Insulation PIR vs Wood Fibre

[stamas01]

Hi all,

So I bought a period property in Scottland that was built around 1900. I want to insulate the internal walls (I cannot insulate the outside as it is in a Conservation area). I did my research, and I am now considering two options.

Option 1: Celotex GA4100 General Purpose PIR Insulation Board - 2400 X 1200 X 100mm
Option 2: STEICO Therm Wood Fibre board

PIR:
Advantages:

• Much better R-Value (the higher, the better): According to the Celotex datasheet, a 100mm insulation achieves 4.5 R-Value (m².K/W) .
• Easier installation, as I can use a plasterboard over it.

Disadvantages:

• According to Scotland-specific research (Guide to Energy Retrofit of Traditional Buildings), PIR-type insulation must only be used if a complete condensation analysis confirms that it can be used. Unfortunately, for that reason, the study excludes PIR insulation and does not compare it to the other options.
• Given the wall type, we must maintain a well-ventilated air gap behind the insulation. As it is not trapped air, does it not somewhat decreases the insulation capability compared to wood fibre, which does not require an air gap?
• If it is not airtight, there is a severe risk of condensation.

Wood fibre:
Advantages

• Breathable. This approach does not require ventilated air gap.
• It does not have to be airtight, and if cracks appear over time, it won't cause worry.

Disadvantages:

• Much lower R-Value (the higher, the better): Documentation states it achieves 2.5 R-Value (m².K/W) at 100mm.
• Hard to install as I have to lime plaster over it.

Our wall is about 600mm "mass masonry wall" and is internally finished with lath and plaster. Given that plaster is not breathable, I can't see much difference between PIR with airgap and lath and plaster or is there? Also, I find it difficult to translate the R-value difference to the money spent on heating during the winter. Can someone comment on this as well? Based on your experiences and expertise, would you choose the PIR or wood fibre option, and why?

Thanks for all the help!

 

[malcolm]

It's a tricky one. Where I've used Celotex on the inside I've had an air gap ventilated to the roof space. I used it because I was replacing a timber frame forming a bedroom wall and there was someone using the bedroom at the time. I think it is important the air space is ventilated and that means the only insulation value to the wall is from the Celotex itself. I worry a bit about the Grenfell tower effect especially as the consumer unit is mounted on one of those walls but I have a decent fire alarm system.

I've been tending to use woodfibre board direct to lime plastered walls elsewhere. I've got bricks and the R value of bricks is about twice as good when they are dry and a bit of heat passing through helps keep them dry once the problems outside are sorted so they also provide a bit of insulation. You also get an indication when things have gone wrong that you won't get with Celotex.

I think keeping the outside walls dry and knowing there isn't any mould is the main reason I'm using woodfibre. I feel it will last longer when people mess with it at a later date. Though I think both systems are valid. In this age of expensive I'm planning to heat only one room with a woodburner and have the rest of the house tolerable.

 

[stamas01]

It's a tricky one. Where I've used Celotex on the inside I've had an air gap ventilated to the roof space. I used it because I was replacing a timber frame forming a bedroom wall and there was someone using the bedroom at the time. I think it is important the air space is ventilated and that means the only insulation value to the wall is from the Celotex itself. I worry a bit about the Grenfell tower effect especially as the consumer unit is mounted on one of those walls but I have a decent fire alarm system.

I've been tending to use woodfibre board direct to lime plastered walls elsewhere. I've got bricks and the R value of bricks is about twice as good when they are dry and a bit of heat passing through helps keep them dry once the problems outside are sorted so they also provide a bit of insulation. You also get an indication when things have gone wrong that you won't get with Celotex.

I think keeping the outside walls dry and knowing there isn't any mould is the main reason I'm using woodfibre. I feel it will last longer when people mess with it at a later date. Though I think both systems are valid. In this age of expensive I'm planning to heat only one room with a woodburner and have the rest of the house tolerable.

Thanks, @malcolm . This is precisely the type of reply I was looking for! I appriciate it.

 

[Lacan07]

Hi all,

So I bought a period property in Scottland that was built around 1900. I want to insulate the internal walls (I cannot insulate the outside as it is in a Conservation area). I did my research, and I am now considering two options.

Option 1: Celotex GA4100 General Purpose PIR Insulation Board - 2400 X 1200 X 100mm
Option 2: STEICO Therm Wood Fibre board

PIR:
Advantages:

• Much better R-Value (the higher, the better): According to the Celotex datasheet, a 100mm insulation achieves 4.5 R-Value (m².K/W) .
• Easier installation, as I can use a plasterboard over it.

Disadvantages:

• According to Scotland-specific research (Guide to Energy Retrofit of Traditional Buildings), PIR-type insulation must only be used if a complete condensation analysis confirms that it can be used. Unfortunately, for that reason, the study excludes PIR insulation and does not compare it to the other options.
• Given the wall type, we must maintain a well-ventilated air gap behind the insulation. As it is not trapped air, does it not somewhat decreases the insulation capability compared to wood fibre, which does not require an air gap?
• If it is not airtight, there is a severe risk of condensation.

Wood fibre:
Advantages

• Breathable. This approach does not require ventilated air gap.
• It does not have to be airtight, and if cracks appear over time, it won't cause worry.

Disadvantages:

• Much lower R-Value (the higher, the better): Documentation states it achieves 2.5 R-Value (m².K/W) at 100mm.
• Hard to install as I have to lime plaster over it.

Our wall is about 600mm "mass masonry wall" and is internally finished with lath and plaster. Given that plaster is not breathable, I can't see much difference between PIR with airgap and lath and plaster or is there? Also, I find it difficult to translate the R-value difference to the money spent on heating during the winter. Can someone comment on this as well? Based on your experiences and expertise, would you choose the PIR or wood fibre option, and why?

Thanks for all the help!

Are you saying the plaster on the lath & plaster isn't breathable? Surely it's lime, and in lieu of that retro fitting wood fibre would probably be easier than battening a wall that's not likely to be plumb or flat. 60-80mm wood fibre is about thick as you dare go with a solid brick wall, you have to allow some heat through to warm the brick during the cold winters.

If you choose to baton with PIR make sure its airtight, tape all joints, bridges, flaws and fixings.

 

[stamas01]

Hi @Lacan07.

I think the building regulation states that I need to achieve 0.25 W/m2K in Scotland. I doubt that 100mm wood fibre will achieve it let alone the 80mm option. How serious should I take this building reg?

 

[noj]

I guess that depends on if you involve building control. If it's done a room at a time then it's not an area that's notifiable...

Actually if you're not removing/replacing the existing wall covering, but adding to it then I understand it's not notifiable at all?

 

[paulc]

Hi @Lacan07.

I think the building regulation states that I need to achieve 0.25 W/m2K in Scotland. I doubt that 100mm wood fibre will achieve it let alone the 80mm option. How serious should I take this building reg?

Under English building regulations, there is a little passage that states "where technically possible and with a simple payback of 15 years" - If Scottish rules have a similar comment, then you could argue that 80-100mm is about the limit without sacrificing too much interior floor space.

 

[Lacan07]

Hi @Lacan07.

I think the building regulation states that I need to achieve 0.25 W/m2K in Scotland. I doubt that 100mm wood fibre will achieve it let alone the 80mm option. How serious should I take this building reg?

Here in the UK they don't enforce it with old buildings, my BC officer was more than happy with my plan for 60 - 80mm wood fibre on the ground floor. It depends who the officer is I guess but if they understand old buildings then they should realise that's as good as it gets.

Why is there a BC officer involved in your project anyway? A bit of insulation doesn't warrant a notice.

 

[skinnymalinky]

@stamas01 Can you point to where the regs say 0.25 W/m2K? I think it's 0.3 for renovations - but I'm sure there is room for pragmatism e.g. UK gov guidance says:

the walls should achieve an ‘improved’ thermal transmittance (U-value) of 0.3 W/m2K, or where this is not technically or functionally feasible, then the walls should be upgraded to the best U-value possible, but not higher than the ‘threshold’ U- value of 0.7 W/m2K.

Have you looked into aerogel? HES suggest this as the best way to insulate over an existing lath and lime wall. There is a company just up the road from you who produce aerogel products - might be worth speaking to them? https://www.proctorgroup.com/products/spacetherm - but perhaps you already discounted this though based on the cost?

Given that plaster is not breathable

If you have a 1900s building in a conservation area, you will have lime plaster, which is breathable.

 

[erikwoolf]

@stamas01 Can you point to where the regs say 0.25 W/m2K? I think it's 0.3 for renovations - but I'm sure there is room for pragmatism e.g. UK gov guidance says:



Have you looked into aerogel? HES suggest this as the best way to insulate over an existing lath and lime wall. There is a company just up the road from you who produce aerogel products - might be worth speaking to them? https://www.proctorgroup.com/products/spacetherm - but perhaps you already discounted this though based on the cost?


If you have a 1900s building in a conservation area, you will have lime plaster, which is breathable.

Do you know if it can be applied directly on solid walls?

 

[paulc]

@stamas01 Can you point to where the regs say 0.25 W/m2K? I think it's 0.3 for renovations - but I'm sure there is room for pragmatism e.g. UK gov guidance says:



Have you looked into aerogel? HES suggest this as the best way to insulate over an existing lath and lime wall.

With a u-value of 0.015W/mK, you'd only need a layer 30-40mm thick. But at around £1,000 per sheet, it is a very, very expensive solution. Also haven't seen any data with regard to vapour permeability, so don't know if it is suitable for solid walls.

 

[skinnymalinky]

@erikwoolf @paulc it's worth watching the webinar video on the link I posted above - while it is marketing material, it is also informative.

Historic Environment Scotland recommend its usage - in particular for applying directly to existing lath and plaster. Proctor Group quote an SD value of 0.05m for their product.

Interesting synopsis of the HES solid wall guidelines here: https://www.engineshed.scot/building-advice/common-problems/how-to-insulate-stone-walls

 

[fernicarry]

Given the wall type, we must maintain a well-ventilated air gap behind the insulation. As it is not trapped air, does it not somewhat decreases the insulation capability compared to wood fibre, which does not require an air gap?

This is the key. If your internal wall construction is like mine the laths will be on studs dooked into the stone wall. This creates an air gap between the plaster and the inner face of the stone to allow penetrating water to escape. Its a basic form of cavity wall construction. This cavity is very handy for running services but you certainly notice the air circulation around electrical fittings etc.

Other parts of the UK (Scotland *is* still part of the UK... ) plaster straight onto the masonry ("on the hard") as they are not subject to the same levels of driven rainfall. I'm west coast you are east so you might be different from me. Have you had a look inside a wall to see what you are dealing with?

The rooms I've retrofitted so far I've built new studwork stood off the wall and insulated between the studs with up to 80mm wood fibre (option 4. on the engineshed link). I also put wood fibre between the floor joists and roof joists if I have access. That's more awkward since old timbers tend to be all over the place so every piece has to be perfectly templated to fit. I've only done this in rooms that have already been bashed around and had no period details left to save. I don't expect to ever meet modern regs but its much better than it was. I live in hope that one day sane minds will prevail and we'll come up with an energy strategy that suits these sorts of buildings.

If you were to put the wood fibre straight onto the inner face of the masonry I think you'll find its so far from being level you'll still have gaps and probably a very wonky inner surface to work with. I suppose you could dub out the inner face but that could involve an awful lot of material.

You might want to look at this product too: https://www.indinature.co/ They have an application guide that mentions traditional Scottish construction. They do also advocate going straight onto the masonry with natural insulation but don't say anything about what to do when the surface isn't particularly level.

 

[a twig]

If you were to put the wood fibre straight onto the inner face of the masonry I think you'll find its so far from being level you'll still have gaps and probably a very wonky inner surface to work with. I suppose you could dub out the inner face but that could involve an awful lot of material.

I put wood wool directly onto uneven rubble walling without any dubbing, using large fibreglass washers and large screws / rawlplugs. Provided a pretty flat surface that was easy to skim over

 

[malcolm]

For my kitchen I wanted the walls to be straight so I could fit worktops. I used lime render to provide a straight surface (my mix is normally sharp sand mixed with builders sand and then 2.5:1 sand to lime but you can use just about anything so long as it is breathable). Woodfibre board on top of that and a lime skim. The result is very much less green in colour than the previous tanking. I sorted the water ingress on that wall before I started.

An air gap can be handy so long as it's ventilated to an area at a similar temperature (outside if you are insulating). I went for a ventilated air gap behind a timber frame. The frame was going to leak as it was built in green oak which would shrink. The ventilation gets the water away and I could use modern materials on the inside.

For me it's about water management. Water will always get into a wall. The trick is providing a way for it to get out again. With brick buildings built with lime mortar where the ground levels outside are a couple of bricks lower than internal floors you can normally rely on the water getting out to the outside. Stone buildings are more fiddly. But if there is no damp to start with then breathable insulation shouldn't cause damp. The same principles apply for stone but (especially in Scotland) the stones are bigger than bricks and the mortar joints are often much thinner so there is less evaporation. The points above are valid and it's worth looking at conservation advice in your area. Never look at mainstream advice as that will be wrong and result in something as mouldy as I found my kitchen.

Also forget building regs. Work out what you want to do and do that. Don't tell anyone.

 

[stamas01]

Thanks for all the replies! Everyone was really helpful and I appreciate it!
I will most probably go for an 80mm woodfibre option. I will keep you posted how it goes

 

[erikwoolf]

Those with woodfibre insulation on masonry, what kind of lime mix have you been using between the masonry and the woodfibre board? Some of the commercial systems seem to need an adhesive in addition to a layer of lime plaster to flatten the masonry surface. I am trying to understand if this is really needed or what is the added benefit of using an adhesive in this case?

 

[a twig]

I personally haven’t - I used screws, rawl plugs and large fibreglass washers to attach to the walls, no adhesive or mortar or plaster underneath anywhere.

The mechanical fixings are solid, yes you end up with a wibblier wall as the fixings pull the boards into the wall but it’s solid.

If you want flat walls, dub out the masonry with lime plaster/ mortar then affix

 

[Lacan07]

Those with woodfibre insulation on masonry, what kind of lime mix have you been using between the masonry and the woodfibre board? Some of the commercial systems seem to need an adhesive in addition to a layer of lime plaster to flatten the masonry surface. I am trying to understand if this is really needed or what is the added benefit of using an adhesive in this case?

Just use a standard base coat lime mixture to flatten the masonry in preparation for the wood fibre boards, you won't need adhesive if the plaster is nicely flat. You'll most likely need 2 coats to flatten the masonry wall depending on how flat and true the wall is at the beginning. Sponge float your base coat to a nice flat finish, this also leaves an open texture which is more breathable. Then use the necessary screws, raw plugs to attach the boards, (as Twig indicated above).

Use as breathable a plaster as you can find, i.e, lime putty based rather than NHL.

 

[stamas01]

Just use a standard base coat lime mixture to flatten the masonry in preparation for the wood fibre boards, you won't need adhesive if the plaster is nicely flat. You'll most likely need 2 coats to flatten the masonry wall depending on how flat and true the wall is at the beginning. Sponge float your base coat to a nice flat finish, this also leaves an open texture which is more breathable. Then use the necessary screws, raw plugs to attach the boards, (as Twig indicated above).

Use as breathable a plaster as you can find, i.e, lime putty based rather than NHL.

Hi @Lacan07,

I was about to post this question as a followup today but you were quicker. I just need a few more details before I start this.

So:
1. What is a standard base coat lime mixture? Could you also please link example products for each component? I believe it is 1 part lime putty and 2.5 part sharp sand?
For example is this lime putty good? Or am I better of with simply using a readily mixed Lime mortar like this?
2. Do I need to do a "Harl Coat" to give my stone masonry wall key? If so is "Harl Coat" just a base coat diluted to the extreme?

Here is an image of a wall I am planning to insulate with woodfibre.



Thanks