The lambda value of a material indicates its ability to transfer heat: this property is therefore very important in the design of highly performing buildings and passive houses.
The information commonly available is unfortunately quite confusing: with this article, we'll try and shed some light on the topic.
In Europe, the lambda value [λ] of a material belongs to the data that the manufacturer has to include in the data sheet of a product, in order to be allowed to put it on the market. This counts for any kind of material (cork, rock wool, but also concrete, glass, steel and so on), but it is of particular importance for insulation materials, which we described in a previous article.
In the US, on the other hand, this value is not very widespread: insulation products are organized by R-values, that describe the thermal resistance of an insulation panel of a given thickness.
If the energy analysis is not very accurate, using the R-value alone can be enough, limiting the calculation to a monodimensional heat flow simulation. In this case, you don't need to know the lambda value of the material (which has already been used to calculate the declared R-value). For bidimensional or tridimensional heat flow calculations, however, you do need to know the lambda values of the products you are simulating. This is the case, for example, of a finite elements calculation of a thermal bridge.
In the design of the thermal envelope of any building, and even more so of a passive house, one of the first steps the designer needs to take is to establish the design thermal properties of the materials used in the project.
It goes without saying that such an important value can become the object of barbaric marketing by manufacturers of insulation materials.
HOW IS THE LAMBDA VALUE DETERMINED?
The reference norm is ISO 10456, that includes the calculation methods to determine the declared lambda value of a material, and the design lambda value. The norm also include data tables of design lambda values for standard construction materials (wood, plastics metals and so on), which are less performing from a thermal point of view, and therefore less subject to changes in their overall performance due to installation conditions.
Overall, the lambda value of a material (that is, its ability to transfer heat) varies depending on three main factors:
- the actual temperature of the material installed;
- its humidity content;
- its aging process.
DECLARED LAMBDA VALUE
The declared lambda value, [λd], represents the thermal conductivity of a material as tested in a lab.
Its value is calculated statistically by the manufacturer, on the basis of multiple lab tests.
These tests are carried out with constant conditions of temperature and humidity. The aging of the material is already included in the declared lambda value, whereas the influence of installation temperature and humidity conditions need to be calculated by the designer, according to the specific building conditions.
For the declared lambda value to be acceptable, it has to be based on a minimum number of tests, usually 10.
As mentioned, the declared lambda value is calculated statistically from the test results, with ISO 10456. The declared lambda value ensures that the result is valid for 90% of the produced material, with an accuracy level of 90%. This is why the declared value can also be described as [lambda 90/90].
DESIGN LAMBDA VALUE
The design lambda value represents the thermal conductivity of a material as installed in a building.
This value needs to be calculated by the designer, according to the specific internal and external design conditions (temperature, humidity), which influence the performance of the material once installed. As previously described, the aging of the material is already included in the declared lambda value.
Depending on different installation conditions, one material can have several design lambda values, even within the same building.
The design value is calculated by the designer (or energy consultant), starting from the declared lambda value included in the product data sheet.
CAN I USE THE DECLARED LAMBDA VALUE AS DESIGN VALUE?
Many questions come to mind:
- as designer, do I have to calculate the lambda value for each different installation conditions?
- can I use the declared lambda value, calculated by the manufacturer, as my design lambda value in the design of the building?
- if I use the declared lambda value, is the manufacturer liable for it?
At least in an European context, the answer to those questions is easy: the designer is required to assess the performance of the installed material.
The manufacturer is liable for the material performance in lab testing;
The designer is liable for the performance of the material installed.
As designer, you can use any lambda value you want, as long as you are ready to answer for it.
The lambda value used in the energy design has a direct effect on the estimated energy performance of the building, for both energy demand and heating/cooling loads.
ONE-OFF CALCULATION OF THE DESIGN LAMBDA VALUE
The calculation of specific design lambda values for insulation materials can be very time intensive. Within the same project, the same material can have different design lambda values depending on the installation conditions (slab on ground; roof and so on).
If we implement the practical approach of the DIN4108 norm, you can do a one-off calculation of the design lambda value of a material, as follows:
Design lambda value = declared value + 20%.
Such an increase is conservative relative to an accurate ISO10456 calculation, and it allows to always use the same design value regardless of the installation conditions.
In the case of a rock wool insulation panel, for example:
Declared lambda value: 0,035 W/mK (0,020 BTU/h*ft*°F);
Design lambda value: 0,042 W/mK (0,024 BTU/h*ft*°F).
In case the manufactures subscribes to third-party quality control, the quality of the production can be assumed to be more consistent, and the increase of the lambda value can be limited to 5%.
Rock wool insulation panel, with third-party quality control:
Declared lambda value: 0,035 W/mK (0,020 BTU/h*ft*°F);
Design lambda value: 0,037 W/mK (0,021 BTU/h*ft*°F).