When your SAP Assessor states that your FEES are too high, this is nothing to do with cost rather the Fabric Energy Efficiency Standard element of the SAP calculation.
Since the 2006 changes to Part L1A it has been a requirement to check each dwelling design against the DER/TER test (Dwelling Emission Rate/Target Emission Rate); essentially checking that the predicted carbon emissions of the proposed dwelling are no worse than a notional dwelling of the same shape and size but with minimum acceptable standards for thermal and system performance.
We are not going to debate here whether the ‘design’ and ‘actual’ dwellings perform in anything like the same manner or whether we should use compliance software as a design tool, that’s for another blog.
What we are going to discuss is that since April 2014, DER/TER test has acquired a new friend, the DFEE/TFEE test. The process is similar to the DER/TER test and the same software checks both elements. The DFEE (Dwelling Fabric Energy Efficiency) must be lower than the TFEE (Target Fabric Energy Efficiency). Whilst the units are new to the UK Building Regulations (the FEES is measured in kWh/m2/year) they will be familiar to those with Passivhaus experience. In essence the FEES demonstrates the amount of energy lost through the fabric per square metre of floor area per year and makes it easier to compare similar housing types (eg. flats, detached houses). Crudely, it can be thought of as a U-value for the whole building.
To date, virtually every house design that has winged its way to the Whole House Energy offices has failed the DFEE/TFEE test, which means if your design also fails that you are in good company.
So the question is, “are there any do’s and don’ts?”
Well, here is our Top Ten list:
1) If space allows look at 150mm cavities to masonry construction.
2) Apply 400mm mineral wool to loft areas.
3) Look at each thermal bridge carefully and try to eliminate it.
4) Ensure that the design has at least 100mm of rigid insulation to the floors.
5) Consider triple glazing.
6) Check the g-value of the proposed glazing carefully.
7) Composite doors can have very low u-values and may be worth investigating.
8) Consider the best air tightness rate that the builder could achieve (bear in mind Part F and also untested dwellings on a large site).
9) Lots of open fires/ wood-burners? Then look to reduce them.
10) If you are really struggling sometimes it is worth calculating the Thermal Mass Parameter.
The key thing to remember is that the TFEE changes as you amend some of the SAP model elements, sometimes in a surprising way (change all the windows to face North for instance). Therefore, if you are like us, Passivhaus consultants, thinking in a Passivhaus way will not work necessarily help you. For instance decreasing the building volume is unlikely to help as is changing the shape of the dwelling or its orientation.
Modelling in SAP is becoming an ever more complicated process and often paying a bit more for your SAP calculation to optimise it can save you £’000s in build costs, which is where specialists such as Whole House Energy come in. Join us for a Free one hour CPD Webinar (see our training page) for more information on how to beat the SAP system or call for a chat with one of the team.