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Energy Assessment: Realistic SAP Calculation

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    • CommentAuthorvord
    • CommentTimeOct 10th 2019 edited
     
    I want to have a realistic SAP calculation done for building regs relating to work on an old building (it's unusual - mostly Victorian ish built around a Tudor structure).

    The previous SAP calculation used Build Desk and over-estimated energy usage by about 6 times (even after correcting to my degree days). That makes it not at all useful to me for prioritising improvements that might result in a real reduction in energy use. Instead it forces me to spend my budget insulating the mistakes in the calculation rather than work on the important stuff.

    Is it normal for officially recognised SAP calculations to be so far off the mark?

    Can anyone recommend a way to have a SAP calculation done that accurately represents the build level of the house at the moment. Some it insulated to passiv haus standards and some is still single glazed. It needs to be accepted by building regs but otherwise I would like it to be as accurate as possible. I'll provide a spreadsheet with dimensions, wall constructions, and the Lambda values for materials I have used.
    •  
      CommentAuthordjh
    • CommentTimeOct 10th 2019
     
    Posted By: vordIs it normal for officially recognised SAP calculations to be so far off the mark?

    I'm prejudiced but even so I don't believe it's normal for them to be so far off. I do believe they can easily be far enough off to make using them as you suggest a complete waste of time though.

    It's just something you have to pay for and have done in order to tick a building regs box, IMHO. A complete waste of money. For design work, I would recommend one of the well-known dynamic modelling systems or possibly PHPP if your target is to achieve something close to EnerPHit. And somebody who knows how to use the software properly.
    • CommentAuthorvord
    • CommentTimeOct 10th 2019 edited
     
    Thanks @djh - Sadly it's not a tick box. I'm doing a change of use and am in a bit of a hole with it. I'm letting go of the private building regs guy so I can try my luck with the council. It's supposed to be 15 years payback on insulation and it would be brilliant if I were allowed to throw that money into a pot to help insulate people's lofts and be done with it so I can focus on doing stuff to make the house warm.

    For that an accurate SAP would be a really useful starting point. If I can direct work towards things that make the house warm and cosy I would be happy.

    I figured out the maths and made my own spreadsheet to guide me, so in that sense this is a tick box exercise. Money isn't really an object - an accurate SAP would save me tens of thousands while also allowing me to do work that might be kinder to the environment.

    Do you have recommendations for people who can do good SAPS?
  4.  
    Maybe it's just down to BCO's interpretation, but the only time I was (almost) involved with a change of use the BCO's requirements seemed looser, not more stringent, than 'standard Part L'. If I remember rightly they were invoking the threshold U values for retained thermal elements (0.7 for walls, IIRC), and also allowing area-weighted U values so that the client could retain un-insulated external walls (say U = 1.7 - 2.1) in the ex-commercial building by stuffing the stud walls forming the flats with insulation to, say, 0.2 (over a greater area), giving an average of 0.7.
    • CommentAuthorvord
    • CommentTimeOct 10th 2019 edited
     
    I think it is the BCO's interpretation which is why I am going for signoff for works completed so far and why I plan to move to the council for the rest of the work.

    I would rather be able to donate 15 times current heating bills into a pot to help people insulate their lofts so I can be done with it and get on with doing things properly, but rules is rules and I need to follow them whatever the environment thinks.

    I just want a representative SAP. I want to use it to help me direct the work to stuff that might be a tiny bit environmentally friendly while still being able to tick boxes.
    • CommentAuthorGreenPaddy
    • CommentTimeOct 11th 2019 edited
     
    Hi Vord,

    after many years of working with SAP, I've concluded it gives very accurate wrong answers. It goes to the Nth degree to work out the internal mean temperature, but gets so much else wrong. It's just a way to try to get some sort of a level playing field, to have a pass mark for buildings, however the playing field is a rugby pitch for most people, but at the top end of insulation and low energy, it needs to be a putting green.

    Having said that, it is only as good (bad) as the info you put into the model. The guys I use for doing all my SAP calcs really only follow my direction as to lambda's, thicknesses, materials, Uw's, Psi values, extent of coldbridging, efficiency of systems, ventilation, etc, etc. I'd guess that most of the SAP results I get, versus my and my clients' actual energy usages, as being around 150% of reality. So I'd say the blame for your 600% value may well lie with the info given/assumed by the SAP person.

    If you have a real mix of different conditions for say walls, then the SAP person will have to make that number of "wall types" in the SAP model. Like wise for all the structural elements, and especially for Psi values for cold bridging. SAP costs about £300, so they don't have many hours allocated for doing all that. You will likely have to give them lots of drawings with all the detail for every part of the structure, and pay them more for their time to model all that into SAP.

    I always run through a draft of the full SAP calculation, and usually pick up a number of errors or mis-interpretations by either myself or the SAP guy, but you need to have a good understanding of the building in question and its build up in every area, and also what SAP is looking for. If you have a good understanding of the building, then you'll know its weaknesses, so there's nothing useful SAP is going to tell you. You're just looking in a mirror.

    If you want a reasonably good SAP number to pass BRegs, that's one thing. If you want to know what remedial action to take to reduce your carbon footprint, that's something different.
    • CommentAuthorvord
    • CommentTimeOct 11th 2019
     
    Thanks @GreenPaddy, I think the original SAP was very oversimplified and that has probably caused a lot of the over estimation. I sent plans for that one, but think I should send data to type in this time.

    150% of actual energy use would be brilliant if I can get a SAP that says that. It is just for building regs and will hopefully help direct work to stuff that actually needs doing and help me avoid some work with little benefit that would make the building wet.

    I'm getting a spreadsheet together with actual dimensions, wall structures and Lambda values, so hopefully I'll get a more accurate representation.
    •  
      CommentAuthordjh
    • CommentTimeOct 11th 2019
     
    Posted By: vordwill hopefully help direct work to stuff that actually needs doing and help me avoid some work with little benefit that would make the building wet.

    I'd love to see an explanation of how SAP makes you do work that makes the building wet? And for that matter makes you prioritise stuff wrong?
    • CommentAuthorvord
    • CommentTimeOct 12th 2019 edited
     
    Posted By: djh
    I'd love to see an explanation of how SAP makes you do work that makes the building wet? And for that matter makes you prioritise stuff wrong?

    Easy when you have an inflexible building regs inspector. I'm moving to the council so hopefully it will be more sensible, but I want to be armed with a good SAP. It's an old house and there are a couple of short walls with impervious coatings, no damp course and high exterior ground levels that I want to put off until much later as I could use the budget more sensibly.

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