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Green Building Bible, Fourth Edition
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    • CommentAuthordaveking66
    • CommentTimeJan 16th 2021 edited
     
    I’m looking for advice on the most appropriate and easy to install insulation system for the pitched ceiling sections of the eaves rooms in a solid brick Victorian property.

    The (vertical) external wall of the rooms will be insulated with a modest layer (40 or 60mm thick) of wood fibre insulation and finished with a breathable lime plaster. I’m now trying to work out which (Breathable) system will be best for the adjacent pitched section of ceiling measuring 1100mm (from top of wall to edge of the (horizontal) ceiling).

    The pitched ceiling section is original lathe and plaster. Behind the lathes are the rafters and a replacement, artificial slate, roof underlined with roofing felt.

    There’s plenty of headroom, so I’m not worried about losing space by insulating internally, and am wondering whether the simplest solution is to remove the top layer of paint from the ceiling and cover the area with the same Steico Therm boards I’ll be using for the wall (and just continuing up with the Lime Plaster Finish)

    Or do I need to remove the lathe and plaster first, installIng something direct onto the rafters? (Similar to how I have removed all render on the walls to expose the brickwork)

    Would be grateful for any suggestions
  1.  
    Your sloping sections are pretty short, so you may not have the sort of loading issues that I often have when doing rip-outs and re-fits of Victorian attic bedrooms (original bedrooms, not loft conversions. Nevertheless you will need to give at least some regard to the rafter size (strength) and fixing. The bottom of each rafter sits on a wall-plate, but what sort of support is there to the length of rafter above ceiling height? Is there a purlin? What is the timber section? 3" x 2"/ (75 x 50)? If you use rigid W/F can it take the additional load. In attic bedrooms the spans of sloping ceiling may be longer and, while I am not necessarily recommending it to you, this is why I often use PIR, as rigid W/F at the depths I'd like to use would just be too much of a load on the rafters, particularly if the client is considering PV as well.

    Do some weight comparisons to see what the weight difference would be between rigid W/F and flexi W/F in a sort of 'Larsen Truss' (spaced stud (/rafter)) arrangement.

    If you use the latter you will need to take down the exg ceiling. If the former you can choose, but I usually do.

    I also usually 'oversail' the insulation into the apex void (tricky, and involving some filling, because each rafter has a joist nailed to it) where it can act as a 'stop' for the quilt insulation 'on the flat', allowing you to stuff the quilt in tight without risking blocking the ventilation gap (probably 50mm in your case) which you have left.
      Larsen Trusses.jpg
    • CommentAuthordaveking66
    • CommentTimeJan 17th 2021 edited
     
    Posted By: Nick ParsonsYour sloping sections are pretty short, so you may not have the sort of loading issues that I often have when doing rip-outs and re-fits of Victorian attic bedrooms (original bedrooms, not loft conversions. Nevertheless you will need to give at least some regard to the rafter size (strength) and fixing. The bottom of each rafter sits on a wall-plate, but what sort of support is there to the length of rafter above ceiling height? Is there a purlin? What is the timber section? 3" x 2"/ (75 x 50)? If you use rigid W/F can it take the additional load. In attic bedrooms the spans of sloping ceiling may be longer and, while I am not necessarily recommending it to you, this is why I often use PIR, as rigid W/F at the depths I'd like to use would just be too much of a load on the rafters, particularly if the client is considering PV as well.

    Do some weight comparisons to see what the weight difference would be between rigid W/F and flexi W/F in a sort of 'Larsen Truss' (spaced stud (/rafter)) arrangement.

    If you use the latter you will need to take down the exg ceiling. If the former you can choose, but I usually do.

    I also usually 'oversail' the insulation into the apex void (tricky, and involving some filling, because each rafter has a joist nailed to it) where it can act as a 'stop' for the quilt insulation 'on the flat', allowing you to stuff the quilt in tight without risking blocking the ventilation gap (probably 50mm in your case) which you have left.
      http:///newforum/extensions/InlineImages/image.php?AttachmentID=7770" alt="Larsen Trusses.jpg" >


    Many thanks for the detailed response - I’d not considered loading implications on the rafters.

    If I go down the breathable route, will I need to install something stiffer (and therefore heavier) than steico Threrm?

    Should I be considering a PIR System for the Sloping section?

    Can anyone advise whether the risks of interstitial condensation forming on the rafters are comparable to the risks when insulating a solid brick wall?

    Thanks
  2.  
    Assuming that the existing roof felt is not breathable? So there will need to be a good ventilation gap between the insulation and felt, and so then there is no need for the ceiling to be breathable, infact better if it is not to avoid condensation under the felt. There is also no need or excuse to target a poor insulation value on this section, do a proper warm job (but only once!)

    In that situation in our last house, we* removed the lath and plaster and replaced with two layers of foiled PIR deep enough to meet the legal insulation standard (was a bit more than 100mm of PIR in Scotland back then). The first layer being whatever depth would fit between rafters while preserving ventilation gap, the second layer beneath the rafters to make up the depth. The PIR manufacturers' websites tell you how deep. Then cover with plaster or plasterboard of your choice. There are different schools of thought how to fix through the insulation into the rafters.

    Totally recommended, it transformed the rooms. Good luck with your project!


    In one part of the roof, the rafters were unusually deep, so I slid/wriggled sheets of PIR down into the gap from above. Lots of cutting and sticking and poking to get round joist ends and fill gaps. In retrospect I wouldn't do it that way again.


    *Well actually the previous owners had already botched up the lath+plaster with some sections of plasterboard. It is a messy job to remove it but not as difficult as the render you did.
  3.  
    ''If I go down the breathable route, will I need to install something stiffer (and therefore heavier) than steico Therm?

    Should I be considering a PIR System for the Sloping section?

    Can anyone advise whether the risks of interstitial condensation forming on the rafters are comparable to the risks when insulating a solid brick wall?''

    *Nothing like as risky. Most IWI situations are directly on the wall, without a vent gap, and the risk of interstitial cond'n is always present, and needs mitigation. In the roof sitation you have, if the roof ventilation is right, a howling gale ready to whisk away any water vapour which dares to get into the vent path.*

    *For breathable you could use Steicoflex or Pavaflex flexi W/F, but you'll need about 250mm of it to get the required U value of 0.18W/m2K.*

    *No, if you don't like PIR and the above (and headroom) is not a problem, no need to use PIR.*
    • CommentAuthortony
    • CommentTimeJan 17th 2021
     
    you should decide on design philosophy and target U-value first
    • CommentAuthordaveking66
    • CommentTimeJan 22nd 2021 edited
     
    Cheers both for your comments,

    I’m now leaning towards PIR boards for the pitched ceiling section for their increased performance. Does anyone know if the switch from woodfibre on the walls to PIR on the (pitched) ceiling will present any problems from a plastering perspective? The Woodfibre boards will need to be lime plastered, can I continue with lime plaster on the plasterboard PIR boards? Or will they require a modern pink plaster system?
    • CommentAuthordaveking66
    • CommentTimeJan 22nd 2021
     
    Posted By: WillInAberdeenAssuming that the existing roof felt is breathable

    The first layer being whatever depth would fit between rafters while preserving ventilation gap, the second layer beneath the rafters to make up the depth. The PIR manufacturers' websites tell you how deep. Then cover with plaster or plasterboard of your choice.


    How much of a gap is needed to provide adequate ventilation?
    • CommentAuthorbhommels
    • CommentTimeJan 22nd 2021 edited
     
    50mm.
    It can be 25mm in case you have a ventilated roof space with a defined air flow path (soffit/fascia ventilation and a dry ridge or similar), but I' d stick with 50mm just to be safe. I nailed small wooden blocks to the sides of the rafters to push the PIR boards against to make sure the gap is what it is supposed to be.
    • CommentAuthordaveking66
    • CommentTimeJan 22nd 2021
     
    Posted By: bhommels50mm.
    It can be 25mm in case you have a ventilated roof space with a defined air flow path (soffit/fascia ventilation and a dry ridge or similar), but I' d stick with 50mm just to be safe. I nailed small wooden blocks to the sides of the rafters to push the PIR boards against to make sure the gap is what it is supposed to be.


    Ok great thanks for that
    • CommentAuthordaveking66
    • CommentTimeJan 23rd 2021
     
    Can anyone advise whether woodfibre board can be fitted on top of PIR insulated plasterboard?

    My plan is to fit PIR insulated plasterboard onto the rafters, then add a layer of wood fibre board over the top. This strange sounding plan is to avoid having two different substrates (woodfibre on the walls and PIR plasterboard on the pitched ceiling) to Lime plaster.
    • CommentAuthorJulesB
    • CommentTimeJan 23rd 2021
     
    Can't help with your question about wood fibre over the top of plasterboard I'm afraid but I'm very interested in this thread as I'm doing the same thing in a small eaves space at the moment. I've got PIR between the rafters and then a layer underneath the rafters and then horizontal battens hold the insulation in place. I'm planning to plasterboard the ceiling by fixing plasterboard to the battens and then was going to get someone to skim the ceiling.

    I've got two external walls and two internal and was planning on using wood fibre on the external walls and lime plaster. I hadn't considered any problem of going from a gypsum plastered ceiling to lime plastered walls. Are you thinking gypsum plaster on the ceiling and lime on the walls would look odd/be difficult to blend?

    Thanks,

    Jules
    •  
      CommentAuthordjh
    • CommentTimeJan 23rd 2021
     
    Posted By: daveking66Can anyone advise whether woodfibre board can be fitted on top of PIR insulated plasterboard?

    My plan is to fit PIR insulated plasterboard onto the rafters, then add a layer of wood fibre board over the top. This strange sounding plan is to avoid having two different substrates (woodfibre on the walls and PIR plasterboard on the pitched ceiling) to Lime plaster.

    You mean to avoid having to put lime plaster over both plasterboard and woodfibre? I don't know how hard it is to skim plasterboard with lime but I don't see why that can't be done. I'd be more concerned about likely cracking at the angle between the two (which can just be filled of course so isn't really a problem). But why put the PIR plasterboard there in the first place? Why not put PIR up and then woodfibre, or even woodwool for better adhesion?
    • CommentAuthordaveking66
    • CommentTimeJan 23rd 2021
     
    Good point re doing just PIR - my thinking was clearly in a (plasterboard lined) tunnel!

    The lime plasterer I have found to do the finish has suggested it might be best if I use the same material for the pitched part of ceiling as the walls to avoid any potential differences how they might react to the coat of lime plaster. I can’t see that using a layer of wood wool board would be that much of a difference compared to Woodfibre board though? As you say, Wood wool would provide better adhesion. I’ve just googled wood wool Boards and found Savolit boards. Is this the kind of thing you were referring to?
    • CommentAuthordaveking66
    • CommentTimeJan 23rd 2021 edited
     
    Jules,

    It’s just the pitched part of the eaves room ceiling I’m referring to. I’m not (currently) planning to touch the main flat bit of the ceiling, apart from along the edge where it joins the pitched section. This edge might need to be sanded back a bit, and taped to the adjoining pitched section before plastering. And the main flat bit of the ceiling is Standard gypsum plasterboard.
    •  
      CommentAuthordjh
    • CommentTimeJan 24th 2021
     
    Posted By: daveking66I can’t see that using a layer of wood wool board would be that much of a difference compared to Woodfibre board though? As you say, Wood wool would provide better adhesion. I’ve just googled wood wool Boards and found Savolit boards. Is this the kind of thing you were referring to?

    I would listen to your plasterer if he's good with lime. He'll know far more about what will work well and what won't. Yes, Savolit is one brand of board, there are others. They're hard wood and cement boards.
    • CommentAuthorJulesB
    • CommentTimeJan 24th 2021
     
    Ah I see. 👍
    •  
      CommentAuthordjh
    • CommentTimeJan 24th 2021
     
    Posted By: JulesBI've got two external walls and two internal and was planning on using wood fibre on the external walls and lime plaster. I hadn't considered any problem of going from a gypsum plastered ceiling to lime plastered walls. Are you thinking gypsum plaster on the ceiling and lime on the walls would look odd/be difficult to blend?

    There's no particular problem with gypsum on ceilings and lime on the walls. That's what we have. They all look much the same once they're painted. You can expect some cracking along the corner edge, if our experience is anything to go by. But I've no idea how much is due to different materials and how much is inherent in joining two structures. We were going to get the cracks filled with decorators caulk last year, but circumstances, yada yada.
    • CommentAuthorJulesB
    • CommentTimeJan 24th 2021
     
    Thanks for this. Very helpful to hear that it doesn't look odd. I'm still weighing up gypsum on plasterboard or lime plaster on wood wool board for the ceiling so handy to know.

    I'm not too concerned about cracks in the corner of the room to be honest - our whole house has got cracks in the corners and along wall/ceiling junctions!
  4.  
    I have used 5cm wide plastering mesh in-bedded into the joints which seems to stop the cracks - or at least keeps them unnoticeable.
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