More than 800 people registered for Building’s online seminar on cracking the Code for Sustainable Homes, and between them they asked 125 questions. Here, our experts tackle some of the queries that there wasn’t time to address on the day.

Subject: Materials

Are sustainable materials the most difficult issue to get right?

Simon Jones The materials section is critical for code 5 and 6 houses. If you want flexibility in design, then this should follow after you have worked out how to maximise the credits for the energy performance of the home.

When calculating the credits for materials, is the overall score rounded down or is it rounded down for each specific element?

SJ The total credit score is rounded down, but each element has a direct influence on this. Try to balance your credits per element. The key is to become familiar with the MAT 1 worksheet used to calculate the credits.

What guides or manuals would you recommend to find appropriate products and materials?

Scott Sanderson First, the BRE’s Green Guide, which is the benchmark document to be used with the code. The Green Building Bible is another useful reference.

Is it true that timber frame houses overheat more than traditional construction?

SS Not if solar gain is properly considered in the detail design.

Subject: skills

Do you believe that we have the correct skills base at trade level to turn a theoretical house into an actual house? Will this work on a large development where many houses are built?

SS There is definitely a huge skills, process and build quality challenge in delivering to the upper levels of the code and there must be real concern that, as a whole, our industry is not yet geared to deliver to these standards.

For this reason, we welcome the staged implementation of the code. Events and initiatives, such as English Partnerships’ Carbon Challenge, will continue to be significant in driving the agenda forward and in providing tangible, accessible examples for others to learn from.

Subject: Energy

How many solar panels are needed to produce electrical generation sufficient to meet code level 6?

Katherine Holden A rule of thumb is that you need half the floor area in panels for an electrically efficient home. For example, a two-storey house would need to use the whole of the roof for the panels, which need to face south.

Is biomass likely to be a key feature of zero carbon homes?

KH Yes. At the moment, it’s the most cost-effective solution for heating and for combined heat and power. There is just about enough available land in the UK to produce 10% of our buildings’ heating from biomass. There is plenty of biomass available in Canada that could be shipped to the UK, for a relatively small additional use of energy.

How do you suggest meeting CO2 reductions for levels 5 and 6 in flats where biomass is not feasible?

KH Potentially, you could use a ground source or air source heat pump to provide heating. They are a better low carbon technology than gas boilers, and can produce up to about 20% savings in CO2 emissions. However, you would need to provide even more renewable electricity to power the heat pump, which is an expensive form of power. Heat pumps could be used as an interim means of heating until zero carbon or “waste” heat district heating is available. Air source heat pumps are being used on a level 6 project that we are involved with and the intention is that these homes will be connected to district heating systems in the future.

What simple measures can you take to raise your code rating without resorting to expensive bolt-on renewable energy technologies?

SS At all levels of the code, the focus should first be on passive design, such as improving envelope performance and sensitive consideration of solar gain and control. At code level 3, the energy standards will usually require a renewables element, unless perhaps when biomass is specified. At code level 4 and above, the renewables element becomes key to code compliance. As long as the code continues to focus on energy assessment at a development level, other than perhaps looking at changes to infrastructure supply or remote off-site generation, these products will be essential features.

Subject: Air tightness and ventilation

Is there a conflict between requirements for air-tightness in the code and the minimum background ventilation required by Building Regulations?

SJ No, good air-tightness cuts unintended air loss. The ventilation strategy should consider the level of air-tightness and be designed to complement this. The best practice recommendations of the Energy Savings Trust suggest an air loss rate of 3m3/hr or less and the use of mechanical ventilation with heat recovery as the ventilation strategy.

How is air leakage through electrical sockets dealt with?

SS The improved air permeability targets really increase focus on service penetrations. One approach is to include a service zone on the inside of the external walls, which allows services distribution without penetrating the vapour and air check membrane.

To achieve a heat loss parameter of 0.8 [fabric, thermal bridging and ventilation heat losses translated into watts lost per m2 of floor area per °C], what U-values have been used?

KH The walls, roof and floor at the Kingspan Lighthouse at the BRE have U-values of 0.11W/m²K with 284mm wide Kingspan Tek panels, glazing U-values of 0.7W/m²K, which is with triple glazing, very low emissivity coatings and argon-filled cavities, and even an insulated door with a U-value of 0.35W/m²K.

Is whole-house mechanical ventilation really necessary in a mild climate like the UK?

KH MVHR isn’t really necessary for the UK, unless you need to attain a very low heat loss parameter. Passive stack ventilation or simple, natural ventilation with trickle vents and intermittent extract fans actually produce less CO2 than MVHR.

Where is a good resource for architects’ details for air-tight junctions?

SS The technical guidance associated with the Passivhaus standards is very helpful. BRE offers detailed support on air-tightness, as do specialist product suppliers such as Dupont.

Meet the experts

Simon Jones is a senior consultant at BRE. He helps clients deliver all levels of the Code for Sustainable Homes including those built at BRE's Innovation Park.

Scott Sanderson, associate director of PRP Architects, has managed the build programme of the Stewart Milne Sigma home to code level 5 at BRE.

Katherine Holden is an associate director of Arup who works on energy efficient, sustainable building design and was involved in the design of the code level 6 Kingspan Lighthouse at the BRE.

To view the webinar, register at