Understanding Insulation Values: WLS, Lambda Value and Thermal Conductivity Classes

Anyone comparing insulation materials quickly stumbles over a series of numbers: WLG 035, WLS 032, lambda 0.024, plus a U-value for the finished building element. The terms come from different eras and sets of rules and partly mean the same thing, partly something entirely different. This article sorts out what lies behind these figures, where their significance ends and at which point specialist planning has to take over. It is intended as orientation and expressly does not replace an energy consultation.

What the Lambda Value Actually Measures

The lambda value (λ) is the thermal conductivity of a material, given in watts per metre and kelvin, that is W/(m·K). It describes how much heat flows through a one-metre-thick layer of the material when there is a temperature difference of one kelvin between the two sides. The smaller the value, the more slowly heat travels through and the better the material insulates. But that is all the lambda value says: it is a pure material property and says nothing yet about a building element.

Two further quantities are needed for the building element. The thermal resistance R of a layer results from its thickness divided by its lambda value; it therefore increases with thickness. The U-value then describes the entire construction with all its layers and the heat transfer at the surfaces, and is the reciprocal of the summed resistances. Only the U-value says something about the wall, the roof or the basement ceiling. A very good lambda value in a thin layer can end up performing worse than a mediocre insulation material in sufficient thickness.

The difference between two numbers that can both appear on a data sheet is also important. The nominal value λD is the value the manufacturer declares as part of the CE marking and which was determined under standardised laboratory conditions. The design value is the value used in calculations; through conversion and adjustment factors it takes into account that an insulation material in its installed state is exposed to different temperatures and moisture levels than in the test laboratory. The basis for this is set out in DIN 4108-4, the German standard on hygrothermal design values. Anyone comparing products should take care not to set the nominal value of one against the design value of the other.

A common misunderstanding concerns bulk density. A heavy insulation material is not automatically the better one, and a light one not automatically the worse. Bulk density relates to compressive strength, dimensional stability and acoustic properties, but only indirectly to thermal conductivity. Comparing by the packaging is equally unhelpful: whether a pack weighs ten or twenty kilograms says nothing about the insulating effect. What counts is the stated value and the thickness in which the material is actually installed.

WLS Classes at a Glance

WLG stands for Wärmeleitgruppe, thermal conductivity group, and is the older German classification. It groups insulation materials in steps of five: WLG 030, WLG 035, WLG 040. The number corresponds to the lambda value multiplied by 1,000, so WLG 035 means 0.035 W/(m·K). Because values were always rounded up to the next step, differences between products within the same group disappeared.

WLS stands for Wärmeleitstufe, thermal conductivity level, and has largely replaced the WLG. Here the classification is in steps of one: WLS 031, WLS 032, WLS 033. An insulation material with a lambda of 0.033 W/(m·K) falls into WLS 033, but under the old system was assigned to WLG 035 and thus presented as worse, in calculation terms, than it is. For comparing two products, the WLS is therefore the more precise figure – and if in doubt it is worth looking at the numerical value itself rather than at the class.

The common families of insulation material can be broadly sorted, although the ranges vary considerably depending on the product and manufacturer. PUR and PIR rigid foams sit at the lower end of the scale and insulate the most per centimetre, which makes them interesting where little space is available. Polystyrenes such as EPS and XPS as well as mineral wool sit in the middle range. Wood fibre, cellulose and other insulation materials of plant origin usually lie above them, but bring other properties with them, for instance in summer heat protection or in dealing with moisture. Which value applies to a specific product is stated in the data sheet and the declaration of performance, not in a table in a guide. If you are weighing up two specific products, do get in touch with us – we will consult the manufacturers' documentation.

In any case, the lambda value is only one of several product properties. Equally relevant are the fire behaviour according to the Euroclasses of DIN EN 13501-1, the compressive strength, the behaviour towards water and water vapour, acoustic properties and dimensional stability. An insulation material with the better WLS may simply be unsuitable for the intended installation location – for instance because it would be exposed to moisture there or would have to carry loads it is not made for.

Anyone standing in a builders' merchant wanting to assess a product will find reliable information in three places. The packaging states the application type, the thickness, the key value and the fire class. The manufacturer's declaration of performance records the declared properties to which it commits itself legally. And the technical data sheet describes how the material is to be processed, which substrates are permissible and which system components belong with it. It is also worth looking at details that come back to bite you on site later: edge formation with or without a rebate, panel format, dimensional accuracy and the question of whether the product is part of a tested system or sold as an individual component.

Why Thicker Insulation Is Not Automatically Better

The relationship between insulation thickness and heat loss is not linear. The first centimetres bring the biggest jump; each further layer acts on an already improved building element and therefore contributes correspondingly less. Doubling the insulation thickness does not halve the heat loss. At what thickness the gain no longer bears a sensible relationship to the effort depends on the initial condition of the building element and can be calculated, but not estimated.

Then there are thermal bridges. A carefully insulated surface is of little use if window reveals, roller-shutter boxes, balcony slabs, parapets or the junctions with the roof and plinth remain untreated. With heavily insulated building elements, the share of these weak points in the total loss is in fact relatively greater than with uninsulated ones. The detailed planning of the junctions therefore often determines the outcome more than the last two centimetres of insulation on the surface.

The third point is moisture protection. Insulation shifts the temperature profiles within the building element and thus the point at which water vapour can condense. This is particularly delicate with internal insulation, because the old external wall behind it becomes colder. Whether a build-up works is a matter of verification – depending on the case via a procedure in accordance with DIN 4108-3, the German standard on moisture protection, or via a hygrothermal simulation. That belongs in expert hands and is not a subject for rules of thumb from the internet.

Finally, the construction and its surroundings set limits: the existing rafter depth in the roof, the roof overhang at the façade, the clear height above a basement ceiling including door heights, boundary distances to the neighbouring plot, the reveal of existing windows and the load-bearing capacity of the substrate for fixings. Which thickness makes sense follows from the interplay of these constraints and from the requirements that apply to the particular project. How material decisions affect the service life of a building beyond this is something we have described in our article on sustainable building materials.

And then there is the workmanship. An insulation material only performs as well as it is installed. Open joints between the boards, voids behind the insulation through which room air can flow, surfaces not bonded across their full area or incorrectly placed fixings cost more performance than two additional centimetres would ever bring. This applies particularly to insulation between rafters: rolls and batts have to sit tightly and without gaps between the rafters. Anyone who calculates the thickness but economises on care pays twice – once for the material and once in the heating bill.

Typical Areas of Application: Façade, Roof, Basement

So that it is clear what an insulation material is actually intended for, DIN 4108-10 designates so-called application types with short codes. They appear on the packaging and in the data sheet and are often more important when buying than the lambda value:

• DAD – external insulation of a roof or ceiling, protected from the weather, beneath the roof covering
• DAA – external insulation of a roof or ceiling, protected from the weather, beneath waterproofing
• DZ – insulation between rafters in a pitched roof as well as insulation of non-accessible floor structures
• DI – internal insulation of the roof or insulation applied to the underside of ceilings, for instance on the basement ceiling
• DEO – insulation beneath screed without acoustic requirements
• DES – insulation beneath screed with acoustic requirements
• WAB – external insulation of the wall behind cladding
• PB – external insulation beneath the ground slab against the soil

On the façade, essentially four routes compete: the external thermal insulation composite system, the ventilated rainscreen façade, blown-in insulation into the cavity of a twin-leaf masonry wall, and internal insulation. Internal insulation is the most demanding route in building physics terms, but comes into play when the appearance has to be preserved – with slate façades, half-timbering or listed buildings, which occur regularly in our region. What else needs to be clarified in such cases is set out in our article on bathroom renovation in old buildings.

On the roof, the existing construction usually decides. Insulation between the rafters uses the space that is there anyway, but is limited by the rafter depth and is therefore often supplemented by a layer beneath the rafters. Insulation above the rafters lies continuously over them, avoids thermal bridges through the timber and is the more demanding solution, because the roof covering has to be renewed for it. In both cases the airtight layer is at least as important as the insulation material itself: leaks transport moisture into the construction, and considerably more than diffusion would ever bring in.

In the basement, insulating the basement ceiling from below is often the measure with the best ratio of effort to effect, because it manages without intervening in the living spaces. What has to be checked is the remaining headroom, pipes and ducts beneath the ceiling and the junctions with walls and staircases – it is precisely there that the thermal bridges otherwise arise which diminish the effect. Building elements in contact with the ground, by contrast, require insulation materials that can permanently withstand moisture and absorb practically no water. Here the application type is the decisive criterion, not the lambda value.

A special case that is often overlooked is the top floor ceiling above an unused loft. It is usually accessible, can be insulated without scaffolding and separates heated from unheated space – frequently the least complicated measure of all. What matters here is whether the surface is to remain walkable. Walkable versions require insulation materials that can take compression and a load-distributing covering; for non-walkable ones, softer materials or loose fill are sufficient. Here too the application type provides the answer, and here too the edge junctions with gable, chimney and loft hatch are the places where careful work is needed.

Where Specialist Planning Is Needed Rather Than a Guide

This article sorts out terms. It deliberately makes no statement about which insulation thickness or which U-value is correct or required for a particular building. The Gebäudeenergiegesetz, Germany's buildings energy act, sets requirements for new buildings and for building elements that are altered in existing stock; whether and to what extent they apply in an individual case depends on the building element, the scope of the measure and possible exemptions. Checking this belongs in an energy consultation or specialist planning and not in a blog article.

Specialist planning also delivers what a guide cannot provide: the U-value calculation for the specific build-up of the building element, the moisture verification, the assessment of thermal bridges, coordination with the fire safety requirements at the façade and – with insulation above the rafters or additional loads – the structural assessment. For funding programmes, involving a suitably qualified expert is generally a prerequisite in any case. Which requirements currently apply is something to clarify before the measure starts, not afterwards.

A consultation appointment becomes considerably more productive if a few things are available: the year of construction and – where available – documentation on the existing building, information on measures already carried out, photographs of the building elements and junctions concerned, information on the existing heating system and an honest account of what is due to be done anyway. If a façade has to be re-rendered or a roof re-covered in the next few years, that changes the calculation considerably, because scaffolding and set-up time then only arise once. Looking at insulation in isolation almost always leads to poorer decisions than placing it within an overall plan.

Once the planning is settled, it comes down to products: application type, lambda value or WLS, fire behaviour, thickness, system affiliation and availability. That is where we can help. Alongside tiles, bathroom and sanitary ware, Schindler GmbH also stocks insulation and building accessories; more about our company and our range can be found on our about page. If you already have a plan or a materials list, you are welcome to send it to us in advance – simply get in touch and we will discuss which products suit the planned build-ups.