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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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    • CommentAuthordelprado
    • CommentTimeJan 3rd 2018
     
    Is it as strict as any different in thermal condtivity of two adjoining materials? In other words, is a wall junction that has a u vlaue of 0.5 a thermal bridge if the surrounding wall is 0.3, even though they are both "low"?

    Or is it more (in practical terms) a phrase to refer to moderate or massive differences in conductivity, ie a steel beam extending out to the outside of an internally insulated house?

    Or somewhere in between?
    • CommentAuthortony
    • CommentTimeJan 3rd 2018
     
    It is an area or component of an object which has higher thermal conductivity than the surrounding materials.

    so, yes even though they are both low the higher one is considered a bridge.

    In practical terms there are lots of different types of thermal bridges which have a very wide variety of different consequences for both heat loss and risk of condensation.
    • CommentAuthordelprado
    • CommentTimeJan 3rd 2018
     
    Is there any general conditions which provide guidance as to when it is a problem and when not? Like for example a percentage differential in u value?
    • CommentAuthortony
    • CommentTimeJan 3rd 2018 edited
     
    Nope, it is well complicated and dependant on several variables

    My rule of thumb is where there are surface temperatures of below 12C in a lived in house there is a big risk of mould and condensation.

    All thermal bridges are bad news but the degree of the problem is difficult to define.
    •  
      CommentAuthordjh
    • CommentTimeJan 4th 2018
     
    It's actually to do with the conductance (or equivalently the resistance) rather than the conductivity. An area of thick wall with poorer insulation is not a thermal bridge alongside an area of thinner wall with better (i.e. lower conductivity) insulation, for suitable choices of thickness.

    And it's not necessarily to do with the U-value of a flat surface either. The so-called 'geometrical' thermal bridges at external corners are some of the most important. External corners of rooms are some of the most likely places to find condensation and mould in bad cases. Window reveals are another case where external corners cause difficulties.

    As to whether something is or is not a thermal bridge, that's a matter of definition by whatever standard you are using to measure the performance of the building. There's a good article explaining how PHPP handles the question at https://elrondburrell.com/blog/passivhaus-thermal-bridge-free-construction/

    Note that the aim in PH design is that there is no more than 3°C difference in surface temperature on external surfaces. So when the house is at design temperature of 20°C, the coldest points on the walls, roof, floor, windows etc will be 17°C or greater.
    • CommentAuthorSigaldry
    • CommentTimeJan 5th 2018 edited
     
    A thermal bridge can be defined as an area that has greater heat transfer than adjacent areas. When calculating the energy and emissions performance of a building, types of thermal bridges that are considered include:

    - repeating (linear and point)

    - point (non-repeating); and

    - linear (non-repeating)

    Repeating thermal bridges occur where there are regular interruptions within the building fabric, by materials with poorer insulating properties – for example, timber studwork in a timber-frame wall construction or steel framing in an SFS wall. The typically additional heat flow incurred by repeating thermal bridges is accounted for in a U-value calculation for the building element containing the bridges.

    Repeating Point thermal bridges are typically used as an adjustment to a U-value for an element, to take account of fixings or fastenings for example (heat losses are calculated for one 'penetration' and then the number per square metre of those penetrations is used to adjust the U-value).

    A non-repeating point thermal bridge could arise where a flue passed through a wall construction for example.

    Linear (non-repeating) thermal bridging occurs at the junctions of a building’s planar elements (i.e. between roofs, walls, openings and floors) and can add significantly to a building’s total fabric heat loss. Higher heat flows occur at junctions due to complex geometries, or from the use of materials with a higher thermal conductivity than the adjacent materials. This can cause localised reductions of internal surface temperature, which could lead to surface condensation and mould growth problems where the internal temperatures are particularly low, particularly where moisture generating activities are higher.

    The differing heat flow attributable to the thermal bridge is typically, but not always, greater than that through the adjoining plane elements. This is the linear thermal transmittance of the bridge, measured in W/mK, referred to as a ψ-value (psi-value). The lower the ψ-value, the better the performance. ψ-values are not taken into account in U-value calculations, but they are included separately in the calculation methodologies used to assess the operational CO2 emissions of buildings. In the UK, SAP is commonly used for domestic buildings, and SBEM for non-domestic buildings.

    Psi values can differ depending on whether they are based on internal measurements (Psi internal) used for SAP or sBEM, or if using external measurements (Psi external) as is the case for Passivhaus details.

    Thermal bridges tend to refer to point, or linear bridges, whereas different performing wall or roof or floor areas would usually be accounted for by including each in an overall calculation of losses for a building - the junctions between such areas might be considered linear bridges depending on where and how they come together.
    • CommentAuthordelprado
    • CommentTimeJan 9th 2018
     
    Sigal - what a fantastic response, thats exactly what I needed to get my head around it :)

    So I guess when someone says "thermal bridge free construction" they mean if the target is 0.2 u value, or whatever, then it means it! Even 0.3 is a thermal bridge in that context.
    • CommentAuthorSigaldry
    • CommentTimeJan 9th 2018 edited
     
    Thermal bridges are typically additional to the overall losses from the other elements yes (although 'thermal bridge free' would mean no additional losses from the junction over and above that going out through the adjoining elements).

    Because Passivhaus uses external measurements, they typically account for all but the worst junctions.

    If you think of a normal corner junction, by taking the external measurements for walls, passivhaus will have included the losses through the corner twice, whereas internal measurements (e.g. as used in SAP) wouldn't account for the losses from the corner when considering wall area multiplied by U-value; and so thermal bridging losses are needed to be considered (of course for an inverted corner, the opposite applies, hence why for SAP purposes, this is usually a negative value).

    When considering thermal bridging for a house, all of the junction losses and lengths are added up and are added to the other 'fabric' losses, i.e. those from 'planar elements' (roofs, walls, floors, windows and doors) and the losses from intentional and unintentional ventilation.
    • CommentAuthordelprado
    • CommentTimeJan 9th 2018
     
    Out of interest, in a cavity built house that has cavity wall insulation, presumably the wall junctions are not thermal bridges because the internal wall doesn't ever touch the external? The internal wall only ever touches the internal skin?
    • CommentAuthorgyrogear
    • CommentTimeJan 9th 2018 edited
     
    Posted By: delpradothe internal wall doesn't ever touch the external


    well, it does if there are wall-ties !
    the latter then become repeating-point thermal bridges as explained previous...
    Also, the inner leaf touches the floor, and the latter (presumably...) touches the outer wythe, so more bridging there...

    (whence the utility of EWI...).

    gg
    •  
      CommentAuthordjh
    • CommentTimeJan 9th 2018
     
    Posted By: delpradoOut of interest, in a cavity built house that has cavity wall insulation, presumably the wall junctions are not thermal bridges because the internal wall doesn't ever touch the external? The internal wall only ever touches the internal skin?

    The subject under discussion was corners, not places where internal walls meet the external wall. Corners have the same geometry for any wall type.

    Places where internal walls meet the external wall are a separate problem and can be thermal bridges in the case of internal wall insulation. Not usually in the case of EWI or cavity fill though, as you say.

    Posted By: gyrogearthe inner leaf touches the floor, and the latter (presumably...) touches the outer wythe

    Not usually. The floor only extends to the inner wythe. The two wythes are separate from the foundation up. In PH type builds the outer wythe is built on a separate foundation.
    • CommentAuthorgravelld
    • CommentTimeJan 9th 2018
     
    Lots of examples with CW houses. E.g. in cold lofts, the inner leaf at a gable.

    Also depending on the way the cavity is closed in various places (at the top, at window reveals etc) these can be bridges.
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