Monday 17 August 2015

Fabric First, but not Second and Third

3kWp integrated solar system on new build home.  Image: Viridian Solar

There’s an interesting shift going on in attitudes to solar in low carbon building at present.  Solar panels, previously seen by many in construction as a necessary evil, something to get you over the line if you couldn’t quite reach energy performance targets, are starting to feature more significantly in developments.  Instead of just one or two panels per house (0.25 - 0.5 kWp), we are seeing more and more developers installing solar arrays of a size someone might actually pay to have on their own home (typically in the range of 2 - 4 kWp)

Previously, voices in the sector have encouraged designers to ‘build in performance’ by using as much insulation as possible.  The argument, pitched under the catchy slogan ‘Fabric First’, goes that ‘bolt-ons’ such as solar panels can easily be removed from the building and only insulation can ensure the performance of the building for its lifetime.

This line of argument ignores the possibility that the performance of insulation can deteriorate over its lifetime (possibly a subject for a whole other blog, but see links 1, 2, 3 for starters).  It assumes also that when a solar array reaches the end of its life, or even before, the householder won’t replace it with a new one (with the benefit of 30 years of technical improvement to the technology in the meantime). 

It also ignores the inconvenient finding that some of the benefits of higher levels of insulation are lost to ‘temperature takeback”  (where people just run the house hotter so they can walk round in their underpants in winter, thus offsetting some of the expected energy efficiency gains). By contrast, the people with generating technologies such as solar have been shown to be more engaged in monitoring and managing their own energy use.

Building ever-more airtight homes has also raised concerns about indoor air quality and over-heating in summer.  For sure, there are technical fixes to both these issues, but doing it properly adds yet more to the cost of achieving very high levels of thermal performance.

And there’s that word, the thing the construction industry focuses on with relentless intensity, the thing that matters above all others. 


When it comes to reducing the carbon emissions from a building, the cost structure of solar is very different from the cost structure of insulation, and it’s all down to the different way their cost-benefits change with the amount you use on a building.

Cost Curves for Insulation and Solar

Double the thickness of your insulation material and you halve the heat loss conducted through it.  If your starting heat loss was 100 units the new heat loss is 50, a saving of 50.

Double it again and the heat loss goes from 50 to 25, saving 25.  The saving from the next doubling is 12.5, the next is only 6.25.  By now our insulation is 16 times thicker than our starting point.

In real life, of course, the insulation is only a part of the wall, floor or roof build up, with other elements making up the total U-value.  Kingspan Insulation provide an online U-value calculator to have a look at this effect in real walls and roofs.

You can see that doubling the insulation thickness reduces heat loss by only 30%.  Doubling a second time knocks a further 26% off. 

Insulation is a relatively cheap material, but the more you have the more you need to add to make a difference.  This is a classic example of a diminishing return.

Thicker insulation doesn’t just mean a higher spend on insulation, for example different wall ties are needed for thicker insulation in traditional cavity wall construction.  Window reveals become larger with cost implications.  Rooms become smaller for the same building footprint.

The cost structure for solar is very, very different.

First of all, the energy output from a solar array is linear with size.  If you double the number of panels and scale up the inverter, you’ll double the annual output of electricity - right up until the point you run out of available area on your southerly roof pitch and start having to use less good orientations.

Granted, doubling the energy output doesn't necessarily double the benefit for the homeowner - until battery storage technologies become more common, the larger the solar system, the more electricity will be exported to the grid.  However for the house-builder, the important measure is most often the treatment in the building regulations.  Currently the energy calculations (SAP) give exported electricity equal benefit to that used in the building, both for the carbon production and for the energy costs calculations.

Secondly, solar follows a diminishing cost per unit.  The larger you make a solar array the lower is the cost of the next increase in power.  The spend on the solar panels themselves will scale linearly with the size of the array, but other costs do not. 

Equipment costs per unit of output fall with system size.  The cost per watt-peak of power of a solar inverter (the electrical equipment that converts direct current solar generation to alternating current  for use in the home) falls as the units power rating increases.  Other equipment costs such as for generation meter and isolation switches are the same for any size of domestic system.

As an example, looking at the Viridian Solar price list, the cost of an electrical kit suitable to connect a 4kWp (16 panel) system is about 3 times the price of a kit suitable for a 0.5kWp (2 panel) system, despite offering 8 times the annual energy yield and carbon savings.

As for the labour costs, once you’re up on the roof the extra time to fix down a few more panels is small and the time to wire up a larger array is only marginally more than for smaller one.

Implications for Design

Until recently, a designer aiming for a particular energy performance would typically increase the level of insulation up to what they consider to be a practical limit and then turn to solar to get the last little bit of performance for the building.

However, it is becoming clear that building designers are starting to understand that the different cost structures of insulation and solar mean that considering embodiments with a larger energy contribution from solar may yield a more cost-effective overall design. 

The diagram above illustrates the concept.

The cost of solar power has fallen spectacularly in recent years and continues to trend lower. Cost-effective roof integrated solar systems have improved the aesthetic qualities of solar.  Is it time to reconsider your approach to low carbon design?