Go to Forum Home PHPP, SAP & Software The PHPP and ‘natural’ materials?

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    • #30999

      Hi there,
      I've never used PHPP but am considering having a go…
      However, I firstly had a couple of questions I hoped someone could answer;

      Does the software consider thermal mass?

      Does the software consider hygroscopicity?

      Does the software account for potential movement in some materials?

      Would it be possible to accurately model the PH standard using more 'natural' materials and products eg timber, earth, straw or their products eg fibreboard, clayblocks, strawboard, etc? Has this been done?

      Essentially, does the PHPP allow the characteristics of these materials and products to be modelled?

      Thanks alot,

      Matt Bloom

    • #35532
      Mark Siddall

      I've no direct experience of PHPP but I can offer the following:

      1) Yes

      2) No

      3) No (design issue not calculations issue)

      4) Yes provided that you know the conductivity of the materials (check out the thread on the S-House in the Building Design section of the forum). NOTE: A key design issue would be to ensure that the materials and technology are suited to airtight design (the results of your airtightness are critical to PH design but you don't model airleakage of specific details in PHPP as this is a technology/design/workmanship issue, again refer to of other thread in the Building Design section of the forum).

      5) Depends what you mean by “characteristics.” For energy/thermal characteristics yes other characteristics no.


    • #35533
      Tom Foster

      I'd say the answer to the point of matt's Q is No – first, Mark says PHPP assumes airtightness and I'd guess (?) struggles with the controversial vapour-breatheability concept too; second Mark says PHPP doesn't model hygroscopicity (nor presumably capillarity nor rate and density of moisture uptake) so won't model the effect of all that in-depth humidity buffering that 'natural' materials claim if used 'breatheable'.

      I feel that this is a technical issue that the industry is ignorant of, at its peril – just as it was ignorant of interstitial condensation until the fabled Arena TV programme blew the whistle and ruined timber-frame's reputation for the next 20yrs.

      As well as sheep, hemp etc, remember several non-natural materials are good for this too.

    • #35534

      Thanks Mark and Tom,
      PHPP models conductivity and thermal mass (an improvement on SAP!), but doesn't model hygroscopicity or account for breathable construction (which I believe are some 'natural' materials forte, and can reduce ventilation loads).
      Not that this precludes these materials from PH's – as is demonstrated by the S-House. which achieves airtightness with a continuous laminated timber envelope, which theoretically will not move enough to let heat out in the future. No vapour membrane!
      Typically though, PHs do use a vapour membrane to ensure airtightness and gaining the PH standard? Even when using 'natural' materials eg the Cardiff PH.


    • #35535
      Tom Foster

      Can we really call what PHPP does 'modelling'? Isn't it just a spreadsheet that's been empirically tweaked to agree with the results of FEA simulation, within defined limits? Isn't this the difference between old Hot2000 and its successor Hot3000? Given modern desktop computing power, why does anyone bother to develop what can only be a limited bag of pre-defined tricks, requiring much effort and verification to extend? Isn't do-anything number-crunching FEA the future?

    • #35536
      Nick Grant

      Hi Matt

      Mark's summary is correct.

      PHPP is a building energy model, you can use whatever materials you like, as long as they work and you know the U value of the proposed construction.

      For moisture modelling you can either design out the problem or where this is not possible (eg renovation or old buildings) then software such as WUFI can be used. However with all software garbage in garbage out. It is always necessary to make simplifying assumptions and that is part of where the skill lies.

      As usual I have to disagree with Tom.

      For me the real beauty of a 'simple' spreadsheet type model such as PHPP is that you can see where the numbers come from, it is physics, you can look under the hood.

      As it happens, PHPP shows that the steady state model is surprisingly good especially once you get to low energy use so temperature swings are minimised. In terms of computing power I'd give my money to a good building physicist with a calculator from the pound shop any day.

      IMHO fancy front ends and cad integration can hide all manner of assumptions.

      Designing low energy buildings requires a deep understanding of basic principles. No amount of software will do the job for you any more than an electronic guitar tuner will help you play jazz, as I have discovered to my bitter disappointment.

      On the ventilation front PHPP will let you model a lower ventilation rate if you believe that nasties will be magiced away through the walls but see else where for discussion of air-tightness, breathability and ventilation. However the PHPP default is already lower than typically used for small UK homes.


    • #35537
      Tom Foster

      As usual I have to disagree with Tom.

      I didn't know that.

    • #35538

      As usual I have to disagree with Tom.

      I didn't know that.

      Well I have to agree with Tom's excellent post.

    • #35539
      David Olivier

      AFAIK, of the order of 80% of Passivhaus buildings in Germany, Austria and Switzerland have clay block, concrete block, calcium silicate or concrete walls, which don't need a vapour barrier unless they have some unusual and vapour-impermeable materials on the outside. Vapour-breathable constructions can also be airtight but they don't reduce ventilation loads.

      PHI did large amounts of energy monitoring showing as expected that many heavy building materials – wood does the same, but there are fewer m3 of it in a building – absorbed a great deal of water vapour from the interior at some seasons of the year; e.g., autumn, and gave up water vapour to the interior at other seasons of lower humidity; e.g. spring. This changed the monthly energy balance but of course made no difference to the annual energy balance. Special “magic” materials aren't needed for this to happen; it happens in most buildings except possibly a steel-frame one with closed cell foam insulation.

      So in effect, yes, PHPP does deal adequately with hygroscopic materials. There's a note in the manual about this, commenting that monthly heat demands may be slightly different than predicted by the monthly method but the overall annual demand will not be altered.


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