Warm air under the roof

Warm air is lighter than cold air, so in a heated hall it rises and collects under the roof. Without anything to move it, the air gets warmer the higher up you go. In a heated hall the rise is typically 0.3–0.5 K per metre, and more with warm-air heaters. At 0.4 K per metre, the air under the ridge of a 9 m hall is 3.6 K warmer than the air where people work.

That warm layer does nobody any good, and it costs. The roof loses heat in proportion to the difference between the air under it and the air outside, so every extra degree under the roof is paid for on the heating bill. The upper part of the walls loses more for the same reason.

Large, slow ceiling fans (HVLS) push the warm air gently back down and mix the hall to one temperature. That has two effects:

The roof and the upper walls see air at the temperature of the floor instead of the warm layer, so they lose less.
The floor gets the warm air back, so the thermostat can be turned down 1–2 K for the same comfort.

Stratification and mixed air

Height to the ridge

The highest point under the roof. Equal to the wall height for a flat roof.

Temperature rise per metre of height

K/m

How much warmer the air gets for each metre up, without fans. 0.3–0.5 K/m is typical for a heated hall; with warm-air heaters it can reach 1 K/m.

Lower setpoint with mixed air

Once the air is mixed, the floor is warmer, so the thermostat can be turned down. 1–2 K is typical.

More settings

Temperature where people are

°C

Width

Length

Wall height

Roof

Insulation

Sets typical U-values for the age of the building. You can change each one below.

U-value, roof

W/m²K

U-value, walls

W/m²K

U-value, floor

W/m²K

U-value, windows

W/m²K

U-value, skylights

W/m²K

Windows, share of the walls

Skylights, share of the roof

Free heat

Degrees kept warm by people, lighting, machines and the sun.

Ground temperature

°C

Degree days

K·d

K·days below 17 °C over the heating season. Typical for the chosen country.

Heating season

days

Heating

Price of heat

EUR/MWh

Per MWh of heat delivered, boiler or heat pump efficiency included. Typical for the country and heating.

CO₂e of heat

kg/MWh

Air under the roof22.9 → 18.0 °Cat the mean roof height, 7.3 m

Heat saved13.4 MWh30 % of the envelope’s loss

Saved1,070 EURper year

CO₂e avoided1.01 tper year

Air temperature from floor to ridge, stratified and mixed

Heat lost per year by surface, kWh
	Stratified (today)	Mixed air (HVLS)
Roof	15,000	9,870
Walls	14,100	10,300
Windows	9,380	6,860
Floor	7,060	5,040

Heat lost per year by surface, kWh

Indicative calculation based on the assumptions shown. How we calculate · Contact us for a calculation of your building.

The higher the hall and the stronger the stratification, the more there is to save. In a low hall with little stratification, most of the saving comes from the lower setpoint.

How we calculateLast subject-matter review: 20 October 2026

Formulas

The transmission loss of each surface is calculated as in U-values and heat loss, twice:

Stratified (today): the floor at the setpoint $T_\text{set}$ , the roof at $T_\text{roof} = T_\text{set} + g\,\bar h$ where $g$ is the gradient and $\bar h$ the mean roof height, and the walls at $(T_\text{set} + T_\text{roof})/2$ .
Mixed air (HVLS): every surface at $T_\text{set} - \Delta T_\text{sp}$ , where $\Delta T_\text{sp}$ is the setpoint reduction.

The saving is the difference between the two over the heating season:

\Delta Q = \sum_i \left( Q_i^\text{stratified} - Q_i^\text{mixed} \right)

Assumptions

The temperature rises in a straight line from floor to roof before, and the fans mix the hall completely after. In practice a small gradient remains with the fans running, so for the same inputs the saving is an upper estimate.
The free heat (people, lighting, machines, the sun) is the same before and after, so it changes the totals but not the saving.
The fans’ own electricity, ventilation and cooling in summer are not included here.

Range of validity

Heated halls with a ceiling height of 4–25 m. The gradient is an input: measure it, or use 0.3–0.5 K/m for a hall heated with radiators or floor heating and up to 1 K/m with warm-air heaters.

Standards and sources

U-values of building components: EN ISO 6946. Thermal comfort and draught: EN ISO 7730.
The method follows the Nordicco template for energy calculations, the same calculation the Nordicco sales team uses for quotes (engine crm-energy-1.7).

Results

Formulas

Assumptions

Range of validity

Standards and sources

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