Friday, 17 July 2015

Are our homes very high energy performance yet?

The government's decision to drop the zero carbon homes standard is a serious backward step but in my view energy efficiency is a more important target than carbon emissions as such. After all, the less energy we use the less we need to worry about where we get it from. Also, with our current electricity generation mix, a carbon target favours gas heating but in the longer term our electricity will become cleaner than gas. An energy efficiency target is at least neutral on which is the most appropriate fuel. Fortunately, our building regulations have an energy efficiency target as well as a carbon emissions target and it is not too bad - but it could be a lot better.

This chart shows fabric energy losses from a standard size semi-detached home with different constructions and regulations through recent history. It starts from a Victorian solid brick house and goes through to the present day: under current regulations and what can be achieved with the Passivhaus standard.

Improvements in fabric heat loss and ventilation loss in Watts/C for a semi detached home with the same size and shape but different constructions (details in the table at the end of this post). The heat loss is not directly related to heating bills because there are other important factors:  heat gains and hot water are discussed below; internal temperature - older homes are often  less well heated - and your actual bills depend on your heating system fuel and efficiency.  Data from [1][2][3][4]

The current standard is better than before but Passivhaus is still twice as good.
Passivhaus is twice as good as current regulations, but we have already come a long way. The table at the end of this post shows the typical constructions my chart is based on. My house was originally Victorian but by insulating the walls, floor and roof and upgrading the windows I have brought it up to about 2006 level. It would be very expensive to improve it further.

At Passivhaus level, most of the space heating comes from appliances and people, but there is still heating demand for hot water.
You don't have to go to zero heat loss to get zero heating bills, because some of those losses are balanced by heat gains. These include solar gains, people and appliances. The chart below shows heating demand in January with a chilly 4°C outside and 20°C inside. Gains supply 2/3 of the Passivhaus heating. But we also need hot water, and that doesn't count for heat gains because it mostly goes down the plug and heats the sewers instead of our home. Households vary but you could easily use 1500 kWh/year for hot water, an average of 180W (not counting losses). Very efficient homes usually need more energy for hot water than for heating. Even so, the gas bill for a Passivhaus homes is typically about a fifth that for a traditional style house [6][7].
January space heating requirements (4°C outside, 20°C inside) set against typical heat gains: solar gains through windows (100W), lighting and appliances (400W), and 2 people (100W for 14 hours/day, average 120W). The Passivhaus needs very little more. Gains estimated using [5]

Some (very efficient) homes are prone to overheating, due to inadequate ventilation.
In the winter time if you get too warm you can cool down by opening the windows a little bit, but in the summer this does not work so well and it is often said that very energy efficient homes are prone to overheating. I totally agree that some of them are, but this is mainly a matter of poor design and poor regulations (see Why are we still designing buildings for yesterdays climate). Besides which, overheating is not restricted to energy efficient homes as I can vouch for personally. Our home improvements have reduced heat loss and also reduced overheating.

Research shows that overheating is most common in flats with windows on only one side so it is difficult to get cross ventilation, or where it isn't possible to open windows because of traffic noise or safety issues. This makes it difficult to cool them down overnight. During hot days, when it is warmer outside than in, opening the windows doesn't help! However, you can still do a lot with shading: using shutters, external louvres or trees (see Coping with Solar Gain). Also, you can avoid running power hungry appliances during the day to reduce the gains - run the dishwasher and do the hoovering in the cooler evening. Using energy efficient appliances will also help.

The extra costs of Passivhaus are a matter of debate

The building industry claims that tighter fabric efficiency standards are too expensive but this is a matter of debate - see Kate de Selincourt's review of the costs of Passivhaus homes. Cardiff University has just built a carbon negative home for £1000/m2 - just within the normal cost range for social housing. This doesn't necessarily meet the Passivhaus standard because it has a lot of solar panels to offset some of the heating energy but it is probably close. As more homes get built to tighter standards the costs come down because there is more competition and economies of scale.

Zero carbon homes usually means solar panels but these aren't the most cost effective renewable heating
Under the zero carbon homes standard, however little energy you use it still has some carbon emissions so you need to offset that with renewable energy. That is usually achieved with solar panels. However domestic sized systems are not as cost effective as large scale renewables, and solar panels are most effective in the summer when we don't need heating. Finally, people don't necessarily use their solar panels to best advantage. This isn't a problem for solar electricity as any extra will be exported to the grid but with solar hot water what you don't use is wasted (see Living with Hot Water Panels). (It is possible to misuse an energy efficient home by opening all the windows and turning up the heating but you don't need my advice on how to do that.)

The EU standard Nearly Zero Energy Buildings is an energy based standard rather than a carbon emissions standard (Zero Carbon Hub has a comparison here) and this makes sense because it is more cost effective to generate renewable energy elsewhere.  Nearly Zero Energy Buildings needs to be in place by 2020. It doesn't set a specific target for energy consumption - that is up to national governments. It does say the target should be very high energy performance.

Never mind the Zero Carbon Homes standard. We need to push for an excellent Nearly Zero Energy standard.
Targetting carbon emissions in buildings encourages tick box solutions that don't really help: locking us into fossil fuels, and installing costly domestic solar panels that don't work well at times of peak demand. For these reasons it is much better to set standards in terms of energy efficiency. Never mind the Zero Carbon Homes standard - what concerns me is that we dropped the next level of energy efficiency targets too. The building industry is now in limbo waiting to see how our government will interpret the Nearly Zero Energy Buildings requirements - do we already have 'very high energy performance' standards? I think not.

Constructions used in the charts
These are the construction values I used to generate the charts. I modelled a semi-detached dwelling, 5.7 x 8.5m with each storey 2.6m. I assumed no heat loss through the party wall. All figures are U-values (W/C/m2) except where for ventilation, expressed as air changes per hour.
Walls solid brick:
unfilled cavity: 1.0filled cavity: 0.35 0.210.15
Windows single glazing: 4.8basic double glazed: 3double glazed with argon fill and low emissive coat: 21.150.5
Doors solid: 3solid: 321.150.5
Roof no insulation: 2.360mm insulation: 0.6300mm insulation:
Floorsuspended floor: 0.7solid floor:
Thermal bridging
Ventilation (Air changes per hour)draughty: 1.5 a bit draughty:

1These U-values are 15% larger than the values in table 4 of the building regulations part L1A because the target fabric efficiency calculation allows an extra 15%. This doesn't have to be spread evenly through the whole envelope though so in practice some elements might be worse than this if others are better.
2 Passivhaus does not specify actual U-values for the elements of a building: the limit is for the overall energy requirement. These U-values are typical. For ventilation, 0.5 ACH is the minimum needed for health of the building and its occupants, but Passivhaus homes have heat recovery ventilation so they have a lower heat loss for the same level of ventilation. I have estimated 0.2 as the effective heat loss, taking into account some leakage and the power for the ventilation system too.

[1] Building regulations Part L1A Conservation of fuel and power in new dwellings.
[2] Passivhaus Technical Requirements 
[3] Green Building Store Ultra timber windows and doors
[4] Control of energy use in Building Regulations (ppt)
[5] Cambridge Housing Model
[6] Passivhaus Buildings: Case study evidence for reduced whole life costs (Bere Architects)
[7] Actively cutting energy bills in Oldham  - welcome to the Passivhauses (Guardian) Nov 2013

1 comment:

  1. Nice piece, and cogent arguments. Omitting heat loss through party walls is contentious though, and makes a big difference for energy-efficient homes, see


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