Green Building Forum - Home Energy Model

Home Energy Model

djh — Fri, 15 Dec 2023 09:32:39 +0000

Will in Aberdeen spotted the Home Energy Model initiative: https://www.gov.uk/government/consultations/home-energy-model-replacement-for-the-standard-assessment-procedure-sap but can't start an appropriate thread. So here is one.

Home Energy Model

djh — Fri, 15 Dec 2023 09:37:19 +0000

Posted By: WillInAberdeenBut problems are still there:

- software has to cope with design phase where key inputs are not known yet, like airtightness results, shading, occupancy for new insulated homes where solar/incidental gains make big difference to whether heating is needed or not.

Every modelling system for new builds has to deal with that issue; it's the nature of the beast. Shading and solar gains are indeed important but they are known in advance so aren't a problem. Occupancy and airtightness are dealt with in PHPP by standardisation and requirement respectively; I expect HEM could do similar - I haven't looked yet.

Home Energy Model

WillInAberdeen — Sat, 16 Dec 2023 12:03:23 +0000

Thank you!

I said previously:
SAP is to be abandoned and replaced by something called the Home Energy Model... The new Home Energy Model software that is going to replace SAP is apparently more detailed and has been tested against PHPP..

But problems are still there:

- software has to cope with design phase where key inputs are not known yet, like airtightness results, shading, occupancy for new insulated homes where solar/incidental gains make big difference to whether heating is needed or not.

- same software has to cope with EPCs for old houses where key inputs are not visible, like AT or insulation buried in walls or under floors

Apparently they tested the new software against real world data measured from a passivehouse, it did ok until the occupant started closing the blinds, so keeping the sun out.

Elsewhere they said that SAP10, PHPP and Home Energy Model performed pretty much the same, when fed with the same standardising assumptions. However they all have different default assumptions built in, which they say explains the differences seen between SAP and PHPP.

Home Energy Model apparently doesn't yet deal with blinds and curtains, maybe to be added later (as they are on many houses!).

One change is that Home Energy Model makes energy balances over half-hour slots, versus SAP which balanced per month. So daytime PV will be less useful, as cannot be used so easily to offset evening electricity use. Thermal mass will be more important to model correctly to carry daytime gains into the evening.

They also seem to have increased the Primary Energy Factor of grid electricity from 1.5 to nearly 2, which is a penalty on choosing direct electric heat or DHW, compared to gas and coal at 1.1. However the grid CO2 intensity has been halved, and when heatpump CoP is applied on top of that in the notional house spec it sets an ultra low TEr, pretty much impossible to meet with fossil or direct electric heat or DHW.

Home Energy Model

borpin — Sat, 16 Dec 2023 15:05:49 +0000

Posted By: WillInAberdeenOne change is that Home Energy Model makes energy balances over half-hour slots, versus SAP which balanced per month. So daytime PV will be less useful, as cannot be used so easily to offset evening electricity use.

Batteries of course solve that!

Posted By: WillInAberdeenHome Energy Model apparently doesn't yet deal with blinds and curtains, maybe to be added later (as they are on many houses!).

But that is not simple as (for instance) it depends on the aspect of the house. If the 'public' rooms are on the North, then passive solar will have less impact on the rooms often heated the most.

Anything that depends on the sun/passive gain is always fraught with danger.

For a mass builder, with a standard house design, that could be sited in any orientation, something like PHPP will never work.

[edit]
There is also a thread running on Twitter of a CFSH5 certified (I think) flat that has so many flaws in the insulation is is a struggle to heat. It might be airtight, but if the insulation behind the AT membrane is faulty, it will still be cold (as shown by the TI images).

Home Energy Model

djh — Sat, 16 Dec 2023 15:49:11 +0000

Posted By: borpinBatteries of course solve that!

But at a cost. Both monetary and embodied carbon.

For a mass builder, with a standard house design, that could be sited in any orientation, something like PHPP will never work.

Then such a builder needs to adapt to the real world and start to design buildings that bear the orientation in mind. Much as there is already pressure to orient roofs to have north and south slopes for PV.

I'd be interested in the twitter thread or links to any external references it contains that explains how it came to be certified with such defects. Where is the flat? And who certified it? And so on.

Home Energy Model

WillInAberdeen — Sat, 16 Dec 2023 17:40:08 +0000

>>>"Both monetary and embodied carbon."

Home Energy Model doesn't consider capital cost, or embodied carbon, neither do SAP or PHPP.

The question is: why don't they?

In particular, the carbon from heating under the HEM input factors, has reduced to a very low level, so the focus on carbon impact of buildings needs to shift to include embodied carbon.

Building regulations are going to have to start regulating embodied carbon, as are private standards such as PH and AECB. To do that they're going to need a standardised software model to compare the embodied carbon of different designs against a standard. The new HEM software should (eventually) be the single point software, we don't want two incompatible software products for regulating energy carbon and embodied carbon.

Example based on LETI primer and HEM assumptions:

Carbon from heating over building lifetime 35kWhth/m²/a ÷ COP3 x 0.09kg/kWhe grid intensity x 100 years life = 100kg carbon per m²

Carbon embodied from construction - now typically 1000kg per m², 2030 best practice target 300kg/m²

-> embodied carbon is 3x to 10x more significant than heat losses.

Home Energy Model

borpin — Sat, 16 Dec 2023 17:57:55 +0000

Posted By: djhBut at a cost.

Everything has a cost. I simply pointed out a solution to the problem presented.

Posted By: djhI'd be interested in the twitter thread

Went back and it wasn't passive but CFSH5 and supposed to be Zero Carbon. https://twitter.com/jimmybb/status/1733051913027949027

Posted By: WillInAberdeenHome Energy Model doesn't consider capital cost, or embodied carbon,

My bug bear. Why do we still use brick and block skins? Because Planners very often demand it. I've said it before, if planners had been around longer, we'd still be in mud huts.

Home Energy Model

djh — Sat, 16 Dec 2023 18:31:09 +0000

It's unreasonable to expect an energy model to consider costs. It is reasonable for a designer or client to consider costs, and in particular buying on-site batteries to apparently shift PV generation time may very well be not justified in many circumstances at present. So they are an alternative rather than a solution. The energy should allow for batteries being present in the model in so much as they change the values it does care about.

It's similar to the heat leakage from a hot water cylinder contributing to the warming of a building.

Posted By: borpinWent back and it wasn't passive but CFSH5 and supposed to be Zero Carbon. https://twitter.com/jimmybb/status/1733051913027949027

Whew So it's another demonstration of why PHPP certification is valuable, rather than some ramshackle gov.uk scheme

In a PH the internal surface temperature should be at least 17°C, which is checked in the design by the materials and components used, and that it was built to design by the certifier using photographs and other records.

Home Energy Model

djh — Sat, 16 Dec 2023 18:33:49 +0000

My bugbear is that PHPP should be allowed as an alternative model, or preferably adopted instead of developing yet another UK-only model. How they account for pre-existing buildings is a whole different kettle of fish whichever way they do it.

Home Energy Model

WillInAberdeen — Sat, 16 Dec 2023 20:03:47 +0000

Home Energy Model will apparent allow both Heat batteries and Electrical batteries and neither require PV to be fitted. So you can import off-peak electricity, or self generate, and use it later. I think storing your PV will be very favourable to your score in HEM, because the Primary Energy Factor for consuming electricity yourself (1.97) is set much higher than the PEF for exporting PV (-1.0) and reimporting it later. So using a battery would score highly, and allow you to score less highly in other areas (insulation etc) and still pass.

If HEM calculated the embodied carbon of the PV and batteries (it doesn't) then it would be easy for the designer to run the same house design with and without batteries, and check which has the lowest net carbon.

Likewise the designer could run the house with and without extra layers of polystyrene insulation, or concrete thermal mass, and see which thickness has the lowest net impact. As it is, HEM will favour infinite thicknesses of both.

Home Energy Model

WillInAberdeen — Sat, 16 Dec 2023 21:32:42 +0000

Edit to add - PHPP works the same, but more so afaik, so storing and self consuming PV (primary energy factor = 0.9 iirc) scores very much better than exporting it and reimporting it later (2.7). I don't know if PHPP explicitly recognises battery storage yet, or only heat storage in cylinders or slabs.

I'd be happy to see PHPP used for building regs compliance but can't see why PHI would want to agree to that, as gov would presumably want control to tweak it.

At some point as DJH mentioned the software will have to spit out EPCs for existing houses, based on 20-minute 'surveys', so it will need a lot of robust assumptions and default values.

Home Energy Model

djh — Sun, 17 Dec 2023 16:21:12 +0000

Scoring PV consumed directly versus exported to the grid battery and then re-imported rapidly gets complicated, with all kinds of exceptions and what-ifs. At one point I think I had understood it (the PHPP version?), but I no longer can remember the details and I'm not sure I can be bothered to re-learn it. It all feels like angels dancing on a pin.

IIUC (If I understand correctly) the way HEM is organised is as a base physics model, with various wrappers around it to incorporate policy (or political) judgments. PV export scoring ought to be part of whichever wrapper IMHO. I don't know whether it is?

Clearly the PHI would not allow the UK government to control it, which is all to the good IMHO. But I don't see that as a valid reason for the UK government (edit:) NOT to adopt it in building regs, as some in the Scottish parliament are trying to do.

Home Energy Model

fostertom — Mon, 18 Dec 2023 09:30:31 +0000

Posted By: WillInAberdeenthe focus on carbon impact of buildings needs to shift to include embodied carbon

or even make it the primary consideration - this is really the message of the moment, according to brilliant and authoritative new book
https://www.amazon.co.uk/Build-Beyond-Zero-Carbon-Smart-Architecture/dp/1642832111/ref=sr_1_1?crid=1ALNK5MEHXU0Q&keywords=build+beyond+zero&qid=1702891707&sprefix=build+beyond+zero%2Caps%2C193&sr=8-1
quite hard to swallow for all us eco-builders who've been thinking energy-in-use for so long.

Home Energy Model

WillInAberdeen — Mon, 18 Dec 2023 10:20:49 +0000

Think I've been saying that a couple of years here, not sure how well I went down!

The best time to build a passivhaus with thick insulation and thermally massive floors, was probably ten years ago, when saving heating was a very good way to save carbon. The new round of regulation for 2025+ are for a different time, when electric pumped heat has very low carbon (per the new HEM carbon intensity data) but concrete and polystyrene are still high carbon.

It's actually quite breathtaking how fast the world changed, and the draft regs have not really kept up - still requiring U values 0.1-0.2 with carbon-intensive insulation, when U= 0.5 might be the new sweet spot for lowest lifecycle carbon. Unfortunately the new HEM model is another Energy Model, not an Embodied Carbon model or a Lifecycle model.

For something like a house which will last (hopefully) 100 years, a question is whether it should be heavily optimised for the situation of the next decade (low carbon energy, high carbon materials), so risk becoming sub optimal a few decades in future (-> poss demolished like much 60s/70s high rise). If not, then what should it be? Adaptable, modifiable, extendable, perhaps.

Home Energy Model

tony — Mon, 18 Dec 2023 11:59:56 +0000

I would rather look at thing in terms of energy used/saved and the costs and make sensible decisions based on economics.

For me u-values below 0.1 for walls and ceilings/roofs and 3g as standard air tightness as Canada

Home Energy Model

borpin — Mon, 18 Dec 2023 13:22:03 +0000

Posted By: WillInAberdeenstill requiring U values 0.1-0.2 with carbon-intensive insulation, when U= 0.5 might be the new sweet spot for lowest lifecycle carbon.

I like the look of straw as the insulation material.

If I was going again (I wish) I think insulated slab, Straw insulated walls, recycled plastic 'weather board' or the panels made from Volcanic rock.

Does that offer the lowest embodied carbon footprint I wonder?

The 12M swimming pool might be a problem though...

Home Energy Model

djh — Mon, 18 Dec 2023 16:42:13 +0000

Posted By: borpinIf I was going again (I wish) I think insulated slab, Straw insulated walls, recycled plastic 'weather board' or the panels made from Volcanic rock.

The reason for lime render has a lot to do with fire resistance. But certainly Ecococon panels or similar look interesting. I'd also look again at a timber floor structure if I was building again, although a lot depends on the site. Looking at the heave I see in our garden, I'm glad our floor is pretty solid!