Wood fibre over wood wool

[AureliaB]

Morning all,

Long time visitor first time poster. Basically we've moved into an old house and it needs pretty much gutting. We've sorted the roof out so now onto the bedrooms, ive purchased 40mm wood fibre boards for the walls. When we have come to strip the walls we've found some of the walls have wood wool on them with lime plaster behind. Are we ok to screw the wood fibre ontop of the wood wool into the wall?

Hopefully so, because it will save us stripping replastering etc them walls. I can't see it being an issue, just want to check.

Also we have sloping ceiling in one corner of one room where we are thinking of pir boards between joist and wood wool over the top for plastering. If this is a good idea, whats the best method to seal the pir insulation between the wood joists?

Thanks
A

 

[fernicarry]

Wood wool doesn't have much thermal performance, its function is as a carrier for the final plaster finish. You'd be better of stripping the wool boards too so you lose a little less floor area when you put on your wood fibre boards. Are you sure they are wood wool though? Seems strange for it to be fitted over existing plaster and it won't be strong enough to hold a screw.

Why do you want to use PIR? Because of its better performance than natural products for same thickness? Its non-breathable so you run the risk of it trapping moisture against the rafters. Particularly if you use the typical builder approach of sealing around it with gun foam. Better to use something natural and breathable. Some products have a bit of give in them so they will take up irregularities. You can add further insulation below the rafters on the sloping wall if needed with negligible impact on usable space.

 

[AureliaB]

Yeah all ive read it wood wool is just a carrier. But its flat, seems like effort to remove. Will have a look how easy it is. It looks like the plaster under is just to straighten the wall.

As for the pir, we have no eaves so i thought leaving a 20mm gap between pir and roof felt would give sufficient air flow to keep it dry. Then leave a 10-20mm gap and put wood fibre onto the rafters.

 

[fernicarry]

Maybe I didn't understand the current situation correctly. Has someone started a renovation and put wood wool onto the wall but not finished it off? If the plaster was to level off the masonry then the boards may well have been bedded into the wet plaster to give a good surface contact. That's a good approach but will make them a bit of a pig to get off and you may need to parge again before putting the wood fibre on. On that basis maybe let them be and accept the extra depth. Do you know if thermal plaster was used?

So far as the rafters are concerned whatever you put between then you shouldn't have a gap in front. Definitely have a gap on the outside face to ventilate the back of the roof covering but if you allow air flow on both sides of the insulation it won't insulate. If you insist on using PIR them make sure you have a vapour barrier on the warm side. I'd still be concerned about how the PIR will trap water should there be a defect in the roof in the future.

 

[AureliaB]

So yes we have moved it, started upstairs in the loft, insulated etc and now onto the bedroom. We purchased 40mm wood fibre boards thinking its crappy plaster to knock off but on some of the walls that look decent they look to have used wood wool. So it's finished just 20mm wood wool. I thought about knocking the top layer of plaster off and screw the wood fibre boards onto the wood wool into the wall and then plaster over it. I thought as you said looks solid on the wall so ideally dont want to be messing with it.

My plan was to leave an air gap roof side and no gap inside like you said. I do have concerns about pir and old roof but equally want something that's a good insulator because no point sticking 40mm on solid wall to have the heat go up the sides . Anything else suitable to use?

 

[fernicarry]

How deep are the joists? I'd also use wood fibre (or something similarly natural and breathable) between the rafters. My next room has a sloping ceiling on one side and I'll be using Inditherm on it. I've used it on walls and under the floor and I'm pretty happy with it. Its has a bit of give so you can friction fit it between joists and rafters that aren't quite square and get a good surface contact . Steico have a product that seems to have similar properties but I've not used it.

Unless your rafters are particularly weeny you should be able to get 80mm or so in between them whilst leaving the air gap. Aim for a depth that gives you a U-value similar to the wall so that the sloping ceiling isn't the coldest part of the room. If you can't get the depth between the rafters then overboard on the room side with more wood fibre. I'll also be putting crosswise slats behind the insulation so its kept in contact with the back of the internal wall.

I've just this second finished the first coat of plaster on a wall that I've built up with thermal plaster, then 50mm or so of Inditherm then wood wool boards. So far the temperature difference I'm seeing with one of those point and click laser thermometers between the insulated area and bare stone is pretty encouraging. My plastering skills still leave a lot to be desired however...

 

[AureliaB]

Our rafters are 7.5cm deep, so yes pretty crappy . How would you build up into the roof, just screw ontop of each other?

I thought about running so wood horizontal across the top and bottom of the rafters and fitting boars inbetween and then have wood to screw wood fibre boards too

 

[fernicarry]

3", that's not very substantial... So realistically max of 40mm between the joists whilst still allowing for some support behind and an air gap.

The insulation should stay up with just a friction fit. If it doesn't then you have air gaps along the sides and you don't want that. You may need to use a more flexible product that what you are putting on the wall because the rafters aren't going to be regular enough for a rigid product. The battens and slats are just to stop it being pushed too far into the gap and to keep it pressed against the back of the ceiling.

 

[AureliaB]

Yeah, i am looking at increasing the joint depth either via a frame or just screwed down the length and the joists. Its almost dead space internal so dont care what i use up.