Hello again,

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!

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.

I 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?

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!



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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 - 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.

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.

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.

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

Thanks SimonD, but that's as I have always thought - fixing *through* the WF to the masonry, not fixing *into* the WF.

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.

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.

If you use vertical wooden cladding not horizontal the solves most of the load bearing issues as it will bear it's own weight.

You could hang the vertical battening from the top and also have the battening sitting at the bottom. This would work for horizontal cladding but if you added horizontal battens then vertical cladding could also sit at the bottom thus reducing the load further. A bit like build a self supporting wooden shell around the building. I have never done this with a house but I don't see why it wouldn't work. Just an idea though. Not suggesting it a good one though

What do you support it on? by definition, on a EWI'd building the cladding is well outboard of the foundation wall, tho may just perp down to the edge of the strip found. Are you going to build up a found just to suppotrt the cladding?

Huge amount of work and cost, when suspending from above (a roof is an easily cantilevered element) is straightforward, or can be if designed for.

Posted By: JontiMaybe I was over thinking it Tom

No, 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.