U-values and heat loss

A U-value tells you how many watts pass through one square metre of a building part for every degree of difference between the air on the two sides. A roof with a U-value of 0.20 W/m²K loses 0.2 W per square metre per degree. Multiply by the area and you have the part’s conductance in watts per kelvin; add up every part and you have the heat loss coefficient of the whole envelope.

Over a heating season, what counts is not the coldest day but how many degrees colder than indoors it is, day after day. That is what degree days measure. Together with the U-values and the areas, they give the heat the envelope loses in a year.

In a hall, the roof usually loses the most. It is the largest surface, often larger than the floor under a pitched roof, and it sits where the air is warmest. Windows lose far more per square metre than walls, but they are a small share of the area.

Heat loss through the building envelope

Insulation

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

Height to the ridge

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

Temperature where people are

°C

More settings

Width

Length

Wall height

Roof

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

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.

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

Heat lost through the envelope45.5 MWhper year

Cost of that heat3,640 EURper year

CO₂e3.41 tper year

Heat loss coefficient769 W/KΣU·A of the envelope

Heat lost per year by surface, kWh
	Heat loss
Roof	15,000
Walls	14,100
Windows	9,380
Floor	7,060

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.

Try changing the insulation to a building from the 1970s: the walls go from 0.30 to 1.00 W/m²K and become the largest loss. Then raise the height to the ridge and watch the roof and gables grow.

The figures cover heat lost through the building envelope only. Ventilation, open gates and air leaks add to it; the sun and the heat from people and machines take some of it back, which the calculation covers with the free heat setting.

How we calculateLast subject-matter review: 20 October 2026

Formulas

Each surface i has a conductance $H_i = U_i A_i$ in W/K. Roof, skylights, walls and windows lose heat to the outside air through the heating season:

Q_i = \frac{U_i A_i}{1000} \cdot 24 \left[ G + D \,(T_i - T_\text{free} - T_\text{b}) \right] \quad \text{kWh}

where $G$ is the degree days at the base temperature $T_\text{b} = 17\ ^\circ\text{C}$ , $D$ the days in the heating season, $T_i$ the air temperature at the surface and $T_\text{free}$ the free heat. The floor loses heat to the ground at $T_\text{g}$ through the whole season, never a negative amount:

Q_\text{floor} = \frac{U_\text{floor} A_\text{floor}}{1000} \cdot 24\, D \cdot \max(0,\ T_\text{floor} - T_\text{free} - T_\text{g})

The air temperature rises by the gradient $g$ per metre: the floor is at the setpoint, the roof at the setpoint plus $g$ times the mean roof height, the walls halfway between.

Assumptions

A rectangular hall heated as one zone; the walls include the gables, windows are a share of the walls and skylights a share of the roof.
The tabulated degree days are shifted linearly to each surface’s temperature, which is exact when every day of the season is a heating day.
Typical U-values by building age follow the Danish building regulations; U-values for other countries’ regulations will follow.
Climate, prices and CO₂e factors are typical for the chosen country and can all be changed.

Range of validity

Heated halls with a ceiling height of 4–25 m and a floor area of 100–50,000 m². Not for buildings with large internal heat sources, cold stores or rooms with several temperature zones.

Standards and sources

U-values of building components: EN ISO 6946.
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

Ask Nordicco