Green Building Forum - Wood fibre EWI, worth it?

Green Building Forum - Wood fibre EWI, worth it? Sun, 19 Apr 2026 03:20:23 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284340#Comment_284340 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284340#Comment_284340 Tue, 03 Nov 2020 19:01:59 +0000 teach_glas
I started a thread a few months back about timber cladding on 200mm EPS EWI with single-leaf masonry blockwork as the substrate. There were a few possible solutions raised but it means getting a bit experimental and could be difficult to get engineer to sign-off on the building.

I am considering using a 200mm wood fiber board instead now as this provides much more stability to the fixings and simplifies the cladding design (you can fix the cladding batten with EWI fixing).

Only problem is now is that the projected U-value of the wall is now around 0.18 which is pretty poor considering target was 0.14.

Advantages seem to be: sustainability, fire resistance (compared to EPS), and structural rigidity for cladding.

Not sure if this is worth compromising my overall insulation... it would be a simpler build.. maybe I could use an insulating render inside, but this messes with my 'storage heater' design!

Any thoughts would be much appreciated!

:smile:

]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284341#Comment_284341 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284341#Comment_284341 Tue, 03 Nov 2020 19:15:41 +0000 tony Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284343#Comment_284343 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284343#Comment_284343 Tue, 03 Nov 2020 20:00:18 +0000 Peter_in_Hungary Posted By: tonyNot fail safe, better with nom biodegradable insulation
+1

Posted By: teach_glasI am considering using a 200mm wood fiber board instead now as this provides much more stability to the fixings and simplifies the cladding design (you can fix the cladding batten with EWI fixing).

What sort of cladding are you considering - Is the cladding because wood fibre needs cladding?

EPS can be got with fire retardant although my information to date is that EPS with the standard thin film render system is fairly fire resistant anyway and by the time the render has burnt through you have more problems than the burning EPS. Also my info at the moment that the Grenfell disaster (which has brought the fire risk of EWI into focus) was aided and abetted by the chimney effect between the cladding and the insulation (someone correct me if I am wrong on this) and there is no chimney effect with the standard EPS EWI.]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284394#Comment_284394 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284394#Comment_284394 Thu, 05 Nov 2020 18:58:08 +0000 teach_glas Posted By: Peter_in_Hungary

What sort of cladding are you considering - Is the cladding because wood fibre needs cladding?

EPS can be got with fire retardant although my information to date is that EPS with the standard thin film render system is fairly fire resistant anyway and by the time the render has burnt through you have more problems than the burning EPS. Also my info at the moment that the Grenfell disaster (which has brought the fire risk of EWI into focus) was aided and abetted by the chimney effect between the cladding and the insulation (someone correct me if I am wrong on this) and there is no chimney effect with the standard EPS EWI.

No the cladding is an aesthetic choice, I'm just a bit wary of creating a tinderbox of EPS and larch cladding.
Thanks for the info about fire retardant EPS, didnt know about that!]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284396#Comment_284396 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284396#Comment_284396 Thu, 05 Nov 2020 19:17:15 +0000 Nick Parsons
I do not understand how you feel the fixing is easier with WF than EPS. No more difficult, perhaps, but why easier, do you think?]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284397#Comment_284397 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284397#Comment_284397 Thu, 05 Nov 2020 19:19:30 +0000 teach_glas Posted By: tonyNot fail safe, better with nom biodegradable insulation

I had actually read your thoughts on this before.. but is it really that susceptible?

From what I can see its relatively dense (for insulation) wood fiber impregnated with resin and paraffin wax. The wood gets heated enough during manufacturing to dissolve the natural protein in the wood, rendering it inedible to pests..

Are there any case-studies about this stuff degrading.. I mean, its been popular in Germany since the 1990s, you'd think we would be aware of any glaring issues by now?]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284406#Comment_284406 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284406#Comment_284406 Fri, 06 Nov 2020 09:43:06 +0000 SimonD Posted By: teach_glas

I am considering using a 200mm wood fiber board instead now as this provides much more stability to the fixings and simplifies the cladding design (you can fix the cladding batten with EWI fixing).

Only problem is now is that the projected U-value of the wall is now around 0.18 which is pretty poor considering target was 0.14.

Advantages seem to be: sustainability, fire resistance (compared to EPS), and structural rigidity for cladding.

Any thoughts would be much appreciated!

https:///newforum/extensions/Vanillacons/smilies/standard/smile.gif" alt=" :smile:

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When I went to Pavatex to get the technical designs done for my house, they took our target of 0.16 to 0.20 suggesting that the woodfibre insulation will make up for it due to better y-value. However, this could be questioned as we're talking about ewi and thus will there be a real world thermal bridge difference in a well detailed ewi installation regardless of the material? I don't have enough experience of each of these materials to make the comparison. Perhaps someone else on the forum can share their knowledge?

I'm personally somewhat dubious about our obsession with u-value as I think it also depends on how the material behaves, not just in terms of keeping us warm, but also in terms of keeping us cooler. In this regard I think woodfibre lies ahead of eps.

Each of these insulations will behave differently which I think is where you may benefit most in doing your research to make the decision.

FWIW I'd take the woodfibre over EPS anyday :bigsmile:

- simply handling the two is a totally different experience. And if you're looking to clad it, then the woodfibre board can readily support this. I don't know about EPS as it's not my area of interest.]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284408#Comment_284408 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284408#Comment_284408 Fri, 06 Nov 2020 10:17:26 +0000 Nick Parsons Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284410#Comment_284410 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284410#Comment_284410 Fri, 06 Nov 2020 11:02:39 +0000 fostertom Posted By: SimonDif you're looking to clad it, then the woodfibre board can readily support thisHow is it different to EPS? Neither AFAIK is strong enough to 'support' cladding (other than direct render or stik-a-brik fake brickwork) i.e. to prevent long through-screws from having to take the load in bending - niether EPS nor wood fibre will support the shaft of the screw long term, and neither is uncompressible enough to support the weight by friction.]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284419#Comment_284419 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284419#Comment_284419 Fri, 06 Nov 2020 15:35:27 +0000 teach_glas Posted By: fostertomhings. Again, I go not know the likely effects of wind (direct action and suction) and how that, too, would affect the

Posted By: Nick ParsonsI would suggest a scratch coat of render before the cladding, as a fire-resistant layer.

I do not understand how you feel the fixing is easier with WF than EPS. No more difficult, perhaps, but why easier, do you think?

I'm currently talking to Steico about this but it appears they know of fixings that are suitable to hold a batten on 200mm of insulation:

"..you can also use STEICOprotect dry L at 200mm thick with an external timber cladding system as well as long as you create a ventilated airspace behind the cladding and use a sufficiently large batten to hold the insulation in place. Fixings would need to go through the batten, through the insulation layer and at least 40-50mm into the blockwork so a significant connection. Fixing spec is generally provided by the relevant fixing manufacturer based on localised loading requirements and the density of the insulating layer which we can of course provide."

Have emailed fixing manufactures today and await reply...]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284425#Comment_284425 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284425#Comment_284425 Fri, 06 Nov 2020 18:39:34 +0000 SimonD Posted By: Nick ParsonsSimonD, you write, re wood-fibre, '' And if you're looking to clad it, then the woodfibre board can readily support this.'' I'm not contradicting this, just questioning. Are you saying that the cladding could be fixed *only into the W-F*? If so, that surprises me. My main use of W-F has been as IWI, not EWI, but my experience suggests that light things can be fixed, but not heavier things. Again, I go not know the likely effects of wind (direct action and suction) and how that, too, would affect the connection.

Posted By: fostertom
Posted By: SimonDif you're looking to clad it, then the woodfibre board can readily support this
How is it different to EPS? Neither AFAIK is strong enough to 'support' cladding (other than direct render or stik-a-brik fake brickwork) i.e. to prevent long through-screws from having to take the load in bending - niether EPS nor wood fibre will support the shaft of the screw long term, and neither is uncompressible enough to support the weight by friction.

As I said, I don't know about EPS but re the wood fibre take a look at the Pavatex ewi clad solution including for masonry walls here:

https://www.natural-building.co.uk/system/external-wall-insulation-clad/

It even has BBA certificate and suitable down to u-value of 0.10

hth.]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284427#Comment_284427 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284427#Comment_284427 Fri, 06 Nov 2020 19:18:46 +0000 Nick Parsons Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284431#Comment_284431 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284431#Comment_284431 Sat, 07 Nov 2020 10:48:12 +0000 SimonD Posted By: Nick ParsonsI have had a quick read of that BBA cert and cannot see how the cladding fixings work with masonry, or any ref to the battens being fixed only into the WF. Have I missed something? Anyone?

It's all in the technical manual available for download on that page.

the TLDR version is:

for timber battened cladding: Basically you put on a levelling coat onto the masonry, put the board to the wall and hold them in place with a couple of wall fixings. Then you install the battens which are fixed against the woodfibre board using long screws that go right the way through into the masonry.

I think there was a question above about screws bending under load: because the screws are in tension against the battens and woodfibre board, there isn't a bending force, I think it's actually a sheer force which is reduced due to the friction forces created by the material connections.

hth]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284433#Comment_284433 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284433#Comment_284433 Sat, 07 Nov 2020 11:17:31 +0000 fostertom Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284436#Comment_284436 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284436#Comment_284436 Sat, 07 Nov 2020 15:04:15 +0000 Nick Parsons Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284445#Comment_284445 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284445#Comment_284445 Sat, 07 Nov 2020 18:17:02 +0000 tony Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284446#Comment_284446 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284446#Comment_284446 Sat, 07 Nov 2020 19:16:19 +0000 SimonD Posted By: tonyHow long are the fixings?

IIRC the Pavatex clad system recommends 70mm embedment depth tbc by either their technical department or structural engineer depending on the substrate.]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284447#Comment_284447 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284447#Comment_284447 Sat, 07 Nov 2020 19:17:27 +0000 teach_glas Posted By: SimonD
I think there was a question above about screws bending under load: because the screws are in tension against the battens and woodfibre board, there isn't a bending force, I think it's actually a sheer force which is reduced due to the friction forces created by the material connections.

hth

Steico mentioned the compressive strength of the Protect L board when discussing my cladding requirements...

Posted By: tonyHow long are the fixings?

I will know on Monday, they are putting a proposal and quote together for me :bigsmile:

]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284449#Comment_284449 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284449#Comment_284449 Sat, 07 Nov 2020 20:14:46 +0000 djh Posted By: teach_glasSteico mentioned the compressive strength of the Protect L board when discussing my cladding requirements
That's odd. From the data sheet for 'protect dry':

Compression strength [kPa] 200 (H) 100 (M) 50 (L)

so the L is the most compressible and is weaker than even standard EPS75, let alone EPS200 or EPS300. So if they were concerned about compressibility I'd have thought they would recommend H or M rather than L.

There must be more to the story. Do let us know.]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284451#Comment_284451 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=284451#Comment_284451 Sat, 07 Nov 2020 21:11:47 +0000 tony Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286594#Comment_286594 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286594#Comment_286594 Thu, 21 Jan 2021 07:51:40 +0000 Jonti Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286595#Comment_286595 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286595#Comment_286595 Thu, 21 Jan 2021 08:50:50 +0000 fostertom Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286597#Comment_286597 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286597#Comment_286597 Thu, 21 Jan 2021 09:16:30 +0000 Jonti ]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286602#Comment_286602 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286602#Comment_286602 Thu, 21 Jan 2021 10:52:48 +0000 djh Posted By: Jontithen vertical cladding could also sit at the bottom
I used horizontal cladding because I didn't like the thought of all the boards in vertical cladding rotting at their bottom ends together because of splashback. A fortiori I wouldn't want the lower ends of vertical boards resting on anything instead of being surrounded by free air. But that's just IMHO. :smile:

]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286605#Comment_286605 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286605#Comment_286605 Thu, 21 Jan 2021 12:33:27 +0000 fostertom Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286606#Comment_286606 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286606#Comment_286606 Thu, 21 Jan 2021 12:53:14 +0000 djh Posted By: fostertomWhat do you support it on? by definition, on a EWI'd building the cladding is well outboard of the foundation wall
If you did want to do it, it would be quite possible to have a brick dwarf wall at the bottom, outside the EWI, and stand the cladding on that. Yes, it would need some sort of foundation.]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286608#Comment_286608 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286608#Comment_286608 Thu, 21 Jan 2021 15:30:28 +0000 fostertom Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286612#Comment_286612 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286612#Comment_286612 Thu, 21 Jan 2021 18:06:44 +0000 Jonti Posted By: fostertomHuge amount of work and cost, when suspending from above (a roof is an easily cantilevered element) is straightforward, or can be if designed for.

Then do it that way. Maybe I was over thinking it Tom :confused:

Posted By: djh
Posted By: fostertomWhat do you support it on? by definition, on a EWI'd building the cladding is well outboard of the foundation wall

If you did want to do it, it would be quite possible to have a brick dwarf wall at the bottom, outside the EWI, and stand the cladding on that. Yes, it would need some sort of foundation.

Or even a rail system needing just occasional support.]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286615#Comment_286615 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286615#Comment_286615 Thu, 21 Jan 2021 18:28:06 +0000 fostertom Posted By: JontiMaybe I was over thinking it TomNo, it needs thinking thro, because as a small detail it's surprisingly fundamental to what can be a fairly standardised approach to EWIing. Of course if rendered or brick slips finish, then no problem, but if clad then first thing to look at is a way to hang it from overhanging rafters, incl creating same if not existing. Creating an overhang may come for 'free' if you want overhanging eaves (and verge) anyway. If not, then that's a good reason to think of something other than cladding as finish.]]> Wood fibre EWI, worth it? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286623#Comment_286623 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16778&Focus=286623#Comment_286623 Thu, 21 Jan 2021 22:22:41 +0000 djh Posted By: JontiOr even a rail system needing just occasional support.
I don't follow that? I'm imagining a fence rail floating in the air (supported at intervals) which seems like it would look decidely odd. So I suppose I have the wrong picture?]]>