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SAP for off-grid no-combustion new build

As most will have gathered by now, my house plans are for off-grid with solar-only (or mostly) heating with a large thermal store in the north of Scotland. Calculations [1] say this won't work but that the shorfall is not as large as some might initially assume. More importantly, there's a lot of uncertainty in how the systems will perform overall so I want to take an iterative approach of doing an initial construction which I'm fairly confident will work survivably then tweak to make comfortable.

What it will likely not do at first is meet the requirements of provably being able to keep one apartment at 21 ° and the rest at 18 °C.

(But then I don't think I've ever lived anywhere that's met that requirement without silly-expensive amounts of heating. Even the house I lived in in Zambia for a year as a child probably cooled below 21 °C on at least a few nights a year in the dry season - it had no heating and the “house boy's” grandmother remembered once seeing frost.)

Originally my house designer was going to do the SAP rating for the house. He thought he could do it in a way that would meet the requirements. Unfortunately, the council now require somebody with some certificate or other to do the job so he's subbed it out. The new person has taken it all rather seriously and “escalated” questions up the certification chain. AIUI, she thinks there's a get-out clause if she can calculate that the PV and solar thermal will be sufficient for the job.

Maybe she or somebody up the chain to BRE can come up with a creative calculation which means I can get on with things. If not, though, any suggestions?



  • I never did a SAP for my house though I did do a thermal model.

    If the house is really low energy then there is a provision which says something like you don't need SAP calculation.

    Building inspector couldn't believe that I was using so much insulation, 3g, airtightness etc so let me off. I have got a completion certificate.
  • Thanks, tony - just been through the Scottish Domestic Handbook sections on EPCs, etc. Only possibly relevant get out I can see is for buildings with less than 50 m² floor area - pity mine's roughly 60 m² finished floor area - and that might only be for ancillary buildings. Obviously the English rules might be different or might have been at the time your house was built.

    Can anybody think of any other relevant recent new builds? Eg., anybody remember that straw bale passive house in Wales or the borders which had a blog and was, IIRC, discussed here in relation to the thermal mass in the floor. Couldn't think of the right keywords to get it into the first few pages of a search result the other day.
  • I didn't need an 'as designed' SAP before I started the build because the BCO accepted my PH calculations. I have now had an 'as built' SAP done which is still subject to the air pressure test, when I have that carried out.
  • My BCO also accepted the PH calcs which are in my case proven to be much closer to reality than SAP which was way off the mark. Do the Scottish rules really say that you have to have a SAP rating of such and such or do they say that a SAP rating will prove that the house is efficient enough. The English regs are written in such a way that if you go back to first engineering principles you can normally prove the case with Passive houses.
  • edited July 2014
    Posted By: pmusgrove: “Do the Scottish rules really say that you have to have a SAP rating of such and such or do they say that a SAP rating will prove that the house is efficient enough.”

    They say that the DER must be no more than the TER:

    Ideally, they'd just say “no fossils being burned, DER = 0, TER > 0, therefore DER < TER, QED.” In effect they (the assessors assessor, I think it was) accepts this - the puzzle is how to write it in the right way.

    Interesting that PH can be accepted. Bit late to change horses and try that, though. Might have done from the beginning if I'd have known (and checked it would be).
  • I would write to them ans ask the question "if I can get close to passive house or code 6, MINERGIE heat losses will you allow me to be considered as a special case and not need calculations, my U -values will be .....
  • an alternative - can you "design it" with electric space heaters - if the insulation is so high that the heat load will be close to zero then it still may pass the calculations. (although the smaller the property, perversely the harder that is to do)

    Appreciate that using one in an off-grid situation isn't likely to happen, but if the point of the exercise is to satisfy the bureaucrats it may well suffice.

    You can always borrow a fan heater for a few days if you need an "as built" inspection..

  • The problem may be the requirement from European legislature:

    Under the Energy Performance of Buildings Directive (EPBD), all EU member states must promote improvement in the energy performance of new and existing buildings.

    This means a requirement to provide an Energy Performance Certificate (EPC) to any prospective buyer or tenant of a property when it is either sold or rented out.

    Check out:


    (The EPC comes from the SAP Calculations).

    For design stages, PHPP calculations to show Passivhaus levels of performance may well be accepted for design stage compliance, but you will likely still need a SAP to get your EPC.

    There's no specific SAP rating that needs to be achieved, it's all about the TER/DER calculation and bettering the limiting values for compliance - that and the requirement for the EPC at the end of it all.

    (not sure on the grey area of living in it and not selling or renting it, I guess if you didn't get one, then you would need an RDSAP based EPC (DEA produced) if you chose to sell it or rent it out at some future time).
  • edited July 2014
    Posted By: tonyI would write to them ans ask the question "if I can get close to passive house or code 6, MINERGIE heat losses will you allow me to be considered as a special case and not need calculations, my U -values will be .....
    Yep, would like to just talk it over with the local BCO. House designer is not too optimistic about getting that conversation though he's (the BCO) seemed willing to discuss things with other people.

    Synggapa: yes - that was the house designer's original intention: write it up as electrical heating driven by the PV without getting into if the PV is sufficient. It wouldn't be, of course, in long-term steady-state winter conditions but the house should work with the large thermal store. The problem is that the full-time assessor is worrying about this.

    Essentially the problems are:

    1) SAP doesn't take account of the thermal store.

    2) It doesn't have any mechanism to allow solar thermal to contribute to space heating.

    3) Fundamentally, it doesn't allow for an experimental and incremental approach - you have to prove up front using their rather constrained calculation methods that the house will “work”. Fair enough for an estate house which is going to be handed over to people who'll just live there but a bit tedious for an owner/occupier who's taking a bit more interest.
  • edited July 2014
    I linked to my energy budget page above but somebody would have to be really rather interested to go through that in detail. Here's a tl;dr version with rather round numbers for December and January:

    Total heat loss from the house would be 1 kW.

    That's from heating degree days from Wick (about 20 miles north) using room temperatures as required for SAP (actually, 21 °C for bathroom, kitchen, living room and study, 18 °C for both bedrooms and both lofts - SAP only requires one room at 21 °C but kitchen, living room and study are open plan to each other and, realistically, I'd keep the bathroom warm anyway).

    Heat gains would be about 150 W from the PV, same again from the solar thermal, 100 W from a not-yet-dead body in the house and 100 W through the windows giving a gain of 500 W so a net loss of 500 W.

    The heat capacity of the 10 m³ water thermal store would be 42 MJ/K so 500 W net loss would run it down at the rate of 1 K every 84000 seconds (or about 1 K/day as a day is 86400 seconds).

    Dropping 60 °C in the two worst winter months is way too much. It might be acceptable for a very compactly shaped store with good insulation but that's not my design.

    Adding in a 1 kW wind turbine which you might reasonably hope to generate 350 W in those months on a hilltop overlooking the Moray Firth reduces the net loss to 150 W which means the thermal store now takes three and a quarter days to drop 1 K so now the temperature drop for those months is 20 °C which is much more realistic.
  • edited July 2014
    But isn't the EPC just a bureaucratic worthless piece of paper, so why not just do the minimum required to get the piece of paper and then throw it in the bin. If you ever move you will need a new certificate anyway, as they are only valid for ten years.

    Recent experience has not been good and as far as I can see there is nothing to be gained by worrying about the actual figure, as its very much rubbish in rubbish out, particularly when my assessor would only input things he could verify, and as he could not verify himself the thickness and type of the insulation he used a default value.
  • Absolutely Triassic. That's why I haven't taken any interest at all in the process up until now; the house designer seemed confident that he could put something together which would get a building warrant and that was sufficient for me - I certainly wouldn't care about the actual rating.
  • Hi Ed,

    DHW will be a bigger issue from the sounds of it than the building heat loss. I wouldn't worry about it, just get the building modeller to put in electric space heaters and immersion in the model if there are any issues with cold days. I can understand why you want to avoid all this but I can also see the council having a standard system whereby they can ensure buildings and their relative heating/DHW systems are fit for purpose and can keep their inhabitants safe and warm at a reasonable cost.

    Triassic, SAP models for building regs purposes in Scotland are accurate in material build ups, windows sizes/orientations etc. Once the blower door test is done that will be added too. This isn't a general guesswork game like those for rental properties.
  • Sorry Willie but I think you're rather missing the point: the building modeller seems not to be willing to [b]just[/b] put electric space heaters and immersion into the model. Still, being able to point to a clause somewhere which says they can do that would help, though.

    Yes, DHW is a bit of a problem but it's a subset of the space heating problem in the sense that the intention would be to try to recover as much heat as possible from DHW for space heating. The envisaged PV would be able to lift 50 litres of water per day by 47°C on average. If the bottom end of that range is done by the solar thermal (so the PV just heats the already warm water at the top of the tank) then all should be good.
  • edited July 2014
    A forum member in who does SAP ratings, etc, in England and Wales has emailed me to point out that part L contains an exemption “if your dwelling does not use energy to condition the space”. The Scottish equivalent seems to be limitation d to Standard 6.1:

    which says:
    Standard 6.1

    Every building must be designed and constructed in such a way that:

    a. the energy performance is estimated in accordance with a methodology of calculation approved under regulation 7(a) of the Energy Performance of Buildings (Scotland) Regulations 2008 and

    b. the energy performance of the building is capable of reducing carbon dioxide emissions.
    I don't see how a new building can reduce carbon dioxide emissions except, I suppose, if it replaces another building which causes more emissions. Whatever, it then goes on with:

    This standard does not apply to:


    d. buildings, which will not be heated or cooled, other than by heating provided solely for the purpose of frost protection or
    Clearly taking things absolutely literally this means a building which is kept at an exactly constant temperature throughout its life (if its temperature changes then it is being heated or cooled). I don't think they mean this.

    I also doubt they mean to exclude buildings which are sometimes warmed by sunlight coming through the windows. So what about buildings which are heated by sunlight falling on PV and solar thermal panels mounted on their roofs? Or by heat leaking out of a thermal store which was itself heated by light falling on the PV and solar thermal on the roof? Where does the line get drawn here?
  • Ed,

    If your designed dwelling is 100% off grid, then by default your DER/CO2 emissions are 0.
    Have you explained this to your 'BCO'?

  • Just emailed my house designer saying do we really need a SAP rating and, if we do, why can't we just set the DER to zero and that we really ought to talk this over with BC.
  • Posted By: willie.macleodThis isn't a general guesswork game like those for rental properties.
    I my recent experience was a refurbishment and the assessor would not accept anything he could not verify and photograph for audit purposes, so as the insulation was covered with plaster board and could not be seen, he used default values.
  • That is the difference between a 'SAP' Assessment+EPC , and an rdSAP Assessment+EPC.
    rdSAP is used for existing dwellings, and the 'rd' means 'reduced data'. It uses lots of defaults, whereas SAP has a higher level of data input.

    Neither allow for non-grid tied dwellings tho'!
  • Nor large amounts of thermal mass.

    (House designer seems pretty confident that SAP lady will produce something plausible soon. Breath bated.)
  • Ed,

    SAP does allow for the Thermal Mass Parameter (TMP) to be input accurately, and will reflect this.
    Whether this gives you the result you are after is another question....:smile:
  • edited July 2014
    Posted By: DarylPNeither allow for non-grid tied dwellings tho'!
    Maybe worth asking Paul (camillitech) who prepared the SAP for his new off-grid house?,15679.0.html

    Though I think he has a wood burner (in addition to PV, wind & hydro), so maybe that simplified things?
  • It would appear that the issue comes from Regulations and SAP having standardised requirements and your and Govt's/DECC's expectations of how a house should be able to perform.

    i.e. your house will still be there once you've sold it / shuffled off this mortal coil. You can't therefore assume that a replacement occupant will be happy to live at 15.5°C ( just above the point of potential condensation issues).

    The heat losses are fairly easy to calculate if you know all your areas, lengths, constructions and their junctions performances, air tightness you can achieve and proposed ventilation parameters.

    The standardised metabolic gains etc will be based on the house's floor area, because you will not necessarily always be the occupant.

    Likewise hot water requirement will be based on typical number of occupants taken from the floor area.

    Solar hot water can be entered and may well fulfill much of your requirements, but the hot water cylinder is going to need supplementary immersion at some point in the year if the solar isn't fulfilling your needs

    Solar gains and really good insulation levels will take you a long way, but a back-up heating system (which could be classed in SAP as your main heating, would likely be necessary for at least a few months of the year, especially as the solar gains won't be very high during the period when you will be needing the most heat (equally the benefits from solar hot water and PV will also be at their lowest in the period when you most need it).

    There's nothing to stop you assuming a dual electric immersion cylinder, with solar hot water input, nor to stop you putting an electric room heater as your main heating, or a closed log burning room heater if you prefer, as long as you can prove TER/DER compliance and better than limiting values.

    SAP honestly is fairly robust compared to any other tool these days (especially SAP2012, although with Scotland, compliance is still using SAP2009, which is still pretty reasonable). There's a few issues (especially regarding lighting still tied to floor area), but overall it's not so bad.

    The design stage SAP is required because too many houses were just being built and then had awful performance when assessed at the end. Understandably, modelling what you propose to do and then changing things at that stage is the best way to go, not least because it spares you from expensive c**k-up's!

    A knowledgeable assessor and a open minded Building Standards officer should be able to sort it.
  • Posted By: DarylPSAP does allow for the Thermal Mass Parameter (TMP) to be input accurately,
    It allows for some. Maybe it's changed or they're mistaken but the Hockerton people certainly seem to think it doesn't take account of large amounts:
    SAP assumes that an element of mass thicker than 100mm has no additional thermal capacity is flawed (SAP2009 Table 1e: Heat capacities for some common constructions), contrary to evidence at HHP.
    Posted By: skyewrightMaybe worth asking Paul (camillitech) who prepared the SAP for his new off-grid house?
    Good point - how could I have missed that? Still, as you say, he's got a wood burner and fossil power backup.
    Posted By: Sigaldryi.e. your house will still be there once you've sold it / shuffled off this mortal coil. You can't therefore assume that a replacement occupant will be happy to live at 15.5°C ( just above the point of potential condensation issues).
    Indeed. And I would not be happy to live at 15.5 °C either - might have to put up with it for a few weeks for the first winter or two as I learn what's needed and get it all sorted out but definitely not a long term plan.
    There's nothing to stop you assuming a dual electric immersion cylinder, with solar hot water input, nor to stop you putting an electric room heater as your main heating, …
    Yep, that was plan A - just that the SAP lady (or somebody up the food chain) seems to want to determine whether the PV could actually provide that.
    A knowledgeable assessor and a open minded Building Standards officer should be able to sort it.
    Again, yes. But being armed with a few hints to nudge them in the right direction can't be a bad thing.
  • None of the regulations require an SAP assessment. The regulations require you to build a reasonably efficient property.

    -The secretary of state will accept a property meeting the TER as assessed by SAP as being reasonably efficient.

    -A Passive House is also reasonably efficient and some more enlightened local authorities will accept this.

    -Technically you are also free to provide your own evidence that the property is reasonably efficient, but good luck in your endeavours without a brand name behind you.

    In the absence of a formal heating system, SAP assumes electric fan heaters and instantaneous electric hot water. Provided DER
  • Is there a requirement for space heating in building regulations?

    There is for hot water but not AFAIK for space heating.
  • Posted By: markocosicNone of the regulations require an SAP assessment.
    Standard 6.1, which I quoted above, seems to in most cases (this is not England). AFAIK SAP is the only method approved for regulation 7a of the EPC.
    Throw a little Honda genny into the mix. You've now got unlimited electricity at a 'known' carbon intensity.
    Good point. Yes, I have thought that I might well finish up with one if only for providing site power during construction but hadn't really thought that its emissions would be low enough. Could be, I suppose.
    Is there a requirement for space heating in building regulations?
    In effect, yes:
    3.13.1Every dwelling should have some form of fixed heating system, or alternative that is capable of maintaining a temperature of 21°C in at least 1 apartment and 18°C elsewhere, when the outside temperature is minus 1°C.
    The following section covers systems like my proposal so, in principle, things should be fine - the question that looks like being resolved after a flurry of email to BRE and the like is how to convince the SAP software to allow it.
  • Ed, the Scottish tech standards procedures are designed not to be too prescriptive. You can submit what is known as an 'alternative approach' application if you feel feel that what you are proposing meets the spirit of the regulation in a way not specifically mentioned in the handbook.

    I have used this method for fire escape provisions on buildings and backed up the application with a design statement.

    The BCO was happy to accept this in the end and was more than happy to discuss along the way.

    Having dealt with Highland Council in the past they are one of the most approachable authorities, providing you are speaking to someone with enough authority to make a decision about alternative approach. (Junior staff members will invariably fall back on the crutch of the regs).

    My advice would be get yourself an appointment with the senior guy in Wick (I'm guessing that's your closest office) and chat through the problem - arm yourself with all the research and info you have.

    You have everything to gain and nothing lose from speaking to them.
  • Hairydude, yes, Highland Council have, so far, been pretty good to me. My pre-application enquiry and actual planning application were dealt with about as quickly as the various consultations and so on allowed and in a helpful and positive way. The roads department were also very sensible and helpful with sorting out my road opening permit for access to the site.

    As you say, there's lots of room in the standards for alternative approaches. The last section of:

    is quite explicit about that for heating.

    What it's less clear about is how a SAP rating should be done when such alternatives are used. As I mentioned above, I'm no longer convinced the house actually needs a SAP rating at all but for the moment I'm leaving the SAP assessor and her food chain to mull it over while BC look at the rest of the building warrant application. If they can't come up with a SAP assessment that fills the bureaucratic gap then, yes, the next step would be a talk with the senior guy in Wick.
  • Typically, If you'll be selling or renting it, then you need an EPC and to get an EPC, you need a SAP. The EPC is a report produced from SAP, It's not something separate.

    Most Building Control's / Building Standards agencies require an EPC before they'll sign off on completion.


    "Following completion of the proposed building, the Energy Performance Certificate which is to be submitted to the verifier with the completion certificate, should be produced using approved software which implements the current edition and version of the relevant calculation methodology. This ensures that building owners are presented with the most current and relevant information practicable."

    The energy assessment (SAP) if correctly entered gives you an idea of the performance of the property, the SAP worksheet can help you to ascertain losses, gains and allows you to compare options - it's not *just* a bureacratic hindrance (It's not a design tool, but that's often how it's used).

    SAP2012 allows more flexibility in how things are entered, but SAP2009 is still the current for Scotland for another year or so.
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