# EPC calculation algorithm. Anyone know what it is ?

#### jonnyhifi

I am a landlord. I am very hands on with a Good working knowledge of engineering as well as trade skills (I am a qualified electrician and plasterer amongst other things), with a good science background and a couple of patents. But EPCs seem to be beyond me. Lol

I understand.how to calculate heat losses for buildings from first principles , what U values are , how they are related to but different to k values, how to calculate them, what thermal bridges are etc etc. None of this seems to help with understanding how an EPc is calculated as the energy assessors seem to “tick boxes” in their opaque software to come up with a number and letter result.

Eg my energy assessor - where I’ve been asking about a single glazed window I’m going to replace, and I wish ti chose one so as to improve my energy assessment number as much as possible - that windows are classified purely as “single blazed, double glazed, triple glazed, made after 2002”.

The heat loss of the window doesn’t matter. So - as the govt. wishes I have been incentivised to order a triple glazed sash window, which given the unit dimensions will perform slightly worse than a double glazed unit - but remember kids, the EPc isn’t about energy loss - it’s about regulatory control.

So my question is - what is the algorithm that is locked in the software ? Making design choices to
Minimise heat loss doesn’t necessarily result in better EpC rating (I have plenty more examples …)

Without understanding what an ePC is actually scoring - how can we build / refurbish to it ?

Does anyone know where there may be an online calculator that I can put numbers into like an assessor would do so even if I don’t know what is being calculated I can at least “pull the levers” and see how this effects things ?

There are obviously A lot of bent EPCs about (just look at blocks of flats from the 1920s for instance
And see how EPCs massively Vary on the public register between flats when you can tell from the summary that the walls are still uninsulated for instance and the glazing units are the same and the surface area the same -) I don’t want to have a bent EPC but at least I should try and hit this weird non energy related target that has been set for us to hit, but to do so I need to know what the target is.

Anyone know where I can find the algorithm / calculation explicitly laid out?

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In an assessment based on broad categories, how is the replacement of a single window going to significantly alter a rating of whole property?

This is a small property - a 1 bed flat. The units in the double glazing in the lounge have gone (condensation inside) and so I’m replacing them with triple glazed units too. So - all the windows will be at the “max” EpC wise.

I need to get a “c” for this to be lettable in some years time. At the moment it’s a d -
And obviously if the marginal cost is low -
Whenever working on an element I need to “upgrade it” . I was told the dry lining we did 10-15 years back (2 inches or so of polystyrene with battens …) doesn’t count as I don’t have an invoice from the installer. So the assumption is the walls are uninsulated (despite assessor being able to see it at the margins, where sockets are etc). I was told because I run a property company and we do our own maintenance and works - that none of it would count as only stuff done via contractors counts !!!!

I guess the energy knows if a contractor installed insulation or someone else …

I do agree with your point that windows are only one element of the whole, but taken To an extreme that would be an argument for never upgrading any element, and indeed not being worried to what specification an element is improved to. I’m wanting to understand what is meant to be measured,
And how that is added to the overall calculation and how the eventual rating is effected by these sub components.

You need to be a trained assessor to prepare an EPC, even if you got hold of the formula calculation.

You may be better off looking at the energy assessment methods used in Part L1, Building Regulations. This is generally for building works, but you can tweak the inputs and scenarios to see what effects the outcome. This won't count as an EPC, but will let you see what changes get the best benefits.

I am a landlord. I am very hands on with a Good working knowledge of engineering as well as trade skills (I am a qualified electrician and plasterer amongst other things), with a good science background and a couple of patents. But EPCs seem to be beyond me. Lol
>>Snip<<
Anyone know where I can find the algorithm / calculation explicitly laid out?
This is a small property - a 1 bed flat. The units in the double glazing in the lounge have gone (condensation inside) and so I’m replacing them with triple glazed units too. So - all the windows will be at the “max” EpC wise.

I need to get a “c” for this to be lettable in some years time. At the moment it’s a d -
And obviously if the marginal cost is low -
Whenever working on an element I need to “upgrade it” . I was told the dry lining we did 10-15 years back (2 inches or so of polystyrene with battens …) doesn’t count as I don’t have an invoice from the installer. So the assumption is the walls are uninsulated (despite assessor being able to see it at the margins, where sockets are etc). I was told because I run a property company and we do our own maintenance and works - that none of it would count as only stuff done via contractors counts !!!!

I guess the energy knows if a contractor installed insulation or someone else …

I do agree with your point that windows are only one element of the whole, but taken To an extreme that would be an argument for never upgrading any element, and indeed not being worried to what specification an element is improved to. I’m wanting to understand what is meant to be measured,
And how that is added to the overall calculation and how the eventual rating is effected by these sub components.

My sympathies. EPC's are, in my experience, an evolving, changing black art never to be known to the (ripped off) Great British customer. I guess I was lucky in that I found a local assessor who was amenable, friendly and helpful.
Any work that is carried out should be counted if you can show receipts - even just for materials and photo's of the various stages; I did and they were accepted. What disappointed me was that in twelve months secondary glazing was counted on the first assessment but discounted on the second one.
Internal insulation is not difficult, AFAIK just have to prove you do not have any missing spots. It sounds to me that the assessor you chose is being a PIA rather than helpful.

I found a program on the web some years ago that took you through determining the EPC of a building. In my opinion the whole scheme is unrealistic for any property that is not a square Box connected to gas with a boiler. Our house, insulated 9" cavity walls, double glazed, Loft insulated but built as Electric only but know with a gas connection is at best a D. We still have basic NSH as background heat source with a Balanced Flue gas fire. As a person brought up in the countryside I know that you always have two means of providing warmth in a house. Neighbour, similar house but they only have gas (central) heating is a C. Their bills are higher than mine but the EPC is supposed to give an idea on the heating costs for a house.

Yes your story is similar to mine. Sadly the Epc does not indicate energy usage. Instead it incentivises sometimes bizarre alterations to a building - unless as you say you have the simplistic of structures a simple cube - a cartoon house. Up till now I’ve viewed this with amused detachment as it doesn’t really affect anything - however it will very soon as the government has said a residential dwelling will not be legally lettable unless it is a C or above at which point this has suddenly become a very serious business for us who are reputable landlords . Now we have to worry about the EPC - and stop worrying about making our products (homes) energy efficient -
And desirable places to live in -
As that’s not what we’re being asked to do.

The assumption that we will have bills relating to energy works is so unrealistically simplistic. I have a 2,000 sq foot warehouse full of tools and materials as we move from project to project. I’ll never dispose of left over materials if we can use them later. There will never have been a direct 1-1 correlation between bills and materials for the flat in question - and it was done 15 years ago - and since so much time has elapsed,
As there’s been no need to keep my commercial accounts from that period- I doubt I have the audit trail anyway. Why would I ?

Re looking at building regs .

It’s a nice idea except that the regulations are framed in fundamentally different ways , so you can’t use one to second guess the other.

The EPC is not based on sound physics. Instead it’s a mish mash of “half heard account of how it might work” watered down to make the inspection performable in 45 minutes. To that end there’s no way it could ever tie up with building regs which seem to be based on sensible specifications, albeit the requirements are watered down to be palatable (Eg why aren’t we necessitated to build to passive haus standards now rather than all this green washing that the govt loves ?!?! Why aren’t double glazing units outlawed - they are no better than a non insulated cavity wall- and you’re not allowed that so why are we allowed the windows ? Everything should be (proper) triple glazed minimum.

From what I’ve seen much of the building regs are still being flouted in regards energy performance of building (though things are getting better) and we have had a history of appalling adherence over the decades in the Uk, which sticks in the gullet all the more as I’ve always cared about this stuff,
But it seems the details I’ve agonised over, over the years don’t matter a jot. (Recent anecdotes - a window fitter recently told me the assessors he’s spoken to who check for ventilated losses, first go round the new builds and tape over all the sockets / window vents etc with parcel tape - perform the test then take the tape off as otherwise none of them pass. My second is I’ve just finished a dormer conversion largely timber frame -
And worked out from the building regs that the timber frame surrounding the window isn’t sufficiently insulating (I forget the detail but I think the u value has to be .7 or .8 or below) and some simple calculations showed the only way to achieve this is to line the opening with insulation as wood is not insulating enough. Some googling - and others have worked out the same thing. The building inspector had never come across this - nor had the FENSa window guys. The building inspector to give him his due was interested and took note and said he’d remember for the future … but clearly no one fitting windows my way for building regs is doing it right - but get signed off. EpCs will accept a building control sign off that something is built to current standards - rather than assessing it. But mine where they can see insulation isn’t good enough. What flippin nonsense.)

Sorry for the rant.

Question still stands. Anyone know if the software referred to by the kind commenter above is out there somewhere ?

Thanks hive mind !

Become an assessor yourself, and then you will know!

I'm sure you would not be allowed to assess your own properties, but you would learn the top secret wizardry that the experts use.

There are two methods of producing an EPC. One is the SAP method which requires entry of a multitude of details including specific individual windows, doors, etc. That method is normally used for new builds but can also be used for existing properties that have been altered or updated over the years.

The other method (the standard one used for most existing properties) is known as RDSAP - which is 'reduced data' SAP. For that method you input the dimensional details, location, orientation etc. and the program applies the building regulation standards for the date of the build. As said above it is very broad stroke but you can change certain details such as updated insulation or double glazing. However, it remains broad stroke and will not account for individual windows.

If you really want to account for your upgraded window you could have a full SAP done. Cost is normally in the region of £400 to £500.

There are two methods of producing an EPC. One is the SAP method which requires entry of a multitude of details including specific individual windows, doors, etc. That method is normally used for new builds but can also be used for existing properties that have been altered or updated over the years.

The other method (the standard one used for most existing properties) is known as RDSAP - which is 'reduced data' SAP. For that method you input the dimensional details, location, orientation etc. and the program applies the building regulation standards for the date of the build. As said above it is very broad stroke but you can change certain details such as updated insulation or double glazing. However, it remains broad stroke and will not account for individual windows.

If you really want to account for your upgraded window you could have a full SAP done. Cost is normally in the region of £400 to £500.
Thank you Jed’s - that is truly helpful. I’ve found a day course to explain how EPCs are calculated aimed at landlords for about £100 this Friday - I’m thinking I will do that. - but your comment alone is worth lots as that gives me a way into the is conceptual maze. It sounds as though if the govt does bring in their c hurdle - it will be worth going down the path of spending money on the Rd sap rather than energy efficiency measures, to hit the target. So sad the govt has changed incentives away from energy efficiency to hitting this bizarr target.

Ive got the same problems with one of my houses 200 yr old 15" thick stone walls,decent D/G no gas so will never be a "C" . Councils/ Govenment are building the perfect storm with the housing crisis if they continue down this road. I think I will go on a cheap EPC course then self certify my own f**k them

Some of it is BS my current house got 4 stars for the heating. Back boiler!!!
The one I sold new freshly completed loft conversion all signed of but wouldn't accept insulation met standards as they couldn't check.
Same as solid wall internal insulation and floor boards loads of pictures etc but that doesn't count

The recommendations for improvement on an EPC are often ridiculous, for example:
Floor insulation (solid floor) Typical installation cost £4,000 - £6,000 Typical yearly saving £61.
Why would anyone contemplate installing that with a pay back period between 65 and 98 years?

Or we could have Solar water heating Typical installation cost £4,000 - £6,000 Typical yearly saving £36
I would have to live for another 111 to 166 years to recover the cost of that!

Ive got the same problems with one of my houses 200 yr old 15" thick stone walls,decent D/G no gas so will never be a "C" . Councils/ Govenment are building the perfect storm with the housing crisis if they continue down this road. I think I will go on a cheap EPC course then self certify my own f**k them
Well I nearly went on a one day course on Friday that was advertised for landlords to understand EPCs given by a private training Company via Eventbrite. They were very good at answering my questions about it - but in the end I didn’t do the course as i was told it was being given by a chap who trains DEA ie energy assessors - and even on their training - they won’t be told how the software calculates the results it gives, and indeed the trainer himself did not know this -
As it’s apparently within the software one can only get access to by being a registered / qualified assessor. So - that seems as canonical as I’ve been able to find so far - landlord or indeed anyone else is unable to work out how our residences perform other than pay someone to assess it. This is completely unlike an MOT for example - where the points being tested are published and we
Can all go over our vehicles prior to the test and look at them or prepare them
In Line with that. What a joke !!

They echoed what Jed’s said above that a full SAP is another way to
Proceed - but that could cost £500 … and arguably doesn’t help me out in the way I want one jot.

The people holding the course do them every 3 months or so but they said that might not happen going forward as there was apparently “rowing back” from the govt in regards introducing this mandated C or above thing for properties to be lettable.

Haven’t had a serious google session on that yet to see if that’s so or not …

But seems like the greenwashing and incompetence of how this is set up combined with the drawing realisation this isn’t workable with the amount of Victorian, 1930s etc housing stock out there - might mean sense will dawn governmentally in regards this one.

I googled and the govt are apparently putting things back to 2028 for landlords with a real palm plant on forehead in regards how it’s being spun - as if with a few more years all these old properties will somehow be up to snuff. I don’t think they get it - it’s not a time barrier it’s a technical one for most people. Eg https://blog.goodlord.co/your-guide...rease-the-minimum-energy-efficiency-standards

But they can spin that they’ll hit the zero carbon govt target - for 2050 (?) and if this pc being a c thing doesn’t work it’s another govt that will deal with it. Meanwhile - landlords of old properties will be left thinking “eh”? as nothing has basically changed other than kicking this down the road a bit. What a mess !

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