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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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    • CommentAuthorgravelld
    • CommentTimeAug 31st 2017
     
    Really dumb question but... couldn't find where this was explained in the manual.

    Is the section for TBs only for TBs that aren't otherwise handled in average by the U value sheet?

    For example, consider an insulated ceiling with insulant only between the joists. Should the TBs for the joists be included in the "U value supplement" on the U Values sheet, or should the ceiling be considered "perfect" and they be entered on the TB area of the Areas sheet?

    If the former, what's the TB area for - "exceptional" TBs? e.g. balconies bridging insulation etc?
  1.  
    gravelld,

    my understanding is that the TB sheet is for any junctions with a psi-value of more than 0.01W(mK), in other words if you have a 'thermal bridge free' envelope you wouldn't need to include anything in this section. As you say, balconies that penetrate the insulation are a good example. The requirements are fairly onerous - you're likely to need a THERM model for each junction, although this is free to download.

    I'm yet to find out how this works in practice as my file currently has nothing in this section for the reason that I believe my junctions are generally following good practice. File has been submitted to certifier but I'm anticipating some further interrogation of my junctions to prove this is the case.

    To answer your example, the way we were taught to deal with repeating elements like joists is to use the adjacent cells in the u-values section. If your joists are, say 15% of the average 1m² area that the u-value represents, you can enter this in the fields below each column and use the second column to put a line item in for joists at the appropriate layer (row). Where this gets tricky is that each layer/row only has one entry for thickness, so you may need to split your joist entry across several rows if the adjacent material is a different thickness or vice versa.

    Note also that window thermal bridging (around frame) is dealt with in the 'components' sheet. You should be able to get this data from the supplier.

    I recommend the 'PHPP Illustrated' book from RIBA publishing. More expense, but it covers each worksheet with screenshots of the relevant cells and a worked example showing elements in 3D etc. Quite a good reference if the manual gets a bit dry.
    • CommentAuthorgravelld
    • CommentTimeSep 1st 2017
     
    Thanks.

    Ok so it sounds like one I thought of - the inner leaf of a cavity wall gable bridging into a cold loft is about 2.32 - http://www.nhbc.co.uk/NHBCPublications/LiteratureLibrary/Technical/TechnicalExtra/filedownload,44601,en.pdf - so definitely needs incorporation.

    Are these calculations very sensitive - do I have to run a new calculation for each measurement or just go with that as a rule of thumb?
    •  
      CommentAuthordjh
    • CommentTimeSep 1st 2017
     
    Be careful using published numbers from UK bodies, since they (SAP etc) use a different basis of calculation to PHPP.

    PS I was impressed by the quality of results for a search using "phpp thermal bridge calculation" without the quotes.
    • CommentAuthorDarylP
    • CommentTimeSep 1st 2017
     
    Yes to the above IIRC PHPP uses the o/s external measurements, SAP uses internal measurements for Psi-values...:wink:
    • CommentAuthorgravelld
    • CommentTimeSep 1st 2017
     
    Ah yes, forgot that; there's a methodology to convert in the manual.
    • CommentAuthorgravelld
    • CommentTimeSep 2nd 2017
     
    Thanks all. Been playing about with THERM tonight. Appears pretty straightforward to begin with, modelled my first bridge and came up with a similar number to the above (internal measurements) but like PHPP I suspect there's a lot of detail and I'm lacking background knowledge which might make my early efforts invalid... still, only one way to fix that.
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