Welcome to the AECB Forums Building Design Ψ Values – Thermal Bridging

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  • #30634
    Mark Siddall
    Participant

    Is anybody aware of any software that will assist with calculating Ψ Values /thermal bridges?

    There's a whole raft of guidance, see below, but what is really needed is a simple user-friendly tool to assist the everyday designer, rather than just the researching building scientist. Furthermore, the U-value software I have come across does not assist with non-repeating thermal bridges it simply focuses upon an elemental repeating thermal bridge.

    Guidance includes:
    EN ISO 9346 (1996) – Thermal insulation – Mass transfer – Physical quantities and definitions
    EN ISO 10456 (1999) – Thermal insulation – Building materials and products – Determination of declared and design thermal values
    BS EN ISO 10211 Thermal Bridging Pt1 + Pt2
    EN ISO 13789 (1999) – Thermal performance of buildings – Transmission heat loss coefficient – Calculation method
    EN ISO 14683 Thermal bridges in building constructions. Linear thermal transmittance. Simplified methods and default values.
    BRE IP 106 Assessing the effects of thermal bridging at junctions around openings
    BS EN 1745: 2002: Masonry and masonry products. Methods for determining design thermal values (London: British Standards Institution) (2002)

    Mark

    #33642
    David Olivier
    Participant

    Mark

    Have you tried THERM? Free from the USA, thanks to their enlightened Dept of Energy (Lawrence Berkeley Laboratory, Univ of California).

    I used to use FRAME – free from Canada (written by Enermodal Engineering for the govt.) – but may have been overtaken by THERM. Was very user-friendly though.

    David.

    #33643
    Mark Siddall
    Participant

    David,
    I had a play with THERM a while back, very user friendly. I was surprised. The trouble with the software is that it does not calculate the Ψ Values/fRsi in accordance with ISO 10211, or any of the supporting, or related, documents. I contacted LBL about this that they gave the following response: –

    “First, we don't anticipate adding the ISO 10211 calculation to THERM in the near future (i.e., the next few years), but I will add it to our features list.

    Second, even though THERM doesn't report these numbers directly, THERM can be used to perform the simulations to calculate these numbers. For the linear thermal transmittance (psi value), you need to perform 2 separate simulations, one with a foam panel in place instead of the glazing system.
    For the Condensation Risk, I assume that all you need are temperatures are certain points.”

    I’m not sure how to use the data from the “2 separate simulations” to derive the Ψ Values. I was, perhaps lazily, hoping for a European version. A point worth noting is that THERM is really designed for designing windows, as a result the library of materials is not so great.

    I have investigated the range of software from Physibel (Bisco, Trisco etc.) but each package costs about £2-4.5k, which is a little steep for the use it would get. Physibel did work on Eurokobra (a digital atlas of thermal bridging detail) I have had close a look at this software as well and found that:

    · Though you can customise existing details to some extent it does not allow you to construct new details and then test them. This is a great limitation (this also relates to a number of the subsequent points.)
    · Many of the details offered by the software are more suited to domestic use than to larger developments.
    · A good number of the details are continental i.e. they rely upon concrete frame structures, whereas in the UK the predominant primary structure is steel frame.
    · We tend to find that many contractors prefer to over board secondary steel with cementicious board, or similar, rather than blockwork, again this is not accounted for in the database of details. No details for this form of construction are available. Again this relates to the software not reflecting aspects of the UK construction industry.
    · It is a shame that it does not calculate U values in accordance with BS EN ISO 6946 – Building components and building elements (noted in the User Manual).

    (Oh, and by the way if you visit the Eurokobra website, the BRE doesn't distribute the software anymore, EURISOL the insulation trade organisation may still do so but last I heard it cost £250, which means, to my mind at least, it isn’t worth the money.)

    I’ll investigate FRAME, though I suspect that like THERM it will not calculate Ψ Values/fRsi in accordance to ISO 10211. If anyone else has any pointers they would be appreciated.

    Mark

    #33644
    Mark Siddall
    Participant

    Great work Rod. Thanks for posting that up for review. Looks like you've had hours of fun 😉

    Have you looked at placing windows at different locations within the thickness of the wall? One of the Stamford Brook .pdf files suggests that the optimum position for the window is in line with the insulation (between 100 and 150mm offset from the face of the masonry wall.)*

    How do you get THERM to give you the U-value? Its a while since I used the software and I don't recall the feature. Is it just on a simple pull down menu?

    Mark

    * Paragraph amended 30.04.07

    #33645
    David Olivier
    Participant

    The Thermotech fixing detail is aimed at more standard North American walls which are quite thin. One wouldn't ideally use such a detail in a house with thicker walls designed to minimise thermal bridging. I did my modelling about 10 years ago using FRAME.

    David.

    #33646
    Mark Siddall
    Participant

    Chris,
    I have just had a quick look at WIS myslef. WIS is run through MS-Access so it seems that it's more of a calculation tool than 'modeling' tool. As you say you can import THERM models.
    I was informed by a chap from Soutwall Technologies that WIS was similar to THERM, perhaps he ment this is just with regard to calculations rather than modeling….
    Oh well, back to the search for the ultimate Psi value calculation tool….it seems that Rod's got the best methodology to date.

    Mark

    #33647
    Mark Siddall
    Participant

    This link from elsewhere on the forum has a few interesting notes on permissible/realistic Ψ Values (courtesy of Nick, David O and Peter Warm).
    https://www.aecb.net/forum/index.php?topic=602.msg2060

    NOTE: The AECB Standards don't appear to offer any advice on minimum Ψ Values (at least I haven't spotted it). As a consequence I've done some digging on the the internet. The results suggest that the Ψ Value for a Passive House, and therefore presumably Gold standard, has to be =< 0.01 W/(mK)! (In the UK the convention is measure heat loss based upon the internal area rather than a specific weakness in the thermal design. I'm not sure whether the same method of measurement is used in the PH standard. As this will have an impact upon internal/external corners etc it would be helpful if someone could clarify how the PH standard operates with regard to this matter?)

    As there are no AECB standard details available as yet the thermal modeling of details, in order to derive the Ψ Value, seems to be critical. Not only because its the only way to be sure that energy performance will be achieved, but also to ensure that the designs can be achieved in a suitably economic fashion i.e. using UK Robust Detail Ψ Values could result in additional costs due to additional “un-required” (though never the less useful) insulation being specified.

    To this end can anyone verify Rod's Ψ Value modeling and calculation methods?

    Mark

    P.S. Just had a look through some of my library. My earlier posting on the 0.5m isotherm zone was a little off beam. The 0.5m dim relates to how you mimic thermal bridging in ESP-r.
    BS EN ISO 10211 Thermal Bridging suggests that a 1m plane of symmetary should be used at the respective detail i.e. the legs of the “L” formed by the ground floor/wall junction should be 1m.

    #33648
    Mark Siddall
    Participant

    It would appear that the required PassivHaus Psi value of 0.01 W/mK relates to external locations rather than internal locations. This is (as I understand it) contrary to UK methods whereby the convention is to measure heat loss based upon the internal area rather than a specific weakness in the thermal design.

    Having contacted the BRE they have confirmed that they will be looking into this over the coming months as they prepare technical guidance on Passive House construction.

    A new version of BRE's Information Paper IP01/06 was updated in February 2007 and states the reasons why internal locations are used, but there is currently no guidance on if there is a direct relationship between the two values (so that we can work out a conversion factor) – this is something which they anticipate will be tackled in the near future.

    In the meantime within PHPP the BRE are using external Psi values where available (as this is what the package has been designed around), if thermal bridging information is not available then they assume accredited construction details values and whilst this does not give true benefit for the construction type (you have to work a bit harder elsewhere) it is considered to be a pragmatic approach in the meantime, but obviously something which needs remedying.

    Mark

    #33649
    David Olivier
    Participant

    The AECB is preparing both.

    PHPP says that if you measure areas using external dimensions (defined) you may ignore small linear non-repeating thermal bridges with a psi value of < 0.01 W/mK (many listed by PHI).

    David.

    #33650
    SimmondsMills
    Participant

    Peter Warm has worked out a conversion method for turning psi internal into psi external – used for the Silver Detail design guidance document (soon to be published!!!)

    #33651
    David Olivier
    Participant

    The method for doing this is also set out in the PHPP Manual and one can use PHPP to convert one to the other.

    David

    #33652
    Nick Grant
    Participant

    I've split this topic and moved the excellent discussion of window installation details to here:

    https://www.aecb.net/forum/index.php?topic=1015.0

    #33653
    Mark Siddall
    Participant

    Thermal Bridging Catalogue by Swiss Federal Energy Office:
    http://www.lesosai.com/download/Warmebruckenkatalog_f.pdf

    Mark

    P.S. Helps if you can read French, though the diagrams and tables almost explain themselves. (If you don't do French then try Googling Bablefish.)

    #33654
    Mark Siddall
    Participant

    The thing that confuses me is these negative psi values. I appreciate that they are used in PHPP but why? I've been thinking about it and this is the best that I have considered to date: –

    For example a “normal” external corner of a building is has both a positive internal psi and a negative external psi. In terms of building physics I appreciate the fact the external corner has a greater surface area, that as a consequence the average distribution of energy across this area therefore lower than through a planar wall and that for this reason the internal temp of the corner inside the building is, due to geometry, lower than the surrounding areas of wall. One could be forgiven for thinking that this imposes a heat load on the heating system. But does it? If the psi value is measured externally it appears that, relative to the corner, the insulation per m2 increases. On this basis you can appreciate that the corner will lose less energy per m2 than is suggested by the internal psi value.

    Whilst the suggestion is that the a negative psi signifies additional insulation properties (reduced heat loss) if we consider a “negative” corner (corner flexed in toward the living space) we find that due to geometry the conditions have become the inverse to the description above.

    This cursory description of the physics suggests that depending upon building geometry the heat load is either reduced when measured externally or increased when measured internally. It can be seen that these two methodologies for various geometrical conditions can lead to some quite considerable differences in thermal bridge assessments. As we tend to have more “normal” external corners on a home than “negative” corners, depending upon how the psi value is calculated, the calculated/predicted performance of a building can be affected quite seriously.

    Whist appreciating that one should just use the methodology of the calculation system that is required PHPP or SAP/iSBEM. I can safwely say that this matter is most perplexing!

    When handling a negative psi value in PHPP do you actually enter “zero?”
    Does anyone with more experience have any comments?

    Mark

    #33655
    David Olivier
    Participant

    Mark

    They're negative simply because the actual heat flux is less than that calculated using external areas – i.e., the sum of all the UAs. By contrast, the actual heat flux is more than that calculated on the basis of just the internal areas.

    If you measure externally, use good details and assume psi = 0.00 W/mK, you avoid a lot of work on a small building and you still get a precise enough result. To be exact, the result tends to be conservative – rather than representing a chronic underestimate of heat loss, as has been the case in the UK for several decades ever since we started using insulation.

    Have you read the PHPP Manual?

    David.

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