How to fill gaps around IWI Woodfibre Boards

[Mulder]

As per title. What is the recommended approach for filling gaps around woodfibre boards?
I've got a bit of a wavy timber beam on the ceiling. I've scribed the boards but, alas, not perfectly. Same for the sides where the walls are not plumb, there are a few gaps, maybe 15-20mm and the depth of the board (60mm).

I'm planning to just fill with lime plaster. Will this cause cold bridging issues? Or should I use the dreaded spray foam, or shove some rock wool in there? It feels like soggy rock wool sandwiched between a timber and lime plaster may do more harm than good. But equally an air void is also bad too, right?

 

[Kearn]

No to rock wool and spray foam, asking for trouble.

That’s not a huge gap, just wrap the plaster round to fill the gap, nice and hairy so it’s got some body.

On my panels, I’ve intentionally left similar gaps, to do just that, as it helps manage and moisture from wind driven rain ingress.

 

[Zebra]

I think I mixed up some fluff (from cutting the boards) with some PVA glue and used that to fill gaps. I think I also used gripfill (not very traditional) to fix all the joins so they didn't move and crack.
Kearn, you're talking about wind driven rain but I think Mulder is talking about boards used internally?

 

[Kearn]

I am, but the principle is no different, lime will fill the gap adequately in this case and be the easiest and most appropriate thing to use against timber.

 

[Zebra]

Perhaps with hair in yes. Without I think it would just crack along the joints.

 

[Mulder]

Is it a case of layering the lime up over a few days to prevent cracking?
I think I'll cut slithers and fill the gaps where possible, then lime.
Strange how this isn't covered in any of the IWI guides I have found.

 

[Kearn]

Perhaps with hair in yes. Without I think it would just crack along the joints.

Indeed, I definitely said hairy! But there would be no joints, the base coat would wrap the edges in a continuous pass, much like a nib through lath.

 

[Kearn]

Is it a case of layering the lime up over a few days to prevent cracking?
I think I'll cut slithers and fill the gaps where possible, then lime.
Strange how this isn't covered in any of the IWI guides I have found.

Depends what you’re using, some hemp lime lime be fine going in all the way, and is nice and sticky over the woodfibre so often used as the base coat.

That way you can just wrap it round, meaning no joints in plaster.

If you’re using something else, make sure it’s got reasonably sized aggregate in it to minimise shrinkage, I love Eden Nosterfield which is up to 4mm, and very forgiving and flexible in its uses, happily can be used in one pass at 8-10mm depth. And mix with hair for good measure.

 

[Zebra]

Sorry, not being clear. I meant the joints between boards could crack the plaster, not joints in the plaster.
Not if they're tongue and groove but if they've been cut and butted up.

 

[Lacan07]

As per title. What is the recommended approach for filling gaps around woodfibre boards?
I've got a bit of a wavy timber beam on the ceiling. I've scribed the boards but, alas, not perfectly. Same for the sides where the walls are not plumb, there are a few gaps, maybe 15-20mm and the depth of the board (60mm).

I'm planning to just fill with lime plaster. Will this cause cold bridging issues? Or should I use the dreaded spray foam, or shove some rock wool in there? It feels like soggy rock wool sandwiched between a timber and lime plaster may do more harm than good. But equally an air void is also bad too, right?

I used Steicoflex which is the friction fit equivalent of wood fibre board, but only because I happened to have some from another project.

Kearn and Zebra both have good solutions.

 

[Kearn]

Sorry, not being clear. I meant the joints between boards could crack the plaster, not joints in the plaster.
Not if they're tongue and groove but if they've been cut and butted up.

Plastic mesh over the joins, same as with wood wool boards. Ideally 10mm gap size, as the 5mm variant is a pig when you encounter some 4mm aggregate when finishing!

 

[Zebra]

Yes, plastic mesh, although with both woodwool and wood fibre I got away with omitting the mesh internally, using gripfill / PVA & fluff as above.

 

[Mulder]

Thanks for the replies.

I'm following the TyMawr IWI recommendations using Schneider woodfibre boards (t&g) with Secil 'Lime' (despite the simple name, appears to be specifically for sticking to woodfibre boards, and not for use as a general lime) as bonding coat and a scratch coat on top of the boards. Finished off with a coat of Secil Reabilita CA.

I think I'll pack the gaps with offcuts of boards as best I can, maybe some fluff mixed in too, then daub out with Secil Lime.