Forget house prices, where you’re going on holiday and the benefits of cosmetic surgery - SAP is what everyone’s talking about at parties right now. This crash course in sustainability software explains why

There used to be a guaranteed way of getting rid of bores at parties: tell them you worked on compliance software for Part L of the Building Regulations. Not any more. After the events of last week you’re more likely to find yourself surrounded by a crowd of people open-mouthed with interest.

The software used to calculate compliance for new homes is called the standard assessment procedure, or SAP for short. First came the news that the latest version of it, which is a necessary precursor to the latest iteration of Part L, would probably not be available until September. This was a serious problem, because Part L was due to come into force on 1 October, and housebuilders say they were promised six months to work out how best to implement it.

Hard on the heels of this came a report from the Zero Carbon Hub - the body set up to deliver low-carbon homes - that said the whole approach taken by SAP wasn’t up to the job of delivering low-energy homes and needed to be rethought.

Never has such a humble piece of software attracted so much interest. So in case you find yourself at a party where the main topic of conversation is SAP, we’ve put together a quick guide to help you understand what the fuss is all about …

Why should we care about SAP?
On the face of it SAP is pretty dull stuff - it’s just the tool housebuilders use to check that the homes they are designing meet the carbon reduction targets of Part L. But stop and think about this for a minute: the assumptions in the software will determine how well the completed houses are likely to perform against reality. If SAP isn’t up to the job, houses could use more energy than predicted in the winter or overheat in the summer. Also housebuilders base design decisions on what type of products are likely to satisfy SAP. That means this collection of computer code affects a market worth hundreds of millions of pounds every year.

What’s wrong with SAP?
According to the Zero Carbon Hub, SAP isn’t broken. It works for the 2006 version of Part L that is in force now, and the updated version will work for Part L 2010. The Zero Carbon Hub even said SAP could calculate heating needs as accurately as more sophisticated energy-modelling tools. The problem is that what it is measuring is changing: low and zero-carbon homes are quite different to the present product. For example, an inspector using the SAP program can make up to 100 data entries for space heating but a maximum of 10 for hot water. This was fine in the days of badly insulated houses, when space heating was the main energy consumer, but in ultra-low-energy homes, hot water is a bigger issue.

Which bits of SAP need improving?
The hot water element of SAP will have to account for heat loss from tanks and pipes, as well as the water that runs down the drain while you wait for the water to heat up. SAP also needs to be able to take more account of sophisticated control systems and technologies that cut energy use. The procedure could also be tweaked to warn of air quality problems, which is likely to be an increasing issue in airtight homes. The Zero Carbon Hub thinks SAP should better model the likelihood of homes overheating. It also believes the basis of calculating carbon emissions requires an overhaul.

Why is that?
The Zero Carbon Hub is proposing two changes. The first relates to the carbon content of the fuel used to generate electricity, which must be worked out each time SAP is updated and is used in its calculations. Because the carbon content of electricity should decrease as the grid is decarbonised, the Zero Carbon Hub believes this should be predicted for the next 15 years and adjusted as it changes. This will give designers and manufacturers more confidence when choosing which solution to adopt.

The second major change is scrapping the existing carbon reduction targets. So instead of saying homes built after 2010 must emit 25% less carbon than those that complied with the 2006 regulations, there will be an absolute figure expressed in kilograms of carbon dioxide per square metre per year. That would eliminate a glaring weakness in the current system, which rewards inefficiency - it is much easier to reduce the carbon emissions of a wasteful building by, say, 25% than one that is efficient to start with.

What about overheating?
The Zero Carbon Hub says the overheating element of SAP has many shortcomings. For example, it doesn’t take sufficient account of temperature variations across the UK or the heat islands in cities. Nor does it take account of temperature extremes - it only considers monthly averages. Finally, there is virtually no data on how design strategies affect the way a home responds to high temperatures. This needs to be researched and the results fed into a SAP bolt-on tool. Designs would be checked against this to see what the risk is of these leading to overheating in the year 2050. Designs with a high risk of overheating would be sent back to the drawing board - and this could become a Building Regulations requirement.

Who’s going to pay for these changes? And how long will it take?

The Zero Carbon Hub estimates that upgrading SAP could cost £1m up front and the same again to update each year. If the industry is (very) lucky the government will come up with that money, but a more likely scenario is that the industry will have to pay, possibly through a levy on its use. Because the Zero Carbon Hub thinks the new tool should be in place ready for the 2013 revision of Part L, work needs to start immediately.

Anything else?
Yes. SAP is set to become all things to all people, so it is going to tackle the thorny issue of design quality, system performance and workmanship by using “confidence factors”. For example, a design that is submitted complete with detailed drawings and evidence of a robust design process by qualified people would be awarded a high confidence factor. This would be incorporated into the SAP calculation according to the principle that a good design would be much more likely to meet the SAP targets so it wouldn’t affect the basic calculation. On the other hand, a team with a low confidence factor could be given a tougher target to hit to compensate for the greater probability of the home not performing as predicted. Housebuilders would be given confidence factors based on their quality of workmanship.

Systems could also be given confidence factors. Individual components are always tested but how well they work together affects performance.

What do the housebuilders think?
Stewart Dalgarno is product development director of housebuilder Stuart Milne. He is also the project leader of the AIMC4 consortium, which aims to develop a house that meets level four of the Code for Sustainable Homes but costs as much as a level three unit. He agrees with the Zero Carbon Hub’s criticisms. “SAP is effective, but it’s not the best tool for assessing low-energy homes,” he says. He therefore welcomes the move towards measuring absolute carbon emissions, and points out that this could be easily expressed in energy units too. “It’s a way of measuring energy efficiency that the customer will understand and that will help stimulate demand for low-energy homes,” he says. “People understand miles per gallon and CO2 emissions when buying a car, so I would like to see this used for homes too.”

What about housebuilders being given confidence factors? “We would like to be favoured in some way compared with someone who has just entered the market,” he says. “Clearly there are people who are going to deliver more robustly than others and that needs to be recognised.”