer friend to prepare my samples and he commented that it was no more difficult to use than a gypsum skim. The surface was slightly rougher than gypsum but not noticeably so. The hemp fibres were so fine that you really couldnt tell they were there. According to my back ground research the hemp was the vital ingredient.
I hope this is of some interest

JwD

Can't thank you enough for all the great advice I am reading! Apologies it is taking me a while to digest all the information and render it into a plan.

Posted By: Nick Parsonsthey recommend that any non-breathable insulant should be used on IWI *only* where a fully ventilated air-space of at least 25mm can be 'engineered' between it and the wall.

Posted By: number_thirty_threesheepswool (cheaper than wf), battened out to create the ventilated void behind - but depending on the depth you're going for this may not give you an amazing overall u value.

So in this case, it seems I can then install 25mm battens vertically then insulate + tile on top? Tadelakt seems really interesting though, but if we can make the tile route work that should be a winner for us.

When you say "ventilated" air-gap, does that mean I need to install some ventilated bricks to let air through the gap? I worry about infestations as this would be an unsupervised space once we are done with it.

Posted By: number_thirty_threeWhat do you plan to do with the pipes that run along that external wall? Will you be moving them inboard of the insulation or are they going somewhere else?

It's not clear from the pic if this is a ground floor or first floor?

This is a ground floor bathroom with screed floor (the only screed floor on the ground floor)
As for the water pipes I thought I would have them behind the insulation? I read on some WF manufactures installation guides that services should go behind the insulation (between the WF and the brick).
For the discharge pipes they will have to go through the wall to the outside, so I think it might create a small cold bridge?

Also Mike1, jwd and djh, thank you too for sharing all your wisdom! :)

Posted By: bgasparottoFor the discharge pipes they will have to go through the wall to the outside, so I think it might create a small cold bridge?

Yes any pipe going through the external wall will create a cold bridge. Use plastic rather than copper to minimise conduction. Put a bend of some kind on the outside (e.g. turn a horizontal overflow downwards) to minimize airflow. Combine as many pipes as possible into one inside the building. All this assumes you can't go down through the floor of course, which is a better choice where possible.

Posted By: Nick ParsonsNot sure I would want (by which I mean I definitely would not want!) services behind the insulation (on the 'cold side'). Not least if you end up with a VCL (or at very least an air-tightness layer) you don't want to bu**er that up when you need to reach the pipes or cables.

Hmm that is an interesting point! I am planning to do as such because this is the recommendation I found from a few suppliers! Here is one (please see page 2: precautions): https://www.ecomerchant.co.uk/pub/media/productattachments/files/Ecomerchant_IWI_installation_guide.pdf

That said, we are getting the house rewired in a week, so I asked our electrician to install all wall cables inside of oval plastic conduits to help with maintenance if we need it in the future. If I top this up with installing the pipes in front of the insulation (but hidden under cupboards, etc), do you think it would make it better?

Posted By: djhYes any pipe going through the external wall will create a cold bridge. Use plastic rather than copper to minimise conduction. Put a bend of some kind on the outside (e.g. turn a horizontal overflow downwards) to minimize airflow. Combine as many pipes as possible into one inside the building. All this assumes you can't go down through the floor of course, which is a better choice where possible.

Not sure I can fit them under the floor, but I can definitely bend them horizontally on the outside. Thanks for the advice!

The easy answer (but maybe not so easy to do) is remove the existing solid floor, unless you're pretty sure there is good insulation under it. You can then get good perimeter insulation at the slab level, which would then match the IWI, giving continuity of insulation.

It would be such a shame to put UFH everywhere except the bathroom. You could then inset drains etc into the slab (eg. for shower tray) and not start off with compromises.

I think you said the floor level is quite a bit above the outside ground level, so ground based moisture/DPM's will be less of a risk factor.

Again, don't see an issue with parts of the bathroom and kitchen walls being non breathable, as so much of the house area will be breathable, and local extraction for peak vapour events (showering/cooking).