There is a strong demand for using renewable energy solutions in new houses at the moment; however we should not forget the benefits of reducing the energy demand of a building at source. If a building has very high levels of insulation to the floor, walls, roof and windows/doors, and is well sealed to reduce heat loss through unwanted ventilation, then the space heating required can be very small.
Some people take this to the levels achieved under the PassivHaus scheme, where no space heating is required at all, however this involves a very significant commitment by the client. As well as paying the additional insulation costs, the client also has to pay the specialist fees associated with the PassivHaus scheme.
Once you achieve very high levels of insulation, then the size of the renewable energy equipment needed to provide the residual heat input is much reduced. These does save capital cost, however because the savings per year start from a lower energy demand, the payback time for renewable equipment can become longer.
The kind of insulation levels needed to get to this point well exceed the current building regulation standards. A sensible standard to aim for is a U-value of 0.15 W/m²K for floors, walls and roofs. Current building regulation standards call for U-values of 0.18 for floors, 0.20 for walls and between 0.16 and 0.18 for roofs.
It is commonplace now to be able to achieve a U value of around 1.4 for a high quality double glazed unit and in the south east it is difficult to make the case for the current extra cost of triple glazing which reduces the U-value down to around 0.8 W/m²K. However as the glazing technology improves triple glazing costs will fall and this may well become the much more attractive.
Making a house airtight does not mean you can’t open the windows. Most new houses are built to a reasonable standard but have small gaps around windows, doors, roof and wall junctions that allow warm air to escape from the building. This directly increases the space heating load to replace the warm air lost.
Airtight buildings stop this unwanted ventilation. The ventilation is instead provided in a controlled way to all rooms using fresh air that is pre-heated using heat recovered from stale extracted air from bathrooms and kitchens. This is known as a mechanical ventilation heat recovery system (MVHR). The system runs continuously moving small amounts of air over a long period and this enables them to run almost silently. On warm days it’s easy to turn off the system and throw open the windows. There is also a boost mechanism that will increase the ventilation to bathrooms or kitchens when needed for a short period of time.
A cold bridge is a small zone of the structure that is not adequately insulated and therefore allows unwanted heat loss to occur. These are often small areas, but add up significantly over the whole house and the lost heat costs money to replace. These can be designed out at detailed drawing stage and avoided by good quality construction work on site.
Fundamental Design Decisions
There are some design decisions at the start of a project that can make significant differences to the energy costs of a building:
- Is it possible to design the house with a compact building shape to reduce the external wall and roof areas to the minmum? Clearly define the thermal (heated) envelope and the airtight layer.
- Is a southerly orientation (±30°) and large south-facing window areas possible? Are there any trees that may shade the building preventing the use of solar gains? Minimise winter shading by; garden walls, vegetation, balconies, roof overhangs and outbuildings.
- Can compact service zones be designed by placing bathrooms above or next to kitchens, etc? Consider routing and space for ventilation ducts and plant. Design short ventilation ducts with cold air ducts outside the heated envelope, warm ducts inside.
- Plan in enough space for building technology. Make sure there is space and access for regular maintenance.
Instructions For Owners
Create a user manual in a binder with user instructions, technical manuals for equipment, warranties and contact details for service and maintenance functions. This encourages the owners to use the technology in the building correctly which in turn should mean that the designed energy savings are achieved in long term use.
The payback period is usually defined as the number of years is takes to recover the initial capital cost through annual savings on energy costs in use. Because the actual energy savings are so difficult to predict, it is equally difficult to have any certainty about payback periods.
Another factor is the lifespan of the energy saving equipment. Insulation generally has a lifespan equal to the building so the saving can be assumed to continue every year. However the lifespan of equipment such as air source heat pumps and solar thermal panels, as well as more conventional plumbing items, generally has a life span of around 15-20 years and so payback periods that exceed this do not make sense.
A final factor is that the better a building is insulated, the longer the payback period because the savings per year are lower.
It seems therefore that high insulation levels and good air tightness are an ideal first design step to reduce energy demand, with relatively low capital cost and good payback.
Solar thermal panels (hot water) are well worth considering as they are inexpensive and offer real energy savings. Panels are now readily available and a typical 2 panel package costs around £3,000 to install and they are easy to connect up to hot water cylinders with a built-in solar coil.
However the case for renewable energy equipment that has a higher capital cost such as air source heat pumps and ground source heat pumps needs careful analysis for each individual building to see whether it makes good sense to ask clients to pay the additional capital cost.
However wealthy or environmentally committed, every client has a budget, and it is our job as Architects to help them spend their budget wisely.