Whole-house ballpark

Whole-dwelling heat-loss estimator

A quick, indicative whole-house design heat loss on a BS EN 12831-1 steady-state basis — fabric, ventilation and thermal bridging, broken out. It models the dwelling as a single simplified box, which is exactly why it’s a sense-check for early conversations, nota sizing document. The same underlying engine runs PipeworkHQ’s room-by-room surveyor, which is what a real quote should use.

Indicative guidance for competent persons

This tool gives indicative guidance only. It is not a substitute for the engineer’s own on-site assessment, the official standard, or the judgement of a suitably qualified, registered competent person. Gas work must only be carried out by a Gas Safe registered engineer. Heat-loss outputs here are indicative: plant and emitter sizing must come from a room-by-room survey of the actual property, with measured fabric details, by a competent person.

Total floor area (m², all storeys)

From the EPC or by measuring. A typical UK 3-bed semi is around 85–95 m².

Storeys

Average ceiling height (m)

2.4 m is typical for modern UK homes.

Glazing area (% of floor area)

Typical UK homes are around 15–20%. Indicative — editable.

Glazing type

Airtightness

Wall U-value (W/m²K)

Typical, editable: solid brick ≈ 2.0; uninsulated cavity ≈ 1.6; filled cavity ≈ 0.55; post-2010 build ≈ 0.3.

Roof / loft U-value (W/m²K)

Typical, editable: minimal loft insulation ≈ 0.7; 100 mm ≈ 0.4; 270 mm+ ≈ 0.16.

Ground-floor U-value (W/m²K)

Typical, editable: uninsulated suspended timber ≈ 0.7; insulated solid floor ≈ 0.25.

Design external temperature (°C)

MCS design temperatures for UK locations are commonly between -1 and -5 °C; -3 is a common default.

Internal design temperature (°C)

21 °C is the usual whole-house design figure.

Basis: Steady-state design heat loss per BS EN 12831-1 (UK practice): transmission Σ(A·U·f·ΔT), ventilation 0.34·n·V·ΔT, plus a thermal-bridging surcharge on the SAP 10.2 simplified y-method (default Y = 0.20 W/m²K on the exposed envelope). Default U-values and air change rates are typical figures — every input here is editable, and a surveyed value always beats a typical one.

The room-by-room version lives inside PipeworkHQ

The same rules engines behind this free tool are built into PipeworkHQ, saved to the job and the certificate trail. Start free — no card, no contract.

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