How we calculate: insulation and ventilation

Extractor fan size calculator

Open the extractor fan size calculator

1. Part F mode: the minimum intermittent extract rate for the room type from Approved Document F (England)
2. Air change mode: m³/h = room volume × air changes per hour
3. l/s = m³/h ÷ 3.6; air changes a fan gives = fan m³/h ÷ room volume

Box ratings are at free air: long or narrow ducts and bends cut the real flow, so keep ducts short and choose a fan with some margin.

In a bathroom the fan's IP rating and position depend on the zones in BS 7671 Section 701. Wiring in a bathroom is notifiable work in England and Wales.

This calculator has no built-in defaults: every figure comes from you or your product's data.

Insulation thickness calculator

Open the insulation thickness calculator

1. Resistance of the other layers R_other = Rsi + Σ d/λ + Rse
2. Thickness d = λ_insulation × (1 ÷ target U − R_other)
3. When the insulation sits between studs or joists, the thickness is found by trying values until the combined-method U-value (BS EN ISO 6946) equals the target
4. Then buy the next thickness up and check its U-value

Corrections for air gaps, wall ties and fixings (ΔU) aren't included: allow a little extra or ask the maker for a full calculation.

U-value calculator

Open the u-value calculator

1. Each layer's resistance R = thickness d (m) ÷ thermal conductivity λ (W/mK)
2. Total resistance R = Rsi + Σ R + Rse; U-value = 1 ÷ R (W/m²K)
3. Bridged layer (studs or joists): the average of the upper and lower limits of R, from the share of the layer taken by the timber (BS EN ISO 6946 combined method)
4. The check uses the unrounded U-value, so 0.181 is over a 0.18 limit even though it shows as 0.18 to 2 decimal places

Corrections for air gaps, wall ties and fixings (ΔU) are not included, and cavities and ground floors aren't modelled: the insulation maker can give a full calculation.

Material λ values are still being checked: use the maker's declared λ.

Condensation risk calculator

Open the condensation risk calculator

1. Dew point (Magnus): γ = ln(RH ÷ 100) + a·T ÷ (b + T); Td = b·γ ÷ (a − γ), with a = 17.62 and b = 243.12 °C
2. Inside surface temperature θsi = θroom − U × Rsi × (θroom − θoutdoor), with Rsi = 0.25 m²K/W for condensation checks
3. Temperature factor fRsi = (θsi − θoutdoor) ÷ (θroom − θoutdoor)
4. If the surface is at or below the dew point, water condenses on it

Mould can grow before water appears: surfaces only a little above the dew point stay damp. Ventilate and heat steadily.

Condensation inside the wall or roof (interstitial, Glaser method) isn't covered: insulation makers offer a condensation risk analysis.

Loft insulation calculator

Open the loft insulation calculator

1. Rolls per layer = loft area ÷ roll coverage (m² on the pack), rounded up
2. Total depth = insulation already there + layer 1 + layer 2
3. Lay layer 1 between the joists and layer 2 at right angles across them, so the joists are covered

Our 270 mm target is a commonly quoted figure for mineral wool that we're still checking; a different material needs a different depth for the same U-value.

Don't squash insulation: if it ends up deeper than the joists and you want to board the loft, raise the boards on loft legs.