As mentioned previously, heating accounts for 60% of the energy consumption of a house, with the other 40% going on all the electrical appliances, including washing machines, TV’s and computers. So before we resort to renewable sources for the electricity we use, we should minimise the energy that we need to heat our homes.

Heat loss happens in three principle ways, through the fabric of the building itself, through gaps in the fabric and through weak points. In clothing these are comparable to the jumper you wear when it’s getting chilly, the jacket you put on when it’s getting that bit windy as well, and the hat and gloves you were when it’s seriously cold. The insulation manufacturer Isover recently did a study and found out that 60% of the heat loss is through the key building elements, with 30% going to air-tightness and 10% to thermal bridging. In this way, insulation is the priority, then air-tightness and then thermal bridging.

Insulation

So the main fabric of your house needs to be insulated so that the walls, floors and roof lose less heat. The diagram below shows the heat loss from a typical house, and although very general, it gives an idea of the heat loss in a house in relation to the different elements, which gives us an idea of how to prioritise the areas which most need insulation. The greatest heat loss is through the walls, with the roof a close second, and the floor and windows being less of a priority (I have ignored doors as this is really air-tightness which is a separate issue). We can then look at a cost:return payoff for each element to see where the budget should be allocated, for instance loft insulation is very cheap and is usually straight forward to install and so is a no-brainer whereas double glazing will have a much smaller impact on heat loss.

The easy solutions

The easiest solutions is to insulate your loft which will only cost you a couple of hundred pounds.

If you have a suspended timber floor you can lay mineral wool between the joists with a net below to hold the insulation in place. This can be done for a couple of hundred pounds as well.

Cavity wall insulation is the next priority, because it will cost around £500, although you will need specialists to install it.

Double glazing can cost £5,000 for UPVc and a lot more for timber or aluminium, but these will still have a significant impact on heat loss. Even in a solid-walled flat, replacing the old windows for double glazing has a significant impact on reducing heat loss, and can make the difference between an uncomfortable environment in winter to a very warm one.

Traditional construction

Insulation is more complex when your house is a traditional construction, with solid brick or stone walls which needs internal or external wall insulation. This can cost £10-15,000 for internal insulation and £15-20,000 for eternal, and both are more intrusive. External insulation changes the look of your house, and internal insulation impacts on the use of your house. However, if they have not been carried out they will have the most significant impact on heat loss from your house.

Concrete floors are the most difficult to insulate because you have to raise the level of the ground floor. This has an impact on the internal floor level and stairs, needing steps and ramps to get around them, and all the doors will need to be altered. It would be very unusual to do this in all but the most extensive refurbishment projects.

Building regulations

The current Scottish Building regulations apply differently to a new-build house and an extension. A new-build house should achieve a u-value of 0.22 in walls, 0.18 in floors, and 0.15 in roofs, with windows achieving 1.6. In an extension, if the existing building is un-insulated, then significantly more onerous u-values apply, (0.17 for walls, 0.15 for floors & 0.11-0.13 for roofs, with windows required to achieve 1.4) whereas if the building is insulated (i.e. with cavity wall insulation etc) then the same u-values apply as for new-build. Although quite easy to achieve, the SAP calculation for a new-build house means that in practice, you will have to significantly improve on these values, or put a lot of solar panels etc on your building to bring it up to scratch, although an extension just needs to meet these minimum values.

Air tightness

Preventing air leakage from all the small gaps in a building’s construction is the next priority. This takes the form of an airtight material, which could be cast concrete, glass, plaster, plastic, or thick OSB. A concrete floor is inherently air-tight so it does not need a separate air-tight layer, but blockwork and timber are not air-tight so they do. In timber kit a plastic-type sheet is used (VCL) and in blockwork a coat of plaster is used (parge coat).

It is important that the air-tight layer is continuous, or else it fails to achieve it’s aim of air-tightness, so when a VCL is used it is important that all the sheets are taped at joints and at junctions with windows, doors, ceilings and floors. If looking at a cross section of a building, you should be able to trace the continuous line of air-tightness on the building without lifting your pen from the page.

The building regulations currently require quite a low target for air-tightness (around 10 air changes per hour) but as the SAP calculations get more onerous this is having to be improved. A typical modern house in Scotland with well-detailed air-tightness will achieve around 5 air changes per hour and flats often achieve 4. The average in England is still around 7 which is a result of the less onerous requirements. In contrast a traditionally-built houses will expect to achieve around 20 air changes per hour, which shows you just how much heated air is being lost.

Thermal bridging

The final piece of the puzzle is thermal bridging. Until recently this has not been considered a major issue, and was not essential. However, since the introduction of the 2015 building regulations we have had to pay attention to this. We cannot rely on the insulation within the timber kit any more, but need to apply a thin sheet of insulation to the inside face of the timber kit as well as into the window reveal to stop heat leaking out at the locations of solid timber structure.

This has been brought about through the gradually increasing requirements in terms of energy efficiency. Accredited details were introduced in Scotland a while ago, as a guarantee of a certain level of performance. As the technical requirements became more onerous, it became necessary to use these details, to the extent where even the most ardent speculative house builders who did their best to avoid the additional cost of the materials required, consented and started to build using these details.

The main change is the installation of a thin strip of insulation board across the whole face of the timber kit to prevent the heat passing through the solid timber structure, but all the specific details also need this insulation to be applied, i.e. the window and door heads and jambs (sides) The insulation needs to be of a certain performance, i.e. PIR rather than EPS or similar.There are similar details for block-work houses.

This additional insulation has the added bonus of preventing condensation within your house. Condensation occurs where warm, moist air comes into contact with a cold surface causing the water within the air to condense, so it is liable to occur where the structure is un-insulated, i.e. around your window and doors. A thin layer of insulation in these areas prevents this.

Mechanical Ventilation with Heat Recovery (MVHR)

This drive towards warmer and more air-tight homes does create a new issue which was unheard of in traditionally-built homes, which is that or air quality. Buildings always inherently allowed air (and heat) to pass through relatively easily, but the prevention of air movement can lead to the build-up of smells and generally stale air. This can be a particularly issue in winter when the windows are not opened. We also continually replace the Oxygen in the air around us with CO2, which needs to be allowed to escape and be replaced with oxygen rich air. Whilst there is not any danger of suffocation in a modern home, as no house can be made that air-tight, this does need to be avoided, and ideally without opening the windows and letting out all that carefully heated air!

The solution is MVHR. This extracts the warm stale air from the kitchen and bathrooms, and draws in cold dry air from outside. It passes the two streams of air very close to each other so that 90% of the heat is transferred from the outgoing to the incoming air. This can be done with over 90% efficiency with modern units. A mechanical system is not generally required unless a high level of air-tightness has been achieved, and it is generally accepted that 3 air changes is the cut-off for these being required. This is also generally viewed as the point at which MVHR becomes efficient in terms of paying for itself because if the air-tightness is any less then it is a waste to have an electric system trying to do heat exchange on extract air when it is leaking out all over the building in an uncontrolled way.

Passivhaus

The logical conclusion of this drive for preventing heat loss is building a house which is so efficient that it has no need for a central heating system at all. A German and a Swedish scientist thought that this was possible so set up a set of standards which would make central heating obsolete. This is a complex set of spreadsheets taking account of everything from the orientation of your windows, the insulation in your walls, to the size of the window frames and the amount of over-shadowing from trees. There might seem like a vast amount of data required, but the result is a mathematical model which can accurately predict the energy use of your heating system, and the internal temperature, from the coldest day in winter to the hottest day in summer. It even tells you if you will need to open the windows in summer because it gets too hot.

This is a lot more complicated than a SAP Calculation, so it is a lot more complicated than purely minimum U-values, but these are a part of it. Walls, Floors and Roofs all generally need to achieve as close to 0.10 as possible. All the details need to be designed (and mathematically modelled!) to minimise heat loss, and a stonking air-tightness of 0.6 air changes/hour needs to be achieved. A super-efficient MVHR system needs to be added as well as usually triple glazing. You also need to minimise the number of windows on the north side of the building (as these lose heat over a year) or else you will need to increase the insulation elsewhere in compensation.

A lot of these have been built, but they have mainly been in German-speaking Europe (Germany and Austria) where there are 20-30,000, and there are still only a handful in the UK, although they are getting increasingly popular. The best thing about the Passivhaus standard for us as designers is that it doesn’t enforce a particular style or construction method on the Home-owner or Architect, it just sets out a set of standards, and it is up to you how this is achieved. This is the gold standard in sustainable housing design, however it is not easy to achieve. The design team (and ideally the builder as well) need to have experience in this form of construction, because it is a complete shift in mindset. Everything is affected by the drive for heat loss. No more standard specification documents! Any consultant taking on one of these projects is likely to have to devote double the usual time to it, so that they can re-assess every part of what they do to meet the stringent requirements of the standard.

The image above is of a Passivhaus designed by Bere:Architects. It is both a Passivhaus and a Zeo Carbon house, meaning that not content with ultra-low heating bills, they also wanted to generate their own hot water and electricity, which has to make it the ultimate Eco-House!

Unfortunately all this does not come cheap. Passivhaus-accredited windows cost double what standard windows cost. The extra insulation (generally about a foot thick) is a significant expense, as well as the accredited MVHR system. All of these together mean that the cost of building to the Passivhaus standard increases the build cost by around 15%. For a £150,000 house, this would be an extra £22,500, which is not far off the cost of some renewable technologies, and the reward of a yearly fuel bill under £100 is an enviable reward! This could amount to a saving of 500/year compared a house built to current standards, which is easily the greatest saving compared to any other renewable technology. However, the downside is that it does not qualify for the RHI, which means that the payback period is not very inspiring at around 45 years. This is frustrating for those who believe that this is the future of sustainable design, and we hope that the government will change their position on this soon.

However, there are other ways to look at it which improve the apparent savings of achieving Passivhaus. Firstly, if we assume that the price of fuel continues to rise at the rate above inflation that is is at the moment (around 6% a year) then the savings will be far more enviable in 25 years. We can also consider the impact of taking the savings off the effective mortgage repayments, i.e. if that £500/year saving was spent on the mortgage, it would but you an extra £8,000 in capital borrowing across the lifetime of the mortgage, so you could take this off your effective expenditure, which reduces the £22,500 spend to £14,500, which is comparable to the other renewable technologies. However, Passivhaus won’t break down, gradually reduce in performance or suffer from a change in government funding priorities. In Germany the government offer a low interest loan to cover the additional cost of a Passivhaus, which can be paid off like a mortgage, which might encourage the take-up of the standard in the UK.

So, if I was asked my professional opinion on what is the best way to make your building sustainable, and you had a budget of £15-20,000, I would say it has to be to build a Passivhaus. It addresses all of the issues raised above and does it all to a very high standard. Unfortunately this is a significant investment for anyone, and financial limitations mean that although this is the ideal, most of us will have to settle somewhere along the way.

The first steps to getting an Eco-House are to make sure that the essentials are in place, insulation, air-tightness and thermal bridging, and if the level of air-tightness warrants it, then MVHR would be the technology to use. After this there are a variety of other things which can be done. Refer to my next post for these.

Eco-Minimalism: Going beyond the envelope