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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorjfb
    • CommentTimeJan 15th 2019
     
    I am converting an old barn into an office space and am considering the best option for insulating the pitched roof at rafter level. A new roof was put on it just before I bought it to ensure that it wouldn't fall foul of a planning timescales. So I am very reluctant to reroof but that makes it harder to work out a sensible insulation plan.

    There are no plans for cooking facilities or shower but there will be a toilet/basin.

    At the moment the structure is:
    new clay tiles (tight fitting)
    battens
    breather membrane (2 types, don't know what sort, but modern, not felt, so I presume is breathable)
    rafters (100mm)
    purlin (minimum 150mm, max 200mm)

    Rest of structure is 500mm thick stone walls where I will put on lime render/wood fibre board and a limecrete floor.

    For the pitched ceiling I was hoping to do something like this:

    tiles
    battens
    breather membrane
    rafters (100mm full filled with fibre glass)
    graphite eps (75mm)
    intello airtight/vapour control layer
    batten (25mm)
    plasterboard +skim (15mm)

    This leaves all the purlins exposed as I want and as much insulation as can be had but doesn't have any ventilation under the breather membrane. Except that the build up is breathable and there will be some ventilation through the tiles.
    Even if I try and reduce the rafter insulation so it is not fully filled air getting in at the eaves wouldn't have any where to ventilate without installing some sort of tile/ ridge vent. Even then there isn't any cross ventilation that you would have with counter battens.

    In an ideal world I would have the roof off, cover with 100mm eps, breather membrane, counter battens, battens, tiles.
    But that is going to be so much more work building up walls, planning issues, reproofing that I would really like to find a simpler solution.

    Any thoughts appreciated.
      pitchedroofsmall.jpg
  1.  
    Too late now but I would have thought counter battens would have been needed under the tile battens. Why weren't they put in?

    For the walls I am assuming that the lime render/wood fibre board is going on as EWI. I would use standard EPS EWI rather than anything organic. I have used EPS EWI on 50cm stone walls (with rubble infill) without problems. Internally lime render would be a good choice.
    • CommentAuthorjfb
    • CommentTimeJan 15th 2019
     
    the wood fibre is IWI - planners wouldn't allow EWI. At least IWI will be easier to join up with the roof insulation.

    No counter battens.
  2.  
    I was advised against IWI on my stone / rubble walls because the wall would suffer by being left out in the cold and including it within the heated envelope would be better. Over here all stone / rubble walls are rendered any way so whether it is just render or EWI then render is of no issue.
  3.  
    Was the roof structure run past a SE? I showed the picture to son in law SE over lunch and he asked if was a concrete ring beam put on the wall (standard practice here) if not you probably need joists 1/3 (minimum) down the rafters to prevent side thrust on the walls. Depending upon the length of the roof he did not think that the purlins would mitigate the side thrust imposed by the roof.
  4.  
    Even with a breather membrane I am more comfortable with a 25mm ventilation gap, and particularly so in this case you do not know the makes and 'models', so you don't know whether their certificates allow for full fill. As others have hinted, counter-battens are often/usually required with full fill.

    You can have cross-ventilation (from eaves to eaves) if you simply install a bit of dropped ceiling (and P-in-H suggests probably more than I was thinking, for other reasons). In my experience you often do not need ridge vents if you have the equivalent of something like a 15-25mm 'slot' all along both elevations at the eaves. If you want to keep at least half of the purlins visible I would suggest Pu/PIR - 75mm between rafters and 75 below. This will get you a clear 0.18 U value (probably nearer 0.16, although I have not done the 'timber fraction' calcs). That complies with the regs, which 100 GF between/75 pEPS below doesn't, quite, although it's close, so I suspect your BCO would not quibble. The Pu/PIR option is, of course, not breathable, which might be a deal-breaker for you.
    • CommentAuthorjfb
    • CommentTimeJan 15th 2019
     
    nick I'm not quite sure what you mean by a 15-25mm slot along both elevations at the eaves. Do you mean at the gable ends? The gable ends are bedded in with mortar right now so no real gap there for cross ventilation. I can't see how I could get a slot there. There is plenty of space for ventilation at the eaves on the non gable walls at rafter level right now.
  5.  
    No, I didn't mean the gable ends, I meant the eaves. The cross-sectional area of ventilation provided at the eaves of each roof slope should be equal to (15-25mm x width of eaves). So for a roof 10m (or 10,000mm) wide, the cross-sectional area of ventilation provided should be between 15,000 and 250,000mm2.
    •  
      CommentAuthordjh
    • CommentTimeJan 15th 2019
     
    Posted By: jfbNo counter battens.

    If there are no counter-battens then probably the membrane is the type that needs to be draped to allow water to escape under the battens. In which case full-fill insulation underneath may be problematic. I'd also check exactly what the membrane materials actually are - they may be breathable but then again, they might not be.
    • CommentAuthorSprocket
    • CommentTimeJan 23rd 2019 edited
     
    Wind is surely going to bypass most attempts to insulate below that.
    We had one done a bit like that from around 2004... it was 100mm celotex between rafters and T&G board below, all sat on a new (at the time) green oak roof (big open trusses and purlins, no ceiling). Despite our best efforts it was fairly draughty and we got mice, flies, the odd bat, and a small bird or two managing to find their way in.

    Then in 2009 we seriously got the eco-build bug. All our clay-tile roof builds since then have used a sarking board roof, even when we have a ceiling.

    Last year, finally sick of the cluster flies and wren poop, we re-roofed by lifting ALL the tiles, carefully fitting a layer of 18mm T&G sarking board over the rafters, then new membranes, battens, counter battens, and old tiles back on top. It has made a *huge* difference.

    You'll still have to careful how you deal with the wall-plate though if you want to avoid draughts, that'll be the weak spot then. Plugging all those wall-plate to rafter gaps with new nogging will be pretty fiddly... really need to remove the bottom tiles at least. I assume you have guttering to consider there too?

    I'd have liked to see a decent roof truss in there to hold that purlin. Is that timber framing the only thing supporting it?
    • CommentAuthorCWatters
    • CommentTimeJan 24th 2019
     
    <blockquote><cite>Posted By: Nick Parsons</cite>No, I didn't mean the gable ends, I meant the eaves. The cross-sectional area of ventilation provided at the eaves of each roof slope should be equal to (15-25mm x width of eaves). So for a roof 10m (or 10,000mm) wide, the cross-sectional area of ventilation provided should be between 15,000 and 250,000mm2.</blockquote>

    See top right....

    https://www.swishbp.co.uk/design/building-regulations/
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