Part O & HEM Methodology

Property Geometry & Data Collection

We capture the geometric and construction data needed for both Part O and HEM calculations — orientation, opening areas, construction types and thermal properties.

• ▸Room-by-room geometry: floor area, ceiling height, room volume and window/opening positions
• ▸Orientation captured using compass bearing — critical for solar gain calculations
• ▸Opening schedule: every window and door recorded with dimensions, type and openable area
• ▸Construction build-up: wall, roof and floor U-values estimated from construction age and verified with thermography
• ▸Ventilation strategy: existing extract fans, trickle vents, background and purge ventilation provision
• ▸Shading factors: external obstructions, overhangs and adjacent buildings that reduce solar gain

Part O Solar Gains Check (Simplified Method)

We calculate the solar gains through all glazed openings and compare against the Part O limiting values to determine overheating compliance.

• ▸Solar gain calculated per opening: G = Area × g-value × Solar Irradiance × Orientation Factor
• ▸Total solar gain summed for each relevant room and the whole dwelling
• ▸Limiting solar gain values applied per Part O Table 1.4 based on cross-ventilation availability
• ▸If total gain ≤ limiting value: compliance demonstrated by simplified method
• ▸If total gain exceeds limit: mitigation required (reduced glazing, solar control glass, external shading) or dynamic thermal modelling needed

Part O Minimum Free Area Check

We verify that openable window areas meet the minimum free area requirements using the discharge coefficient and equivalent area method.

• ▸Equivalent area (Aeq) calculated for each opening: Aeq = Cd × A_physical
• ▸Discharge coefficient (Cd) applied based on opening type: top-hung, side-hung, pivot, sliding
• ▸Minimum free area checked against Part O requirements: typically ≥ 1/20th of room floor area
• ▸Cross-ventilation path assessed: does air have a clear route from one opening to another?
• ▸Night-time ventilation capability assessed for bedroom overheating risk
• ▸If free area insufficient: additional openings or larger openable areas specified

HEM Four-Metric Assessment

Properties are assessed against the four Home Energy Model headline metrics, each decomposed into contributing factors for actionable improvement.

• ▸Fabric performance metric: calculated from element U-values, thermal bridging and air-tightness
• ▸Heating system metric: efficiency and carbon intensity of the primary heat source modelled against demand
• ▸Smart readiness metric: capability assessed across energy management, monitoring and demand-side response
• ▸Energy cost metric: modelled annual energy cost using standardised occupancy and energy price assumptions
• ▸Each metric benchmarked against the 2027 HEM target values for compliance readiness
• ▸Gap analysis: difference between current score and target quantified, with measure-level impact breakdown