Their total energy consumption is averaging at 11.93kWh/day.  At that rate, their annual consumption will be 4,356kWh.  However, the heaters are on at the moment – averaging 2.27kWh/day (pretty low!) – so we’re hoping the annual use will be lower than this, as these will not be on all year.  It will also probably require more heating this winter as the house has been a building site for 6 months, and not able to build up a store of solar energy from the summer.

So how does this compare to their previous energy use?  Well, looking at 18 months of bills prior to the retrofit, they were averaging 3,241kWh/yr of electricity and 18,972kWh/yr of gas.

So down from an annual energy consumption of 22,213kWh to 4,356kWh – not a bad start; long may it continue …

Front of the completed properties

The rear of the completed properties

The existing tenants of number 30, Mr & Mrs Morton, have already moved back in and are enjoying their new home.  Newark and Sherwood Homes are advertising for tenants for number 28, which was previously void.

The houses will now be monitored for 2 years to see how they really perform.  The monitoring includes:

• Temperature and relative humidity in 3 rooms of the house (sitting room, bedroom 1 & bedroom 2)
• External temperature
• CO2 level in the sitting room
• Total water and electricity consumption
• Electricity consumption on 8 individual sub-circuits

All this data is being monitored automatically and transmitted to a central database at 5 minute intervals, and we can view it real-time via a website.

We are also hoping to do additional monitoring on the properties, and are in discussion with a couple of universities about the possibiliity of collaborating on this.

Meters, meters and more meters!

We hope you’ve enjoyed our blog so far, and watch this space for more news of post occupancy evaluation …

Significant levels of insulation in the loft

Although there is no access to the loft, the houses now additional storage areas in the sun space and porch.

Insulated thermal store

The hot water in the houses is provided by a super-insulated thermal store heated with an electric immersion heater.  A thermal store is like a traditional hot water cylinder, but the key difference is that the hot water in the cylinder is not the hot water used, instead it simply acts as a heat storage mechanism, hence the term thermal store.  The hot water delivered to taps and showers, is actually cold water directly from the mains supply, which is then passed through a large copper coil (heat exchanger) within the thermal store, and in doing so extracts energy from the hot water in the store to heat it.  As the water in the cylinder is not being used directly, it does not need to be heated to 60 degrees to kill legionella bacteria; instead it can be kept at aorund 45 degrees, significantly reducing energy consumption.  The hot water delivered is around 40 degrees, more than adequate for washing and showering.

Super insulated thermal store - it is in there, honest!

1. Passive solar gain, both directly into the house and harvested from the sun space, which will be absorbed and stored in the mass of the walls and floor, being released as the air temperature falls;
2. Incidental gains from cooking, hot water and appliance use – these all produce heat as a by-product that will help to heat the house, again by being absorbed and stored in the mass of the walls and floor;
3. Metabolic gains from occupancy – we all give off body heat, the amount varying depending on what activity is being undertaken, but again this heat will be absorbed and stored in the mass.

We have however made provision for top-up heating in the properties via provision of electric radiators in the sitting room and dining room, and electric towel rails in the bathrooms.  If the house performs as expected though, these will require very little use.

Programmable timer and thermostat on one of the radiators

The heaters are on a separate electrical circuit which is being monitored, so we will be able to see exactly how much top-up heat has been provided by the electric heating.

The houses are now hopefully very airtight – the actual level of airtightness achieved from the retrofit will be tested in the autumn and compared to the value beforehand – and as the design principles dictate not opening windows during winter, to minimise heat loss, then there needs to be provision for a fresh air supply, and extract of moisture from the kitchen and bathrooms.

We have therefore installed a mechanical ventilation with heat recovery unit, or MVHR for short.  These devices use fans to extract the warm, moist air from kitchens and bathrooms, to prevent the build up of condensation and potential mould growth, and at the same time bring fresh air in to other rooms from outside.  Both these air streams, the outgoing warm, moist air, and the incoming cooler, dry air, then pass through a heat exchange unit in which the latent heat in the air being extracted helps to pre-heat the cooler air being brought in from outside.  The units we have installed have a heat recovery efficiency of 90%, i.e. 90% of the heat in the air being extracted is recovered to pre-heat the cooler air being brought in, so overall heat loss from the house is minimised.

The MVHR unit in the bathroom, which contains the fans and heat exchanger

The MVHR unit has one additional smart feature that will help to keep the house warm.  We have worked with the manufacturers, EnviroVent, to modify the unit’s control logic and ducting, to optionally take air directly from the sun space when the house is cooler than a preset temperature, and the sun space is warmer than the house; this is effectively automating the harvesting of passive solar energy from the sun space, so the occupant doesn’t have to worry about opening doors to do this manually.

Triple glazed door unit to the sun space being installed

All the glazing on the house, including that into the porch and sun space,  is triple glazed, krypton filled and with a low-emissivity coating; this makes the windows very energy efficient, with a U-value of 0.8 – the lower the U-value, the less heat is lost.  The average U-value for double glazing is around 2.

The glazing on the sun space is double glazed; this doesn’t need to be as energy efficient as the glazing on the house itself, as the house and sun space are thermally separated with the triple glazed units.  The sun space however is still well insulated, probably as well insulated as most new housing being built today, so will still retain heat reasonably well.  As it is also sheltering a reasonable area of the rear of the house, it will help to further reduce heat loss from that area of the house fabric.

Sunspace glazing installed

Sunspaces under construction

The sunspaces to the rear are one and a half storeys high, with flat roofs containing two Velux roof lights, one opening and one fixed.  The sunspaces are thermally separated from the main house, i.e. there are external quality triple glazed windows and doors between the house (kitchen/diner) and the sunspace. 

The sunspace will harvest passive solar energy, and when the sunspace is warmer than the house, and the tenant wants to heat the house up, the windows and doors can be opened to the house and this heat will then transfer in and get absorbed into the mass of the floor and walls; it will then get given up by the mass when the air temperature cools.  We are also planning to have this harvesting automated by a modification to the mechanical ventilation and heat recovery unit, such that it will take its air supply from the sunspace instead of outside, when the sunspace is warmer than the house, and the air temperature inside the house is below a pre-defined minimum.

The sunspaces also include a downstairs bathroom, with a shower, hand basin and toilet, which adds lifetime homes benefit to the houses.

Front porch

The porches help to separate the internal environment from the outside;  they effectively act as an airlock to the house.  In winter only one door should be open at any one time, this will help reduce heat loss from the house, and thereby help to keep them warm.

Wall insulation showing in new cavity at front door

The insulation is between 250-300mm thick; the variance is due to the fact that the original walls had shifted slightly over time, so in some places the cavity was slightly wider than others, and we took every opportunity to get in as much insulation as possible!  You can also see in the photo above that the cavity extends down below ground level, to the bottom of the original wall foundations, and insulation was packed down here as well to minimise thermal bridging from the internal floor to outside.

Wall insulation being packed into the cavity at roof level

The photo above shows the insulation being packed into the cavity just below the existing eaves.  The insulation is then continued up over the top of the original wall, between the rafters and into the roof space to again minimse thermal bridging at the top of the original walls.  This is the weak point of the overall insulated envelope however, as the void over the orignal wall between the rafters only allowed for around 150mm of insulation.  The roof was then extended by two rows of tiles to meet the corbelled external skin.

Wall cavity at an opening showing wall tie

The photo above shows the newly created cavity at one of the windows, and if you click on the photo to expand it you can just see the wall tie spanning the cavity four brick courses down.  These ties were secured into the original wall by drilling a hole and inserting the tie with a mortar fill and they then span the cavity and are secured in the mortar layer of the external skin.  The ties used are Teplo Ties from Ancon, which can be cut to length; traditional ties do not come in sufficient length for this sized cavity.

Floor insulation being installed, which will be below the new concrete floor

The new floor slab will act as a heat store to help keep the house warm through winter, with heat loss from the slab to outside of the house minimised by the insulation below and around it.  The heat then only has one way to go; back into the house as the air temperature drops below that of the floor.

The new concrete floor slab being poured over the insulation layer

In summer the floor slab will act in reverse as a cooling mechanism, because as the air temperature rises above that of the floor, the excess heat will be absorbed into the dense concrete.

The mass of the original solid walls will act as a heat sink in a similar way, helping to cool the house in summer and heat it in winter, once it is fully encased in insulation, which is the next major phase of the retrofit.

In highly energy-efficient houses that combine high thermal mass and significant levels of insulation to remove the need for a space heating system, it is vital that the occupier appreciates the design concepts and how to live in the house; if you leave a window open in winter the house is going to get cold!

Therefore we will be working with the tenants moving into the retrofitted houses, to help them understand how the houses are designed, and more importantly how they need to occupy the house to ensure this design is effective.

This subject was covered during a seminar at the recent National Energy Management Exhibition (NEMEX), and included a name check for Hockerton Housing Project.