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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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    • CommentAuthorlineweight
    • CommentTimeDec 10th 2018
     
    Without going for anything too esoteric, what do folk reckon to be the best wall buildup at this point in time, to achieve decent insulation but with minimum overall thickness?

    In this case I would be aiming for something a little better than building reg minimum and it would not need to have any kind of onerous loadbearing capacity.

    Is it likely to be something like SIPS panels?
    • CommentAuthorgoodevans
    • CommentTimeDec 10th 2018
     
    Presumably this is a external dwelling wall with a requirement to comply with building regs? and by "not onerous load bearing capacity" you mean it still has to be load bearing.

    With a rain-screen layer (e.g. cladding). Thinnest would be timber frame with insulation between (focus here on keeping the base of the timber frame warm to prevent rotting).

    There are timber frame solutions with a render finish.

    Or 140mm block with EWI and render finish (engineer signoff for wall required for 140mm wall or 190mm thick wall for deemed to satisfy - may be 200mm).

    I looked at sips - and like timber frame I had a problem with confidence that the base would be too cold and rotting. In the end that why I went for Block and EPS - structurally fairly bullet proof - you still have damp/cold bridge issues but not liable to rot.
    • CommentAuthorlineweight
    • CommentTimeDec 10th 2018 edited
     
    Posted By: goodevansPresumably this is a external dwelling wall with a requirement to comply with building regs? and by "not onerous load bearing capacity" you mean it still has to be load bearing.


    Yes. Although it might be interesting to consider what the possibilities would be if it didn't have to be loadbearing other than to support its own weight between floors (ie with structural frame separate and inboard)

    The base of any timber frame would be raised off the ground on a masonry plinth to avoid concerns about damp.
    • CommentAuthorphiledge
    • CommentTimeDec 10th 2018
     
    When we did our timber frame build 89mm×38mm C16 timber frame was specified for a 2 storey house with concrete tiled pitched roof. Assuming that is still a valid size then anything over 89mm that gives the insulation value you want/need. On top of the frame thickness you need to add interior finish, sheathing ply at 9.5mm, rain screen at 40-50mm
    • CommentAuthortony
    • CommentTimeDec 10th 2018 edited
     
    Yes but not about condensation on top of the DPC (which is in contact with wood) due to it being cold due to thermal bridging


    When you ask about simplest do actually mean to,ask about cheapest?
    • CommentAuthorlineweight
    • CommentTimeDec 10th 2018 edited
     
    Posted By: tony


    When you ask about simplest do actually mean to,ask about cheapest?


    Slimmest - not simplest.
    • CommentAuthorlineweight
    • CommentTimeDec 10th 2018
     
    Posted By: tonyYes but not about condensation on top of the DPC (which is in contact with wood) due to it being cold due to thermal bridging


    If this is an issue...then it would be a problem with most of the standard details shown in my copy of TRADA's 'timber frame construction' book.
    • CommentAuthorJulio
    • CommentTimeDec 10th 2018
     
    Out to in.

    20mm timber cladding
    25mm batten
    Breather membrane
    40mm t/g woodfibre board
    150mm timber frame - insulated with woolen insulation
    12mm racking board - osb or panel vent
    15mm woodfibre board or plasterboard
    internal plaster

    Total thickness -about 265mm, not bad
    U value - 0.177

    Timber frame is insulated by the woodfibre board so reducing risk of condensation.
    • CommentAuthorlineweight
    • CommentTimeDec 10th 2018
     
    Posted By: JulioOut to in.

    20mm timber cladding
    25mm batten
    Breather membrane
    40mm t/g woodfibre board
    150mm timber frame - insulated with woolen insulation
    12mm racking board - osb or panel vent
    15mm woodfibre board or plasterboard
    internal plaster

    Total thickness -about 265mm, not bad
    U value - 0.177

    Timber frame is insulated by the woodfibre board so reducing risk of condensation.


    It ought to be possible to improve upon that, thickness-wise, by using better performance insulation though.
    • CommentAuthorIan1961
    • CommentTimeDec 10th 2018
     
    Mine was 260mm total:

    Out to in

    20mm horizontal T&G cedar weartherboarding
    25mm treated batten
    breather membrane
    9mm OSB3 racking
    140mm treated timber frame
    Insulation full fill between studs
    25mm foil faced PIR insulation board across inside face of frame
    25mm batten to form wiring zone
    15mm p'board & skim

    Depending on what you use for the insulation between the timber studs the 'U' value can range between about 0.17 and 0.21
    •  
      CommentAuthorfostertom
    • CommentTimeDec 11th 2018 edited
     
    Posted By: goodevansengineer signoff for wall required for 140mm wall or 190mm thick wall for deemed to satisfy - may be 200mm
    deemed to satisfy is 190.

    Alternative to say 150 insulation between 150x50 studs, 100 insulation between 100x50 studs plus not less than 25 insulation across the face or the studs would give about same U value for less thickness, by mitigating the major cold bridging of studs exposed to hot/cold both edges.

    For near-PH, 100 insulation between 100x50 studs plus 150 insulation across the face or the studs is unbeatable for very modest thickness.

    Acrylic thin coat render straight onto board insulation is as thin as you can get, compared to cladding/rainscreen.

    You don't need thickness (and expense) of a battened-off inboard service void if there's no inboard (or any) VCL, instead a 'breathable' construction. Airtighness by putting 11 OSB3 as racking board and airtightness on the outside not inside of the studs.

    The buildup then is:
    100x50 studwork
    Blown cellulose between the studs
    Plasterboard & skim direct on inside of studs - services free to penetrate as it's not airtight or vapour tight
    11 OSB3 on outside of studs
    150 EPS, like EWI, over the outside of the OSB
    thin coat acrylic render.
    Total thickness 291mm, near-PH performance.

    If settling for just Regs-plus performance, reduce the outboard EPS to 75.
    • CommentAuthorlineweight
    • CommentTimeDec 11th 2018
     
    So... in this case the wall U-value I need to achieve is 0.28 as it's an extension.

    (I know, that's a rubbish U-value but I'm using this to investigate the slimmest possible buildup. Also, there are valid arguments that in this particular case, a high level of insulation is not actually needed)

    Fiddling around on the Celotex calculator suggests that this can be achieved with a 100mm stud frame, insulation between studs, and then only about 15mm insulation on either the inside or outside of the frame. Going for an internal plasterboard laminate and allowing for a ply/OSB racking board means an overall thickness of something like 140mm, plus whatever thickness is required for battens plus cladding system.

    Using a SIPS system, it looks like it might be possible to do away with the insulated plasterboard but it's only saving 15-20mm of thickness. Probably more expensive than on-site timber frame?

    The other option is single skin blockwork. Assuming I can get an engineer to OK 140mm blockwork, then it looks like something like 50mm phenolic EWI might do it. So say 210mm plus cladding system (or render)

    It looks like it would come down to structural requirements... if a 100mm timber frame is possible then timber beats single skin blockwork. If 150mm studs then it doesn't beat single skin block by much, unless structural requirements mean that it would have to be 200mm block.
    • CommentAuthorgoodevans
    • CommentTimeDec 11th 2018 edited
     
    I like Toms setup because at least half the insulation is outside the structural timber, no vapour barrier for simplicity, airtight at the OSB layer so no wet plaster required, the cellulose adds bulk (for sound insulation, heat and humidity buffering - or heavy rockwool cold do a similar job except for the humility buffering).

    If timber is nearer than that to the outside (e.g. sips , or less EWI outside Toms osb layer) I would like to see a rain screen and ventilated 25mm cavity. The last thing you need to see is the bottom of the structural elements rotting.

    Toms minimal U value make-up is 216mm (291-75) - that's difficult to beat with a U value around 2 (and I'm wondering why I didn't consider it). Easy to get right, easy service installation and I suspect you could get the structural design signoff and delivery for the walls and roof complete with the required connectors from a roof truss supplier.
    • CommentAuthorlineweight
    • CommentTimeDec 11th 2018
     
    I agree there are advantages to Tom's suggestion.

    Compared like-for-like with the celotex version, it adds about 70mm extra to the buildup.

    As the wall encloses a pretty narrow space, it may or may not turn out that that 70mm is critical - ie. worth sacrificing some other advantages for.
    •  
      CommentAuthorfostertom
    • CommentTimeDec 11th 2018
     
    Celloltex won't be breatheable (as well as being environmentally dodgy) so I have no opinion (other than yuk) on its safety. EPS/cellulose every time for me.
    • CommentAuthorgoodevans
    • CommentTimeDec 12th 2018
     
    Tom, How would you fix the EPS to the OSB (OSB has a waxy finish so I'm not sure of the adhesive options).

    would the fixing method assume the eps to be supported at the bottom?
    • CommentAuthorlineweight
    • CommentTimeDec 12th 2018 edited
     
    Posted By: goodevansTom, How would you fix the EPS to the OSB (OSB has a waxy finish so I'm not sure of the adhesive options).

    would the fixing method assume the eps to be supported at the bottom?


    These systems usually have a kind of 'starter rail' mechanically fixed at the bottom (and ends) whether the EPS is adhered or mechanically fixed. The rail also functions as a stop bead for the render layer.

    I did something similar to what Tom suggests recently - but with celotex between the studs instead of cellulose or mineral wool. Am now trying to remember whether the EPS itself was just adhered or mechanically fixed (there was about 200mm of it). That was onto ply rather than OSB sheathing though.
    •  
      CommentAuthorfostertom
    • CommentTimeDec 12th 2018 edited
     
    EPS is easily glued to OSB, ply or CPB (cement particle board) alike - either non-expanding 'foam gun' glue, or contact adhesive - tho that mustn't be a continuous spread (or spray) if it's to be breatheable. Bottom support def not necesary - it sticks like s**t.

    Bottom rail IMO opinion non neceassary, an expense and faff and better without. Why on earth think of stopping the EWI short of the ground, so needing a bottom stop bead? EPS doesn't wick water up, so no DPC break needed. Better to run it down to just below GL, or better still, right down to bottom of foundation in a trench, which can be made as a french drain, esp applicable to extg buildings, cures rising damp, wet underfloor void.
    • CommentAuthorlineweight
    • CommentTimeDec 12th 2018
     
    Posted By: fostertom Why on earth think of stopping the EWI short of the ground, so needing a bottom stop bead?


    In my recent case, to stop the render getting bashed and dented along a narrow passageway where people are frequently bringing bicycles, wheelbarrows, etc etc.

    Because the internal ground floor level was somewhat higher than the outside ground, the external render could still overlap with the insulation in the floor, and stop short of the ground. We had to have a suspended floor because of ground conditions.
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