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Fabric First, up to a Point- Viridian Solar

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

Written by  Stuart Elmes, Chief Executive at Viridian Solar  (Guest Blogger) 

I’ve started to detect a small, but significant shift in how the construction industry views solar panels for low-carbon construction.  It always seemed to me that for many people solar was seen as a kind of necessary evil, something to help you get over the line if you couldn’t quite get to your energy performance targets. However, we are starting to see solar feature more prominently in developments.  For sure on many schemes you still see just one or two solar panels per house (giving a power or 0.25 - 0.5kWp), but now we are also seeing more and more new homes being built with solar arrays of a size closer to those that people pay for as retrofits (typically in the range of 2-4 kWp). 

What’s causing this shift? The advice for low carbon construction has always been for designers to ‘build in performance’.   The alliterative slogan ‘Fabric First’  has been used to exhort designers to use as much thermal insulation as possible and to avoid using ‘bolt-ons’ such as solar panels that could easily be removed from the building. 

Only insulation can ensure the performance of the building for its lifetime, the thinking goes. Of course, this argument discounts the notion that insulation performance can drop over its lifetime, possibly the subject for a whole other blog, but these papers 1, 2, 3 suggest an unsafe assumption.  The implication that a significant number of homeowners would remove a solar installation at the end of a 30-year life (or before) except to replace it with a newer or larger system could also be challenged. There’s also a rather inconvenient finding that some of the benefits of higher levels of insulation (this study finds 14-17%) are lost to ‘temperature takeback’, a phenomenon where residents of homes with higher levels of insulation tend to keep the indoor temperature higher, resulting in higher heat losses than expected. Owners of renewable generating technologies such as solar have by contrast been found to be more engaged in monitoring and managing their energy use than those without. Concerns about indoor air quality and over-heating in summer have also been raised as homes have become more air tight.  

It is possible to apply technical fixes to these issues, for sure, but there are costs associated. And here we are.  We’ve reached the thing that seems to matter above all others in construction. Cost The cost structure of solar differs greatly from the cost structure of insulation, and it’s all down to the way their costs-benefits change as you increase the amount you use on a building. Doubling the thickness of an insulation material will 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. Doubling again halves the heat loss again, this time from 50 to 25, so the saving has halved for adding twice what you did the first time.  The saving from the next doubling is 12.5.  By the time our insulation reaches 16 times thicker than our starting point we just added 8 times the thickness of the first step and only saved 6.25 units. This is classic illustration of the law of diminishing returns.

Of course, in the above example we’re only considering the conducted heat through the insulation and ignoring the thermal resistance at the surface interface (which doesn’t scale with thickness).  Also the insulation is only a part of the wall, floor or roof build up, with other elements contributing to the total U-value.  I used an online calculator to have a look at real walls and roofs.

Insulation thickness(Rigid Foam Insulation board) U Value Heat Loss as a proportion of 25mm Insulation
25mm 0.34 W/m2K 100%
50mm 0.24 W/m2K 70%
100mm 0.15 W/m2K 44%
Cavity wall with brick outer leaf and lightweight block inner leaf finished with skim coated 12.5mm plasterboard.

You can see that doubling the insulation thickness reduces heat loss by only 30%.  Doubling a second time knocks a further 26% off. Insulation materials are cheap, but you don’t have to double a small number many times before it starts to become a big number.  Using thicker insulation doesn’t just mean higher costs for the insulation material itself. 

As you increase insulation thickness longer wall ties are needed in traditional cavity wall construction.  As walls become thicker, window and door reveals become larger with knock-on cost implications.  And if you’re having to keep to an overall building footprint, the living space decreases. An economist would say that insulation has an increasing marginal cost.  The more you use, the more the next improvement costs. By contrast, solar has a diminishing marginal cost.  The more you use, the cheaper the next bit of improvement costs. The energy output from a solar array is linear with size.  Double the number of panels and scale up the inverter to suit and it will produce twice the annual output of electricity. 

Well, right up until the point you run out of available area on your most favourable roof pitch and start having to use less good orientations, that is. Secondly the cost per unit drops as the array size increases.  

The cost of the solar panels themselves will scale linearly with the size of the array, but other costs do not. The cost per watt-peak power of the inverter (the electronic device that converts the DC solar generation to AC for use in the home) drops as the units’ power rating increases.  Costs such for the meter and isolation switches for a domestic system of any size are the same.

On the Viridian Solar price list, the cost of an electrical kit suitable to connect a 16 panel (4kWp) system is about 3 times higher than the price of a kit for a 2 panel (0.5kWp) system, but will produce 8 times the annual carbon savings and energy yield. As for the installation itself, once the installers are up on the roof the extra time to fix down a few more panels is small and the time taken to wire in a larger array is only marginally more than for smaller one.

Designing for Energy Performance

A designer targeting a particular energy performance would, until recently, increase the level of insulation up to what they (or their client) considered to be a practical limit and only then turn to solar to get the last bit of energy performance to get the building to the target. Signs are emerging that building designers have got to grips with the different cost structures of insulation and solar.  More and more understand that by considering designs with a larger energy contribution from solar, they may find a more cost-effective overall design. The diagram illustrates the concept. 3kWp integrated solar system on new build home. You’ve heard it before, but the cost of solar power has fallen spectacularly and while the pace of price decline has slowed, costs continue to trend lower. Cost-effective roof integrated solar systems have greatly improved the aesthetic qualities of solar.  Is it time to reconsider your approach to low carbon design? Stuart Elmes is founder and Chief Executive of Viridian Solar, a UK manufacturer of integrated solar photovoltaic and solar thermal panels.  He writes as ‘the solarblogger’, where this article first appeared.  

Author: Stuart Elmes - Guest Blogger

This article was published by Stuart Elmes - Guest Blogger on 01.09.2015.