It’s fair to say that, given time and endless budget, the majority of us would prioritise improving the energy efficiency of an existing home during a major improvement project (or a new build for that matter). However, improving the thermal efficiency of a home and how it’s heated tends to come down to a question of cost-effectiveness — what measures will strike a balance between initial capital cost and future running costs, and what measures will simply prove uneconomical?

A key part of my role involves devising energy strategies for homeowners. In basic terms, an energy strategy is a detailed plan for optimising the thermal performance of the house, with the aim of lowering the heat demand and making the house a more comfortable place to live, in the first instance. Secondly it involves recommending the most suitable and cost-effective heating system for the homeowner’s budget and lifestyle.

When it comes to creating a strategy, a homeowner’s future plans are a vital consideration. No two homeowners, their lifestyles nor their priorities for a home are quite alike. For some, the aim of a renovation project is to create a forever home, while for others, it’s a stepping stone — a home for a couple of years, but a means of climbing up the next rung on the property ladder.

Here, I’ll be assessing two properties (both off mains gas) for homeowners with different priorities.

  • The first project will see a 1960s house undergo an extensive extension and remodel to become a ‘forever’ (or at least, long-term) home for a family. Future running costs are a priority and a portion of the project budget will be invested accordingly in improving the thermal efficiency of the house, as well as a new heating system utilising renewable energy.
  • The second project will see a 200-year-old cottage renovated and extended. The couple behind the project will be undertaking most of the work themselves, with the aim of renovating for profit.

Project One — the Forever Home

The clients are refurbishing and extending a 1960s three bedroom house to upgrade the accommodation and to add further bedrooms.

Property Details

  • Two storey house
  • Sits on a large, relatively flat plot with good access and has a number of outbuildings
  • There’s some 205m2 of floor space — the extended building will bring the floorspace up to 322m2
  • Currently the house is draughty, cold and difficult to heat.
  • The house is of cavity wall construction with various timber, stone and brick outer skins
  • Features a mix of flat and pitched cut roofs under tiles. The fenestration is fairly old double glazing in PVCu frames. The ground floor is concrete

Current Insulation and Heating
There is considered to be no insulation to the walls or ground floor, and only 100mm of mineral wool to the roof. An oil-fired boiler, which is around 15 years old, is the current principal heat source. The boiler has a capacity of 26kW and, as new, had an efficiency of >80%. The boiler is non-condensing and efficiency is likely to have dropped to below 60%.

The existing fuel oil consumption is said to be between £1,500 and £2,000 per year. Taking the higher figure, that would equate to some 5,000 litres (at today’s exceptionally low prices) with energy potential of 53,500kWh. Assume 60% boiler efficiency and deduct for domestic hot water (around 3-4,000kWh) and that figure then equates to a space heating energy consumption of around 142kWh/m2/yr.

Minimising Energy Consumption
This is a 1960s building that has had no significant upgrade to its insulation or airtightness since being built. Clearly then, in terms of an energy strategy, this is the place to start:

Loft:
Increasing the 100mm of mineral wool in the loft to 300mm (or 120mm PUR insulation) would reduce the U value from 0.3W/m2 to 0.14W/m2.

Walls:
The walls are currently uninsulated, with a U value of 1.16W/m2. A couple of bricks could be taken out from at least two walls at DPC (damp-proof course) level to enable an inspection of the cavity for mortar or other accumulated debris. If the cavity is clean, injecting foam insulation will reduce the U value to 0.4W/m2.

The homeowner has enquired about the use of 50mm-thick rigid foam insulation externally for the walls. Standard phenolic foam is not suitable for external use, so guidance will need to be taken from the manufacturer. In any event, at 50mm thick it will reduce the wall U value to 0.36W/m2. Using this together with cavity insulation would reduce the U value to 0.22W/m2. However, the effect would be to reduce space heating energy consumption by just 825kWh/yr — at a value of around £35 per year. It would be difficult to justify the extra cost with that level of return.

Floor:
The floor is currently uninsulated with a U value of around 1.8W/m2. There are no plans to take up the solid floor. Therefore, the only option is an insulating underlay, such as Sempafloor, or an insulating primer like Therma-Coat — or a combination of both products. They can be installed under carpet and will reduce the U value to around 0.9W/m2. More importantly it gives a warmer feel to the floor.

The floor to the extension can be fully insulated with 100mm rigid foam to achieve a U value of 0.16W/m2.

Windows and doors:
New windows will be installed as part of the project, with a U value of 1.4W/m2. New doors with an assumed U value of 1.4W/m2 are also to be installed.

The extended building will have some 104m2 of glazing. Triple glazing was discussed as some of the larger windows and doors are north facing. However, there will be more benefit in directing attention to the airtightness of the building rather than spending funds on triple glazing.

Airtightness:
This is the key issue. Given the age of the original construction it is likely to be fairly ‘leaky’. In addition, there are open fireplaces drawing heat from the house. Taken together it means that the airtightness is likely to be in excess of 30m3/hr (current Building Regulations call for no more than 10m3/hr).

The insulation measures recommended will have an impact on airtightness, but more importantly, sealing the chimneys and installing woodburning stoves will have a greater effect.

An initial airtightness test should be carried out, and a smoke stick used to find out where existing air leaks. A target of 10m3/hr would be acceptable (if not good). It might be possible to improve on that figure but it needs to be considered that at 7m3/hr a mechanical ventilation system would be needed and installing such a system is not really an option. A second test will be needed at the completion of the works and a figure of between 7m3/hr and 10m3/hr would be ideal.

It perhaps goes without saying that trickle vents will not be needed but extractor fans to the bathrooms and kitchen will be.

Heat Load and Energy Demands

Using the proposed insulation and airtightness levels, and including the planned extension, the probable energy loads are:

  • Space heating: 20,616kWh per year
  • Hot water: 3,000kWh per year
  • Lighting and power: 5,000kWh per year

Remaining with heating oil and Grid electricity the annual running costs will be:

  • Heating: £1,269 per year (at 46p/litre and assuming 80% boiler efficiency)
  • Power: £775 (at a tariff comparison rate of 15.5p/kWh)

It’s worth noting that heating oil is unusually and unrealistically low at around 40p per litre, at present. A more typical price would be in excess of 60p. The latter figure would bring heating running costs to £1,655 per year.

Renewable Energy Options
There are very few realistic options available to provide a satisfactory solution. However, the options are discussed here for the purposes of comparison:

Air-source heat pump — The size of the house and the levels of insulation possible mean that an air-source heat pump is not really a viable option. The peak heat load will be in excess of 16kW, outside the scope of a single air-source heat pump, and even if that were possible, running costs would be £1,307 — which is higher than heating oil.

Ground-source heat pump — The peak load of 16kW means that an area of at least 800m2 would be needed to install a ground-source heat pump. The space is not available on this site, so boreholes would be needed (probably three in total) the cost of which is likely to be prohibitive.

The existing heat distribution system will not support a heat pump either. Underfloor heating (UFH) can be installed to the extension, but the existing radiators will need to be replaced with high-efficiency, low-flow temperature radiators.

Capital cost, excluding boreholes, will be around £20,000, plus installation. The running cost will be in the region of £930, to include 50% of domestic hot water. The RHI (Renewable Heat Incentive) payment would be around £3,200 per year. All in all, however, this would represent a long payback period.

Biomass — Wood pellet is currently the preferred option, but they take a good deal of space. There are two potential locations, both discussed during the site meeting, and both needing the installation of a heat main and second buffer tank. The latter will, of course, add to capital cost. Budget £75 per linear metre for the heat main and around £500 for a hot water cylinder.

A bulk fuel store will also be needed. A heating load of 24,000kWh per year indicates a fuel requirement of five tonnes per year. That could happily be supplied by two deliveries each year with a bulk store capacity of maybe three tonnes. There are bulk delivery price breaks and it may be advantageous to increase this to four tonnes, but that needs to be checked with the chosen supplier.

Capital cost would be around £10,000 installed, plus VAT, and the cost of the bulk store. A three tonne bulk store will add some £5,000 to the bill. Running costs will be around £1,200 per year and the RHI payment will be around £1,710.

Solar thermal — A solar thermal array will allow the heat pump or biomass boiler to shut down over the spring and summer months and provide a useful top-up on bright autumn/winter days. Potential locations have been discussed and capital cost would be around £3,500 installed. The RHI returns will be some £400 per year.

The proposal is that the solar panels would be installed to the new flat roof. In which case, companies such as Sonnenkraft offer modules specifically for flat roof installation.

Heating Distribution
There’s a suspicion that the existing radiators are undersized and there is a good chance that they contain a ‘sludge’ build-up. The recommendation is that all the radiators be changed, but what to will depend on the heat source chosen.

Heat pumps need to run at low temperature – 45°C maximum – and the existing radiators would not support that. A biomass boiler will work happily with simple pressed steel radiators, but a heat pump will need something like the Jaga Strada low-temperature radiators.

A more sophisticated control system is recommended. Installing full zone control is not practical but programmable radiator thermostats is possible, and recommended.

Suggested Heating Strategy
A biomass boiler is a better financial option, and will suit the family’s lifestyle. Capital cost is high and will take a significant proportion of the budget, but the price of wood pellets has largely flat-lined over the past five years and there is no reason to suspect that situation will change. Clearly electricity prices will continue to rise, however. A biomass boiler with solar thermal back-up would be my recommendation.

Cost/Benefit Comparison
The running and capital cost comparison for the three viable options based on 2015 prices

Capital cost Annual running cost Incentive payment Five year cost* 10 year  cost* 20 year cost*
Heating oil £7,500 £1,269  £0 £6,345  £15,190 (¹) £32,880 (²)
Wood pellet and solar thermal £25,000 (³) £1,050 £2,000 -£4,750  -£3,500 £7,000
G.S heat pump and solar thermal £23,000 + boreholes (4) £930 £3,600 -£13,350 -£15,900 -£6,600

 * Figures reflect annual running cost and incentive payment (which runs for seven years under RHI), but does not reflect capital cost
(1) Adding capital cost for a replacement oil boiler
(2) Adding capital cost for a second replacement oil boiler
(3) Assumes standard bulk fuel store
(4) Assumes COP (coefficient of performance) of 4.0 ad 15.5p/kWh for electricity, including standing charge and VAT


 Project Two — Renovating for Profit

The homeowners are renovating a 103m2 19th-century, two-bed cottage to bring it up to modern standards and intend to add a small extension to house a third bedroom; this will likely add a further 15m2. The couple are doing most of the work themselves, but the rules remain the same and a well thought-out energy strategy will help focus their attention on the work that really needs doing, make the house more comfortable and more saleable.

Property Details

  • One and a half storeys
  • One bedroom on the first floor
  • The house sits on a large, relatively flat plot with two outbuildings but there’s relatively poor access — the cottage being at the end of a narrow lane
  • Solid stone wall construction
  • 1970s extension of cavity brickwork
  • External finish is pebble-dashed render
  • A frame roof with slate tiles
  • Old single glazing in timber frames
  • The ground floor is concrete

Current Heating and Insulation
The house is heated with electric storage heaters on Economy 7 (overnight) electricity tariff. The couple intend to sell the property in 18 to 24 months, so the capital cost of a new heating system is a big issue. There is no desire to retain the storage radiators, but equally it has been recognised that installing a wet heating system will be expensive.

There is no insulation to the walls, ground floor or roof. As a consequence the Energy Performance Certificate (EPC), produced in October 2014, gives a G rating — as low as it can get! The EPC shows a primary energy consumption (heating, hot water and lighting) of 916kWh/m2/yr, and to say that is huge would be an understatement. But, at the time of the EPC, this was a wholly uninsulated house with solely electric heating, without any form of a control system, which is as bad as energy efficiency can get.

However, examination of electricity bills and calculation indicates that the historic primary energy consumption rate is closer to 120kWh/m2/yr. It is entirely unclear how the figure of 916kWh/m2/yr is arrived at on the EPC. The stated space heating energy consumption is 27,997kWh/yr, but the calculated heat loss actually shows 10,692kWh, or 104kWh/m2/yr. This is still very high by today’s standard but perhaps less misleading than the figures given on the EPC.

On that basis I will use the calculated figure rather than the EPC figure, and extrapolate to include the extension (as no drawings or specification are currently available) and arrive at a total space heating energy demand of 12,250kWh/yr.

Minimising Energy Consumption
This is a cottage that has no insulation or airtightness, so both need to be investigated. My suggestions are:

Loft:
There is no available loft space as the first floor bedroom has a vaulted ceiling. There are 100mm-thick rafters supported by large, attractive timber purlins and A frames. The recommendation is to install 50mm PUR insulation between the rafters and a further 70mm below the rafters. This will leave the purlins and A Frame exposed and provide 120mm PUR, giving a U value from 0.15W/m2. The flat roof section over the proposed extension will have the same level of insulation.

Walls:
The solid stone walls are currently uninsulated, with a U value of 1.9W/m2. The internal plaster is in good condition, as is the external render. The clients are reluctant to remove either, for obvious cost reasons. Installing either external or internal insulation would, at best, reduce the U value to 0.53W/m2, producing an overall energy saving of around 3,900kWh/yr at a value of £214 per year (assuming natural gas could be installed). To achieve that will mean hacking off either the internal plaster or the external render, installing the insulation and replastering or re-rendering. The cost is difficult to estimate but it may be difficult for the energy saving to justify that level of capital expenditure.

Floor:
The floor is currently uninsulated with a U value of around 1.8W/m2. There are no plans to take the floor up, therefore the only option, as with our first property, is an insulating underlay, such as Sempafloor, or an insulating primer like Therma-Coat. Either or both can be installed and will reduce the U value to around 0.9W/m2.

The floor to the proposed extension can be fully insulated with 100mm rigid foam, and achieve a U value of 0.16W/m2.

Windows and doors:
The existing softwood window frames are all in good repair, although a couple of the casements need attention. Installing replacement double-glazed PVCu windows would be the usual option but not in keeping with the property. In this case retaining the frames, repairing the casements and fitting double-glazed units will give a U value of 1.6W/m2 and reduce the cost by at least half. The existing doors are in good repair and can remain (assumed U value 2.1W/m2).

Airtightness:
Usually a key issue but, given the size, age and type of property, in this case it does not have the same significance. The intention is to install a woodburning stove to the existing fireplace. This is all that can reasonably be done.

Heat Load and Energy Demands
Using the proposed insulation and including the planned extension, the probable energy loads are:

  • Existing space heating will be reduced from 12,250kWh, down to 9,800kWh per year
  • Hot water: 3,000kWh per year
  • Lighting and power: 4,000kWh per year

Using heating oil and Grid electricity, the annual running costs will be:

  • Heating: £674 (at 40p per litre and assuming 80% boiler efficiency. Again, a more typical price would be in excess of 60p. The later figure would bring costs to £720 per year)
  • Power: £620 (at a tariff comparison rate of 15.5p/kWh)

Renewable Energy Options
In this case there are very few realistic options available to provide a satisfactory solution:

Air-source heat pump — The size of the house and the levels of insulation possible mean that an air-source heat pump would work at marginal efficiency. The peak heat load will be almost 10kW (to include domestic hot water). With a COP of 3.0 (the best that can be hoped for) running costs will be £661 per year. Capital cost would be around £8,000 installed with RHI payments of £633 per year.

Ground-source heat pump — The peak load of 10kW means that an area of at least 500m2 would be needed. That is not available so boreholes would be the only option — like our first property, the cost is prohibitively expensive.

Biomass — There is nowhere convenient for a biomass boiler and the only realistic option is a wood pellet stove boiler, located in the inglenook fireplace. Options include the Bronpi Leticia Hydro 14.3kW, Klover Bifire stove or similar.

A heating load of 12,800kh per year indicates a fuel requirement of 2.6 tonnes per year. Prices for wood pellets vary but this is likely to cost around £650 per year and RHI will provide a payment of £920 per year. Capital cost would be £4,000 to £7,000 installed plus VAT.

Solar thermal — A solar thermal array will allow the heat pump or biomass boiler to shut down over spring and summer months and provide a useful top-up on bright autumn/winter days. Potential locations have been discussed and capital cost would be around £3,500 installed and RHI returns will be some £400 per year.

Cost/Benefit Comparison
The running and capital cost comparison for the three viable options based on 2015 prices

Capital cost Annual running cost Incentive payment Five year cost 10 year cost*(¹) 20 year cost*
Heating oil  £5,500 £674 £0 £3,370 £8,540 £17,080
Wood pellet and solar thermal £9,500 (²) £650 £1,320 -£3,350 -£2,740 £3,760
G.S heat pump and solar thermal  £11,500 (³) £661 £1,033 -£1,860 -£621 £5,989
* Figures reflect annual running cost and incentive payment (which runs for seven years under RHI), but does not reflect capital cost
(1) Adding capital cost for a replacement oil boiler
(2) Assumes a mid-range stove boiler
(3) Assumes  COP of 3.0 and 15.5p/kWh for electricity, including standing charge and VAT

Suggested Strategy
The planned refurbishment is a fairly major project for a young couple and has a very limited budget. In addition, the planning at this stage is to sell-on the property in 18 to 24 months. Installing low-energy (LED) lighting makes obvious sense, as does ensuring all new electrical equipment is at least A rated. So too does insulating the roof. Beyond that there is not much that can be done to reduce electricity consumption.

Illustration: David Stevens

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