Code guidance FAQ

SAP assumptions

I'm struggling to match your scenarios in my SAP software calculations. What are the assumptions you've used to arrive at the figures quoted in your guidance documents?

All of the scenarios provided have been produced using SAP2005 v9.81, this latest version of SAP2005 was introduced in May 2008. Whilst the core calculations have not changed from the first release of SAP2005 (which was v9.80) this latest version does allow benefit to be gained for mechanical ventilation systems modelled through SAP Appendix Q; if your SAP software is not the latest edition you will not be able to match the results provided within our scenarios. Visit BRE's website for a full list of the changes incorporated into SAP2005 v9.81.

Assuming that accredited SAP software is being used, any other differences between the results provided within our Code Solutions and results obtained by readers are due to differences in data entry. For example, the areas of the exposed elements in the examples are based around standard dwelling types which we have not provided to the reader simply because we feel this would over complicate the guidance.

Photovoltaic (PV) sizing

It appears that the area of solar PV panels exceeds the possible roof area on some of your scenarios.

In the scenarios that have used solar PV, we have assumed that a 1kWp array is approximately 6m² in area.

Thus for all of the houses, the maximum specified solar PV arrays can be accommodated on one slope of a duo-pitch roof of about 35^O in pitch, as long as the house is orientated to optimise the use of the solar PV array.

As for the flats, the modelling assumes that the overall floor area of the building will be larger than the individual floor areas of the flats quoted due to flats having communal areas, such as corridors and stairs. Where necessary they could also have mono-pitched roofs to optimise collection areas, or indeed there is no reason why solar PV arrays cannot be wall mounted in addition to be provided on the roof.

The scenarios presented are there to demonstrate a range of options that are available for a designer, however they are not intended to be the only solutions.

Solar thermal sizing

A number of these scenarios seem to greatly oversize the solar thermal hot water panels. Why is this?

The sizing of the solar thermal panels is based upon the SAP2005 v9.81 default values for Absorption Level (n ) of 0.6, and Heat Loss Co-efficient (a1) of 3.0. Also, most solar thermal panels are around 2m² in area, so in the modelling we have used either one or two panels.

We realise that some actual solar thermal panels have parameters that are significantly better than those we have modelled, and indeed on most installations only one panel may well be sufficient. However, it was our aim to demonstrate that some solar thermal installations may actually need to consist of two panels if their performance is similar to that of the SAP2005 v9.81 default values.

PV output assumptions

Can you  confirm the PV output assumptions used in your modelling? Also, what are the carbon dioxide savings for each of the technologies used in the modelling?

These parameters are all SAP2005 v9.81 default figures. The assumed output rating of the solar PV panels are 800kWh/kWp, while the carbon dioxide savings are dependant on the fuel that the technology displaces, i.e. 0.568 kg/kWh for displaced grid electricity.

High airtightness levels

The airtightness you stipulate is very difficult to achieve. Isn't it true that it is possible to achieve the high Code levels with a more relaxed airtightness target?

Our Code Solutions are based on minimum design backstops with a degree of flexibility to allow the designer to choose the route they wish for achieving the final code rating required. Our scenarios are based on demanding airtightness targets, but these are achievable with good design and build quality in all situations. The reason we set this particular standard at this level is that without these fabric standards it becomes increasingly costly and difficult to achieve Code levels 4 and 5. At Code level 6 you also need to achieve a Heat Loss Parameter of 0.8W/m²K or less, and this would be extremely difficult to achieve with levels of airtightness that are low.

Omission of certain technologies

You have omitted certain energy saving technologies from your guidance. Can you explain why you've concentrated on those technologies that feature in your guidance above others?

The scenarios modelled in our guidance are based on commonly used technologies to ensure that the guidance we are providing is readily achievable today and so we are unable to include all technologies. Of course, new technologies and approaches are likely to emerge that can provide equivalent or greater carbon savings than those currently included in our guides. It is therefore our intention to review our guidance regularly in order to keep it up to date with these developments as well as revisions to the CLG technical documents that support the Code.

Use of gas boilers in scenarios for flats

Can you explain why none of the solutions you give for flats use individual gas boilers?

There is no reason why individual gas boilers can not be used in flats for achieving the various Code levels. However, we have only shown individual electric panel heaters as this is the most common type of heating currently installed in new developments which include flats. In each of the Code Solutions guides, one single electric option is presented along side a range of communal solutions to give designers a comparison with what they are currently familiar with.

Replicating  scenarios using different technologies

I'd like to be able to use your scenarios with alternative technologies. Can I do this?

The guidance we have produced is based on firstly achieving the design backstops that set minimum levels of fabric performance. From these it is then possible to follow a variety of routes depending on your own preferences. Using a skilled SAP assessor you can replicate our design backstops and then develop a model for your own preference or unique development. Example of this may be for instance the use of medium scale wind generation instead of solar PV, or the use of air source heat pumps.

Thermal bridging Y value of 0.04

Can you explain how to achieve your thermal bridging (y value) of 0.04 as used in your guidance? SAP only has two default values for y of 0.15 and 0.08.

From April 2008 it has been possible to enter a user defined y value into SAP and so we have produced a set of  Enhanced Construction Details that enable you to use a y value of 0.04 in SAP2005 v9.81. If your SAP software does not allow these values to be entered please contact the software vendor.

This is possible because they have improved on the three poorest performing junctions in the Accredited Construction Details set (Roof/Wall junctions, Wall/ Floor junctions, and Lintels). Using these modelled designs together with the rest of the details at Accredited Construction Detail standards enables the user to achieve the 0.04 y value.

Our guidance

Our 'Energy Efficiency and the Code for Sustainable Homes' is a suite of publications providing guidance and solutions for meeting the energy efficiency requirements of levels 3, 4 and 5&6.

Code for Sustainable Homes level 3

This publication is the first in a suite of guides that provide technical guidance on designing and building new homes that meet the energy requirements of the Code for Sustainable Homes. This guide addresses energy efficiency measures to meet, and in some instances, exceed minimum measures set down in level 3 of the Code for Sustainable Homes.

Find out more about the Code for Sustainable Homes level 3

Code for Sustainable Homes level 4

This guide addresses energy efficiency measures to meet, and in some instances, exceed minimum measures set down in code level 4 of the Code for Sustainable Homes.

Find out more about the Code for Sustainable Homes level 4

Code for Sustainable Homes levels 5&6

This publication is the third in the suite of code guides. This publication will help housing professionals meet energy efficiency requirements of levels 5 and 6.

Find out more about the Code for Sustainable Homes level 5&6