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    • CommentAuthorlineweight
    • CommentTimeNov 12th 2020 edited
     
    I'm not sure if I'm trying to ask an overly general question here - but thought I'd give it a go anyway.

    This relates to what I'd think of as the bog standard loft conversion here in the UK, hundreds or thousands of which are done every year. Commonly they'll be to a Victorian or Edwardian terrace or semi-detached house.

    A few things that most of these tend to have in common:

    1) Existing, timber-rafter pitched roof planes which are to be retained
    2) A new-build "dormer" element which will have vertical faces and a flat roof
    3) New structural steelwork which will be by necessity bearing onto existing masonry (whether party walls or external walls)
    4) The existing masonry walls (in the main body of the house) will be solid with no insulation, and insulating them externally or internally may or may not be part of the project.

    With any project, I like to start out with a basic strategy for the insulation: does it go inside or outside of the main structure. And I think most people here would agree, in principle, it's usually best for it to be on the outside.

    However -

    With a typical loft conversion, it's not that straightforward. There is a conflict between parts of the build where the insulation is better on the inside, and parts where it's better on the outside. I'd break these down as follows:

    Arguments for insulation inside the structure:
    - Any *high level* steelwork, bearing into solid masonry party or gable-end walls, can do so without bridging the insulation layer.
    - On any existing bits of pitched roof which are to be retained, putting insulation inside of the rafters means you can keep the external covering (tiles or slates) and you can keep the external roof plane unaltered and re-use the existing rafters.


    Arguments for insulation outside the structure:
    - Any *floor level* steelwork bearing onto masonry walls can do so without directly bridging the insulation layer
    - It avoids most worries about compromised VCLs and condensation within the construction buildup.
    - Generally works best for any new-build flat roof elements (straightforward insulation on top of deck, easier to build, no worrying about ventilation above insulation layer, etc)
    - In principle it's easier to co-ordinate with any external insulation to the main walls of the house.


    Some potential responses/strategies:

    (A) If you want to do it properly, pay up and do it properly: remove the entire existing roof structure, rebuild the whole thing (including pitched elements) with insulation on the outside, and mitigate high level steel-masonry junctions (eg thermal breaks at junctions) or eliminate high-level steel-masonry junctions (eg by bringing load down to floor level). Then, ideally externally insulate lower walls and ensure a continuous layer.

    (B) Go for an insulation-inside strategy; and accept that there's a compromise in doing flat roof elements this way. Pay maximum attention to creating a reliable VCL throughout. Eliminate floor-level steel-masonry junctions at external walls, or accept them and mitigate as best as possible.

    (C) Some kind of compromise, with transitions between outside-of-structure and inside-of-structure insulation at multiple points, detailed as well as possible.


    Strategy A is often beyond the budget. Strategy B feels like a "least bad" approach. I don't like the fiddly-ness of strategy C, but is it, in the end and pragmatically, just what has to happen in many cases?


    Any thoughts appreciated!
  1.  
    If you insulate on the outside doesn't the roof become mis-matched with the neighbouring houses? Most older houses will be terraced or semi surely.
  2.  
    Have you missed out the 'in-between" insulation strategy for insulation. What I have done is to insulate between the rafters and then increased the depth of the rafters by adding deep battens (50x100) horizontally spaced to accommodate plasterboard. Would this count as insulation inside or outside.
    • CommentAuthorlineweight
    • CommentTimeNov 20th 2020 edited
     
    Posted By: modernvictorianIf you insulate on the outside doesn't the roof become mis-matched with the neighbouring houses? Most older houses will be terraced or semi surely.


    As I mention in the OP, an advantage of doing it on the inside is that the exterior of the roof remains unaltered, even if you keep the old rafters.

    If you do it on the outside, then yes, either you end up with a mismatched roof, or you have to rebuild the whole roof structure, placing the new rafters further inboard than the location of the old ones, to allow the external plane of the roof to end up back in the same place.
    • CommentAuthorlineweight
    • CommentTimeNov 20th 2020
     
    Posted By: Peter_in_HungaryHave you missed out the 'in-between" insulation strategy for insulation. What I have done is to insulate between the rafters and then increased the depth of the rafters by adding deep battens (50x100) horizontally spaced to accommodate plasterboard. Would this count as insulation inside or outside.


    Do you mean battens fixed perpendicular to the rafters?

    This I'd class as "inside" because there's insulation inboard of the rafters but not outboard of them.

    Up to a certain thickness of insulation you can do away with battens and just fix direct through the insulation (eg using insulated plasterboard) to the rafters, minimising cold bridges via the battens and rafters.
  3.  
    Posted By: lineweight
    Posted By: Peter_in_HungaryHave you missed out the 'in-between" insulation strategy for insulation. What I have done is to insulate between the rafters and then increased the depth of the rafters by adding deep battens (50x100) horizontally spaced to accommodate plasterboard. Would this count as insulation inside or outside.


    Do you mean battens fixed perpendicular to the rafters?

    This I'd class as "inside" because there's insulation inboard of the rafters but not outboard of them.

    Up to a certain thickness of insulation you can do away with battens and just fix direct through the insulation (eg using insulated plasterboard) to the rafters, minimising cold bridges via the battens and rafters.

    Yes battens (if you can call 50x100 battens) fixed perpendicular to the rafters - rafters vertical, battens horizontal, this mitigates the cold bridge of the timbering. I have in one case full-filled the space between the rafters and in the other left a nominal 50mm gap under the roofing felt. The difference being the type of roofing felt in place. The insulation used in both cases was glass fiber wool so fixing without battens wouldn't work. The insulation thickness achieved was 200mm and 250mm respectively (150mm rafters).
    • CommentAuthorlineweight
    • CommentTimeNov 21st 2020
     
    Posted By: Peter_in_Hungary
    Yes battens (if you can call 50x100 battens) fixed perpendicular to the rafters - rafters vertical, battens horizontal, this mitigates the cold bridge of the timbering. I have in one case full-filled the space between the rafters and in the other left a nominal 50mm gap under the roofing felt. The difference being the type of roofing felt in place. The insulation used in both cases was glass fiber wool so fixing without battens wouldn't work. The insulation thickness achieved was 200mm and 250mm respectively (150mm rafters).


    What was your strategy for the flat roof portion?
  4.  
    Posted By: lineweightWhat was your strategy for the flat roof portion?

    There is no flat portion of the roof. There are 2 dormers which have pitched roofs with ceilings the at same level as the flat portion of the attic room ceiling and here the insulation matches the loft insulation of the flat portion of the attic room ceiling (as opposed to the skeiling portion of the ceiling where the horizontal 50x100 battens described above are).
    • CommentAuthorlineweight
    • CommentTimeNov 23rd 2020
     
    Posted By: Peter_in_Hungary
    Posted By: lineweightWhat was your strategy for the flat roof portion?

    There is no flat portion of the roof. There are 2 dormers which have pitched roofs with ceilings the at same level as the flat portion of the attic room ceiling and here the insulation matches the loft insulation of the flat portion of the attic room ceiling (as opposed to the skeiling portion of the ceiling where the horizontal 50x100 battens described above are).


    Ah, ok.

    I guess it's not the kind of scenario I'm really thinking about for the purposes of this thread. I'm thinking of the type of loft conversion which is very common in the UK (resulting from a combination of the loft height of a typical Victorian/Edwardian house, and what planning/permitted development rules allow) which involves a substantial dormer which, generally, necessarily requires a flat roof as well as some substantial steel supporting structure. It's those two things that generate the tricky situations as far as insulation is concerned.
    • CommentAuthorlineweight
    • CommentTime2 days ago edited
     
    Having failed to get much traction on this, I thought I'd do some diagrams to try and explain what I'm on about.

    Firstly, here's what I'll call the "insulation outside structure strategy"
      Screen Shot 2020-11-30 at 17.56.53.jpg
  5.  
    Secondly here's what I'll call the "insulation inside structure strategy".
      Screen Shot 2020-11-30 at 17.57.04.jpg
  6.  
    Each has its own problems, which are the points where the insulation layer and structure must cross, creating complex detailing and thermal bridging opportunities. I've indicated these problems points with red arrows.


    My feeling is that on the whole, the "insulation outside structure strategy" is preferable because:

    - It lets me keep the insulation on the outside of the timber frame for the new dormer, and in particular it lets me have a warm rather than cold roof buildup.

    - On the cross section, it shifts the "problem point" from location E (around the eaves and floor/wall junction) to location A (around the ridge). I feel that I'd rather deal with the problem at location A, because more of what I'm dealing with here is new build. In any case, I don't see that location A is *more* difficult to resolve than location E.

    - On the long section, in the scenario where the dormer stops short of the masonry party wall, problem H is changed to problem D. There seems more opportunity with 'D' to at least lengthen the heat path between the masonry and the interior of the dormer. Again, I don't see that D is substantially worse than H, even if it's no better.

    - On the long section, where the dormer runs right to the party wall, I'm assuming some insulation on the internal face of the masonry (because there's no real alternative). Problem points C and G are essentially the same. So, no benefit either way.

    - Again on the long section where the dormer runs to the party wall - situation 'F' is clearly better than situation B, demonstrating an advantage of the " insulation inside structure" strategy. But is it enough to swing things in favour of it? My feeling is that a better approach might be to avoid any high level structure bearing into the masonry, which would mean posts bringing the load down to floor level, creating something essentially similar to the "dormer stops short of party/external wall" end, even if externally visually the dormer reaches the full width. But can the money/space cost of that extra bit of structure be justified?

    If anyone would like to agree or disagree with me on any of the above I would be interested to discuss further. I surely can't be the first to sit down and ponder this (very common) situation.
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