Wood Fibre IWI - plaster removal or reinstate?

[larry]

Hi, 

We are slowly working around our house doing IWI using Wood Fibre insulation.

In our upstairs bedroom  (3 external walls) I am using 80mm Steico Flex, in between wooden studs, with a plan to then counter batten and plasterboard on top. 

In preparing the room I've come up to an upstairs external corner where there was a very old cast iron pipe buried in the wall, presumably supplying an old radiator - it goes about 2/3 the way up the wall. As I pulled it off of course a lot of surrounding plaster has come off and I've got a nice section - perhaps 1 sq m - back to brickwork. 

It looks like the pipe acted as a point for condensation since the area immediately around it is buzzing on a damp meter somewhat more crazily than the wider area around it. Though those have been reducing over the last few days so presumably it is now drying out. 

 

Do I:

a. try and reinstate the plaster prior to putting on the same thickness wood fibre insulation so all the plaster is level (and if so with what? I've got a bag of gypsum browning plaster to use up....)

b. get some 100mm wood fibre insulation for the 'back to brick' section so the area is roughly level with the rest

c. pull all the plaster off the whole wall and then apply the wood fibre directly to the brick

d. do something else

 

And, to help my learning - why?!

 

Wall is solid brick wall with external (cement??) render outside. The original plaster had had a skim layer on top, and then papered and painted. I've managed to scrape off most of the skim plaster so it is otherwise back to the original lime plaster. 

 

No obvious evidence of a damp problem historically or in the time we've been here (5 years, it's a 100 year old house)

 

Thank you all

Edited June 24, 2021 by larry

[jfb]

a. yes : better to use lime but for a small area probably won't matter using gypsum

b. no

c. definitely no

d. no

 

original lime render is performing a role as an airtight layer so bet to leave it in place.

 

The other overall option if the existing render is reasonably flat is to use woodfibre boards that are fixed directly to the walls without any need for studs with a lime render on top. But I am guessing it might be a bit late to consider that.

[larry]

3 hours ago, jfb said:

a. yes : better to use lime but for a small area probably won't matter using gypsum

b. no

c. definitely no

d. no

 

original lime render is performing a role as an airtight layer so bet to leave it in place.

 

The other overall option if the existing render is reasonably flat is to use woodfibre boards that are fixed directly to the walls without any need for studs with a lime render on top. But I am guessing it might be a bit late to consider that.

 

Thanks so much JFB. That's hugely helpful. 

 

It's the second room that we were doing. For the first we had Pavatex Isolair, recommended by Mike Wye, and fixed that directly with the plastic plugs and then a lime plaster system on the top. Was keen to try the other method for this room partly to learn a bit more myself but also partly as the 'headline' price of the Steico Flex was significantly less than the Isolair or similar rigid boards. It also allows a gap for a service duct behind the plasterboard so I can run cables for a new ring, which is very handy. Over time we do plan to do the whole house so I'm treating these first two rooms as a practical classroom!!

 

Thanks again, appreciated

 

[jfb]

I haven't looked at prices recently but i remember when I bought some last that Steicotherm boards (for plastic fixing to walls) were considerably cheaper than other options. But I don't know off hand how they compare to Pavatex Isolair.

And you can still chase cables into the boards, put some capping over and then render.

[larry]

That's a good point about chasing... Hadn't thought of that!!

[Snowbeetle]

The external concrete render will mean any moisture in the walls, (possibly coming up from the ground - depends on foundations and DPC, if there is one; or weather getting behind the external render - render varies in how well it keeps the wet out - age, how well the joints are done etc) won't escape outward and will only escape inward. Wuth lime plaster on the inside it can escape that way and not be forced on to appear elsewhere. 

At that age your building is likely to be single skin brickwork not double with a cavity, so how dry the fabric of the building is will depend on it 'breathing' in line with buildings of it's age. So your lime plaster will buffer moisture (take it in when there is an excess and expirate it when humidty drops) and allow water to escape. 

 

If you put gypsum on, that doesn't 'breathe' so any moisture in the brickwork will not escape and could lead to damp problems. JFB has a point that in a small area the effect will be minimal, so your choice if you want to worry about that or not, for 1 sqaure metre you are unlikely to see issues but moisture finding itself behind the gypsum will have to go sideways/up/down to escape. 

 

For the reasons I mention:

a) You could do, it isn't 'ideal' but it isn't likely to bite you on the bum too hard.

b) I agree with JFB the wood fibre on the walls would not be a good idea, it could end up slwoly getting wet and wood does not dry readily without good ventilation.

c) no

d) You could lime plaster the patch, it isn't that hard but the technique does differ to gypsum and it is a more skilled managing the going off process than gypsum, so you'd need to look it up, or I know an excellent lime plasterer who travels and isn't stupid money, so if you PM me I could give you their details. 

 

I only go into the explanation cos you wondered about the whys. But since you are plasterboarding you are making the walls non-breathable anyway. I wonder if the building will cope with that (deoends on whe way it was constructed and how it stays dry, how it is used, ventilated, affected by external weather etc. 

If you want to look at that sort fo thing more this guy is knowledgeable on older buildings and what helps/doesn't help

Energy Retrofitting - why it doesn't work and why it's causing damp. - YouTube

 

This email is about a slightly different question but the forces at work are the same. 

 

 

 

 

[larry]

Thanks so much for such a helpful reply. Though I'm a little confused, I have read elsewhere on this forum that gypsum based plaster is just as breathable (vapour permeable??) as lime plaster, it's just the stuff like PVA that people add which causes the problem. I am not claiming this as fact ( way beyond my expertise) but is this incorrect or a misunderstanding on my part? 

If so presumably I could patch with gypsum based plaster, no PVA etc, and let the wall breathe away? Or do snares lie in that direction also?!

Thanks again for your advice and expertise, I do appreciate it!

 

[larry]

I should add, the wall (outside in) is cement render, solid brick, lime plaster, paint X 2 layers, gypsum plaster skim, lining paper, two more layers of paint... Which might account for water getting trapped behind I guess??

I'm painfully stripping it back to the lime plaster layer then putting the wood fibre on top of that. Got to be more breathable than before??!

 

Also, same question about the plasterboard... Not breathable??

[Snowbeetle]

Plasterboard/gypsum plaster is not breathable. Likely the paint isn't too. 

So yes your bricks are currently sandwiched between two non permeable layers. 

Whether this actually gives you problems or not will depend on whether water is reaching the bricks. Which as you can imagine could depend on many things. 

 

But the fact is that you can't argue with water, so you either need to very effectively keep it from ever getting to your build fabric in the first place (this is the modern build system. So wall cavities, damp proof courses etc)  or, you have a building which works with the water and  you provide it with a non-damaging escape route (this is the older building method, so materials buffer the moisture variation and expirate it without harm so ultimately the building is dry for the inhabitants because the fabric does the job of buffering and expirating correctly. 

 

Because the older building methods were skills handed down verbally father to son or apprentice the country lost a huge body of knowledge in the world wars and the massive build programme post WWII meant concrete and pre-fab with it's ease of setting was the go-to choice. So now the building trades and modern buildings are much more familiar with the modern systems.

Add to that that modern methods have been applied to old buildings often leading to problems as people unwittingly mashed together water exclusion versus water management systems meant that the traditional methods got a really bad name and you will often find builders who won't touch it with a 10' barge pole. 

 

But although lime/gypsum plaster have the same job as a finish surface, they function and are applied differently so they are kind of apples and pears. There are a lot of people now formally re-developing the body of knowledge and products are becoming better known, but it can be hard to find the information for the DIY builder. 

 

In a nutshell if you try to mix the systems, you could cause problems unless you know what you are doing (i.e in that particular situation what result the relationships between the systems will create - I have a stone barn conversion and the roof has a non-breathable membrane which I have to keep for bat preservation so I have adjusted my methods to account for the interplay between the two), whether you get problems or not will often be down to the luck of the draw as to what water makes it as far as the building fabric. 

You get a lot of people trying to retrofit water exclusion methods to a building constructed with water management in mind, such as people chemically injecting sealants into single skin brick houses. but that often just pushes the water onto the next thing. I have a friend who tanked their victorian basement so they could have a games den... the water it was providing an escape route for then just started creeping up their lounge walls instead so now they have to tackle that. They would have been better putting in excellent ventilation. 

 

You can't argue with water as anyone who has been to the grand canyon knows, so if your building can't keep it out (no foundations / DPC) then you have to let it escape. Or just cross your fingers and hope you don't have a lifetime adressing damp symptoms. 

 

It can be dismaying to find you are trying to bolt on unsuitable methods to an old house, modern materials are often cheaper up front and people are familiar with them. But, if the house is to be your home for years and you want to do it right and do it once, you should educate yourself what you're tackling to make the decision. 

[Snowbeetle]

Good video

 

Problems With Cement in Traditional Buildings - Video | Unity Lime

 

Yes, lime+wood fibre will be breathable. The cement render isn't of course which may or may not be an issue - any moisture getting into your brick can only escape inward but if the volume and rate is low that isn't necessarily going to cause problems. You could consider altering that, but costs / benefits etc all need to work and some bod like me on the internet can't tell you that. All I'm trying to do is flag up potential pitfalls for you. 

My previous house had an unsuitable insulation applied, sadly I didn't have the knowledge then and the advice I was given was ill-informed (I now know). Unfortunately the people who bought it will be suffering the symptoms of that for years (I didn't know when I sold it). It is common unfortunately. 

 

 

[Snowbeetle]

sorry that wasn't the video I thought it was...not actually that educational at all...

[Snowbeetle]

Much better place to acquire information/knowledge

 

Search our Knowledgebase | www.spab.org.uk

[Snowbeetle]

Breathable alternative to plasterboard..

Wood Wool Board | Savolit Plus | Natural Plasterboard (mikewye.co.uk)

 

 

[Snowbeetle]

Incidentally, though wood fibre is breathable you still need to use it in the right way as it is also hygroscopic which will mean it doesn't dissipate the water molecules so whether it is right for your application depends on ventilation and substrates  and what they are doing in the performance of your building. If you haven't checked I would be tempted look into it if I was you. 

[larry]

Thank you so much again for coming back to this, this is a mine of information and hugely helpful. I loved the " you can't argue with water as anyone who has been to the grand canyon knows" quote, I'll remember that one!!  (I haven't, by the way!)

 

I've actually done hours (days) of reading around this - though ultimately I end up feeling like I'm chasing my tail with this - I'm not a tradesperson but have a sciency mind and always like to understand things for myself hence why I appreciate your reply all the more. Some of the things you'd flagged I'd already 'worked through' in my mind. The cement render on the outside has been there a long time, it's possibly ? original but I'm fairly certain cement not lime (Youtube suggested pouring vinegar over a small section to see how much it fizzed, and the lime plaster was much fizzier!). But obviously somebody has added the extra layers to the internal wall more recently. 

 

We've fitted an MVHR system already - and yes I can see the multiple arguments to and fro doing this in an old house, but already we notice improvements in air quality even if it isn't doing loads for 'heat recovery', and as we're doing a lot of work with access to floorboards and ceilings it felt like the right time to do it. So that may hopefully help with the ventilation side. 

 

In terms of the 'is gypsum breathable' or 'is plasterboard breathable' question, you are clear that it is not, but the reason I am so confused about this is because I have read people stating this (agreeing with you) but also the opposite in equal extremes:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9580

https://www.engineeringtoolbox.com/vapour-resistance-d_1807.html

https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/building_materials_property_table.pdf

https://www.greenbuildingadvisor.com/question/gypsum-board-vapor-permeable-or-what ("very permeable")

 

This must be an issue of fact, surely? Or there must be other elements of the equation that I am missing?

 

In any case, the Mike wye plasterboard definitely looks something to look into - I haven't seen that so thank you for sharing/suggesting that link

[larry]

And this one: https://strawbalebuildinguk.com/wp-content/uploads/2017/09/breathability.pdf

[Snowbeetle]

I think the issue is in the definition of 'breathable'. Can wet pass through it... Yes, but will it actively facilitate it's movement...no. So for instance, wool insulation will let wet go through it. But I it doesn't actively assist. Imagine a soggy tweed wool jacket. It will stay wringing wet until you hang out by the fire or get a nice breeze going over it. So it its not that it is impermeable like a sheet of plastic so much that it isn't dissipating and moving the water on and out. Wood is the same. You can use them but where you know that draughts it warmth will stop them from sitting soggy. 

Gypsum doesn't like water, it tends to soften the plaster and cause it to fail if it is moist often or stays moist. So it isn't like a plastic sheet but it isn't happy acting as a member of the team in a breathable system. 

Lime plaster acts more like blotting paper, put a drop on it and it will disperse and keep dispersing until it is down to the molecular level and easy to vaporise and disappear. This causes the line no harm. It's on the team as it were. 

 

The problem lies in the fact that this field of expertise is still going through a process of formalising and professionalising (as I mentioned up thread), so some of the terminology is used interchangeably and doesn't always have consistency of meaning. 

 

Things like wood and wool can still be used but you would need to keep in mind their properties and not put them somewhere where the hygroscopic nature is going to be an issue. 

 

There are some minerals etc you can add to lime plaster to create an insulating effect, or to buffer and manage water even more effectively than stand alone i.e hemp or certain minerals... Ty Mawr in Wales do some courses which are for folk like us which you might find interesting. 

 

It seems like a dark art, it's not, it's just that the development of a body of expertise is relatively limited and not as easy to summarise due to the historical factors. It is much better than it was though. 

[Snowbeetle]

Hope you can decipher the autocorrect garble. 

[Snowbeetle]

Thanks for the links, I haven't referred to those for my reply as not got time to look just now. I'm always interested and curious though (much like you) so I'll check them out later. 

If you want the details of the lime plasterer eventually let me know. 

[offthepiste]

Not sure if this topic thread is dead but ....  a daft question.

 

Definitely going to do IWI.

 

Current state of plastered walls is sound. They are painted with emulsion ... probably Dulux .. possibly trade but maybe not.

 

I would prefer to retain plaster as it provides largely good airtight barrier .. where any cracks or gaps etc can be resolved. It would feel perverse to hack it off and then either do a parge coat of lime plaster say, or worse stick that blue membrane I have seen in renovation photos. My gut feel is that sticky tape, however industrial, may not last.

 

So if I retain and make good exisiting plaster then I suspect the emulsion is not breathable.

 

My guess is then to steam and scrape off rather than replaster.

 

Any advice, cheers, thank you in anticipation

 

 

[larry]

On 16/10/2021 at 14:53, offthepiste said:

Not sure if this topic thread is dead but ....  a daft question.

 

Definitely going to do IWI.

 

Current state of plastered walls is sound. They are painted with emulsion ... probably Dulux .. possibly trade but maybe not.

 

I would prefer to retain plaster as it provides largely good airtight barrier .. where any cracks or gaps etc can be resolved. It would feel perverse to hack it off and then either do a parge coat of lime plaster say, or worse stick that blue membrane I have seen in renovation photos. My gut feel is that sticky tape, however industrial, may not last.

 

So if I retain and make good exisiting plaster then I suspect the emulsion is not breathable.

 

My guess is then to steam and scrape off rather than replaster.

 

Any advice, cheers, thank you in anticipation

 

 

 

I've used a scraper with a Tungsten Carbide blade recently, screwfix had one for about £9 and it has been absolutely incredible, though more for gloss than walls. Buy a few spare blades if you're doing a large area though 

 

Your logic all seems sound to me but I am definitely not commenting from a position of expertise.

What sort of plaster is it? If gypsum skim then I guess quite likely to have a PVA coating underneath which will not be breathable even if you strip the paint off. 

 

[SimonD]

On 16/10/2021 at 14:53, offthepiste said:

Any advice, cheers, thank you in anticipation

 

 

What is the full buildup of your walls?

 

Recent research on gypsum shows that it is indeed 'breathable.' It provides some vapour permeability and it has very good moisture buffering values. The problem tends to be if it get too wet. But as you've mentioned the paint poses some questions and as @larry says you may have a dollop of pva in there too. Lime is actually not very good at all from a moisture buffering capacity, but functions very well in vapour permeability. The difference is down to pore structure with lime having macro-pore structure and gypsum having both micro and macro-pore structure, with larger macro-pores. Gypsum also has higher pore volume than lime. SPAB now explicitly differentiates between the functions of vapour permeability and moisture buffering/capillary action of materials and where they might work best in old buildings.

 

The difference in how the materials work is that vapour permeability works through vapour pressure gradient and with hygroscopicity and capillary materials moisture transport is driven by relative humidity and capillary pressure.

 

It's a thorny topic, mind you.

 

My perspective is that if your whole wall buildup provides for the passing of moisture either as vapour and/or capillary action, then great. If not, look at how the external wall may perform outside of your gypsum layer with potential barriers of paint/pva to ensure moisture doesn't get trapped. Then you can utilise the moisture buffering capacity of your woodfibre and wall finish for indoor air quality - this can actually reduce ventilation demand in the house, but ventilation is nevertheless an essential component part in upgrading thermal performance in your home and should always be done in sync with the upgrades. Just make sure you do a condensation analysis on the proposed buildup before going ahead.

[larry]

On 21/10/2021 at 12:05, SimonD said:

 

What is the full buildup of your walls?

 

Recent research on gypsum shows that it is indeed 'breathable.' It provides some vapour permeability and it has very good moisture buffering values. The problem tends to be if it get too wet. But as you've mentioned the paint poses some questions and as @larry says you may have a dollop of pva in there too. Lime is actually not very good at all from a moisture buffering capacity, but functions very well in vapour permeability. The difference is down to pore structure with lime having macro-pore structure and gypsum having both micro and macro-pore structure, with larger macro-pores. Gypsum also has higher pore volume than lime. SPAB now explicitly differentiates between the functions of vapour permeability and moisture buffering/capillary action of materials and where they might work best in old buildings.

 

The difference in how the materials work is that vapour permeability works through vapour pressure gradient and with hygroscopicity and capillary materials moisture transport is driven by relative humidity and capillary pressure.

 

It's a thorny topic, mind you.

 

My perspective is that if your whole wall buildup provides for the passing of moisture either as vapour and/or capillary action, then great. If not, look at how the external wall may perform outside of your gypsum layer with potential barriers of paint/pva to ensure moisture doesn't get trapped. Then you can utilise the moisture buffering capacity of your woodfibre and wall finish for indoor air quality - this can actually reduce ventilation demand in the house, but ventilation is nevertheless an essential component part in upgrading thermal performance in your home and should always be done in sync with the upgrades. Just make sure you do a condensation analysis on the proposed buildup before going ahead.

 

Blimey Simon you are a wizard. 

You clearly know your onions here. If you fancy writing a 'shoulds and should nots' list for people facing the task of IWI I'm sure a lot of people would benefit. Theres a lot of (at least apparently) conflicting info out there for us DIY but do our bit to save the world types. I've now finished my second room, they were the easy ones! Learnt a lot though already....

[SimonD]

12 hours ago, larry said:

 

Blimey Simon you are a wizard. 

You clearly know your onions here. If you fancy writing a 'shoulds and should nots' list for people facing the task of IWI I'm sure a lot of people would benefit. Theres a lot of (at least apparently) conflicting info out there for us DIY but do our bit to save the world types. I've now finished my second room, they were the easy ones! Learnt a lot though already....

 

Hey Larry, no not a wizard at all, just a DIY self-builder who got sucked into particular building methods only to find a lot of the information is bull, or should I politely say misunderstood and at the very least incomplete? I've just gone into full blown nerd mode muddling through to pick the wheat from the chaff so that I can build my house properly. Previously I've also been through the process of retrofitting a mid-19th century place only to find out that even after adding ventilation measures the house is riddled with new-found mould growth, needing a weekly spray of white vingar, bleach, and other such things all over the place and having wall paper start to peel of the walls in places because of excess moisture. The other side to it is seeing my mother's house in the middle of Sweden, also built in the mid 1800s with timber frame and very well insulated with saw dust (which is typically topped up every decade or so), still function amazingly both when it's cold and during heat waves of 30 deg C - it's the exact opposite of damp draughty UK housing stock - and happily heated using a heatpump. Weirdly it is a house that breathes very well which you can tell by the pleasant indoor air environment when you're in it yet uses gypsum plaster throughout. I've just wanted to understand the building physics around it all.

 

For me, the problem is going to be disseminating all the current techical information and research as it comes out in the hope that it can dispel the myths and is readily understandable for DIYer and builder alike. It's going to be especially important in the next decade or so as we try to upgrade thermal performance of existing house stock.

[larry]

On 23/10/2021 at 10:23, SimonD said:

 

Hey Larry, no not a wizard at all, just a DIY self-builder who got sucked into particular building methods only to find a lot of the information is bull, or should I politely say misunderstood and at the very least incomplete? I've just gone into full blown nerd mode muddling through to pick the wheat from the chaff so that I can build my house properly. Previously I've also been through the process of retrofitting a mid-19th century place only to find out that even after adding ventilation measures the house is riddled with new-found mould growth, needing a weekly spray of white vingar, bleach, and other such things all over the place and having wall paper start to peel of the walls in places because of excess moisture. The other side to it is seeing my mother's house in the middle of Sweden, also built in the mid 1800s with timber frame and very well insulated with saw dust (which is typically topped up every decade or so), still function amazingly both when it's cold and during heat waves of 30 deg C - it's the exact opposite of damp draughty UK housing stock - and happily heated using a heatpump. Weirdly it is a house that breathes very well which you can tell by the pleasant indoor air environment when you're in it yet uses gypsum plaster throughout. I've just wanted to understand the building physics around it all.

 

For me, the problem is going to be disseminating all the current techical information and research as it comes out in the hope that it can dispel the myths and is readily understandable for DIYer and builder alike. It's going to be especially important in the next decade or so as we try to upgrade thermal performance of existing house stock.

It absolutely is, isn't it? It really strikes me that in the same way that we have all become mini epidemiologists in the last 18 months we now need to become mini building scientists. Everybody needs to take an interest in their house and understand roughly how it was built and what that means, particularly anybody living in an older place . This knowledge needs to be taught in schools. And builders also need so much more of this information. It's a huge task for government and it feels like the sense of urgency is just missing. Anyway, I go on...

 

We are retrofitting (1920s with solid walls) but doing it slowly, room by room, so we can still live in it. As I say, huge amounts of learning and so surprised at how much you have to dig to get answers to questions and problems which many people must also have had to ask/answer.