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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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    • CommentAuthorkrishna
    • CommentTimeSep 13th 2018
     
    We need to retile our Edwardian end of terrace house. We have about 100m2 of roof. Currently has concrete tiles over traditional roofing felt held down by 25mm battens. The top floor of the house is essentially in the roof (built that way) and gets very cold. I want to insulate as best as possible but there are some limitations. We are planning to replace the concrete tiles with slate.

    1. Can't insulate internally (so no insulation under the rafters).
    2. The rafters are only 70mm or 75mm deep (varies depending on which part of the roof - there are three pitches.
    3. We are not allowed to alter the ridge height.

    If we put insulation between the rafters with breathable felt on top, I understand from building control we will need to leave a 10mm gap above the insulation, thus limiting us to 60mm of insulation.

    In checking with Kingspan about their Nilvent breathable membrane, they said the alternative is to lay the Nilvent taught and counterbatten above. As far as I can see, this will raise the height of the roof by about 25mm?

    Another possibility I can see from looking at the product brochure for Nilvent, is to lay sarking boards over the rafters, membrane on top of this, and nail the slates directly down, without the need for battens. This would actually be 5mm less than the current set up. This, it seems to me will allow us to fully fill between the rafters with 70mm insulation and, if say we used 20mm Pavatex Isolair sarking boards, this would provide a (small?) contribution to the insulation itself.

    Is it worth doing the latter (or is that even feasible?) or should we stick with 60mm insulation with a gap to allow the breather membrane to drape between the rafters? Anything else I haven't thought of?

    Also, what insulation between the rafters? Building control say I have to use PIR or something else of equivalent thermal efficiency. Would phenolic board make a significant difference?

    Thank you.

    Krishna
    • CommentAuthortony
    • CommentTimeSep 13th 2018
     
    PIR will be better, you need to comply with building control so should take their advice.

    Part L1b says that you should insulate to current regs I always aim to far exceed these rather poor minimum standards.
    •  
      CommentAuthordjh
    • CommentTimeSep 13th 2018
     
    Posted By: tonyPIR will be better,

    Why do you say that, Tony?

    you need to comply with building control so should take their advice

    Indeed, and they apparently said krishna has "to use PIR or something else of equivalent thermal efficiency". Phenolic has better thermal efficiency and is safer in a fire, so I'm not sure what's not to like.
    • CommentAuthorkrishna
    • CommentTimeSep 14th 2018
     
    I'd love to exceed the standards but I am unable to go under the rafters, and won't be able to get planning permission to raise the ridge height. So I am stuck with doing the best I can with the 70mm between.

    I was thinking of phenolic because, as djh says, the thermal efficiency is better and I understood it was safer in a fire. Is there a better option?

    I'm trying to gain what I can from the 70mm available, which is why I am wondering:

    * Will using sarking boards enable me to fill that space fully as opposed to leaving a 10mm gap? (the Kingspan brochure seems to indicate this is the case)

    * If I don't have cross battens or tile battens and nail straight into the sarking boards, this is presumably going to penetrate the membrane. Is that an issue?

    * Is there a minimum thickness sarking board I would have to use to ensure tiles can be nailed securely without actually penetrating the insulation below?

    * Are there other issues or I have not thought about?
    • CommentAuthorgravelld
    • CommentTime5 days ago
     
    Posted By: djhWhy do you say that, Tony?
    Available depth I would guess. 70-75mm is not a lot of depth.

    That's a "desktop study" of course and we all know how realistic they can be... another consideration might be a spray solution which would also ensure near-perfect fitting (difficult with PIR) and also a level of air tightness.
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Posted By: gravelldAvailable depth I would guess. 70-75mm is not a lot of depth.

    What does the available depth have to do with a recommendation for PIR?
    • CommentAuthorgravelld
    • CommentTime5 days ago
     
    It has a lower conductivity than most other insulants - typically 0.02n compared to 0.03n or 0.04n.

    I know there are lots of other factors but I was just guessing.

    I'm coming to the same conclusion for my own project. Enabling works to fit the "correct" (wood fibre, maybe EPS) solution are too expensive, so have to go for something else. Even though PIR is more expensive than other insulants this difference is dwarfed by the labour cost difference.
    •  
      CommentAuthordjh
    • CommentTime4 days ago
     
    Posted By: gravelldIt has a lower conductivity than most other insulants - typically 0.02n compared to 0.03n or 0.04n.
    But not as low as the alternative under discussion - phenolic. Hence my question to Tony. Since he's the one that posted the suggestion, he's the only one who actually knows why he made it. Guessing doesn't help anyone.

    https://www.kingspan.com/gb/en-gb/products/insulation/kingspan-insight/articles-and-advice/types-of-insulation-a-beginners-guide
    • CommentAuthorgravelld
    • CommentTime4 days ago
     
    Sorry, yes.
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