IWI Wood fibre alternative build up using OSB or Woodwool

[George]

Has anyone tried this construction:

 

- solid wall masonry

- levelling lime plaster ~15mm

- wood fibre, say 60mm - 100mm board

- 11mm osb

- 9.5mm plasterboard + skim

 

 

Or substitute the 11mm OSB and plasterboard for a woodwool board and lime skim.

 

The reason I ask is that I've done most of the house with wood fibre + lime plaster with fibreglass mesh. But I've one room left which is a small shower room. Because this will need various fixings to be drilled into the wall, a 'structural' board covering the wood fibre would be very beneficial.

 

I did something similar in one very small part in another bathroom - covered the wood fibre with OSB then had a service void gap, then tile backer board.

[Roger440]

4 hours ago, George said:

Has anyone tried this construction:

 

- solid wall masonry

- levelling lime plaster ~15mm

- wood fibre, say 60mm - 100mm board

- 11mm osb

- 9.5mm plasterboard + skim

 

 

Or substitute the 11mm OSB and plasterboard for a woodwool board and lime skim.

 

The reason I ask is that I've done most of the house with wood fibre + lime plaster with fibreglass mesh. But I've one room left which is a small shower room. Because this will need various fixings to be drilled into the wall, a 'structural' board covering the wood fibre would be very beneficial.

 

I did something similar in one very small part in another bathroom - covered the wood fibre with OSB then had a service void gap, then tile backer board.

 

Why are you using lime and wool board? To ensure breathability of the solid wall i presume?

 

If so, wacking a sheet of plasterboard on is going to undo you good work.

 

Either, ensure the whole buildup is properly breathable, or dont bother, and use normal plaster and PIR, which would be significantly cheaper.

[George]

Yeah solid wall. 

 

As I understand it, gypsum plaster is vapour permeable - it just doesn't react well to water. But being on the inner face of an insulated build up should prevent condensation within the plasterboard.

[Roger440]

30 minutes ago, George said:

Yeah solid wall. 

 

As I understand it, gypsum plaster is vapour permeable - it just doesn't react well to water. But being on the inner face of an insulated build up should prevent condensation within the plasterboard.

 

Gypsum plaster is vapour permeable. Almost everything is. The question is by how much

 

I can assure you, compared to the rest of yourbuild up, not even close.

 

Im my house it acted as an amazing barrier to vapour. the wall behind was sopping wet. If you put gypsum plaster on it, it will be even worse.

 

As i said, if you are using plasterboard, no point spending big on woolboard and lime.

[jfb]

What area of external wall are we talking about? Internal walls don’t need to be breathable so you can use whatever suits (moisture board, hardibacker ). If it is only a small area I’m not sure I would worry too much about it (especially if upstairs room). Could even skip the wood fibre and just use insulated plasterboard or something similar.

 

[jayc89]

I've been going around in circles on something similar. Solid brick walls, pointed in lime, not rendered. I want to insulate it but can't decide what to do. Locally I haven't found any plasterer who will work with lime (so are they really plasterers!?) so my finish needs to be Gypsum.

I've come down to two options;

1) External Wall -> Breather Membrane -> Wool Insulation between studs -> Variable AVCL -> Cross-Batten for Service Cavity -> PB + Skim. The Service Cavity also acts to separate the breathable materials from the PB. Although there will be little ventilation behind there.

2) External Wall -> PIR stuck to wall + taped -> AVCL -> Studs through to External Wall (to act as Mechanical Fixings for PIR + Service Cavity) -> PB + Skim. 

 

Option 1) Is breathable up to the Service Cavity. Using a variable AVCL should prevent moisture passing through the insulation from the inside during winter, which along with friction fitted insulation should help prevent condensation, and allow any moisture in the wall to diffuse into the house during summer. My concern there is it would diffuse into the unventilated service cavity (once Skirting and Architrave is fitted, it's effectively a sealed space, so my assumption is that it would ultimately lead to damp. Whereas option 2) attempts to prevent the passage of all moisture. If moisture does make it to the inside face of the wall (broken gutter, drain, raised ground levels etc). It's effectively trapped forever as the insulation should prevent the heat needed to dry it out from ever reaching it.

 

Option 1) feels like the least worst option as it's 1) easier to monitor (just pop a socket front off and check within the Service Cavity, even pop some moisture probes in there to track seasonal changes for good measure) and 2) easier to remedy any problems then having to strip everything back to brick again...

 

I don't have any answers, but writing it all out was good therapy, so thanks

Edited December 20, 2022 by jayc89

[Roger440]

1 hour ago, jayc89 said:

I've been going around in circles on something similar. Solid brick walls, pointed in lime, not rendered. I want to insulate it but can't decide what to do. Locally I haven't found any plasterer who will work with lime (so are they really plasterers!?) so my finish needs to be Gypsum.

I've come down to two options;

1) External Wall -> Breather Membrane -> Wool Insulation between studs -> Variable AVCL -> Cross-Batten for Service Cavity -> PB + Skim. The Service Cavity also acts to separate the breathable materials from the PB. Although there will be little ventilation behind there.

2) External Wall -> PIR stuck to wall + taped -> AVCL -> Studs through to External Wall (to act as Mechanical Fixings for PIR + Service Cavity) -> PB + Skim. 

 

Option 1) Is breathable up to the Service Cavity. Using a variable AVCL should prevent moisture passing through the insulation from the inside during winter, which along with friction fitted insulation should help prevent condensation, and allow any moisture in the wall to diffuse into the house during summer. My concern there is it would diffuse into the unventilated service cavity (once Skirting and Architrave is fitted, it's effectively a sealed space, so my assumption is that it would ultimately lead to damp. Whereas option 2) attempts to prevent the passage of all moisture. If moisture does make it to the inside face of the wall (broken gutter, drain, raised ground levels etc). It's effectively trapped forever as the insulation should prevent the heat needed to dry it out from ever reaching it.

 

Option 1) feels like the least worst option as it's 1) easier to monitor (just pop a socket front off and check within the Service Cavity, even pop some moisture probes in there to track seasonal changes for good measure) and 2) easier to remedy any problems then having to strip everything back to brick again...

 

I don't have any answers, but writing it all out was good therapy, so thanks

 

Option 3) use a system/build up that is breathable from the base to the surface. That includes the paint.

 

Your option 1 is pointless for the reasons i outlined earlier. Yes, you can check afterwards and see whats happening. Then rework accordingly. Why not do it properly first time?

 

The key decision is, does it need to be breathable in the first place.? Thats going to depend on wall construction, ground conditions and a bunch of other variable unique to your house.

 

If it does, option 3, if it doesnt option 2.

[Roger440]

2 hours ago, jfb said:

What area of external wall are we talking about? Internal walls don’t need to be breathable so you can use whatever suits (moisture board, hardibacker ). If it is only a small area I’m not sure I would worry too much about it (especially if upstairs room). Could even skip the wood fibre and just use insulated plasterboard or something similar.

 

 

That incorrect. If the internal wall is effectively the same construction and has no DPC, then any issues with the outerwalls may well be present, indeed, likely to be on the internal walls. My internal wall was worse than the external as both sides were plastered with gypsum so nowhere for the moisture to go. At least the outsede walls had one exposed face.

[jayc89]

28 minutes ago, Roger440 said:

 

Option 3) use a system/build up that is breathable from the base to the surface. That includes the paint.

 

Your option 1 is pointless for the reasons i outlined earlier. Yes, you can check afterwards and see whats happening. Then rework accordingly. Why not do it properly first time?

 

The key decision is, does it need to be breathable in the first place.? Thats going to depend on wall construction, ground conditions and a bunch of other variable unique to your house.

 

If it does, option 3, if it doesnt option 2.

 

For the reason stated in my first paragraph - It's near impossible finding someone local to plaster in lime. The nearest I've found are based in the Dales and wanted travel + accommodation costs covering (understandably) but ramps the cost of plastering a single room up considerably. 

[Roger440]

1 hour ago, jayc89 said:

 

For the reason stated in my first paragraph - It's near impossible finding someone local to plaster in lime. The nearest I've found are based in the Dales and wanted travel + accommodation costs covering (understandably) but ramps the cost of plastering a single room up considerably. 

 

Get yourself on the traditional and listed building advice facebook group. You will find the people you need on there.

[jayc89]

19 minutes ago, Roger440 said:

 

Get yourself on the traditional and listed building advice facebook group. You will find the people you need on there.

 

I already am, plus others. Asked many times. Managed to find a guy to fit our new window heads and tidy up some brickwork using a hot lime mix mortar, but no one for plastering, unfortunately. 

[SteamyTea]

As this is an engineering problem, do you have a WVTR for all the materials you are considering?

 

Good luck getting it from the lime plaster people. They seem very coy about the numbers.

[jayc89]

20 minutes ago, SteamyTea said:

As this is an engineering problem, do you have a WVTR for all the materials you are considering?

 

Good luck getting it from the lime plaster people. They seem very coy about the numbers.

 

And they rubbish numbers that are published but don't suit their ideals.

 

By WVTR I assume you mean Water Vapour Transmission Rate, which I've generally seen published is the SD value. Anything with an SD value below 0.05m is generally considered acceptable for solid wall structures. 

 

Breather membranes and AVCLs advertise their values quite clearly. Tyvek HouseWrap has a SD value of 0.01m, for example. I've only been able to find anecdotal SD values for Rockwool, but they suggest 0.3m. Interestingly wood fibre is significantly higher; 80mm of Steico ThermDry has a claimed SD value of 0.24m, suggesting it's less breathable than RockWool...

[Roger440]

1 hour ago, SteamyTea said:

As this is an engineering problem, do you have a WVTR for all the materials you are considering?

 

Good luck getting it from the lime plaster people. They seem very coy about the numbers.

 

You can get the SD numbers for lime stuff.

 

[bighouseproblems]

On 19/12/2022 at 16:23, George said:

Has anyone tried this construction:

 

- solid wall masonry

- levelling lime plaster ~15mm

- wood fibre, say 60mm - 100mm board

- 11mm osb

- 9.5mm plasterboard + skim

 

 

Or substitute the 11mm OSB and plasterboard for a woodwool board and lime skim.

 

The reason I ask is that I've done most of the house with wood fibre + lime plaster with fibreglass mesh. But I've one room left which is a small shower room. Because this will need various fixings to be drilled into the wall, a 'structural' board covering the wood fibre would be very beneficial.

 

I did something similar in one very small part in another bathroom - covered the wood fibre with OSB then had a service void gap, then tile backer board.

Hi George, 

 

I myself have been round in circles for quite sometime and have posted previously about the best way forward with our house. I have come to the point where i am going down the route of

Solid Wall Masonry

Lime Plaster

Wood Fibre 60mm most likely

Mesh 

Lime Plaster

 

How long ago have you completed some of your works and how have you found the improvements. I never intended to tear out the whole house so want to hear some experiences of doing it.

 

Also did you insulate your ground floor.

I have mocked up this so far

Wood fibre boards down the wall butted with wood fibre between the joists followed by an airtight membrane which proceeds up the wall before being lime plastered over.

 

Regards

[jfb]

On 20/12/2022 at 13:44, Roger440 said:

That incorrect. If the internal wall is effectively the same construction and has no DPC, then any issues with the outerwalls may well be present, indeed, likely to be on the internal walls. My internal wall was worse than the external as both sides were plastered with gypsum so nowhere for the moisture to go. At least the outsede walls had one exposed face.

I stand corrected. How about if the shower is upstairs - did you think to ask that?

[jfb]

4 minutes ago, bighouseproblems said:

Wood fibre boards down the wall butted with wood fibre between the joists followed by an airtight membrane which proceeds up the wall before being lime plastered over.

Why the airtight membrane after the boards? Normally the render behind the boards would be the airtight layer and so no need for a membrane. 
also less worry about penetrations for electrics. What membrane were you thinking of?

 

I’ve done a couple of house renovations with the buildup you have suggested and have been happy with the results. They were both full renovations with floor dug out and insulated/new slab with Ufh. Harder to get the detailing right if done bit by bit but you seem on the right track (like making sure the IWI goes continuously up the wall at ceiling/floor level).

[George]

Yeah I did the (slightly older) Warmshell build up using mechanical fixings, mesh and lime plaster. 60mm on the south, west & east walls and 40mm on the north face (to allow more heat to keep frost damage reduced). No vapour control layers or anything - as per Lime Green specs (as was). Now they recommend an adhesive to fix the boards I think. 

 

Much of the original lime plaster was sound with only wallpaper on it, so I just scraped that off and fixed the wood fibre in place. One or two rooms need gypsum knocking off - there I did wherever was damp and/or the lower 500-1000mm and plastered in Duro or Ultra if it was an external wall. In a very damp porch/boot room all of the gypsum was taken off. Did a lot of airtightness with loft hatches, new external doors and checked the UPVC windows were sealed and working well.

 

Completed in March 2021 and has performed very well. On pure u-value calculations it has exceeded them based on energy consumption.

 

(This does require quite a few assumptions but actual ASHP energy use is ~3500kWh, floor area 176m^2. Assuming SCOP between 2.7 to 4.2, gives a energy demand 55 to 85kWh/m^2. i mean, there's a LOT of things missing - occasional log burner, effect of the MVHR, heat from domestic activity, people, cooking etc etc but in terms of order of magnitude it's made the house comparable to a non-passiv new build, circa 2005)

 

The only locations I've had salts coming through is where I'd left gypsum in place on internal walls. I think what has happened is the masonry behind was saturated and so with cyclical interstitial condensation, is taking a very long time to dry out (which would be at most 1mm a day / ~ a year at best). Not worried though - I'll just vacuum off the salts and I'll repaint when its settled down. Certainly not related to either lime or the wood fibre. 

 

Extension I redid the floor. I did use glaspor, but with a concrete slab. Thinking being that the old walls would beenfit form there being a capillary break below the slab. With a lime plastered wall I'm not sure I'd be too concerned using modern insulation on floors as breathability is less of an issue with the floor. Rooms with existing concrete slabs I just left them and put in a thick carpet underlay. Central heating system was new and large rooms so having big radiators was OK. In any case, heat loss through floors isn't massive. Rooms with earth floors with original terracotta pavers I (of course) left - indeed, I exposed them and cleaned them up. Put a few rugs down but they're warmer than concrete.

 

Extra insualtion in the loft.

 

If I were doing it again I would do a better job on the inter-floor insulation, that is, around the outside, between the wood fibre boards. What I did was use wool insulation to pack the floor void. It works well (as evidenced by frost marks on the outside of the house) and I was concerned about the timbers getting too cold and causing condensation. But thinking back, continuous wood fibre insulation would have been better. I was working alone with a young child and it was Covid so I'm not too hard on myself...!

 

While I'm here I'd actually also completely change the heating system if I were doing it again. Wha tI have is a single 16kW ASHP to do upstairs and downstairs UFH + radiators and DHW.

 

What I would rather is if I'd used a much smaller ASHP to do hot water and the UFH/downstairs radiators. Then upstairs I'd have air to air multi-split to do heating and cooling for the bedrooms. The (vast) amount of money saved I'd put into a new Eco-design wood burner to give both systems a boost when very cold. 

 

[Roger440]

30 minutes ago, jfb said:

I stand corrected. How about if the shower is upstairs - did you think to ask that?

 

Sorry, not sure what you are asking/suggesting?

[Jilly]

Have you considered clay plaster instead, as it is supposed to be breathable and buffer humidity like lime? Also more expensive than gypsum, but without the risk of alkali burns, which I assume is what puts plasters off, as they do get quite splattered. 

Edited December 21, 2022 by Jilly

[bighouseproblems]

4 hours ago, George said:

Yeah I did the (slightly older) Warmshell build up using mechanical fixings, mesh and lime plaster. 60mm on the south, west & east walls and 40mm on the north face (to allow more heat to keep frost damage reduced). No vapour control layers or anything - as per Lime Green specs (as was). Now they recommend an adhesive to fix the boards I think. 

 

This is the exact method I have landed on. I will most likely use the adhesive and mechanicals with the thermal break (Plastic i believe)

 

4 hours ago, George said:

 

Much of the original lime plaster was sound with only wallpaper on it, so I just scraped that off and fixed the wood fibre in place. One or two rooms need gypsum knocking off - there I did wherever was damp and/or the lower 500-1000mm and plastered in Duro or Ultra if it was an external wall. In a very damp porch/boot room all of the gypsum was taken off. Did a lot of airtightness with loft hatches, new external doors and checked the UPVC windows were sealed and working well.

 

My focus has come to airtightness I am going to visit a supplier in the near future to discuss and see what they can offer. We had triple glazing installed this year which has made a big difference although i need to spend some time doing the details work with some airtight tape as currently we have lots of exp foam exposed.

 

4 hours ago, George said:

 

Completed in March 2021 and has performed very well. On pure u-value calculations it has exceeded them based on energy consumption.

 

(This does require quite a few assumptions but actual ASHP energy use is ~3500kWh, floor area 176m^2. Assuming SCOP between 2.7 to 4.2, gives a energy demand 55 to 85kWh/m^2. i mean, there's a LOT of things missing - occasional log burner, effect of the MVHR, heat from domestic activity, people, cooking etc etc but in terms of order of magnitude it's made the house comparable to a non-passiv new build, circa 2005)

 

Our house is roughly 160sqm not including the loft so of a similar size to yours. I investigated a ASHP pump a number of times but have fallen on the idea of keeping the central heating system in place and opening up some existing fireplaces and having some log burners installed. I have my boiler currently set to 50C and over 2-3 hours it warms up enough. I worked out If i set it to 45C and run it for 10 hours a day at the current gas price it would be around £5 a day. I am hopeful i can achieve this once the property is insulated.

 

4 hours ago, George said:

 

The only locations I've had salts coming through is where I'd left gypsum in place on internal walls. I think what has happened is the masonry behind was saturated and so with cyclical interstitial condensation, is taking a very long time to dry out (which would be at most 1mm a day / ~ a year at best). Not worried though - I'll just vacuum off the salts and I'll repaint when its settled down. Certainly not related to either lime or the wood fibre. 

 

Extension I redid the floor. I did use glaspor, but with a concrete slab. Thinking being that the old walls would beenfit form there being a capillary break below the slab. With a lime plastered wall I'm not sure I'd be too concerned using modern insulation on floors as breathability is less of an issue with the floor. Rooms with existing concrete slabs I just left them and put in a thick carpet underlay. Central heating system was new and large rooms so having big radiators was OK. In any case, heat loss through floors isn't massive. Rooms with earth floors with original terracotta pavers I (of course) left - indeed, I exposed them and cleaned them up. Put a few rugs down but they're warmer than concrete.

 

We have suspended timber floor aside from the main hallway that run front to back about 8 meters long. I will most likely just remove the tiles and resurface it before putting down a hefty underlay. My logic for using woodfibre under the floorboards is simply to use the same products around the property. 

4 hours ago, George said:

 

Extra insualtion in the loft.

 

We had nothing and currently have around 200-300 in some places but i did this on my own and it was hard to fit through to the eaves area so i will definitely be redoing this area again with more care.

 

4 hours ago, George said:

 

If I were doing it again I would do a better job on the inter-floor insulation, that is, around the outside, between the wood fibre boards. What I did was use wool insulation to pack the floor void. It works well (as evidenced by frost marks on the outside of the house) and I was concerned about the timbers getting too cold and causing condensation. But thinking back, continuous wood fibre insulation would have been better. I was working alone with a young child and it was Covid so I'm not too hard on myself...!

 

This is one consideration i have put as quite a high priority. when renovating the downstairs rooms i will be pulling down the ceiling and packing out the floor voids. I look after my young daughter also and time is severely limited in which i can get things done. She will be attending nursery in august next year so that is when the project will kick in to full go.

 

 

4 hours ago, George said:

 

While I'm here I'd actually also completely change the heating system if I were doing it again. Wha tI have is a single 16kW ASHP to do upstairs and downstairs UFH + radiators and DHW.

 

What I would rather is if I'd used a much smaller ASHP to do hot water and the UFH/downstairs radiators. Then upstairs I'd have air to air multi-split to do heating and cooling for the bedrooms. The (vast) amount of money saved I'd put into a new Eco-design wood burner to give both systems a boost when very cold. 

 

I have recently been debating about a wood burning rayburn, They can run up to 10 radiators and my logic is to install it into a UFH system. I am hoping that in the future we will be able to get some solar and feed a HW tank from that.

I looked at a mini split system myself but we have tilt and turn so even in the summer we can get good ventilation.

 

4 hours ago, George said:

 

Just wanted to say thank you for your full and comprehensive response it has given me some good insight and inspiration i am going in the right direction.

 

Did you undertake the work all yourself and what is your background work wise. How did you find the lime plastering and what thickness did you end up with?

 

Regards

 

Leigh

[jfb]

18 hours ago, Roger440 said:

 

19 hours ago, jfb said:

I stand corrected. How about if the shower is upstairs - did you think to ask that?

 

Sorry, not sure what you are asking/suggesting?

 

I think I am just saying that it seems very relevant whether the shower is upstairs (they often are) or down. If upstairs then I wouldn’t worry about internal walls.

[George]

It is a downstairs shower. 

 

Yes I will stick with wood fibre and lime. Will add timber battens for the fixings. 

[Roger440]

8 hours ago, jfb said:

I think I am just saying that it seems very relevant whether the shower is upstairs (they often are) or down. If upstairs then I wouldn’t worry about internal walls.

 

Agreed.