Welcome to the AECB Forums Building Design Passivhaus Design Question

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  • #31739
    Anonymous
    Inactive

    Excuse me if this question sounds to simplistic, but is the aim of the Passivhaus total maximum energy usage of 120 kWh/m2/yr restricted to using the prescribed envelope elemental values or does it allow design flexibility of these values in achieving the 120 kWh/m2/yr?

    I would also like to apply this question to other low energy design guides such as Carbonlite and Code for Sustainable Homes.

    Jean-Marc Bouvier

    #38700
    Nick Grant
    Member

    The main target is the space heating of 15kWh/(m2.a) (or 10W/m2 peak heat demand).

    This is calculated according to PHPP with Passivhaus assumptions for gains etc.

    Please see the document claiming the Passivhaus Standard which can be downloaded from the Passivhaus Trust Website.

    There are no prescribed elemental U values except for windows (climate specific) and that is for comfort. However you will need very good U values (0.15 or better in the UK is typical) to meet the target for most building types, especially dwellings.

    The 120kWh/m2 if total primary energy value.

    Same for Carbonlite, I can't help with CSH.

    Nick

    #38701
    Anonymous
    Inactive

    Hi Nick

    So if a designer can be convinced that a wall with a value of 0.25 can produce the target of 15kWh/m2/yr then its possible to do so?

    Jean-Marc

    #38702
    Mark Siddall
    Participant

    Jean-Marc
    In theory you could achieve the 15 kWh/m2.yr with a fabric U-value of 0.25 W/m2K. You could do this with a suitably large building that has a preferable surface are to volume ratio and/or a suitably warm climate (think the south of France.) As for the UK, it is likely that more often than not a detached house will need U-values in the order of 0.1 W/m2K or so.

    …also, it is not a case of 'convincing a designer', rather it is a case of running suitable PHPP calculations and determining what U-value is required to achieve the 15kWh/m2.yr

    Mark

    #38703
    Anonymous
    Inactive

    Hi Mark

    Does the type of wall, heavy or light, have any bearing on the u-value?

    Jean-Marc

    #38704
    Mark Siddall
    Participant

    In terms of the BS EN standards the U-value is not affected by thermal mass. (This is not to say that that thermal mass is not beneficial. PHPP does factor in the impact of thermal mass upon overall performance and generally tends to favour heavy weight during the winter and summer.)

    #38705
    Nick Grant
    Member

    Worth pointing out that the benefit of mass is modest and that it doesn't 'slow down' the escape of heat as some claim!!

    #38706
    Anonymous
    Inactive

    Does this apply to all types “mass” walls?

    #38707
    Mark Siddall
    Participant

    Please define “mass” walls.

    #38708
    Anonymous
    Inactive

    I will stay with what I know best, insulated concrete forms.

    #38709
    Anonymous
    Inactive

    Nick,

    I would like to see the document/data concerning the modest “slow down” of escaping heat and mass walls, could you point me to where I can get it.

    #38710
    Anonymous
    Inactive

    Nick,

    Since you haven't sent me any info I thought I would send you some of mine. All links are builds with external wall u-values of 0.23, the Canadian office shows the kWh/m2/yr but the Kentucky school and office is in kbtu/sqft/yr so multiply by 3.15 for kWh/m2/yr

    http://www.enermodal.com/pdf/EEL-Kitchener-AGV-1pg.pdf

    http://www.hpbmagazine.org/case-studies/educational/richardsville-elementary-school-richardsville-ky-

    http://www.hpbmagazine.org/case-studies/office-institutional/cmta-office-building-louisville-ky-

    #38711
    Mark Siddall
    Participant

    Jean-Marc
    I fail to see how these case studies address your initial query to Nick. Furthermore, Nick's comments recognised that the reported “slow down” of heat loss arising from the use of thermal mass is an inappropriate, and incorrect, proposition.

    #38712
    Anonymous
    Inactive

    Mark

    May I suggest you look into what ASHRAE have to say about mass in their new Advanced Energy Design Guides. The point I am trying to make is that u values are a poor indicator of how a wall will “perform” and that all the examples I posted are all operating at 70kWh/m2/yr or less with exterior wall u values of 0.23 and that “performance” to PHPP is doable with the added cost of insulation to “tick a box”.

    I could also show you a hotbox test done comparing various wall types in this country showing lag times for maintaing a constant temp with a “stick frame” lag time of 2.5 hrs and an ICF lag time of 27 hrs. And the stick frame had “better” uvalues than the ICF.

    For me, the only pieces of paper that are a true indication of performance are the energy bills and an ICF “outperforms' everything that I've come across worldwide.

    #38713
    Tom Foster
    Participant

    23hrs seems a non-useful lag, re-delivering yesterday's heat at same time today. 8-12hrs is useful, re-delivering daytime heat in the evening.

    To me, simple massiveness added to the walls, floor, partitions etc of a near-PH building is ineffective, being bi-directional i.e. window-collected solar heat In daytime, Out by same route reversed evening – activates only the near-100mm of the mass at best.

    Mass has to be used more knowingly, so heat is input to one face of the mass and travels uni-directional through to the interior. That is the case with wall-collected solar heat, but is defeated by super-insulation, so is hard to see how ICF walls would make any difference either way; U-value alone would count.

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