Do you feel drafts and cold in your house? Or are you going to build a new building and want to make sure that the building is well insulated? Then it may be time to add additional insulation. Additional insulation lowers the U-value and reduces heat loss. In this article you can read more about what U-value is and how it is calculated.
What is U-value?
The U-value measures how well a building material insulates heat and refers to the amount of heat that is transferred through one square meter of the material at a specific temperature. Contrary to many other contexts, insulation improves as the U-value decreases. Therefore, for example, it is wise to choose energy windows with a low U-value if you want to save energy and create a pleasant indoor climate.
What is a normal U-value?
Now we go through what different building parts normally have for U-value and what is a good or bad U-value for the various materials.
U-value for outer roof and attic
Warm air rises naturally, leading to significant heat loss through the attic and roof of a house. Therefore, it is crucial to have a well-insulated attic and roof to achieve energy efficiency.
In older houses, it is common to have unheated attics, also called cold attics, where the insulation is placed on the attic floor. Before the 1950s, the insulation usually consisted of a 10-20 cm thick layer of sawdust or chippings, which resulted in a U-value between 0.5 and 0.9. From the mid-1950s, mineral wool began to be used, which insulates twice as well as wood shavings - but the insulation was still thin. For houses built in the 1960s, with 10-15 cm mineral wool, the U-values were around 0.4-0.5. Over time, the insulation has in many cases been compressed and deformed, further degrading its insulating properties.
In the 1990s, many houses had 20-30 cm of insulation in the attic, resulting in U-values of 0.15-0.2. Today, 50 cm insulation in the attic joist is recommended for new construction or renovation, which can give U-values as low as 0.08. Since it is relatively easy to additionally insulate the attic in older houses, this is a measure that often quickly pays off through reduced heat losses.
U-value for external walls
The outer walls form a significant part of the house's climate shell and have a major impact on the house's heat loss if they are poorly insulated. The original insulation in the outer walls of older houses is usually very thin – sometimes even worse than the insulation in the outer roof.
During the first half of the 20th century, it was common to have a plank frame. Large planks were joined in several layers and held together by horizontal planks. At first they were not insulated, but later the cavities were filled with insulating material, usually sawdust or wood chippings. The U-values for the constructions were relatively high, between 0.6 and 0.9.
The wall constructions changed significantly from the mid-1950s. Among other things, they switched to regular and concrete frames with mineral wool insulation and brick and plaster facades. Even so, the insulation was still poor and the U-value high for windows, walls and doors until the early 1970s. The insulation was usually between 10 and 15 cm thick, resulting in U-values for the walls of 0.4-0.5. Over time the insulation thickness increased and in slightly more modern how the U-values are usually between 0.2 and 0.3.
Today, U-values below 0.15 are recommended for the outer walls. The thickness of a standard insulation in the outer walls is about 30 cm to meet these requirements. A little later we will tell you more about how you can calculate the U-value for your wall.
U-value for windows
The U-value of a window includes the entire window construction, which includes glass, frame and frame. Manufacturers sometimes specify the U-value for the center of the window, as it is often lower here due to better insulation between the panes than in the frame.
So what is the U-value for windows? Traditional linked 2-pane windows usually have U-values between 2.8 and 3.5, while linked 3-pane windows have U-values around 1.8-2.0. Older single windows can have U-values up to 5. After the energy crisis in the 1970s, windows with improved insulation and a lower U-value were developed. Early insulating glass windows had U-values similar to linked 3-pane windows. Modern double-glazed windows usually have U-values around 1.5, while triple-glazed windows are at 0.9-1.2.
By changing an older connected 2-pane window with U-value 3 to a 3-pane window with U-value 1, heat loss can be reduced to a third.
U-value for front doors
The U-value for an exterior door includes the entire structure of the door with both door leaf and frame. Older doors, usually made of solid wood, often have poor insulation and tend to warp over the years. This results in a lack of sealing between the door leaf and frame. Modern doors are usually thicker and have an insulated core. U-values for walls are usually between 0.7 and 1.5, depending on the design and any glass sections in the door.
Which U-value is best?
The choice of the "best" U-value depends on several factors. Among other things climate, purpose of use for the building and available budget. In colder climates, a lower U-value may be more beneficial in reducing heating costs. In a warmer climate, it is more important to have effective insulation to keep the heat out and thus reduce cooling costs. When choosing a U-value, it is also important to consider the costs of insulation materials and installation to ensure that the chosen U-value is optimal for your needs.
How do you calculate U-values?
Calculating the U-value for different building parts in a house is a somewhat complicated process. In practice, specialized computer programs are used. In addition, simpler calculation tools are available where you can make estimates of U-values on your own.
For windows and doors, manufacturers must provide U-values. As regards other building parts such as roofs, walls and floors, it is required to produce values for the thermal conductivity of the various materials included in the building part.
Thermal conductivity, usually denoted as lambda (λ), is measured in the unit of watts per meter and Kelvin (W/m K). It represents the amount of heat (in watts) that passes through one square meter of a material with a thickness of one meter when the temperature differs by one degree between the sides of the material.
For insulation materials, manufacturers must state lambda values. Reference values are usually used for other building materials. The table below contains approximate values for thermal conductivity for various building materials, as well as for air and water.
Material | Lambda (W/mK) |
Aluminum | 200 |
Steel | 60 |
Concrete | 1.7 |
Plaster and mortar | 1.0 |
Facade brick | 0.6 |
Gypsum board | 0.25 |
Wood | 0.14 |
Chipboard | 0.14 |
Plywood | 0.14 |
Lightweight concrete | 0.12 |
Mineral wool | 0.04 |
Cellular plastic | 0.04 |
Air | 0.026 |
Water | 0.6 |
Factors affecting thermal conductivity
The insulating ability of the materials depends to a large extent on their capacity to keep the air still in their pores. The reason is that air has a very low thermal conductivity. Materials used as insulation materials, for example mineral wool (stone wool and glass wool) and various types of cellular plastics, have low thermal conductivity as a result of their high porosity.
In addition, the U-value also depends on the material's thickness and is therefore calculated by dividing the thermal conductivity (lambda) by the material's thickness (d) in meters – U = lambda /d . A thick layer of a material with low thermal conductivity gives a low U-value.
Calculate the total U-value for a building part
Since the building parts of the house are made up of layers of material with different U-values, the different U-values are added to get the total U-value of the building part. Calculating the total U-value is done using the form: 1/U = 1/U1 + 1/U2 + … + 1/Un, – where U1 to Un are U-values for the different material layers . The inverted U-values for the layers are thus summed up to obtain the total U-value.
Calculate U-value when applying for a building permit
A U-value calculation ensures that the thermal insulation is sufficient, which contributes to high resident comfort and minimal energy consumption during the building's use. It is not as complex as a full energy balance calculation but requires information about the insulation in the climate shell and the windows. The U-value calculation is usually carried out during the planning and construction process to ensure that the building meets the national energy requirements and energy efficiency standards. When applying for a building permit and to meet national building standards, it is often required that parts of the building meet certain U-values to ensure energy efficiency and environmental requirements.
Read also: When is a building permit needed?
We help you with the U-value calculation - and the building permit
Konstruktionhjälpen are experts in everything to do with building permits. We help you with your application and ensure that all important documents are submitted - including U-value calculation. In this way, we can guarantee you a problem-free and smooth building permit process . We deliver your U-value calculation within four working days.
For us, it is important that you share as much information as possible about your project. Help us help you with your building permit! Contact Konstruktionshjälpen for more information about your U-value calculation - and the building permit in its entirety.
