Thickness of Float Coat Over Insulation Boards

[aockenden]

Hey when plastering over wood wool or wood fibre boards, I assume everyone is doing 2 coats? A float and a skim?

Assuming that’s correct, how thick are your float coats typically going on? How much cracking are you typically seeing on the float coat as it dries?

 

[Kearn]

Not me!

It depends on your situation of course, and insulation make up, interval vs external etc and the role the plaster has to play.

Given the old sections of our frame are offering up 70-100mm max of panel depth, I’ve maximised the insulation sandwich in the middle and minimised internal plaster.

It means I’m probably getting 8mm max internally onto 15mm wood wool boards.

I’ve opted for a technically non top coat - using Nosterfield pre mix from the guys at Eden. Its aggregate is up to 4mm, so it can go on reasonably thick in one pass without risk of too much cracking. Suggestion is 4mm, mesh, 4mm all in one go. I’m only meshing the joints in boards, not the entire panel. I errored and got 5mm mesh not 10, so trowelling the finish over the mesh can bring bigger aggregate to the surface, but it’s manageable.

Even as a non top coat, it can be trowelled up smooth if required, I’m preferring to trowel close to smooth, then sponge the following day to even up the surface and open it up too.

It’s not requiring much TLC at all frankly, which is nice. Sponge the following day. Possibly extra sponging the day or two after to close up a few cracks. But my heating and ventilation situation is far from consistent currently, so every panel is its own microcosm of conditions!

Woodfibre, is a whole different beast. I’ve moved away from it internally for my roof makeups, opting for wood wool instead as the carrier.

Yes it has insulation, but I’m dealing with this via wood fibre batts, the boards are really render carriers and I’ve heard nothing good about getting muck to stick to wood fibre. Especially above head!
It needs to be very sticky and haired / hemp. I was told to expect half of every trowel to come straight off.

 

[aockenden]

Ok so to clarify, you’re doing a single coat at a max of 8mm directly onto wood wool boards with scrim-taped board joints, troweling it close to smooth and then sponging out any last marks the next day and sponging out any cracking over the next few days?

And what’s in the plaster btw? Looks like it’s non-hydraulic quicklime based, given the name “Eden hot lime”.

 

[Kearn]

Yeah, that would the summary version of my late night addled brain ramblings!

Hot Lime Mortar Mixes - Eden Hot Lime Mortar

Hot Lime mortar enable houses to breathe and to dry out naturally, mixes available for pointing, rendering and plaster skim work.

edenhotlimemortar.co.uk

Nosterfield HLM4
Comes pre mixed in 25kg resealable tubs, I’m getting roughly 1m2 per tub give or take.

It’s nice and sticky, knocks up well. My only gripe is the odd larger (within tolerance) piece that pulls to the surface when trowelling over the plastic mesh. But then I should be using 10mm holes not 5!

But I’ve trowelled up a nice polished finish with it quite happily. It’s just too much time for my project/skills and not the right finish anyway… hence the sponging a day later. Carl at Eden would recommend it over the wood fibre boards too as it’s sticky.

I’m not physically close enough to anyone supplier wise to not need shipping, I’ve had a really good set of experiences with Carl and their pricing is good too compared to some of the usual suspects. Therefore all my mats this last year have come from them…

 

[aockenden]

Cool, that makes sense. Thank you! In your experience is there any risk to making that coat thinner, say 3-4 mm? Assuming you could get proper coverage of your mesh and whatnot. Is there anything intrinsically risky about making that coat thinner?

 

[Kearn]

Cool, that makes sense. Thank you! In your experience is there any risk to making that coat thinner, say 3-4 mm? Assuming you could get proper coverage of your mesh and whatnot. Is there anything intrinsically risky about making that coat thinner?

Many of my panels are certainly less than 8mm, my approach is not exact! The obvious challenge finishing at that thickness is the aggregate size. Especially combined with the mesh which is what usually drives the requirement for the second layer on my panels.

I’d probably opt for a 2mm plaster - gives a bit control for cracking compared to a fine top coat, but shouldn’t drag as much as a 4mm

 

[a twig]

Over wood wool I did 2mm skim, troweled in the mesh then 2mm skim on top of the mesh, admittedly with an NHL based product (Reabilita Cal AC).

The main upside of using the NHL was having to wait less than 2 hours between trowelling in the mesh and putting on the top skim

 

[a twig]

On one section of boards in a different room where I had run out of mesh I did just do a 4mm skim with no mesh or scrim, did get some cracking at board edges which I filled with a lime putty based filler a week or two later, then put a couple of coats of limewash on to help it “heal” before finishing with clay paint. It seemed to last and not crack any further but to be honest I’d probably stick with mesh or scrim in future.