Notional Buildings

Notional Buildings

Demonstrating compliance with Part L- Conservation of Fuel and Power of the Building Regulations is achieved through comparison of the dwelling with a Notional Building across two compliance metrics, the Emission rate and Fabric Energy Efficiency rate. This is expected to expand to three compliance metrics with the upcoming uplift to Part L of the Building Regulations to include the Primary Energy Rate as well.

But what is the Notional Building that produces the targets for these compliance metrics, and how does it impact the design choices for new-build homes?

What is the notional building?

The Notional Building is essentially a benchmark building, with the same geometry as the actual dwelling, but follows an ‘elemental recipe’. This ‘recipe’ is a specification of factors that influence the energy efficiency of a dwelling. The factors included, and the values used to define the notional building currently used for Building Regulations can be found in Appendix R of SAP 2012. Technically, if the actual dwelling is constructed entirely to the specification of the Notional Dwelling, it will meet the TER and better the TFEE rate, thus passing Criterion 1 of Part L1A of the Building Regulations.

In reality, you are highly unlikely to purely follow the specification of the Notional Dwelling just to comply with Part L: economic and aesthetic factors are usually much more important considerations to both the builder and end-client than achieving the highest energy efficiency possible. With the interim Part F, Part L and Overheating regulations due to be published in December 2021, the specification for the notional building will change.

How is the notional building specification changing?

The interim update to Part L of the Building Regulations will see an improvement on the notional building by reducing the U-values of the building elements, thus requiring a more highly insulated building than before. Further to this, the notional building will now have a PV system and also have instantaneous waste water heat recovery (WWHR) systems under all showers with 36% recovery efficiency. As a result, the typical mains gas boiler specification for new builds will likely need renewables added in order to comply.

Essentially, the new notional building will drive the reduction in emissions and improvement in energy efficiency desired by the interim update to Part L by imposing higher standards of thermal performance combined with energy-saving technologies and solar panels as the target for our actual buildings.

Pushing for ever-increasing energy efficiency through higher levels of insulation and reduced air permeability is absolutely necessary, but the impact this may have on other aspects of buildings must be carefully considered, such as ensuring adequate ventilation, and preventing overheating to name but two.

 

Table R1- The notional building specification

The specification for the new Notional Building is presented in Appendix R of SAP 10.1, as seen below:

Element

 

Climate data

UK average

Size and shape

Same as actual dwelling

Opening areas (windows, roof windows, rooflights and doors)

Same as actual dwelling up to a maximum for total area of openings of 25% of total floor area

If the total area of openings in the actual dwelling exceeds 25% of the total floor area, reduce to 25% as follows:

1.       Include all opaque and semi-glazed doors with the same areas at the actual dwelling (excluding any doors no in exposed elements e.g. entrance door to a flat from a heated corridor)

2.       Reduce area of all windows and roof windows/rooflights by a factor equal to [25% of total floor area less area of doors included in 1.] divided by [total area of windows and roof windows/rooflights in actual dwelling].

External walls including semi-exposed walls

U = 0.18 W/m2K

Party walls

U = 0.0 W/m2K

Floors

U = 0.13 W/m2K

Roofs

U = 0.11 W/m2K

Opaque door (<30% glazed area)

U = 1.0 W/m2K

Semi-glazed door (30-60% glazed area)

U = 1.0 W/m2K

Windows and glazed doors with >60% glazed area

U = 1.2 W/m2K

Frame factor = 0.7

Solar energy transmittance = 0.63

Light transmittance = 0.80

Orientation same as actual dwelling

Overshading same as for compliance calculation (average if actual dwelling has very little or average overshading; same as actual dwelling if greater overshading)

Roof windows

U = 1.2 W/m2K (adjustment factor of + 0.3 W/m2K applied to roof window, resultant U value = 1.5 W/m2K).

Overshading factor 1.0

Other parameters as for windows

Rooflights

U = 1.2 W/m2K (no correction applied)

Overshading factor 1.0

Other parameters as for windows

Curtain wall

Curtain walling to be treated as standard glazing and opaque wall with the same areas as the actual dwelling. When the total opening area exceeds 25% of floor area the glazed area to be reduced to 25% as for opening areas above.

U-value of opaque wall = 0.18 W/m2K

U-value of glazing = 1.3 W/m2K (which includes an allowance of 0.1 for thermal bridging within the curtain wall)

Thermal mass

Same as actual dwelling

Living area

Same as actual dwelling

Number of sheltered sides

Same as actual dwelling

Allowance for thermal bridging

Calculated using the lengths of junctions in the actual dwelling and the PSI values seen in Table R2.

N.B. Where the area of openings in the actual dwelling is >25% of the total floor area the lengths of junctions in the notional dwelling remain the same as the lengths in the actual dwelling, even though window area is reduced.

If lengths of thermal bridges are not specified, use a y-value of 0.05 to determine heat losses from thermal bridges.

Ventilation system

Natural ventilation with intermittent extract fans

Air permeability

5 m³/h·m² at 50 Pa

Chimneys and open flues

None of any type

Extract fans/passive vents

2 extract fans for total floor area up to 70m2, 3 for total floor area >70m2 and up to 100m2, 4 for total floor area >100m2.

Main heating fuel

Mains gas

Heating system

Boiler and radiators

Central heating pump 2013 or later, in heated space

Design flow temperature = 55°C

Boiler

If gas or oil combi boiler performing space heating in actual dwelling, instantaneous combi boiler, otherwise regular boiler.

Efficiency, SEDBUK (2009) = 89.5%

Room-sealed, fan-assisted flue

Modulating burner control

No hot water test for combi boiler

Heating system control

1.       For a single storey dwelling in which the living area is greater than 70% of total floor area, programmer and room thermostat;

2.       For any other dwelling, time and temperature zone control, TRVs;

And in all cases:

   Boiler interlock

   ErP Class V

Hot water system

Heated by boiler (regular or combi as above)

Separate time control for space and water heating

Showers and baths

Number of showers and baths same as actual dwelling. If shower(s) specified, shower flow rate to be 8 l/min.

Shower(s) supplied by main water heating system (not instantaneous electric shower)

Waste water heat recovery

All showers connected to WWHR including showers over baths.

Instantaneous WWHR with 36% recovery efficiency

Utilisation of 0.98, waste water fraction 0.9

Hot water cylinder

If cylinder specified in actual dwelling: volume of cylinder in actual dwelling

If combi boiler: no cylinder

Otherwise: 150 litres

If cylinder, declared loss factor = 0.85 x (0.2 + 0.051 V2/3) kWh/day, where V is the volume of the cylinder in litres

Primary water heating losses

Fully insulated primary pipework

Cylinder temperature controlled by thermostat

Cylinder in heated space

Water use limited to 125 litres per person per day

Yes

Secondary space heating

None

Lighting

Fixed lighting capacity (lm) = 185 x TFA

Efficacy of all fixed lighting = 80 lm/W

Air conditioning

None

PV System

For houses kWp = 40% of ground floor area /6.5

For flats kWp = 40% of dwelling floor area / (6.5 * number of storeys in block)

System facing SE/SW, 30° Pitch

Overshading is ‘none’

Connected to dwelling’s meter for houses

Not connected to the dwellings meter for flats

 

 

Table R2- Thermal bridging values

Reference

Junction detail

Ψ (W/m.K)

E1

Steel lintel with perforated steel base plate

0.05

E2

Other lintels (including other steel lintels)

0.05

E3

Sill

0.05

E4

Jamb

0.05

E5

Ground floor (normal)

0.16

E19

Ground floor (inverted)

0.07

E20

Exposed floor (normal)

0.32

E21

Exposed floor (inverted)

0.32

E22

Basement floor

0.07

E6

Intermediate floor within a dwelling

0

E7

Party floor between dwellings (in blocks of flats)

0.07

E8

Balcony within a dwelling, wall insulation continuous

0

E9

Balcony between dwellings, wall insulation continuous

0.02

E23

Balcony within or between dwellings, balcony support penetrates wall insulation

0.02

E10

Eaves (insulation at ceiling level)

0.06

E24

Eaves (insulation at ceiling level- inverted)

0.24

E11

Eaves (insulation at rafter level)

0.04

E12

Gable (insulation at ceiling level)

0.06

E13

Gable (insulation at rafter level)

0.08

E14

Flat roof

0.08

E15

Flat roof with parapet

0.56

E16

Corner (normal)

0.09

E17

Corner (inverted- internal area greater than external area)

-0.09

E18

Party wall between dwellings

0.06

E25

Staggered party wall between dwellings

0.06

P1

Ground floor

0.08

P6

Ground floor (inverted)

0.07

P2

Intermediate floor within a dwelling

0

P3

Intermediate floor between dwellings (in blocks of flats)

0

P7

Exposed floor (normal)

0.16

P8

Exposed floor (inverted)

0.24

P4

Roof (insulation at ceiling level)

0.12

P5

Roof (insulation at rafter level)

0.08

R1

Head of roof window

0.08

R2

Sill of roof window

0.06

R3

Jamb of roof window

0.08

R4

Ridge (vaulted ceiling)

0.08

R5

Ridge (inverted)

0.04

R6

Flat ceiling

0.06

R7

Flat ceiling (inverted)

0.04

R8

Roof to wall (rafter)

0.06

R9

Roof to wall (flat ceiling)

0.04

R10

All other room-in-roof junctions

0.08

R11

Upstands or kerbs of rooflight

0.08

 


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Article published 22/07/21

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