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Will our homes ever be more energy efficient?

Yes: If you buy a new home in a few years time the chances are it will be built to the forthcoming Future Homes Standard 2025 and should be much more energy efficient, all electric and have the latest energy tech included.



UK homes are draughty and poorly insulated. We even install windows with leaks included (aka ‘trickle vents’). Ventilation is of course required or we’ll just asphyxiate in homes built to PassivHaus (or Passive House - but it sounds better in German) standards which is why Mechanical Ventilation with Heat Recovery (MVHR) products are a good idea (more on this below).

If you didn’t know it, your internal doors are supposed to have a 10mm gap at the bottom for ventilation - that’s a feature not a bug! Mine don’t and I don’t fancy cutting them off though.


FHS-25

In 2019 the UK government outlined the requirements for new build standards - referred to as the Future Home Standard 2025 (FHS25) - with the objective of new homes’ carbon emissions reduced by at least 75% compared to 2013 building regulations through better insulation, all-electric systems and higher air tightness (technically known as Parts L, energy consumption, and F, ventilation, of the UK building regulations). At the same time an interim update to Part L was published and came into force in 2022 (31% less carbon emissions compared to 2013).


75% emissions reduction compared to the 2013 baseline refers to both the end of gas boilers and the increased energy efficiency due to insulation and air-tightness and hence lower carbon emissions if the grid itself were still at 2013 levels of carbon emissions.


In late 2022 I, along with 170 other industry experts, contributed to the Future Homes Hub FHS25 input to the government consultation happening this year (2023). Look hard enough and you’ll find me wearing a sling a week after I broke my collarbone mountain biking.


FHS25 won’t be finalised till 2024 but in 2019 the expectation was set as:

  • All electric including heating by heat pumps.

  • Gas ban in all newbuild from 2025.

  • Zero carbon ready (meaning as these are all-electric homes then zero emissions once the grid is zero emissions without any additional changes to the home).

  • Higher insulation (reduction in the U-Value of floors, walls, doors/window, roofs).

  • Triple glazing.

  • Greater air-tightness (air permeability below 5m3 / h.m2) but no mention of MVHRs; instead relying on existing specifications for kitchen and bathroom extractor fans.

  • Oddly no mention of solar panels or home battery storage even though solar is considered for 2021 spec newbuild.

To specify heat pumps, high air-tightness and triple glazing but to omit solar generation and MVHR might have been a reflection of the expectation of energy costs decreasing as renewables increased as the energy crisis wasn’t forecastable. An example of the impact of the unexpected. The analysis of the energy working group showed that this version of FHS25 would increase energy bills by £190 compared to the 2021 Part L version as that includes solar on all newbuild.


With the overarching objective to reduce carbon emissions by at least 75% and in fact be net-zero ready once the grid is fully decarbonised a set of 5 different Contender Specifications (CS1 to 5 in the table image) were defined and analysed for the effectiveness and feasibility to deploy at scale - essentially the more tech and the higher the spec the harder the homes are to build and test to the point the extra decimal point of extra efficiency isn’t justified by the extra cost and time.

Specification 1 is really just homes being built today without gas - so not a ‘Future Home Standard’ at all. Similarly 2 is pretty weak by combining waste heat recovery but at least mandates solar installation.

Specification 3 is a decent target whilst versions 4 & 5 probably exceed what’s needed based on the design requirement to be net-zero in the future although I like the inclusion of triple-glazing. For example we’re already supporting zero-bill homes when homes are designed with the maximum solar install without having to max out the insulation levels or triple-glazing. The working groups found that specification 3 reduced carbon emissions by 95% compared to 2013 homes; far more than the government’s target of 75%. Whilst a heat pump provides an efficiency of 300% to 400%, specification 3 reduces the heat demand itself by half compared to a 2021 home so the heating kWh consumption is around ⅙ of a 2021 build spec.


I’d love to hear if you have an opinion on any of these specifications.


In the rest of this article I refer only to my preferred spec, 3, as this has the right combination of tech without going over the top.


From the Future Homes Hub led work, these homes are likely to cost between 10% and 19% (there's a fair amount of disagreement amongst the builders that participated) more to build (compared to a 2021 spec home), can be built at scale, may slightly reduce the number of homes built on a large site (increased wall thickness), or mean more semi’s and terraced builds. Versions 4 & 5 have a greater impact causing lots more build-scalability issues and space requirements.


The process monitoring of build quality means increased skills will be required - air permeability has to be measured during the build and up-skilling needed for the air-tightness and MVHR installation. All feasible and proven in apartment blocks but still a change for the housebuilder industry. That cost is partly offset by savings from not installing any gas infrastructure and potentially a lower spec electric grid infrastructure.


For the first time there could also be a homebuilding standard that addresses the differences between detached, semi, mid-terraced, apartment types of homes instead of assuming all archetypes have identical energy efficiency - i.e. the performance of a home varies considerably which needs to be taken into account in the design spec.


There’s three main aspects to the #3 specification:

  • Insulation

  • Air-tightness

  • All-electric


U What?

Ok, it’s a given: Insulation makes a home more energy efficient, but here’s the science.


The energy required to heat a home is actually calculated from the heat lost through the floor, walls, windows, roofs, etc. The calculation depends on the materials (glass, brick, insulation, concrete, wood, etc), thickness of those materials, area of the material (a wall 3m by 1.8m for example) and the designed difference in temperature (e.g. 21 inside and -5 outside). That heat loss calculation determines the size of radiators (by water temperature) to meet that heat loss.

The calculation combines all the different materials per room (floor, walls, etc) into one heat-loss requirement. Once repeated for the whole home you have the whole house heating requirement.


k comes before R

Each material (brick, wood, glass, etc) has a different ability to resist heat transfer through the material - for the same thickness a sheet of wood is about 5 times better at resisting heat than brick. This is the R-value of a material and the higher the R the more resistance against losing heat (i.e. higher values are a good thing).


To measure resistance we need to start with thermal conductivity of a material (known as k - note it’s lowercase) which is the measurement of energy in Watts (similar to electricity measured as Watts of energy) that can flow through a metre of material for each degree of temperature difference often based on the Kelvin (K) scale rather than centigrade or fahrenheit; hence we get k measured as W/mK (Watts of energy per metre of material per degree of difference in temperature between the two sides).


Brick material (meaning a whole cubic metre of bricks, not an individual brick) has a thermal conductivity of k = 0.77 W/mK (although this varies by brick material and density) - meaning 0.77 Watts of energy can pass through a metre of brick-material for each degree of difference between each side.


But we don’t build brick walls a metre thick; rather a brick wall (just the visible outer part of a cavity wall) is around 10cm, or 0.1m. The R value (resistance) of a brick wall is therefore 0.1 / 0.77 = 0.13. In units this is 0.1m / 0.77 W/mK which can be rewritten as 0.13 m2.K/W.


And back to U

Although I’ve covered R values, building materials are supplied with a documented Heat Transfer Coefficient, U, which is the reciprocal of R (i.e. 1/R) plus the resistance of the interface (there’s an extra resistance such as that between the air and the wall material, or where two wall materials bridge) and any convection and radiation losses. All building materials therefore have declared U values which must conform to various British Standards - BRE and CIBSE are good sources of info - and buildings must be built to overall U value requirements for floor, walls, roofs.

The higher the R value the better, or, conversely, the lower the U value the better given it’s the reciprocal.

The U value measurement is therefore W/m2.K - Watts of energy that can transfer through the material(s) per square metre of wall/floor/roof per degree of difference between inside and outside temperatures. Or in other words the higher the Watts transferred through the wall per m2 area of wall per degree of temperature then obviously the higher the heating need.


The 2021 regulations for new build are:

Wall: 0.18 W/m2.K (0.35 from 2013)

Roof: 0.11 W/m2.K (0.25 from 2013)

Floor: 0.13 W/m2.K (0.25 from 2013)

Window: 1.2 W/m2.K (2.2 from 2013)

Door: 1.0 W/m2.K (2.2 from 2013)


And for FHS25 version 3 (my preference) the targets are:

Wall: 0.15 W/m2.K

Roof: 0.11 W/m2.K

Floor: 0.11 W/m2.K


What does air permeability mean?

If you have a sealed box tightly shut then you’ve got zero air flow. Air permeability is the measurement of how air flows (well, leaks) from the property such as gaps between floorboards, round window/door seals, joins in building materials, pipework and electrical fittings, loft hatches, etc (but not including controlled ventilation such as trickle vents).


The measurement is the cubic metres of air flow per hour per square metre of floor area at 50Pa (Pascals - meaning air pressure).


Our homes are draughty - properties built from 2013 should have an airflow less than 15m3/h.m2 so older homes will likely have more loss than that. Part L 2021 requires only 5m3/h.m2.


The FHS25 version 3 expectation is 3m3/h.m2 at 50Pa (atmospheric pressure).


That means for a home with a floor area of 100m2:

  • 36,000m3 per day for a home built in 2013

  • 12,000 m3 per day for a home built in 2022

  • 7,200 m3 per day for a home built in 2025 (likely FHS-25 level)

That’s an 80% reduction from 2013 to 2025.


Air flow is measured by directing a large fan through the front door (carefully sealed in place) and using measurement equipment to detect the flow loss. It’s likely that the FHS-25 standard will require this to be measured during build and on completion and signed-off for each house.


To achieve such a significant reduction in flow some form of impermeable wrap is installed between the inner and outer walls, floors, ceiling/roof and all windows, doors, electrical points, pipework, etc with all joints firmly taped. Literally it aims to create as airtight a bubble, sandwiched within the property fabric, as possible. Creating this bubble is hard but it makes a significant difference to the energy efficiency. Tricky areas are the interface between the floor and walls, round doors and windows and in particular around the loft hatch. The loft hatch is still likely to be the leakiest point. It’s surprising that we still have lofts rather than make use of that space and make it habitable in all new build - may that’s for the next generation of Future Home Standard.


How does an MVHR work?

Now we’ve almost hermetically sealed ourselves in the built-in-house-bubble we’re going to need to change the air with some form of ventilation system. But as soon as you start moving air out you’re taking a lot of heat too so MVHR systems are designed to keep that heat whilst also ventilating the property - hence the term Mechanical Ventilation with Heat Recovery.

The MVHR unit extracts air via vents in each room (but particularly bathrooms and kitchen areas) and passes this through a heat-exchanger to warm the flow of fresh incoming air so you have ventilation but with much less loss of heat. Unfortunately MVHR systems are really hard to retrofit though as the home needs really good airtightness and there’s lots of ductwork to install. Hence including them in newbuild is a really good idea.


The FHS25 version 3 expectation is to include an MVHR in all new build. When the working groups compared version 2 and 3 MVHRs accounted for half the energy saving.


How does WWHR work?

As with the MVHR that recovers heat from ventilation it’s obvious there’s wasted heat from hot water - washing, showers, baths, etc.

Unfortunately WWHR technology is still in its infancy when we consider it only applies to showers and only then has an efficiency of 55%. Ideally we need solutions that can recover heat from all hot water waste, store that energy and make it available for later. Current WWHR systems extract heat from the shower drainpipe to pre-warm the incoming cold water to the shower so only start to have an effect on the shower after a minute or so and obviously only whilst having a shower.


Even so the FHS25 version 3 expectation is to include an WWHR in all new build as that increases the hot water utilisation.


Electrifying

FHS-25 is likely to specify:

  • Air Source Heat Pump, district heating or ground source heat pump.

  • MVHR - see earlier

  • WWHR - see earlier too

  • Solar plus battery

The combined spec of insulation, air permeability and electrification will see energy bills around £360 less per year (end terrace example - the full study has the saving for each archetype) based on electricity rates in October 2022 compared to a home built to 2021 version. And this is before any benefit from smart tariffs and flexibility participation.

There’s no expectation of a home EV charger although in likelihood if a home has a driveway this would be included too. EV chargers are already mandated to be online and have default profiles not to charge in the peak evening period. As with the BEIS IOT Security project we’re likely to see more online connectivity of heating and solar-battery systems in order for them to be ready to take part in the growing flexibility markets. Ventilation and waste water heat recovery is unlikely to become online as there’s very little benefit to being online.


Give me more?

Specifications 4 & 5 go to and past the PassivHaus extreme. Less than 2,000 homes have been certified as meeting PassivHaus in the UK and only 2,000 are built a year across Europe. And that’s because it’s hard - much harder. Walls have cavities around 230mm, doors & windows are all triple glazed and the air permeability is down to 1m3/h.m2.

Specification 5 does away with the need for heating altogether although an auxiliary heating unit is included with the MVHR for times the home is empty for several days. Specification 5 ultimately exports more energy so results in a negative energy bill. Interestingly the cost increase is only 17% from a 2021 home (compared to 10 to 19% increase for specification 3) due to not installing the heating system.


These two specifications are unlikely to make it into FHS-25 as the build process, build control, testing and commissioning are probably far too hard for the large scale builders to cope with - as commented in the Future Homes Hub report. However the alternative is much greater numbers of factory-built homes rather than the traditional on-site build up.


There is a concern that super-sealed homes mean more homogeneity of design (bay windows, dormer windows, risk creating insulation and air permeability problems) but the many smaller architects and builders are adept at building great ranges of designs and this is probably more en economic efficiency issue for large builders developing sites of hundreds of cookie-cutter homes (I’m not a fan; and the need for scale isn’t an excuse). Aesthetic and highly variable design across an estate is essential.


What’SAP

Energy efficiency is measured according to the Standard Assessment Procedure (SAP) which is maintained by the Buildings Research Establishment (BRE). The result is a score out of 100 - the higher the score the better - based on all the construction detail I’ve written about above (thermal performance, air permeability, etc).


From the Future Homes Hub report 40% of us know our home’s Energy Performance Certificate (EPC) is ‘very important’ but it’s not obvious that as well as the red/amber/green rating and A to G scale EPC certificates also show this SAP score. Sadly our own house is a D and scores only 66 but that’s prior to loft insulation and our air source heat pump install and removal of gas supply.

Every home needs an EPC when it’s sold or rented and they’re all publicly available - always worth looking your own up. If you own a house it might help figure out what work you could do to improve its efficiency. If you rent then you can check it’s still valid; they expire after ten years and the rental can’t be renewed if it’s out of date or the rating is an F or G (and since 2022 a D in Scotland). In a couple of year’s time the minimum rises to C which is great news for renters.


The SAP is based on Part L of the Building Regulations and therefore needs to be updated in line with FHS-25. Currently we’re on SAP version 10.2 (well almost - there’s still issues even though it’s been in force since June 22) and the next version will be 11 which will measure properties built to FHS-25. There’s concern that the issues with 10.2 will roll forward into 11 causing a delay to FHS-25 coming into force - there’s a bit of a chick-and-egg here as FHS-25 needs to be locked in so that SAP11 can be completed and the assessment tools made available.


Retrofit me

What are the chances of retrofitting my house to FHS-25?

Fixing loft insulation is usually very easy and has a great impact on energy efficiency. Changing the loft hatch (uninsulated hatches impact the U value of the ceiling by as much as 9%) is a little trickier but still quite possible to do. Next on the list is windows and doors - more disruptive, more costly but if you’ve got single-glazed then very worth doing. Installing an air source heat pump, benefiting the Boiler Upgrade Scheme, means the cost is getting close to replacing an ageing gas boiler if it’s coming to the end of it’s life. And installing solar and battery although more expensive than the ASHP (assuming BUS grant) gives the satisfaction of generating your own energy and will instantly reduce energy bills.

But improving the floor, wall, roof thermal resistance to the FHS25 standard, getting really high levels of air tightness, installing an MVHR with all the ductwork and a WWHR under the shower is all much harder and much more disruptive. There are retrofit insulation materials available but they’re costly and you’re effectively having to redecorate either the whole of the outside or all internal side of external walls.


A final thought

The action on the Future Home Standard 2025 is now with DLUHC who will issue a consultation very soon (the Future Homes Hub workshops and report are an input to this). That should result in a firm definition of FHS-25 by the end of this year and allow SAP11 to be completed. That will be followed with some sort of transitional period in 2025 before being mandated for all new build and extensions.

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