Insulation and retrofit calculators.U-values, insulation thickness, condensation risk, airtightness and retrofit savings.

Thermal resistance and U-values

Thermal resistance and U-valuesThermal resistance of a layer (R = d / λ)

R = \frac{d}{1000 λ}

Thermal resistance and U-valuesU-value of a wall, roof or floor (BS EN ISO 6946 layer sum)

R = R_{s i} + \frac{d_{1}}{1000 λ_{1}} + \frac{d_{2}}{1000 λ_{2}} + \frac{d_{3}}{1000 λ_{3}} + \frac{d_{4}}{1000 λ_{4}} + \frac{d_{5}}{1000 λ_{5}} + R_{g} + R_{s e}

Thermal resistance and U-valuesInsulation thickness needed for a target U-value

d = 1000 λ (\frac{1}{U_{t}} - \frac{1}{U_{e}} - R_{a})

Thermal resistance and U-valuesU-value with a timber-bridged layer (ISO 6946 combined method)

R_{a} = R_{s i} + R_{o} + \frac{d}{1000 λ_{i}} + R_{s e}

Thermal resistance and U-valuesCorrections for air gaps and fixings (ISO 6946 Annex F)

A_{f} = \frac{π d_{f}^{2}}{4000000}

Thermal resistance and U-valuesSolid ground floor U-value (BS EN ISO 13370)

B' = \frac{A}{0.5 P}

Heat loss

Heat lossHeat loss through an element (Q = U × A × ΔT)

Q = U A (T_{i} - T_{e})

Heat lossAnnual heat loss through an element (degree-day method)

E = \frac{24 U A HDD}{1000}

Condensation risk

Condensation riskInterstitial condensation check (Glaser method, BS EN ISO 13788)

R = R_{s i} + \sum_{j = 1}^{4} \frac{d_{j}}{1000 λ_{j}} + R_{s e}

Condensation riskInternal surface temperature and fRsi (mould risk, BS 5250 / ISO 13788)

T_{s i} = T_{i} - R_{s i} U (T_{i} - T_{e})

Quantities

QuantitiesLoft insulation rolls for two layers

N_{1} = ⌈ \frac{A (1 + k / 100)}{c_{1}} ⌉

QuantitiesCavity wall insulation: volume and bags

V = \frac{(A_{g} - A_{o}) w}{1000}

QuantitiesRoom floor area lost to internal wall insulation

A_{e} = (L - \frac{n_{L} t}{1000}) (W - \frac{n_{W} t}{1000})

Part L compliance

Part L compliancePart L limiting U-value check for renovation (existing dwellings)

Δ_{t} = U_{e} - U_{t h}

Airtightness

AirtightnessAir permeability q50 and air changes n50 from a blower-door test

q_{50} = \frac{V_{50}}{A_{E}}

AirtightnessConvert air permeability q50 to air changes n50

n_{50} = \frac{q_{50} A_{E}}{V}

AirtightnessConvert air changes n50 to air permeability q50

q_{50} = \frac{n_{50} V}{A_{E}}

AirtightnessInfiltration rate estimate from air permeability (÷ 20 rule, SAP 10.2)

n_{i n f} = \frac{q_{50}}{20} + n_{o p}

Ventilation

VentilationVentilation and infiltration heat loss

H_{v} = 0.33 n V

VentilationWhole dwelling ventilation rate (Approved Document F)

q_{b} = 19 + 6 (n_{b e d} - 1) (13 if n_{b e d} = 0)

VentilationBackground ventilator (trickle vent) equivalent area (Approved Document F)

A_{n} = A_{h} (n_{h} + n_{k}) + 4000 n_{b}

VentilationMVHR heat recovery saving and fan energy

H_{r} = 0.33 \times 3.6 q_{v} \frac{η}{100}

Savings

SavingsAnnual energy saving from an insulation upgrade (degree-day method)

Δ H = (U_{1} - U_{2}) A

SavingsBill saving at your tariff

B = \frac{E p}{100}

SavingsCarbon saving (UK Government conversion factors 2026)

C = E f

SavingsSimple payback and lifetime saving

C_{n} = C - G

Whole-house heat loss

Whole-house heat lossHeat transfer coefficient HTC (fabric, thermal bridges and ventilation)

H_{T B} = y A_{\exp}

Whole-house heat lossHeat loss parameter HLP

HLP = \frac{HTC}{TFA}

PAS 2035

PAS 2035PAS 2035:2023 Retrofit Designer route (Annex A.3)

c = 8 b_{8} + 7 b_{7} + 5 b_{5} + 9 b_{9} + 1 b_{1}

Pricing

PricingJob price with VAT (energy-saving materials relief)

N = M (1 + \frac{m_{u}}{100}) + L_{b}

PricingHMRC mileage allowance (cars and vans, 2026–27)

M_{1} = \min (M, \max (0, 10000 - M_{0}))

Insulation and retrofit calculators.U-values, insulation thickness, condensation risk, airtightness and retrofit savings.

Thermal resistance and U-values

Heat loss

Condensation risk

Quantities

Part L compliance

Airtightness

Ventilation

Savings

Whole-house heat loss

PAS 2035

Pricing

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