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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorLehobbit
    • CommentTimeOct 4th 2021 edited
     
    We are renovating an old granite house high on the Plateau Millevaches in the Limousin mountains in the Massif Central. The property is 810 metres above sea level so cold winters!! Originally for the roof we were considering a Bac Acier Coviso sandwich panel roof (steel) which has 120mm of PIR foam integral. We got planning for this roof and were then delayed due to Covid. These panels have now gone up 33% in price since last year and so we have had a rethink. It is a very large roof of about 154 m2 so we are now considering slate coloured terracotta tiles. Actual slate is still too expensive. The attic will have 200mm of wood fibre insulation between the beams of the floor.

    For the roof we won't be doing the insulation straightaway, but our roofer has suggested we could use a multi layer foil insulation and then add extra insulation later on. I have been scanning French wesites for a breathable foil insulation and the price seems much higher than for non breathable? These foils are very popular in France
    Can anyone advise on these? I have seen the price ranging from about €4.00 per m2 to well over €40 a m2!

    I know a lot of people swear by these multi couche foil systems but we would only use it in combination with thicker insulation such as Steico Flex wood fibre insulation. Apparently multi foils work best in combination with other insulation? If the foil multi layer is installed over the chevrons but under the battens for the tiles then this means there is an air gap between the tiles and the insulation. How important is it for the multi couche to be breathable? There is a huge cost difference? Any thoughts. Note: The attic will not lived in just storage. Eventually we will have 200-250mm of wood fibre insulation in the attic floor with 200mm in the roof plus the multi foil insulation as a membrane. We feel this will be sufficient?


    Alternately, would we be better just specifiying a breathable membrane and then putting the extra cost of a multi foil insulation towards better conventional insulation such as wood fibre semi rigid panels? If we are having a new roof what is the best approach?

    What would you advise?
    • CommentAuthortony
    • CommentTimeOct 4th 2021
     
    • CommentAuthorLehobbit
    • CommentTimeOct 5th 2021
     
    OMG 33 pages!!!
    • CommentAuthorLehobbit
    • CommentTimeOct 5th 2021 edited
     
    Okay so having skim read through the thread that Tony put a link to it seems that these multifoils are not all they are cracked up to be especially when used on their own. I thought as much.
    As I stated when I started this thread, originally we were going with a steel Bac acier roof which would have 120 PIR foam between two steel skins. We are now considering a dark grey terracotta tile instead of which we would need 10 to each m2.

    If we don't bother with the foil insulation and just have a breathable membrane under the tiles battens what would people suggest for the best eco insulation? Given that the attic will not initially be used as a living space and the floor of the attic will be insulated with 200-250mm of wood fire between the framework.
    I know intially we could leave the roof uninsulated but where the house is situated can have very cold winters of -20 and less. I was thinking if we finally end up with 250mm of wood fibre in the attic floor and 200mm in the roof that 450mm in total would be more than sufficient. So what to go with and can the wood fibre or other natural insulation be butted up tight against the underside of the breathable membrane? If we went between the chevrons with 100mm of wood fibre for example (the semi rigid stuff) I presume we can then do a secondary layer over the chevrons of 100mm. Is there a more solid form of the wood fibre for this purpose? Any other suggestions /recommendations gratefully received!!
  1.  
    IMO there would be no point in putting insulation on the floor of the attic and in the roof. Indeed it could even be problematic. Normally when insulation is put on the floor of the attic the space above it is ventilated to remove condensation (warm air coming up from the house and vapour condensing on the roof timbers) Any insulation in the roof at the joist level would need a continuos joining with the wall otherwise any air flow from the outside will negate the roof insulation.

    If you are going to use the loft space as habitable space (I.e. within the heated envelope) then I would be inclined to do the insulation in the roof now rather than do the job twice. (once you use the roof space as heated what happens the the roof floor insulation and you won't have enough at the joist level).

    The makeup of a roof I use is
    tiles,
    tile battens,
    counter battens,
    breathable roofing membrane,
    insulation between rafters,
    counter battens to give desired insulation depth filled with insulation,
    VCL,
    plasterboard.

    I use glass fibre or mineral wool insulation as i find that it is much easier to get a good fit between timbers with a wool type insulation rather than a hard or semi-hard board.

    If you use the right sort of breathable roofing membrane you don't need a gap between the membrane and the insulation.
    You do need counter battens between the tile battens and the membrane

    By chevrons do you mean roof joists? If so putting semi-rigid insulation between the joists and then more over I would be concerned about getting the air tightness between the joists and the insulation and then the insulation over the joists butting together properly without moving over time. You have not said how the tile battens will attach to the roof once the insulation is over the joists.

    For insulation I usually use glass wool because it is readily available, cheaper than alternatives and usually contains a fair amount of recycled glass. I justify the use to myself thinking that given the amount of time it will be doing its job any environmental cost over a more natural insulation is worth the result, but that is just IMO.
    • CommentAuthorLehobbit
    • CommentTimeOct 5th 2021
     
    Thank you for your comments, Peter in Hungary.

    Chevrons are referred to as rafters in English. I mean't about putting insulation (100mm) between the rafters and then another 100mm going across the rafters on the underside (interior side) to double up the thickness. Then we would use lambris (pine cladding) or similar as a finish. I think we will stick with a wood fibre insulation as the glass stuff is horrible to use, but admittedly much cheaper. We have also found a product produced in France which is a waste product from rice straw which grows in the Camargue region. This is formed into panels and it is cheaper than the wood fire panels, with a similar u value. Is a VCL a vapour control membrane?
  2.  
    Yes glass is horrible to use. If I have a lot to install I use a disposable overall, gloves, mask and goggles. If you are installing insulation panels between the timbers then often people will cut them slightly undersize and then foam in at the edges to guarantee a good fit. Is the rice straw insulation fire and bug proofed?

    VCL = vapour control layer (membrane), this can / will also serve as an air tightness layer

    When I use plasterboard then I fit the battens under the rafters horizontally, spaced to fit the PB, this mitigates the small cold bridge that would occur if there was continuos timber with battens and rafters. In your case using pine cladding the direction of the battens will depend upon the direction you want the cladding, either vertically of horizontally. Use spacers on the rafters to get the desired distance for the battens to obtain the depth of insulation you require.
    •  
      CommentAuthordjh
    • CommentTimeOct 6th 2021
     
    Posted By: Peter_in_HungaryYes glass is horrible to use.

    Mineral wool is a whole lot better to handle than glass wool, in my experience. And for smallish amounts there's a similar 'wool' made from plastic bottles that's very nice to handle.

    Rice straw is generally better than wheat straw in its properties, but I don't have figures for fire and bug proofing to hand. With wheat straw, you don't normally worry about fire or bug performance as long as it's enclosed by plastering on both sides. But bales are never used under floors and rarely in a roof. Dunno about processed products made from straw. There is/was a product in the UK called Stramit that sounds quite similar to the panels but made from wheat straw.
    • CommentAuthorMike1
    • CommentTimeOct 7th 2021
     
    Posted By: djh
    Mineral wool is a whole lot better to handle than glass wool, in my experience. And for smallish amounts there's a similar 'wool' made from plastic bottles that's very nice to handle.

    +1
  3.  
    We fitted TLX Gold combined multifoil and breather membrane to our roof. The foil helps with airtightness as it can be sealed with tapes. Rock/glass wool loses a lot of its heat retention capacity when exposed to drafts, so combining multifoil with rockwool would be a good option.
    •  
      CommentAuthordjh
    • CommentTimeOct 11th 2021
     
    Posted By: Pile-o-StoneWe fitted TLX Gold combined multifoil and breather membrane to our roof. The foil helps with airtightness as it can be sealed with tapes. Rock/glass wool loses a lot of its heat retention capacity when exposed to drafts, so combining multifoil with rockwool would be a good option.

    Foil insulation works by reflecting heat, which genrally succeeds with a smallish gap above/below it. Unless the rockwool is somehow constrained I suspect the foil will be in contact with it and thus not work so well. I expect a regular airtight membrane would be cheaper.
    • CommentAuthorEd Davies
    • CommentTimeOct 11th 2021
     
    Wouldn't the actual contact area of the mineral wool¹ be quite small relative to the overall area so the foil would have some reasonable chance to do its reflecting thing?

    ¹ https://www.rockwool.com/uk/about-us/trademarks-and-policies/
    • CommentAuthorLehobbit
    • CommentTimeOct 11th 2021 edited
     
    <blockquote><cite>Posted By: djh</cite><blockquote><cite>Posted By: Pile-o-Stone</cite>We fitted TLX Gold combined multifoil and breather membrane to our roof. The foil helps with airtightness as it can be sealed with tapes. Rock/glass wool loses a lot of its heat retention capacity when exposed to drafts, so combining multifoil with rockwool would be a good option.</blockquote>
    Foil insulation works by reflecting heat, which genrally succeeds with a smallish gap above/below it. Unless the rockwool is somehow constrained I suspect the foil will be in contact with it and thus not work so well. I expect a regular airtight membrane would be cheaper.</blockquote>

    Just to confirm djh the membrane you are talking about is the (usually black) layer of fabric that goes underneath the battens for the tiles? Our roofer is fitting a membrane as part of the roofing work. Are these membranes breathable put also airtight? When the battens are fixed to the roof rafters doesn't this create a lot holes in the membrane?
    •  
      CommentAuthordjh
    • CommentTimeOct 11th 2021
     
    Posted By: LehobbitJust to confirm djh the membrane you are talking about is the (usually black) layer of fabric that goes underneath the battens for the tiles? Our roofer is fitting a membrane as part of the roofing work. Are these membranes breathable put also airtight? When the battens are fixed to the roof rafters doesn't this create a lot holes in the membrane?

    Well some membranes are breathable and some aren't. And some can be installed airtight but often aren't. The ones that are designed so they can be airtight will typically self-seal against nail holes. I expect details would be in the installation instructions. I don't know details of any specific product, sorry.
  4.  
    Posted By: Pile-o-StoneRock/glass wool loses a lot of its heat retention capacity when exposed to drafts, so combining multifoil with rockwool would be a good option.

    Loss of heat retention due to drafts is often called wind wash. multifoil is not needed to prevent this loss a simple thick paper layer will also perform the same function as would a breathable roofing membrane.

    Posted By: LehobbitJust to confirm djh the membrane you are talking about is the (usually black) layer of fabric that goes underneath the battens for the tiles? Our roofer is fitting a membrane as part of the roofing work. Are these membranes breathable put also airtight? When the battens are fixed to the roof rafters doesn't this create a lot holes in the membrane?

    When roofing membranes are put on roofs there will be a counter batten on top and then tile battens on the counter battens.
    So membrane over rafters, counter battens, along the length of the rafter, over membrane, nailed through to rafters then tile battens nailed to counter battens (ideally nailed through to rafters) the nail holes in the membrane will be sandwiched between the counter batten and the joist so this hole is not seen as a problem.

    Some roofing membranes are sufficiently breathable to allow full fill insulation up to the membrane whilst others need a ventilation gap between insulation and membrane. You need to look at the manufactures data.
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