W h i c h i s t h e B e s t T o o l t o A s s e s s Z e r o E n e r g y H o u s i n g ?
195
Tables 5 and 6 show that PHPP slightly underestimates the heat transfer through the
fabric per °K for the Section 6 target values. However when the more taxing PassivHaus
targets are applied it is SAP which underestimates the heat transfer. Both methodologies
do demonstrate the potential for fabric improvements to deliver significant reductions in
energy transfer through the building fabric. SAP2009 also allows the calculation of non-
repeat linear thermal bridging to be avoided by assuming a default of 0.15 x total area of
the building envelop w/°K, for all thermal bridges including those at openings. In the
house design modelled above this is equal to 35.40 w/°K whereas clause 6.1.2 sets a
target of 0.07 x total area of building envelop w/°K (which in our model is equal to 16.52
w/°K). Adding openings to our analysis increased the overall length of non-repeat linear
thermal bridging to 156.8m with an average γ-value of 0.3204 or 50.24 w/°K (Appendix K
defaults) or 0.0871 or 13.65 w/°K (accredited details) this highlights the advantage of
calculating accurately the impact of non-repeat linear thermal bridges. As the
PassivHaus Standards encourage detailing which minimises non-repeat linear thermal
bridging, the true scale of the difference is likely to be greater.
Implications
Distinguishing Between Standards and Methodologies
All 3 methodologies are used to demonstrate compliance with a set of standards. In the
case of SAP and SBEM, these are the domestic and non-domestic building codes in the
UK. In the case of PHPP, these are with the PassivHaus Institute Standards which are
voluntary in the UK. The UK building codes use a target for compliance based on a
notional value whereas the PassivHaus Standard sets a maximum energy use per m
2
and an overall building limit. PassivHaus Certification also requires the adoption of the
PasivHaus design approach. There is however no intrinsic barrier preventing each of the
calculation methodologies from being used with either targets or maximum values. This
begs the question implicit in Reason & Clarke’s 2008 report, which is that even when the
different assumptions and inclusions are factored each calculation methodology gives a
different answer for the same building; so which calculation methodology, if any, is an
accurate prediction across a range of house types, size and construction
methodologies?
Definition of Zero Energy Use and CO
2
Emissions
There is as yet no consensus as to the definition of a zero energy or emissions buildings
(Sartori, Napolitano & Voss, 2012) both in terms of what systems are, or are not included
and over what time frame (McLeaod et al., 2012). Where energy is imported and
exported via an infrastructure grid should they be given the same value when the import
takes place at the time of greatest need and any export at the time of lowest need and is
therefore potentially of less commercial value to the grid operator if indeed it is
deliverable to the grid (Baetens, et al, 2012).
There is debate for example as to whether appliance loads should be included in any
calculations (Reason & Clarke 2008). These are included in PHPP but not in SAP and
SBEM however all 3 calculation methodologies assume some internal gains from
appliances which potentially displaces some of the heating load. Appliances assume
greater importance in the face of increasingly stringent fabric efficiency standards
(especially the Passivhaus standards) which will see a diminishing requirement for space
heating. This presents a future scenario where appliance use diminishes and/or
appliance efficiency increases to the point that there are no longer any useful gains,
which implies a requirement for some form of additional heating. Similarly current
working definitions exclude embodied energy (Hernandez and Kenny, 2010), which
means choices with regard to building materials, components, services and renewable
