Wood Wool Board vs Plasterboard

[jsmac]

Hello again,

As a follow up to my previous post I want to ask about your feelings towards plasterboard. Specifically with using plasterboard and skim for ceilings. As I mentioned previously we have had to do a lot more stripping back to the bones than we intended to get a good look at the main structure of the building. So this meant the laths had to come down from the ceiling (after the paper and plasterboard and in one room, another layer of plasterboard). There's been a lot of bodged repairs over the years and before we decorate we wanted to ensure that we weren't going to be met with any nasty surprises in the coming years. We've already uncovered some 'repairs' to the roof joists that will need some attention.

On the recommendation of the folks at Mike Wye we're going to use wood wool boards on the walls with Pavaflex insulation between the studs. The wood wool boards will be plastered with AdhereVit and Secil Finishing Plaster. We would like to recreate as much as possible the look and finish of the plaster that is still sound and intact in the rest of the house and this finish is what was recommended. Have any of you used these products? Any other recommendations?

I had thought that we would use the same boards and plaster for the ceilings but the expense may preclude us from doing so. So to my surprise I'm now thinking about using plasterboard for the ceilings and having it skimmed. Is this likely to look incongruous next to the lime finish or is it something we're not likely to notice once the ceiling is painted? Are there other concerns that we should be aware of?

As always your thoughts and recommendations of PPUKers are most welcome.

Many thanks.

Scott

 

[MikeG]

Is this an upstairs ceiling? In other words, is this insulation you are proposing the external insulated envelope of the house? If so Pavaflex is a really poor insulant, and I'm struggling to see why you would choose that. The route you have been advised is a very expensive one, and not a very thermally efficient one.

Why are you taking lath & plaster ceilings down? Overboarding is an excellent way of preserving the original structure of the building, and yet getting a decent finish. You could do this with plasterboard and Universal Onecoat plaster if you want a lime look without the performance, or you could use a render carrier board such as Savolit Plus and lime plaster direct to that. If you use Onecoat, you need to tell your plasterer to stop when he has the toolmarks out, and not to wet it down and polish it up. In other words, he has to stop earlier than he would do ordinarily. Onecoat looks an awful lot better than the normal pink stuff.

 

[jsmac]

Hi Mike,

Thank you for your message. I will try to clarify.

The upstairs rear of the house is a timber structure. The walls on the inside were a combination of the original lime and lath plaster, plasterboard and cement. They were a real mess - the result, we think, of years of patchy repairs and water damage.

We want to insulate between the studs as there was nothing between the plaster on the inside and the cladding on the outside. So we chose to strip the walls back to the studs rather than take a patchwork approach, which we were advised would cost us considerably more, prevent us from properly examining the timber structure underneath to ensure that it is sound and prevent us from getting insulation between the studs. It was in these walls that Mike Wye proposed Pavaflex as it can be packed tightly in between the studs, followed by wood wool boards and AdhereVit and Secil Finishing Plaster. Conversely, one plasterer we spoke to recommended Celotex (or similar) and skimmed plasterboard but this works out to roughly the same cost as Mike Wye's recommendation since Celotex is so expensive. And the gains we'd make in thermal performance with the Celotex we would lose with lack of breathability.

If there is a more suitable/more thermally efficient/less expensive solution for the walls then I would very much welcome that advice.

The ceilings were in a similar state to the walls. The plaster had been removed from the laths years ago and the laths were largely broken and in a poor state. They had been boarded over and the plasterboard was soft and crumbly along one wall where we suspect there is moisture ingress from the chimney. Again, we decided to strip everything back so we could get a good look at what we're dealing with.

We are intending to insulate the loft space with 300mm of fibreglass insulation. But we will need to replace the ceiling and this is where I'm wondering whether there is any drawback to using plasterboard and skimming it. I appreciate your comments on using Universal Onecoat plaster. No one has suggested this to us so I'll mention it to our plasterer.

As always any recommendations are greatly appreciated.

Scott

 

[MikeG]

.....If there is a more suitable/more thermally efficient/less expensive solution for the walls then I would very much welcome that advice.

Yep. Good old fashioned mineral wool ("fibreglass"). Cheaper by a mile, and a much better insulator. There is so much utter nonsense drifting around in the world of "green" energy and period property renovations, and these poor performing but flavour of the month insulating materials such a wood wool are just the latest. I'm having a battle at the moment with a roofer who thinks that sheep's wool is the only insulation allowed in listed buildings. The nonsense is pervasive.

The ceilings were in a similar state to the walls. The plaster had been removed from the laths years ago and the laths were largely broken and in a poor state. They had been boarded over and the plasterboard was soft and crumbly along one wall where we suspect there is moisture ingress from the chimney. Again, we decided to strip everything back so we could get a good look at what we're dealing with.

We are intending to insulate the loft space with 300mm of fibreglass insulation. But we will need to replace the ceiling and this is where I'm wondering whether there is any drawback to using plasterboard and skimming it. I appreciate your comments on using Universal Onecoat plaster. No one has suggested this to us so I'll mention it to our plasterer.

As always any recommendations are greatly appreciated.

Scott

Firstly, I would deal with the source of the damp. After that, if you have worries about ongoing damp then Savolit Plus and a haired lime plaster. Otherwise, you can use plasterboard and Universal One Coat for the cheaper (but equally as attractive) finish, but without the performance of Savolit & lime.

Don't stop at 300mm of fibreglass in your loft. The stuff is cheap. Put 450mm in if you can maintain airflow around your rafters/ underside of your felt/ membrane.

 

[jsmac]

Interesting. I had been led to believe that mineral wool wouldn't be appropriate as it would sag and it is difficult to ensure there's an effective air gap.

Do I need to leave a gap for air circulation between the studs and the insulation or can it be packed in tightly?

Separately, would Rockwool (ie. mineral wool rather than fibreglass wool) provide better soundproofing but just as effective thermal insulation? The more sound insulation we can get in the party wall the better.

Good advice for adding more insulation in the loft. There is plenty of space up there and we can't use the loft for storage as the joists are strong enough to hold the ceiling up (evidently) but I wouldn't want to do too much crawling around on them.

Thanks, again.

 

[MikeG]

Unless I'm misunderstanding you, the mineral wool would sit flat on the ceiling. No sag possible there. In the walls, use "batts", a slightly stiffer form, which don't slump.

There should be absolutely no air circulation within the depth of the insulation. The insulation should be hard up against in back face of the internal skin of your wall, and tight between the joists. Ideally, there should be an airgap behind the external render (ie between the back face of the render and the insulation), but if you are working from the inside then this could be difficult to achieve. Use the same stuff for the party wall, as the differences acoustically are marginal. Acoustics are much more about density of material, and gaps. You ideally want a heavyweight wall, which you haven't got. I suggest that if you are stripping this wall that you "double tack" it.....2 layers of plasterboard with staggered joints, or, if you are using Savolit & lime, one layer of say 15mm plasterboard, then Savolit over the top of that.But pay huge attention to the gaps.....particularly at junctions (floors, walls & ceilings), and around services (wiring and plumbing).

 

[jsmac]

Thanks, Mike.

It sounds like for between the studs a high density mineral wool batt will be ideal. So the build-up from the inside out could be:

- Universal OneCoat Plaster

- 2 X Gyroc Soundbloc plasterboard layers

- Knauf Building Slab mineral wool

- external timber cladding

Is a vapour control layer necessary between the mineral wool and the plasterboard? My thinking up until now was to maximise vapour permeability but if I'm using plasterboard and Thistle plaster do I need to be concerned about creating a dew point and moisture problems within the timber wall?

 

[MikeG]

No, no..the double tacking suggestion was just for the party wall. For an external wall you only need the one layer of plasterboard, or render carrier board.

Just a word about appearance. Plasterboard flattens surfaces out. You get a much cleaner flatter look using plasterboard, compared with Savolit (a sort of half-way house) and lath & plaster (much more "interesting"). This is something you need to be cautious of in an old building, whose charm often derives from the inconsistencies, and lumps & bumps. At the very least, try to follow the shape of the walls, and tell you plasterer not to rule the walls off (this is where he runs a straight edge up the wall to skim off the high points).

I'm not going to comment on your VCL, because these are whole-house decisions. Ordinarily, foil-backed plasterboard is fine, but the answer is going to vary from house to house. My 300 year old timber framed house has a whole house mechanical ventilation system and a vapour-open lime render externally, so there is no need for any VCL moderating the penetration of vapour into the walls. I'd need to do a full assessment to begin to comment.

 

[Zebra]

I've done sheepswool between the studs in the walls, as part of a total breathable inside-to-out structure. It may not be as thermally insulating as mineral wool but it is more breathable, I believe. Sound insulation wasn't a concern here so I don't know about it's performance in that regard. It's finished internally with woodwool boards and putty-based lime plaster.
On the ceiling is rigid foam insulation, taped and joined to prevent air gaps, as I figured the roofspace is well ventilated enough, and then woodwool boards with putty-based lime plaster again.
I'm not sure whether to advise that system, I'm nowhere near as experienced as Mike, but am happy to answer any questions about it!
Your question seems to be will it be incongruous to use plasterboard on the ceiling and woodwool on the walls. I haven't used it but I understand that clay paint is a great way to even out the finishes and create a coherent look across gypsum and lime, and is breathable too.
Woodwool board is not that expensive, I recall, although clearly more than plasterboard. You could mix your own plaster to save money.
It's tricky to decide on a system to use, isn't it. I remember poring over U-values and breathability for ages, so I understand your dilemma.