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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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  1.  
    I will be converting my loft as part of my victorian refurbishment. The rafters are in good condition (plenty of ventilation!) as are the original slate roof tiles which are mounted on battons over the rafters. But no roof felt. I would like to insulate using woodfibre batts between joists. What layers would I need?
    • CommentAuthortony
    • CommentTimeJan 31st 2020
     
    Decide on a u value and work back from there, it is a good thing to have no sarking felt but may indicate that the roof is due for redoing
  2.  
    Not sure on your build but when I insulated my Victorian roof for a conversion in hindsight I wish I had insulated over the rafters at the same time I had the new roof...
  3.  
    Not planning on overhauling the roof at this stage due to budget mainly. Also its in reasonable condition. I would definitely insulate over the rafters if I was replacing. Although being semi-detached that could cause problems.

    Do you know if I need to introduce some sort of breathable vapour layer between the tiles and the insulation? I can't see how I'd achieve this save for wrapping it under the rafters.
    • CommentAuthortony
    • CommentTimeJan 31st 2020
     
    No breather layer needed
  4.  
    That's good. I think. I am imagining the scenario of a tile coming off and rain soaking the woodfibre batts...
  5.  
    Posted By: modernvictorianThat's good. I think. I am imagining the scenario of a tile coming off and rain soaking the woodfibre batts...

    Yes it will - and any snow blown under the tiles will also also soak the insulation. Driven rain can also get to the insulation and cause problems. It rather depends how well fitting the tiles are.

    insulation batts ate difficult to put between the rafters without gaps at the edges wool insulation (either from stone or glass NOT INO sheep) is easier to install without bypass gaps at the edges
    • CommentAuthorMike1
    • CommentTimeJan 31st 2020 edited
     
    I'd look at fixing something like Solitex Mento to beneath the tiles, fixed from inside to the sides of the joists. A bit tricky at the eaves if you don't have the ceilings down, but otherwise works well.

    Not only stops rain & snow, but also keeps the wind out of the insulation, which would reduce its effectiveness.
      SolitexMentoRefurb.png
    •  
      CommentAuthordjh
    • CommentTimeJan 31st 2020
     
    What's the layer underneath the tiles in the illustration? I can't think what it might be in modernvictorian's context?

    Also, what's the point of the 'drape' in the Mento, given that the whole thing is on a slope? Or even if it isn't on a slope?
    • CommentAuthorMike1
    • CommentTimeJan 31st 2020 edited
     
    Posted By: djhWhat's the layer underneath the tiles in the illustration?

    Good point. That's a layer of Pro-Clima Intello - airtight barrier / intelligent vapour retarder, which would be air-sealed at the eaves / gables to maintain airtightness.

    The central drape is no doubt to encourage any water to run down to the eaves, rather than sideways to wet the rafter though, as far as I recall, they don't say so.
    •  
      CommentAuthordjh
    • CommentTimeFeb 1st 2020
     
    Posted By: Mike1That's a layer of Pro-Clima Intello

    I don't understand. I'm talking about immediately underneath the tiles. I'd expect Intello to be on the warm side, under the rafters, where the 'dotted line' is.

    The central drape is no doubt to encourage any water to run down to the eaves

    But it's running across, the roof not, down it?
    • CommentAuthorMike1
    • CommentTimeFeb 1st 2020 edited
     
    @djh, the orange is Solitex Mento (on the cold side above the insulation beneath the tiles); Pro-Clima Intello on the warm side below the insulation.

    The section is from gable-to-gable (left-right), so any water would run down to the eaves. Maybe the interlock of the tiles is misleading?
    •  
      CommentAuthordjh
    • CommentTimeFeb 1st 2020
     
    Posted By: Mike1The section is from gable-to-gable (left-right), so any water would run down to the eaves. Maybe the interlock of the tiles is misleading?

    Ah yes, that's what threw me. I thought it was upslope, but now I see that doesn't make sense with the rafters either :devil:

    I still don't understand what the layer immediately under the tiles is though? Is that a tile batten?
  6.  
    MV, neither our previous nor current Victorian houses have any felt under the slates. The slates have been there for many decades without leaking, and there is no sign of rain or snow getting in, even when a few slates blow off each winter, though there's plenty of 'ventilation' as you said! So we didn't disturb the slates to add membrane when we converted the loft.

    I have no experience of woodfibre, we used PIR. To get a worthwhile u value you'd likely need insulation below the rafters as well as between them, you get thermal bridging through the rafters. The better the insulation value of the material, the less headroom you lose.

    We converted to a room-in-roof with insulated flat ceiling (rockwool) and dwarf walls (PIR), so it was only the sloping bit that lost headroom.
  7.  
    Hello MV, forgive me if I am teaching you to suck eggs, but I do not know how much you already know.

    You talk of woodfibre (henceforth WF) 'batts' between rafters. I assume you mean the 'flexible fluff', not the rigid WF. If the former, yes, that's fine as far as it goes, but remember that conventional wisdom says that you need a 50mm ventilation gap between the top of the rafter and any insulation (there are arguments for accepting a 25mm gap where there is no membrane at all, but I usually leave 50 as I am a pessimist).

    Most WF has a lambda ('k') value of about 0.038-0.044W/mK, so let's take a working figure of about 0.040 for ease of calculation. 200mm of WF @ 0.040 would give an R value of 5m2K/W. Assume a 'base case' U value of 2.0W/m2K for the existing roof, giving an R value of 0.5m2K/W, so the R value for the whole sandwich is around 5.5. The reciprocal is 0.181818181.... so you could argue that that *just* creeps through the 0.18 requirement of Part L1B of the Bldg Regs - or you could argue that it doesn't, because my calcs do not take into account the 'timber fraction' - the amount of the 'sandwich' which is rafter, not insulation, in which case, increase to 225mm and you have probably cracked it.

    OK, assume your rafters are 75mm (most Victorian ones I come across are, though the one I am working on at present has 100mm and a rare few have even bigger section). You can, with a 50mm ventilation gap, get 25mm between the rafters (can you get flex WF in 25mm - I perhaps doubt it). Therefore that's 200mm underneath. You could use rigid WF and 250mm fixings, but I would then advise you to retain a structural engineer, as the loadings will be massive. The alternative is a 'Larsen Truss'-type arrangement ('spaced stud') - effectively a secondary rafter, but that will still add weight to a timber section which would be deemed too small today.

    I have not even considered headroom issues.

    I have to say that's why, although I regard it with distaste, I often use PIR.

    I hope that helps, or tells you what you already know, in which case, sorry!
    • CommentAuthorMike1
    • CommentTimeFeb 1st 2020
     
    Posted By: djh
    I still don't understand what the layer immediately under the tiles is though? Is that a tile batten?

    That's my interpretation.

    Posted By: Nick Parsonsconventional wisdom says that you need a 50mm ventilation gap between the top of the rafter and any insulation

    Yes, to dispense with the 50mm gap (and ridge + eves ventilators) you need a vapour permeable membrane suitable for the purposes. Don't try this with the old BS747 Type 1F style felts!
  8.  
    Posted By: WillInAberdeenMV, neither our previous nor current Victorian houses have any felt under the slates. The slates have been there for many decades without leaking, and there is no sign of rain or snow getting in, even when a few slates blow off each winter, though there's plenty of 'ventilation' as you said! So we didn't disturb the slates to add membrane when we converted the loft..


    Will - I am pleased to hear this. The loft is in great condition. Even the old insulation looks pretty clean. I keep being told I need a roof membrane but as you say I am reluctant to change something was has worked well for the last 100 years or so.
    • CommentAuthorCWatters
    • CommentTimeFeb 2nd 2020
     
    The drape is to stop batten rot. If you press the membrane against the underside of the battens then dirt collects above the battens and it can prevent water running down. Pools of water can form above the battens rotting them.
  9.  
    Posted By: Nick ParsonsHello MV, forgive me if I am teaching you to suck eggs, but I do not know how much you already know.


    Preparing for first renovation project so only what I've read! Grateful for any advice.

    The reason I am interested in wood fibre vs PIR is because of the supposed heat buffering effect due to lower thermal diffusivity values. I imagine this might be beneficial in a hot loft in the summer. Also I don't want to fill my house with petrochemicals if I can help it. However the additional thickness required could be problematic.

    My architect has calculated for PIR I would need 150mm between rafters plus 50mm insulated plasterboard to give a final U-value of 0.17.

    For wood fibre 150mm Pavaflex between rafters plus 100mm Pavatherm- combi to give a final U-value of 0.17.

    The rafters don't seem deep enough for this if they are 75mm. Hmmm. I need to get up there and check, Can I get away with not using a 50mm gap at the top of the insulation given I have no roof felt?

    Ultimately if I have to go PIR then so be it.
  10.  
    >> vapour permeable membrane
    Go for the ultimate in vapour permeability: no membrane at all!

    That's how our roof had worked for it's first 150 years, with plentiful ventilation (no membrane) combined with a double overlap of slates that can tolerate a loss of any individual slate without letting in water. Any problems would have revealed themselves sometime before 1900 and been fixed back then.

    Membranes are appropriate for modern and tiled roofs, where you don't have the same confidence that the tiles will stay water tight if one cracks or slips.

    >>woodfibre
    Woodfibre has half the insulation value of PIR so needs to be twice as thick, roughly.

    The buffering thing is about woodfibre buffering humidity, not heat (AFAIK). To buffer heat, you need heavy stuff on the surface, like plaster and stone. But insulating the roof solved (for us) the hot loft problem of heat coming through the slates, you do need to think about how many south facing roof windows you add.

    >> 250mm fixings
    We had a little disagreement with the joiner about long fixings through the insulation layer under the rafters. We wanted the thicker insulation layer to be under the rafters to avoid thermal bridging, but his productivity depending on using his pro tools (collated screws etc) that only fired shortish fixings. We ended up with thicker insulation between the rafters (ours were deep enough) and we fixed horizontal battens under the rafters, thinner insulation fitted between these battens, plasterboard fixed onto the battens.
    • CommentAuthorMike1
    • CommentTimeFeb 2nd 2020
     
    Posted By: modernvictorianCan I get away with not using a 50mm gap at the top of the insulation given I have no roof felt?

    You really do need a membrane under the tiles, no matter what. If the insulation does get wet it won't be as effective, and it could stay wet for some time if the weather's bad.

    The only advisable way to reduce the 50mm gap is to use a technical membrane as above, unless you strip the tiles and put it above the rafters.
    • CommentAuthorrevor
    • CommentTimeFeb 6th 2020
     
    Just been throwing out a load of leaflets. Come across a product called rafter fit manufactured by web dynamics. Have a look it may well suit your situation.

    http://www.tlxinsulation.co.uk/
    • CommentAuthortony
    • CommentTimeFeb 6th 2020
     
    No membrane = vapour permeable membrane and in terms of vapour transfer is probably a lot better

    In the past I have stuffed some lofts, skeiling, dormers and roofs full of insulation with no ventilation and never see any problems, many had old style 1F sarking with string

    Have seen plenty of problems with polythene or perforated polythene membranes and one or two with mouldy breather membranes in contact with quilt
  11.  
    Wrote a post earlier and lost it!

    Mike1 wrote: ''You really do need a membrane under the tiles, no matter what. If the insulation does get wet it won't be as effective, and it could stay wet for some time if the weather's bad.''

    I would say only if the roof is badly maintained or exposed to severe driven rain. My 120-year-old slate roof is in good order and does not leak. Yes, a membrane is 'belt and braces', but you can have perfectly satisfactory performance without. As Tony mentions, even breathable membranes (which vary significantly in their performance) are not immune from problems. I have seen a couple lately with severe mould and standing water on the underside.

    My experience of 'stuffed skeilings' (sloping soffits) has been about 50/50. Some have had no problem at all, and from some I have had to pull saturated mineral wool which I could wring out.
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