Plastering question

[jsmac]

Hello all,

After nearly a year of serious unanticipated structural work we've now finished and can get on with fitting windows, plastering and everything else that we thought we'd be finished by now.

I have a question regarding use of wood wool boards on an external facing brick wall. This wall originally had laths fixed to battens that were fixed directly to the wall. Unfortunately, the internal plaster had to come down - partly because it was in terrible condition having been patched in various places with cement, gypsum plaster, plasterboard and any other poor intervention they could think of over the past 60 years. But also we needed to get a look at the wall behind it for the above mentioned structural surprises.

My question is whether you think it is a good idea to recreate the lath and plaster work using wood wool boards on the wall with the window? We have a bunch of them and so I thought that I could fix battens to the brick wall to create an air gap and then fix the boards to the battens (or rather, through the battens into the wall). The wall could then be plastered in lime as before. Any thoughts on whether this is a sound idea (or not) would be most welcome. What I could use to fix the battens and the boards to the brick wall or if there is a better way of going about this?

In the attached photo of Mrs jsmac having a rest after scutching off the gypsum plaster repairs off the wall next to the window (this will be plastered directly onto the brick in lime) you can see that there is a timber plate that runs above the window. Between the plate and the top of the window box is a several inch gap that was previously covered with lath and plaster that I'm hoping to fill with the wood wool board. Does this make sense? I can see why with this gap above the window that they used laths on this wall rather than plastering directly onto the brick.

Thank you.

 

[plasticpigeon]

I'm afraid I can't answer your questions as i have no knowledge of wood wool boards. I have a house that looks similar to yours and I plastered directly onto the wall. Nice to see another person on here with a relatively "normal" home!

 

[paulc]

What I could use to fix the battens and the boards to the brick wall or if there is a better way of going about this?

For fixing woodwool boards directly to the walls, they do a hammer in and screw fixings complete with a large mushroom head. One example: https://www.fischer.co.uk/en-gb/products/insulation-fixings/etic-fixings-in-general/render-fixing-fif-cs-8/534158-fif-cs-8-80
Do a search for "EWI wall fixings", and you'll come up with a multitude of choices.

For fixing via battens, either wall plugs & screws or hammer fixings: https://www.screwfix.com/p/rawlplug-nylon-hammer-in-fixings-6-x-60mm-100-pack/4901f
With battens, it is a good idea to insert a layer of DPC material between the brick and timber. This will limit the amount of moisture getting in to the wood and prevent it from rotting.

 

[a twig]

I've just done my living room in woodwool boards skimmed with Reabilita Cal AC -
https://www.mikewye.co.uk/product/secil-reabilita-cal-ac/

Boarded out directly onto the walls using Rawl plugs, screws and engineered 60mm plastic washers

https://drive.google.com/file/d/13U2opA67U8GDS3h2y0pag9i-RGMEVUe8/view?usp=drivesdk

Plaster skim with Reabilita Cal AC (Finishing Render) @ 2 mm.

Trowel in 4 mm glass fibre mesh

Final skim with Reabilita Cal AC (Finishing Render) @ 2 mm.

End result:
https://drive.google.com/file/d/13g99AAQEEkziXI-nKI3mEXtBDI2Whdt9/view?usp=drivesdk

It's brilliant, so much warmer and drier and the first skim goes off quickly enough to allow the whole lot in a day. Only had to wait an hour or so after trowelling in the mesh before it was good to go. Wish I'd discovered this system 4 years ago, quicker, easier and more effective than fat lime and all the damping and wetting and eleventy million coats!

 

[Zebra]

I plastered directly onto woodwool boards using plaster made from plastering sand and lime putty - entire bedroom, single coat - and using plaster made from plastering sand and NHL - entire kitchen. Woodwool boards were fixed onto timber frame in bedroom, or onto lightweight block in kitchen. One thing I strongly recommend, is to glue the joints between boards so that the plaster doesn't crack along the joins between boards. Gripfill, although not very "eco," is perfect for this. Squidge it right into all the joins so they can't move. Five/six years on and no cracks. Well, no major ones...

 

[noj]

One thing I strongly recommend, is to glue the joints between boards so that the plaster doesn't crack along the joins between boards

Just wondering, would it be possible to leave the same size gap between boards as there'd be between laths and then plaster through those gaps? A 'normal' lath and plaster wall doesn't crack along the lath lines...

 

[jsmac]

This is all great information (as always). Thank you.

I'm intrigued by noj's suggestion. It's not something that I'd considered but I too wonder whether this would work.

 

[numberwang]

Also used wood wool boards screwed directly to wall at regular intervals with the big washer things (sorry!) medium hemp lime and finishing coat over the top. I didn’t mesh the board joins, but pressed mortar well into them (like lath) - so far so good. My problem if any is the shrinkage away from wood corner beads that needs some remediation with filler.

I’ve not worked out how to post pics yet, but will add if can later. Like others have said it goes up relatively quick and is not cold to touch. Did a different wall with consolidation mortar and eco cork but found that less easy and more messy!

My walls like yours are external so insulation a factor, however minimal, plus ability to meet original depth to meet coving.

 

[a twig]

Just replying to myself so I can fix the broken photos for future reference:

I've just done my living room in woodwool boards skimmed with Reabilita Cal AC -

SecilTek Reabilita Cal AC

SecilTek Reabilita Cal AC is a natural hydraulic lime top coat used for the external renovation of old and new masonry and insulation boards

www.mikewye.co.uk

Boarded out directly onto the walls using Rawl plugs, screws and engineered 60mm plastic washers



Plaster skim with Reabilita Cal AC (Finishing Render) @ 2 mm.

Trowel in 4 mm glass fibre mesh

Final skim with Reabilita Cal AC (Finishing Render) @ 2 mm.



Then two coats of Earthborn clay paint to give this end result:



It's brilliant, so much warmer and drier and the first skim goes off quickly enough to allow the whole lot in a day. Only had to wait an hour or so after trowelling in the mesh before it was good to go.

Wish I'd discovered this system 4 years ago, quicker, easier and more effective than fat lime and all the damping and wetting and eleventy million coats!

 

[CarolineTheChemist]

If you’re keen to insulate I’d recommend wood fibre boards. They’re very much like wood wool boards, only fluffier and better at insulation. They have a bit of flex and you can attach them directly to your bricks.

Chalk Down Lime | Products | Shop online - UK delivery

Lime Mortar & Lime Plaster | Lime Putty, Quicklime & NHL | Sand & Aggregate | NHL Mortar & Render | Riven & Sawn Laths | Wood Wool Board | Limewash & Paint | Other Products

www.chalkdownlime.com

We used them for an entire ground floor on all the solid brick external walls. The difference they’ve made is amazing. Cooler in summer and warmer in winter! The brick was rather rough and uneven after a modern plaster removal. First the wall was given a rough lime plaster coat and then the fibre boards attached to it using washers and stainless steel screws (also from Chalk Down Lime). I think the first plaster coat was only really necessary because the bricks were so beaten up, but I’m pretty certain you don’t want to batten the wall first - I don’t think creating a cavity is a great idea. Take a look at the details in the link. We used the thinner board in a circular room, so it can easily handle some flex. If your wall is flat, I’d go for something thick to keep you toasty. (It’s an Oast house in case you’re wondering why the room is round!)

After the fibre board came our regular lime plaster layers: scratch coat (with render mesh) and smooth second coat. Both layers are done using medium lime plaster. We don’t bother with the fine lime top coat as our plasterer makes such a beautiful job on the second coat.

Incidentally we also get our plaster from Chalk Down Lime. It’s nice stuff!

Lime Plaster (Medium)

Chalk Down Lime - Medium-grade “Wealden” premixed (wet), non-hydraulic, hot-mixed, traditional lime putty plaster for use onto: lime base coats, soft and porous historic masonry and wood wool boards.

www.chalkdownlime.com

They offer great advice too, so I heartily recommend calling them to understand how to apply the fibre board in your case. I’ll also see if I can dig out some photos for you.

 

[CarolineTheChemist]

I found some pics!
You can see the round room with the bare brick.

The first rough sandy lime coat in the adjoining room and arch with the wood fibre boards waiting patiently for their turn…



Then the boards attached to the walls…


followed by the first scratch coat.


The floor is coming up in that last photo as we’ve also put in limecrete.

 

[johnbanshee]

Hi

Quick question.

How are people forming the corners on windows / doorways? Is it regular metal right-angled beading?

Thanks!

 

[a twig]

With the Savolit wood wool boards they were rigid enough where they met at the corners that I didn’t have to use any beading, just a corner trowel to get a nice finish.

If you did want to use beading make sure it is either stainless or plastic to avoid rust / corrosion problems down the track

 

[Stevers]

Sounds like a natural for a 1" (25mm) pine dowel corners as used traditionally - nice and straight yet softly rounded corners, what's not to like?

 

[mh12]

If you’re keen to insulate I’d recommend wood fibre boards. They’re very much like wood wool boards, only fluffier and better at insulation. They have a bit of flex and you can attach them directly to your bricks.

Chalk Down Lime | Products | Shop online - UK delivery

Lime Mortar & Lime Plaster | Lime Putty, Quicklime & NHL | Sand & Aggregate | NHL Mortar & Render | Riven & Sawn Laths | Wood Wool Board | Limewash & Paint | Other Products

www.chalkdownlime.com

We used them for an entire ground floor on all the solid brick external walls. The difference they’ve made is amazing. Cooler in summer and warmer in winter! The brick was rather rough and uneven after a modern plaster removal. First the wall was given a rough lime plaster coat and then the fibre boards attached to it using washers and stainless steel screws (also from Chalk Down Lime). I think the first plaster coat was only really necessary because the bricks were so beaten up, but I’m pretty certain you don’t want to batten the wall first - I don’t think creating a cavity is a great idea. Take a look at the details in the link. We used the thinner board in a circular room, so it can easily handle some flex. If your wall is flat, I’d go for something thick to keep you toasty. (It’s an Oast house in case you’re wondering why the room is round!)

After the fibre board came our regular lime plaster layers: scratch coat (with render mesh) and smooth second coat. Both layers are done using medium lime plaster. We don’t bother with the fine lime top coat as our plasterer makes such a beautiful job on the second coat.

Incidentally we also get our plaster from Chalk Down Lime. It’s nice stuff!

Lime Plaster (Medium)

Chalk Down Lime - Medium-grade “Wealden” premixed (wet), non-hydraulic, hot-mixed, traditional lime putty plaster for use onto: lime base coats, soft and porous historic masonry and wood wool boards.

www.chalkdownlime.com

They offer great advice too, so I heartily recommend calling them to understand how to apply the fibre board in your case. I’ll also see if I can dig out some photos for you.

Thank you for the detailed information. I wonder what the U-Value of the wood fibre boards that you've used in your beautiful property? Also what thickness did you use? Did oyu mechanically fix the boards to the wall? or attached with lime plaster? Approximate cost of using this system for 100 square metres?
Thanks

 

[Kearn]

Thank you for the detailed information. I wonder what the U-Value of the wood fibre boards that you've used in your beautiful property? Also what thickness did you use? Did oyu mechanically fix the boards to the wall? or attached with lime plaster? Approximate cost of using this system for 100 square metres?
Thanks

Speak to any of the usual suspects and they’ll do you a quote if you don’t fancy calculating it yourself. Eden Hot Lime, TyMwar, Mike Wye etc

They sell the fibre boards, and the spec sheets show the thermal values by thickness. It’s common to go over the boards with a thick hemp or other insulated plaster if required. Or you can just skim.

Wood fibre needs a nice sticky plaster scratch coat as it hasn’t got too much mechanical key.

If your brick walls are relatively flat/smooth, you can screw straight onto the walls as the boards have some give. If not, patching out in lime first is sensible to remove air gaps between brick and fibre board.

Don’t confuse them with wood wool boards, which are much thinner and have less thermal capacity as they’re more a plaster carrier than an insulation product.

Wood fibre is a nice easy material to work with though, if very fibre-y when you cut it!