Form Factor (Equation) G#38342

By April 6, 2020May 27th, 2020Calculators, Encyclopaedia, Equations, Files
GBE Whole Building Calculator Form Factor A13 BRM 250520 PNG

Form Factor Equation

GBE > Encyclopaedia > Files > Calculators > Equations > G#38342

Form Factor Equation

Form Factor

  • The ratio of external envelope area to floor area or volume ratios can be used as a design driver to improve building energy efficiency
  • Simple singular cubical design is more efficient than multiple, long thin or sprawling buildings.
  • Complexity of the external envelope will normally reflect badly in the energy efficiency stake due to the high risk of air leakiness and thermal bridging at corners and junctions and in the cost of its construction.
  • Simplicity of the external envelop will normally lead to lower costs but there are always edges and corners in a rectilinear format.
  • A spherical envelope is the most efficient means of enclosure (in terms of surface area to enclosed volume) and will lead to the simplest envelope without complications of edges and corners.
  • The ability to make and purchase materials for spherical construction is limited, the costs will not be cheap and it is difficult to place a spherical building on the ground, better floating in air, space or under water or on the top of a supporting column.
  • Once the form factor is established it should be used to modify the U value (UK) or R value (EU & Int)
  • The higher the Form Factor the Lower the U value, the thicker the insulation, needs to be to make the building a low energy demand building.
  • A correlation between Surface Area Form Factor and required thermal insulation thicknesses is a factor of 100:
    • assuming a lambda k value of 0.044 w/m.K ≡ mineral fibre
    • adjust for other material’s lambda k value
    • adjust for other components in elements
    • and don’t forget Decrement Delay
  • Form Factor SA/FA = 2: Insulation thickness = 200 mm
  • Form Factor SA/FA = 3: Insulation thickness = 300 mm
  • Form Factor SA/FA = 4: Insulation thickness = 400 mm
  • etc.

Surface area to floor area ratio (SA/FA)


Total Area of Thermal Envelop
Total Floor Area

Target ratio:


Surface area to volume ratio (SA/V)


Total Area of Thermal Envelop
Total internal Volume

Target ratio:


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LinkedIN Discussion

Matthew Cutler-Welsh

Education Manager at Pro Clima NZ Ltd
Does the form matter for energy efficient homes?
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Does the form matter for energy efficient homes?

© GBE NGS ASWS Brian Murphy aka BrianSpecMan *
6th April 2020 – 24th April 2020

Form Factor Equation


GBE Whole Building Calculator Form Factor A13 BRM 250520 PNGAbove a snapshot of part of the sheet below

This will be extracted and made as a stand alone Excel file

It uses TBH The Building’s Hub publication The Designer’s Handbook page 8 data for the calculations.

GBE Calculator Whole Building ScheduleAccommodation A01 BRM 100218 PNG, Form Factor (Equation) G#38342

One of many worksheet whole building calculator

© GBE NGS ASWS Brian Murphy aka BrianSpecMan *
6th April 2020 – 27th May 2020

Form Factor Equation
See Also:

GBE Equations

GBE Lockdown Learning

GBE Publications

GBE Datasets

  • ICE Database (Jargon Buster) G#1018 N#1037
  • Timber Species Schedule (Dataset) G#1371 N#1351
  • Generic Materials Schedule (Dataset)
  • Regulations/Design Guides Elemental U values/Airtightness (Dataset)
  • Thermal Insulation Materials properties (Dataset)

GBE Calculators

  • Whole Building Energy Demand
  • Elemental % Comparison (Calculator)
  • Fuel and In Use Carbon (Calculator)
  • Regulations/Design Guides Elemental U values/Airtightness (Dataset)
  • Form factor & U value (Calculator)
  • Insulation v Glazing cost comparison (Calculator)
  • Thermal Insulation Materials (Dataset)
  • Insulation material > U value > Thickness (Calculator)
  • Insulation material > Decrement Delay > Thickness (Calculator)
  • Interstitial Condensation Check (Calculator)
  • Whole Building U value, IUC, EE, EC, SC, LCA (Calculator)
  • Whole Building EE EC SC (Calculator) G#1910 N#1764
  • Whole Building U value (Calculator)
  • WasteCost®lite (Calculator) G#531 N#551
  • WasteCost®lite (Calculator) G#1297 N#1289
  • Loft Insulation U value Raised deck support Psi value (Calculator) G#14582
  • StoreFloor Thermal Bridge (Calculator) G#13287
  • Scale Changer (Calculator) G#19287

GBE Calculators Other’s

GBE Calculators Shop

GBE Datasets Shop

By Others

GBE Jargon Buster

Initials, Abbreviations, Acronyms:

  • AECB
  • C
  • CO2
  • CO2e
  • D&DT
  • EC
  • EE
  • PHPP
  • SAP
  • SBEM
  • SC
  • TSB
 Words or phrases:
  • Building Embodied Carbon
  • Building Embodied Energy
  • Building Energy Demand
  • Building Fuel demand and Carbon generated
  • Building U Value
  • CAPEM Compass
  • Carbon lite
  • Design & Decision Tools
  • Decrement Delay Decrement Factor (Jargon Buster) G#14043
  • Element area U Value
  • Elemental section U Value
  • Embodied Carbon (EC)
  • Embodied Energy (EE)
  • Embodied Water
  • Equation
  • Form Factor
  • Fuel Carbon
  • Innovate UK
  • Lambda k value
  • Passivhaus
  • RetScreen
  • Sequestered Carbon (SC)
  • Shared Earth Building Embodied Carbon
  • Technology Strategy Board (TSB)
  • U value
  • WasteCost®Lite (by GBE)
  • Water Calculator (by AECB)

GBE Links

  • AECB
  • WRAP

© GBE NGS ASWS Brian Murphy aka BrianSpecMan *
6th April 2020 – 25th April 2020

Form Factor (Equation) G#38342 END

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