How you can still benefit from the Green Homes Grant

It’s being reported that many people who wanted to take advantage of the Green Homes Grant haven’t been able to, either because of the highly restrictive requirements, lack of local installers or lack of availability of installers due to the six-month timeline for completion of projects. Heat pumps are currently sold out until January 2021 amongst the big manufacturers.

The company I previously worked for this year has been a part of the Green Homes Grant as a certified installer, and this article will help explain:

  1. each of the grant measures;
  2. what you can still do to improve your home and;
  3. how you can still save money.

I’ll review each of the primary measures below, and I invite you to discuss in the comments if you agree or disagree. This is a very long article because there are a lot of measures to run through.

The reality with this grant is that it was so unexpected that the only people who could have benefitted from this scheme were the people part-way into a deep retrofit of their home that were planning to buy a heat pump or solar thermal system anyway. For them, they have got lucky with timing this year.

For everyone else, we can only view the government greenwashing from afar, but we can improve our homes ourselves safely and effectively, by starting with the small improvements and building up.

Government incentives inflate and deflate industries quickly

The solar industry lost 35,000 jobs within weeks when the government cut the feed-in tariff in summer 2016, with no support for those newly unemployed who had trained to be a solar technician for years. It’s likely that the same jobs boom now for the heat pump, insulation and windows suppliers will also bust after spring 2021, leading to a slight deflation in the market (as it’s still quite bouyant).

Despite this, I’m sure that these Green Homes Grants suppliers are very happy to see this further government subsidy to their industries. Carbon-friendly technologies are already incentivised with reduced rate of VAT and Renewable Heat Incentive (RHI) payments, although MVHR is excluded despite being one of the most energy efficient technologies on the list.

But with increased business comes increased sales administration, a frenetic six months of work without breaks and a sudden drop-off of sales in the spring that they will have to prepare for.

It would have been far better to incentivise these low energy industries with a blanket lowering of VAT, or by subsidising for a broader criteria of homeowners but a lesser amount for each home. As it stands the lucky few stand to save thousands of pounds, but most of the country will lose out. How is that good for the general carbon footprint of the UK?

Green Homes Grant measures must not be done in isolation – or it could make the home worse and the bills higher

All of the installers on the Green Homes Grant have to be PAS2030 or MCS accredited, which allows them to apply for Trustmark status. PAS2030 is the legacy accreditation and guidance from the old Green Deal government energy incentive (which went badly) and the current ECO programme funded by energy suppliers. MCS is the accreditation put in place to ensure fair and ethical selling of renewable technologies, brought in place to stop the cowboys and doorstep salesmen. It is a good scheme.

Membership of these schemes involves teaching installers not to install energy improvements without considering the unintended consequences. A home is a complex system, and improving one part of it can cause detrimental issues with the other parts.

What’s more, by its omission in the measures the Green Homes Grant subtly disregards the importance of improving ventilation alongside improvements in the building fabric and heating system. If ventilation is not improved then the home may have issues with condensation, mould and stale indoor air. An installer needs to be able to advise on this.

Solid wall insulation (internal or external)

External wall insulation (EWI), done properly, is probably the lowest risk option for improving thermal performance, as it’s keeping all of the building fabric wrapped in a nice, warm blanket. And if the bricks, timbers and steels are warm, they are less likely to rot.

However, as one wraps the building in insulation it is making it more airtight. Ventilation must be improved in lockstep to prevent stale, damp air, particularly if windows and doors are also updated with modern airtight versions as part of the secondary measure.

Generally, EWI is unlikely to be a popular option, as it can only really be done on detached homes, and it requires significant remedial action such as moving the roof outwards, the gutters, the gas and electric boxes, etc.

Internal wall insulation

Internal wall insulation is actually a very good option for older homes, but it must be done carefully.

To keep it simple, one would remove the old lime plaster back to the brick and replace it with a new build-up of fresh lime plaster as a bonding/leveller, a layer of 40mm woodfibre insulation and a final coat of finishing plaster. With these natural products the external walls will be able to “breathe”, holding or releasing water vapour should it be needed.

Phenolic insulation (the rigid yellow types with the foil backing) is often used, but I personally wouldn’t recommend it as it is not vapour open. That means water vapour can creep to the cold side of the insulation and sit and damage the brick wall, also known as interstitial condensation.

Foil-backed foam insulation

The challenge is that Part L1B: Conservation of fuel and power in existing dwellings demands such a high thermal performance from the renovated wall and loft insulation that you are forced to use rigid foam insulation, despite the interstitial moisture risks, so it must be installed very carefully.

Rigid foam insulation is almost too thermally efficient as it will make the external brickwork really cold, stopping it from warming with the heat of the home and pushing out any moisture. If the brick is wet and cold the mortar may fracture over time as the water trapped inside freezes and thaws (“spalling”). Brick homes have survived for hundreds of years by being gently warmed by the heat of the home – ie, the central heating is heating the building as well as the occupants, and removing that heat by trying to meet L1B puts those healthy walls at risk.

Cavity wall insulation

Filling an empty cavity with insulation is a dangerous game. As a result a few local councils may have already taken this one off the list of allowable measures.

The issue is that if the insulation is installed in an area where driving rain can soak into the mortar, it may seep into the insulation and across to the internal wall, causing damp and structural damage.

Bridging the inner and outer brick of the cavity wall can mean that the insulation gradually dampens over the years, so at first the home will feel lovely and warm, but over the years (and as the warranty lapses) the inherent damage may reveal itself with mould and damp.

Wet insulation is useless insulation, like wearing a damp jumper in winter.

For this option, consider the weather in the area and also consider either using a brick cream to make the outer brick watertight, or some form of cladding with an air gap to keep the rain off the brick wall (essentially this creates a new cavity on the exterior of the wall).

Under-floor insulation (solid floor / suspended floor)

Again, a good option, but costly and tricky to do unless lifting the floorboards. All of these measures so far are also making the home airtight, so consider improving the ventilation in lockstep.

One new invention to the market here is a little remote controlled car that can be dropped below suspended timber floors to blow polypropelene insulation up at the underside of the floorboards. A risk factor here is that uncured blown phenolic insulation will offgas nasty chemicals (formaldehyde) that can cause serious illness when it gets warm, so make sure the chemicals are very carefully applied.

Loft insulation

If one improves the insulation at loft floor level, it makes the home very warm, but the loft very cold. The danger here is again one of moisture management. Imagine all the warm, humid air rising from the showers and cooking, passing through the new loft insulation and cooling as it enters the loft. As it cools it releases water vapour (as cold air can’t hold as much water vapour as warm air), and it either sits on the underside of the loft floor, or could condense on the rafters. This could rot the timber rafters over the years.

The answer is simple, fortunately: keep the loft well-ventilated, and ideally make the loft floor airtight as insulation is laid to stop the warm air rising. Finally, increase ventilation in the wet rooms of the home so that warm, humid air is captured by the fan/MVHR system before it can rise into the loft.

Flat roof, pitched roof and room-in-roof insulation

Pretty much the same as the loft insulation, so it must be done carefully. Make the insulation airtight by taping the joins or water vapour can creep behind and then condense on the underside of the roofing felt.

With room-in-roof insulation, be particularly careful here as typically this option means the rafters are going to be insulated, moving the condensation risk to the roof tiles. Maintaining at least a 50mm air gap between the top of the (airtight) insulation and the bottom of the roof tiles should allow moisture to be wicked away by the wind.

The following low carbon heating measures are covered by the voucher, but they must be matched to a home that has a very low heat demand, a compatible hot water cylinder and very large radiators or underfloor heating.

Air source heat pump

This should only be installed in a very energy efficient home that ideally has underfloor heating (or plans to). This is because a heat pump works most efficiently at very low temperatures, and if it has to heat up a load of cold water very fast it’ll be forced to flick on its immersion heater, which is like boiling a kettle and therefore a very expensive way to heat a lot of hot water.

With a heat pump, ensure that the controls are set so that it’s ready when hot water is needed so it’s not starting from scratch.

Finally, heat pumps are noisy, so they need to be placed externally somewhere they won’t bother the homeowner or neighbours. Realistically if the home is on the gas grid it’s difficult currently to justifying converting to a heat pump unless there is lots of outdoor room.

Ground source heat pump

Same as Air Source, but the capital costs are significant (£30,000 ex VAT and above) and a big field is needed to run the underground pipework or a borehole. If there is a massive field, now may be the only time it’s worth installing this – but again, make sure the building is already very energy efficient.

Solar thermal (liquid filled flat plate or evacuated tube collector)

A pretty good option for south-facing roof space and a water cylinder that’s compatible with a solar thermal heating coil. It probably has the most “generous” RHI payout and it’s good for the environment, but it carries the usual caveats that someone reputable needs to install it.

Biomass boiler

Maintenance and usage is fairly intensive, so I’d only investigate this if off the gas grid.

Hybrid heat pump

This combines the gas grid with electricity to maximise efficiency. That sounds amazingly clever, but from anecdotal experience in the office it’s difficult to get the hierarchy of controls right to optimise when it’ll use gas and when electricity. Again, an installer needs to very carefully design and install this system.

Secondary Green Homes Grant measures should be the ones done first

If one primary measure is installed, the voucher can be used to help cover the cost of any of the following secondary measures:

  1. draught proofing
  2. double or triple glazing (where replacing single glazing)
  3. secondary glazing (in addition to single glazing)
  4. energy efficient replacement doors (replacing single glazed or solid doors installed before 2002)
  5. hot water tank thermostat
  6. hot water tank insulation
  7. heating controls (such as, appliance thermostats, smart heating controls, zone controls, intelligent delayed start thermostat, thermostatic radiator valves)

Secondary measures can only be redeemed once a primary measure is installed, but these are the measures you should do first, as they have the largest cost-to-benefit ratio of any other improvement.

From a building physics perspective it’s a bit backward that the government would incentivise someone to install an air source heat pump before they replaced their single glazing, but that’s the way round the government has chosen to conduct this scheme.

Fortunately, you can choose it to do it the right way round, without the cold hand of government at your back.

Replacing single-glazed windows and doors for double or tripled-glazed or secondary glazed versions will make a huge difference to the home’s thermal performance and comfort. Condensation will reduce and more of the room can be used as it won’t feel so cold by the windows, gaining space in the home.

Draught proofing, lagging hot water pipes and tank, and installing heating controls such as thermostatic radiator valves (TRVs) and thermostats are easy measures to DIY – a person can learn how to bleed and balance radiators on Youtube and all it requires is a couple of cheap tools.

Once the home is airtight, improving the ventilation is the next vital step. If you’d like to hear more about the best way to ventilate a home, or receive an MVHR quotation bespoke to your home, please contact me here.

Internal wall insulation is also relatively straightforward to complete, and there are lots of videos online showing how to install woodfibre insulation safely and effectively. 40mm of woodfibre insulation costs about £100 ex VAT per m2 of wall. Loft insulation is also straightforward to install, although remember that an air barrier and/or good ventilation is needed from the rooms below.

If you have a really draughty home and want a guided airtightness strategy, you can also consider having an airtightness pressure test of your property.

Please don’t worry if you missed out on the Green Homes Grant after all – there is still so much to do to improve the average UK home.




  1. Hi. Is there absolutely no way for EWI to be done on a terrace house? I will be extending the ground and first floor of the rear elevation so no issues there. But I need a solution for the front. It is a brick wall with no cavity’s and pebble dash (which I want to cover or replace) on the 1st floor.

    • Hi Dewole,

      You can theoretically do it, but it’s all about how the EWI links to the non-EWI walls/roofs either side of the terrace. How would the newly thicker wall marry up with the neighbour’s original wall? How would the roof line be edged out but still on the same plane as the neighbouring roofs?

      The solution for the front would be Internal Wall Insulation, IWI: lime plaster on brick, with wood-fibre insulation and lime plaster skim finish. Not perfect, but good insulation qualities and breathable.

      Hope this helps.

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