Hi all

I'm due to insulate a couple of small external walls (IWI) upstairs and managed to purchase some Steico wood fibre board from a lovely chap in Oxford. He had it left over from the renovation of his old house.

I'm going to apply on existing plaster which is mostly flat but would you advise using any adhesive / plaster on the substrate to eliminate any potential gaps? There's no mention of this on the instructions I downloaded.

Cheers

Posted By: Nick ParsonsIs the plaster lime or gypsum? I would not leave it on if it's gypsum. If it is lime just make good any cracks and gaps and you have your airtight parge coat. I would not fix wood-fibre 'dry' - always on a 6mm - 10mm toothed trowel coat of what you will use for the base-coat on top of the board. That takes out any unevenness in the parge coat.

Thanks Nick

The plaster is lime. I guess that means cutting the boards first, to a certain extent, if your mounting them 'wet'. Incidentally, I found a toothed trowel in Lidl the other day for £2.99, how good it will be is another matter lol.

Thanks again Nick, its 60mm. For the moment I've got 2 small walls upstairs to do, both dominated by very large windows so I'll be detailing the reveals also.

Do you know I never actually thought of applying the plaster to the back of the board, I just assumed I'd apply it to the wall. The boards are the smaller size so no problem there, I may well apply to the back of the board and tooth trowel to get it consistent. I never thought of the issue of getting plaster in the joints so thanks for the tip, I'll avoid going too near the edge. I have lots of Steicoflex left over to fill in obvious gaps but obviously hoping that won't be necessary. For very fine gaps I guess I could wet the flex first, a bit like the rope they used to use on ships? Cant think of the name.

Would you apply mechanical fixings immediately after positioning the board against the wall? I'll have to pre-drill the holes of course. I was fortunate to get some free insulation nail fixings from a local man on Facebook last year. I thought they might come in handy on my renovation. Waste not want not!

Posted By: Peter_in_Hungary

Posted By: kristevaa bit like the rope they used to use on ships? Cant think of the name.

caulking

That's it!!

caulking


That's it!!

Exactly that! I just save the 'fluff' from cutting the rigid boards, mix it with water and caulk it in with a bolster if the gaps are that wide (which they shouldn't be but sometimes are if the wall has a curve) or a filling-knife.

As far as the fixings go (a) you don't need to pre-drill the holes, though I suppose there's no harm. I just go through the lot with the the SDS bit. I do have to pull the bit in and out a bit to clear the fluff, so your way might work better than mine (saves idea for later!). If you leave fluff in there's a good chance that the hammer fixing won't go 'home'. (b) You want at least 50mm in the wall. You don't have to hammer-fix as soon as you have placed the board, but if you do toooo high they could (if T & G) all fall off. Pic attached shows how far you can go without fixing. Don't leave it so long that the lime goes off - you want some movement.

Posted By: jfb

Posted By: kristevaI'll have to pre-drill the holes of course

No need to pre drill. Just get the next board in place and drill straight through and then hammer the fixing home. Takes a bit of getting used to the best drill bit size/depth of drilling depending on the state of the wall. Just try and make sure that when hammered in they feel good and tight.

Personally if the walls are reasonably flat I wouldn't bother with any extra step of buttering the back of the boards. I just don't see the benefit as assuming you are getting the fixings in nicely they pull the boards in well to cope with gentle undulations. Any really obvious gaps behind a board you could use some expanding foam.

How long are the fixings you have inherited? For 60mm boards they would want to be a minimum of 110mm long I would think.

Thanks Jfb, the fixings are about 120mm I think so should be ok.

Would you start off with a drill bit the same diameter as the plastic insert that holds the nail?

Hi Kristeva, it is just the old board waiting to be ripped down and re-fitted. In between the joists I fitted a plinth of 40mm wood-fibre (down to the bottom of the joists) up to which butted the recycled cotton under-floor insulation. (Pic below)