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General: Thermal modelling needed for bespoke psi value

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1 to 25 of 25

  1.  
    • CommentAuthoran02ew
    • CommentTimeApr 13th 2021
     
    Can any help with thermal modelling of some non standard junctions to produce the required psi value for a more accurate SAP calc?
    • CommentAuthortony
    • CommentTimeApr 13th 2021
     
    Probably not!!!! pretty much no one does them. Try searching for building physicists or building physics

    I looked in the passive haus book and mitigated through good design
    '
    • CommentAuthorjms452
    • CommentTimeApr 13th 2021 edited
     
    It's fairly easy but most of the software is pricy - this looks to be free and may be worth a play:

    https://windows.lbl.gov/software/therm

    To gain confidence you'd want to model a standard junction as a check you were getting it right.
    • CommentAuthoran02ew
    • CommentTimeApr 13th 2021
     
    <blockquote><cite>Posted By: tony</cite>Probably not!!!! pretty much no one does them.</blockquote>
    Surly this will change soon, I hear there is a shake up of the current SAP assessments, also of part L of the building regs. So maybe they will look to non standard details of building junctions to help meets these more stringent goals.
    •  
      CommentAuthordjh
    • CommentTimeApr 13th 2021
     
    I believe that as from next year, accredited details will no longer be valid for building control. I'm not sure what qualifications will be required for any calculation performed to be able to be used in a SAP calculation. Also the defaults will be made worse, so the importance of calculations will increase greatly.

    All based on a quick read of various sites belonging to companies who offer such services, so which are presumably somewhat biased.
    • CommentAuthoran02ew
    • CommentTimeApr 13th 2021
     
    Which company’s?
    •  
      CommentAuthordjh
    • CommentTimeApr 14th 2021
     
    Those resulting from a search for "psi value calculation for sap", e.g. Elmhurst, C80solutions, Build Energy
  8.  
    Try MES Building Solutions too.

    I can't wait for this to get more mainstream inclusion in the big AEC software. Much as I love LBNL Therm, the interface is still painful. It would be far better to use natively within drawing software and for the calculations to take advantage of cloud processing.

    Posted By: djhI believe that as from next year, accredited details will no longer be valid for building control.



    I don't think there's currently any proposal for what might replace Accredited Details so I suspect they might last a bit longer (for better or worse).
    •  
      CommentAuthorfostertom
    • CommentTimeApr 14th 2021
     
    Posted By: Doubting_ThomasI can't wait for this to get more mainstream inclusion in the big AEC software. Much as I love LBNL Therm, the interface is still painful. It would be far better to use natively within drawing software and for the calculations to take advantage of cloud processing.
    Too right. Tho, is it compute intensive? Therm runs happily on a v basic computer. What kind of capabilities wd need cloud? You're thinking 3D not just 2D sections? That wd be interesting.
    •  
      CommentAuthordjh
    • CommentTimeApr 14th 2021
     
    Posted By: Doubting_ThomasI don't think there's currently any proposal for what might replace Accredited Details so I suspect they might last a bit longer (for better or worse).

    AIUI, the proposal is that proper psi values calculated for the particular situation replace them, or dreadful defaults. The idea being to force people to use pre-designed systems and components (a la ETICS versus EWI). Whether that's a good idea or not I couldn't say.
    • CommentAuthoran02ew
    • CommentTimeApr 14th 2021
     
    im having Elmhurst, C80solutions and greengage quote for psi calculations for some building junction, i had one of them come back with this coment:

    "I would like to highlight that generally there is a direct correlation between U values and PSI in that the better the U-Value, the more heat loss tends to be attributed to the junction; the better the u-values, poorer the adjacent psi-value"

    Am i correct in assuming that they are suggesting the perfermance will lower because of the thermal modeling?
    that seem counter-productive. i thought the performance certificate would improve if the psi value reflected better the efforts taken to avoid cold bridging as opposed to the standard psi values?
  12.  
    Posted By: an02ewAm i correct in assuming that they are suggesting the perfermance will lower because of the thermal modeling?


    No, I think they are just highlighting the fact that better performance across the whole envelope will exacerbate the effect of cold bridging - in essence concentrating all the heat loss (and therefore condensation risk) at those points.

    It’s not the modelling that will ‘cause’ this, it just shows what happens when the fabric isn’t so lossy. The answer is to improve the junction. Most modellers will (or should) include a portion of their fee for a certain amount of minor adjustment if the junction doesn’t pass on the first go - i.e. swap the materials for less conductive ones or a bit more insulation cover etc.

    You’re correct to assume that properly modelled junctions should be better than the default assumptions because the defaults tend to include a penalty factor as tolerance for the fact that they’re just estimates. Of course it is possible that the modelled junctions are worse than the defaults, in which case you definitely want to amend the design!
  13.  
    Posted By: fostertomWhat kind of capabilities wd need cloud? You're thinking 3D not just 2D sections? That wd be interesting.


    A few of them (Flixo Pro for example) already do this but as I understand it they all rely on imports and conversion from other software, which is an inherently slower workflow. An embedded simulation within something like Revit where material properties are already assigned to layers and the object already ‘knows’ it is a window or door would just make more sense.

    It might not need ‘the cloud’ but I’m betting that would make it much faster at producing the coloured heat flux images we all like...
    •  
      CommentAuthorfostertom
    • CommentTimeApr 15th 2021
     
    Posted By: Doubting_Thomasthey all rely on imports and conversion from other software, which is an inherently slower workflow. An embedded simulation within something like Revit where material properties are already assigned to layers and the object already ‘knows’ it is a window or door
    would be just what we need, as you say. In Therm, the slightest change to the cross section diagram means pretty well starting all over again. Import from Autocad is extremely fussy and tricky - it's easier to do it from scratch in Therm, which is equally tricky.
    • CommentAuthoran02ew
    • CommentTimeApr 15th 2021
     
    Sounds like you’re well tutored in the software Tom?
    •  
      CommentAuthorfostertom
    • CommentTimeApr 15th 2021
     
    I did, incl some tutoring from Peter Warm Associates, but found it dispiriting, as described. Therm's only advantage is it's freeware. If similar was built into CAD as D_T sugests, that wd be different. Similar with Thermal Simulator Tas way back. I had better luck with WUFI.

    Therm esp as I was wanting to push its limits (surprise!) e.g. subsoil as deep insulator under uninsulated slab, with wing or downstand perimeter insulation (you know the thing). Therm sounds ideal for this, except it doesn't like an undefined (infinite) lower (isothermal) boundary so I had to try assumptions for depth below of lower boundary, down to which subsoil played a part - which was so arduous to iterate that I gave up - a project still waiting, but hopefully in better software.
    • CommentAuthoran02ew
    • CommentTimeApr 16th 2021
     
    Yesterday I spent several hours playing Therm, I didn’t get far, it does seem very clumsy, but free, plus I’m sure is the sap assessor will accept my untutored efforts. However I have received today quotes from Greengage and C80 £250-£350 per model.
  18.  
    Try Andy Lundberg https://www.passivate.ie/
    • CommentAuthorjms452
    • CommentTimeApr 17th 2021
     
    Posted By: an02ewYesterday I spent several hours playing Therm, I didn’t get far, it does seem very clumsy, but free, plus I’m sure is the sap assessor will accept my untutored efforts. However I have received today quotes from Greengage and C80 £250-£350 per model.


    I've not used Therm but when thermal FE modelling start simple and just do steady-state conduction. This could be a m2 of simplified wall without your detail at all making it easy to check you get the right hear-flux/U-value. Once you get that working ok add complexity in steps.
    •  
      CommentAuthorfostertom
    • CommentTimeApr 17th 2021
     
    Gd idea
    • CommentAuthoran02ew
    • CommentTimeApr 19th 2021
     
    Posted By: jms452I've not used Therm but when thermal FE modelling start simple and just do steady-state conduction. This could be a m2 of simplified wall without your detail at all making it easy to check you get the right hear-flux/U-value


    Thanks for the advise. with help from youtube i have modled my first simple juction, and thought i would recieve a psi value in a neat little box. but that isnt the case, aaaghhh. does anyone have an idiots guide to squeezing a psi value out of the infomation on Therm?

    i have also considered putting the money i would have spent on 4 or 5 junctions into "The Psi Value Competency course" at Elmhurst energy, using my detials as the course work.
  22.  
    Posted By: an02ewdoes anyone have an idiots guide to squeezing a psi value out of the infomation on Therm?


    WARM have your back. See about halfway down in their downloads section:

    https://www.peterwarm.co.uk/resources/downloads/
    •  
      CommentAuthorfostertom
    • CommentTimeApr 20th 2021 edited
     
    Gd idea Andy - then you can earn consultancy money for others. I did the Warm (classroom) course.

    It's one thing to model a junction, quite another to extract a psi value for it - many rules and conventions to follow, and subtracting figures for surrounding 'plain wall' etc so what's left is just the figure for the junction itself. Did me 'ead in, and I still don't understand what to do with subtracting (as above) a window, which isn't a plain slab but a composite (frame + glass). And the documentation that needs to be saved (esp if it's consultancy) like jpgs, annotated, of the diagram for each variant tried - all goes to explain the time/expense per case.

    Pity you can't spend your "4 or 5 junctions" money on learning an easier to use alternative, incl automatic documentation I'd hope - but those aren't freeware.
    • CommentAuthoran02ew
    • CommentTimeApr 21st 2021
     
    Therm working slot better thank to the add-one from Warm. There psi value calculator need a bit of thought.

    I spoke with the team at Peterwarm today, I’m seriously considering there Therm course , at £200 it seem good value, I’m not sure how much contact I would receive as I was told the company charges £90/hr for modelling work. So I’m a bit concerned I’d be no farther forward as these psi values not easily worked out.
    The other option was a classroom based course run by Elmhurst £800ish
  25.  
    Posted By: an02ewI was told the company charges £90/hr for modelling work.

    That doesn't sound too bad when you consider that a friend was recently charged £115/hr by an electrician for what was basic wiring work (that a few years ago would have been DIY)

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