SYNEGRO · digital construction

Is your building element insulated well enough?

The U-value tells you how much heat escapes through one square metre of the wall, the roof or the floor. The lower the number, the better it insulates. Choose your construction and see the answer straight away. If you are converting or extending, step 5 shows what the building regulations require.

Which building element?

How is it built up?

Which insulation, and how much?

Insulation type

Insulation thickness

245 mm

Drag the slider ·

What does it have to meet?

The Danish Building Regulations 2018, annex 2

Calculated U-value

–W/m²K

Calculated to DS 418 and EN ISO 6946 — the same method as in our project documentation.

Section through the construction

Drag to rotate

Are you converting or extending?

BR18 ss. 271–279

On a conversion, energy savings must be carried out when they are profitable and do not create a risk of moisture damage (BR18 s. 274). They are profitable when annual saving × lifetime ÷ investment is above 1.33 (s. 275). The calculation here uses the construction you put together above — that becomes the "after", and you state what it looks like today.

Insulation today

0 mm

Or: U-value today (if you know it) W/m²K If filled in, it is used instead of the slider. Area being insulated m² Heating Price per kWh of heat DKK/kWh Extra cost of the added insulation (optional) DKK Only materials and labour for the energy improvement itself — not scaffolding, roof covering and the like.

What the rules say about conversion

§ 274. Energy savings must be carried out to the extent that they are profitable and do not create a risk of moisture damage. The requirement can also be met with a renovation class (ss. 280–282).
§ 275. A measure is profitable when annual saving × lifetime ÷ investment is greater than 1.33. If it is not profitable, that must be documented, and it must then be examined whether a smaller measure is. Where there is a cavity, it is first examined whether filling the cavity pays off.
§ 277. If a building element is replaced, the new one must meet table 3 regardless of profitability.
§ 278. Churches and listed buildings are exempt, and buildings worthy of preservation can be exempted.
s. 279 and annex 2, table 3. External walls 0.18 · roof and ceiling 0.12 · floors 0.10 W/m²K.
The investment is only what the energy improvement costs extra: materials and labour for the insulation and the necessary associated work. Scaffolding, a new roof covering and anything else that had to be done anyway do not count.
Lifetimes under the guidance to the building regulations: insulation 40 years · windows 30 years · heating systems and ventilation ducts 30 years · boilers and heat pumps 20 years · lighting and controls 15 years · sealing 10 years.

An extension can be documented in three ways (BR18 s. 271): with the energy frame, with the U-values in table 2 and no more than 22 % windows — or with the heat loss frame in s. 272. Enter the dimensions of the extension and you see which routes hold. The building element you calculated above goes in with its own U-value.

Heated floor area in the extension m² Existing facade that is covered m² The part of the house's facade the extension is built against. U-value of the covered facade W/m²K Half the heat loss through it may be added to the heat loss frame.

External wallsGross, i.e. incl. windows and doors m² W/m²K 0.15

Roof or ceiling m² W/m²K 0.12

Floor m² W/m²K 0.10

Windows and external doorsIncl. rooflights and glazed sections m² W/m²K

How the heat loss frame is calculated

The extension's heat loss is U-value × area for external walls (excluding windows), windows and doors, roof and floor.
The frame is calculated for the same extension with the U-values in annex 2, table 2, and with windows equal to 22 % of the floor area. The windows in the frame may be calculated with your own U-value or with 1.2 W/m²K — the calculator uses whichever gives the larger frame.
Half the former heat loss through the part of the facade the extension covers is added to the frame (s. 272(2)). Heat loss through an existing roof cannot be counted.
Linear thermal transmittance is not included here. The calculator assumes that foundations and windows hold the values in table 2 (0.12 and 0.03 W/mK). In the real calculation they count on both sides.
A ground slab's U-value from steps 1–3 is calculated for a house of 140 m². For a small extension the real U-value to the ground is slightly higher.

How this is calculated

The U-value is calculated to DS 418 and EN ISO 6946. Layers with timber running through — studs, rafters and joists — are calculated with the combined method, so the thermal bridge through the timber counts.
Ground slabs are calculated to EN ISO 13370 for a typical single-family house of 140 m² with a 48 m external wall perimeter. If the house is larger or smaller, the figure changes slightly.
The λ-values are indicative design values. If a particular product has been chosen, the manufacturer's declared value must be used in the final documentation.
Thermal bridges at foundations, windows and junctions are not included. They are calculated separately and can matter a great deal in a well-insulated house.
Step 5, conversion: The annual saving is the difference in U-value × area × 2,906 degree days × 24 hours (the normal year at 17 °C, the Danish Technological Institute). The lifetime for insulation is 40 years as in the guidance to the building regulations. The heat prices are examples from September 2026 — use your own.
Step 5, extension: The heat loss frame is calculated as transmission loss without linear thermal transmittance. For the building case the linear values must be included on both sides.
The calculator is an estimate for getting to know your own building. A building case needs full documentation.

Does the calculation have to be used in a building case? We produce the full documentation — with thermal bridges, linear thermal transmittance, the energy frame and a report the authority can approve. Write to us