Article

Materials, selection, execution – foundation waterproofing (part 4)

Comparison of foils with bituminous felt, details, joints and connections

Foils vs bituminous felt in foundation waterproofing

Rolled plastic materials (foils, membranes) are an ‘alternative’ to bituminous felt in the waterproofing of base slabs and foundation walls. They are easier to apply and less expensive. Let us analyse the arguments for and against foils and bituminous felts.

Tightness and durability criteria

Since tightness and operational durability must be the basic requirements for waterproofing, it is extremely important that the waterproofing material used forms a continuous, tight coating that is resistant to mechanical damage.

Foil insulation – design assumptions

Foil insulation requires a completely different approach than bituminous felt. First of all, the structure itself must be designed in such a way that the use of foil is technically feasible. This requires several conditions to be met. It is necessary to obtain a smooth substrate that will prevent point pressure on the foil and eliminate the risk of membrane puncture. In addition, the impact of shrinkage, thermal movements of the structure, etc. on the coating must be minimised. It is therefore necessary to use protective layers (e.g. geotextile) on both sides of the waterproofing membrane.

Methods of fixing foil and operational risks

Foil insulation can be fixed in several ways: by gluing it to the substrate, by loosely laying it on the insulated substrate, by laying it and fixing it at additional points, or by laying the foil on a suitably prepared formwork/substrate so that the foil bonds with the fresh substrate. The most common method is to lay the foil loosely on the insulated substrate. Spot fixing does not change the nature of the coating and is usually used when the length of the vertical section of waterproofing exceeds 3-4 metres.

What are the consequences of this approach? Any mechanical damage (breakage) to the coating causes uncontrolled leakage impossible to locate. This is unacceptable from the point of view of protecting the structure against moisture and ensuring its durability. Therefore, the correct execution of effective waterproofing with foil always requires the preparation of a detailed technical design and specifications before the membrane is laid on the construction site, taking into account the so-called division into zones or zoning. This allows the location and repair of the damage in the event of a leak.

Zoning and technology planning

However, zoning requires prior planning of several technological activities. The division into sealed sections is usually achieved by concreting (embedding) special tapes into the structure. Their layout must be correlated with the area of the separated zone (100–150 m²) ([1] - [5]), its shape, the shape of the sealed facility/surface, the layout of potential expansion joints, pipe penetrations, etc. Such an analysis should be carried out regardless of the type and function of the sealed object or facility (detached house, public building, underground garage, etc.) and the type of sealed surface (concrete, brickwork), and then the above-mentioned technological design should be developed.

Sealing of joints and joining methods

The critical point of a waterproof membrane coating is the tightness of the joints. For this reason, always check the tightness in accordance with the manufacturer’s instructions. Regardless of the type of plastic used in their manufacture, only foils that can be joined together using system adhesive, vulcanisation or heat sealing may be used. It is unacceptable to use foils that can only be joined by overlapping, as well as dimpled foils (membranes) – regardless of the method of fixing and joining.

Foils glued to the substrate – details matter

Foils glued to the substrate also require the same care. In this case, it is also necessary to carefully consider the sealing concept. Please remember that its effectiveness is determined not only by details such as expansion joints or pipe penetrations, but also by the order and direction of foil bonding at joints, especially at transitions between horizontal and vertical insulation.

Thickness and key parameters of the foil

Another issue is the required thickness of the foil and the key properties of this material. The lack of any requirements set by PN-EN standards means that foils introduced to the market in accordance with applicable regulations are often used without any analysis of their essential characteristics. The thickness of the foil used for waterproofing – depending on the type of material – should not be less than 1.5 or 2 mm [2]. Other required properties and parameters should also be specified, such as: impact resistance, bending resistance at low temperatures, resistance to tearing (nail shank), and shear strength of the joint.

Weldable bituminous felts and their resistance

Full bonding of weldable bituminous felt to the substrate (structural concrete base, slab or foundation wall) means that in the event of damage, leakage is limited to a small area (however, this does not necessarily mean that it will be easy to repair). In the case of foil, even when laid in sections, it is much more difficult to identify the location of the damage. In addition, the mechanical resistance of bituminous felt is higher than that of foil – laying bituminous felt in two layers (with a total thickness of at least 7-8 mm) effectively reduces the risk of leaks, both those caused by mechanical damage and those caused by overlaps. Bituminous felt designed for use in foundation slab waterproofing is made of a very strong glass fabric or non-woven polyester reinforcement, which gives it high mechanical resistance. In addition, the bitumen used in the production of these bituminous felts is modified with SBS, and often also with a 2 × SBS system, which gives the felt very high flexibility and cohesion.

Layout variants and their impact on waterproofing

The foundation construction and insulation diagram shown in Fig. 1 in part III of the series is often modified. This is due to several reasons which may occur together or independently. Firstly: the building has no basement and is founded on a slab – this solution is used in single-family buildings (detached or 2-3 flat segments). Secondly, when the building has a basement but is, for example, a passive building. In this case, waterproofing should be considered in conjunction with thermal insulation, and the latter must also cover the structural slab or floor on the ground.

Thermal requirements and consequences for layers

Technical conditions [6] require adequate thermal insulation for the building. This is achieved by limiting the heat transfer coefficient Uₙc(max) and the primary energy demand. When a building without a basement is founded on a slab, the thermal insulation-waterproofing system must be carefully considered. The foundation depth is usually several dozen centimetres, which means that the waterproofing is designed directly on the thermal insulation. This system is particularly sensitive to damage and errors, as the waterproofing coating laid on a susceptible substrate.

XPS as the only recommended solution in the ground

First and foremost, only XPS should be used for thermal insulation. There are two reasons for this: thermal insulation and compressibility (compressive strength is not specified for this type of material). Since thermal insulation is located in the ground and is exposed to constant contact with water, several very important requirements can be distinguished from the point of view of thermal insulation properties ([7], [8]), the fulfilment of which has a fundamental impact on ensuring the required and long-lasting thermal protection. These are:

water absorption during short-term moisture exposure of part of the surface,
water absorption during complete and prolonged immersion,
resistance to freeze-thaw cycles (by determining water absorption during complete immersion and/or as a result of water vapour diffusion).

Dimensional stability and mechanical parameters of XPS

It is equally important to ensure the dimensional stability of thermal insulation:

compressive stress at 10% relative deformation (compressive strength),
relative creep under compression and total relative thickness reduction,
dimensional stability at elevated temperatures and high relative humidity,
deformation under compressive load at elevated temperatures,
resistance to freeze-thaw cycles (by determining the reduction in compressive stress).

Minimum requirements for thermal insulation materials

The minimum requirements for thermal insulation materials [7] include:

compressive strength or compressive stress at 10% deformation – min. 300 kPa,
deformation at a load of 40 kPa and temperature of 70°C – max. 5%,
water absorption after three hundred freeze-thaw cycles – max. 2%; the reduction in mechanical strength must not exceed 10% compared to dry samples,
water absorption during prolonged immersion in water – max. 0.7%.

Floor layer systems on slabs

Moving on to waterproofing issues, the figures below show two floor layer systems on a structural slab. They differ in the location of the thermal insulation: in variant 1 (Fig. 1) it is located under the structural slab, and in variant 2 (Fig. 2) on the slab.

Thermal insulation on the slab – consequences of repairs

Placing thermal insulation on the slab facilitates the correct installation of the initial waterproofing, but significantly hinders (and sometimes even prevents) any subsequent repair works. The planned location of the waterproofing under the slab is correct from the point of view of water protection (when installed correctly, the slab is dry), but it is very difficult to repair in the event of errors resulting in leaks.

For the variant shown in Fig. 1 (thermal insulation under waterproofing), the substrate under the waterproofing coating is a compressible material (XPS boards) and not a substrate such as structural base concrete or even soil. This makes repairs very difficult (almost impossible) in the event of damage to the waterproofing membrane or errors in its installation.

Injection methods for secondary external insulation without excavation, known as curtain injections (also called ground injections), do not guarantee effectiveness in this case. This injection involves drilling a grid of holes through the partition (in this case, the foundation slab) and injecting a preparation under pressure (not exceeding 10 bar) into the surrounding soil, which forms a surface sealing coating at the partition-soil interface. Acrylic gels or polyurethane resins are most commonly used for this type of works. The presence of thermal insulation under the slab, rather than soil, makes the spread of the gel unpredictable.

Fig. 1 (above):

Diagram of waterproofing and thermal insulation of a building without a basement, founded on a slab, where the thermal insulation is located under the foundation slab.

Fig. 2 (left):

Diagram of waterproofing and thermal insulation of a building without a basement, founded on a slab, where the thermal insulation is located on the foundation slab.

Preferred layout from the perspective of durability

From the point of view of service life and minimising so-called difficult and critical areas, the option shown in Fig. 2 is preferable. On the structural base concrete, insulation is made of two layers of weldable bituminous felt, e.g. using NEXLER PREMIUM PYE G200 S40, NEXLER PREMIUM PYE PV200 S40 or NEXLER PREMIUM PYE PV250 S48 (the bituminous felt can be freely selected, but at least one layer must be made of bituminous felt with polyester reinforcement). The bituminous felt must extend beyond the outline of the thermal insulation so that it can be tightly connected to the vertical insulation. It is also important to remember to insulate under walls standing directly on the slab.

Should waterproofing also be applied to the slab?

At this point, the question usually arises as to whether additional waterproofing should be applied to the slab. There is no clear answer here. If the horizontal insulation under the slab and the vertical insulation are installed correctly, the slab will remain dry. However, any installation error combined with the inability to repair a leak will cause moisture or even water to penetrate the interior of the building. For this reason, it is strongly recommended to also apply waterproofing directly to the foundation slab. It must be tightly connected to the insulation under the foundation walls, and this section of the waterproofing coating should then be connected to the vertical insulation. Floor insulation can be made of bituminous felt or bituminous self-adhesive membrane (if there are no other contraindications) or mineral slurries (in which case a vapour barrier foil must also be used).

Insulation under the slab – installation technology

In the situation shown in Fig. 1, the insulation under the slab must be made of two layers of bituminous felt: the first layer is laid dry on XPS, and then the overlaps are welded (protective strips of bituminous felt can be additionally applied from below in the places where the overlaps are welded). The second layer of bituminous felt should be welded over the entire surface to the first layer. In this arrangement, bituminous felt with polyester reinforcement should be used (e.g. NEXLER PREMIUM PYE PV200 S40 or NEXLER PREMIUM PYE PV250 S48).

For vertical insulation, bituminous felt, bituminous membranes or seamless (no-joint) materials, e.g. KMB compounds, which can be easily combined with bituminous felt, can be used. Such details must be resolved individually. As in the case of the arrangement shown in Fig. 2, waterproofing under the walls standing on the slab and floor insulation must be provided (vapour barrier must not be omitted).

Plinth area and connection details

The plinth area also requires protection. Insulation on the structural wall should be extended to a level 30 cm above ground level. The plaster area on the insulation above ground level should also be protected. The diagram is shown in Fig. 3.

Coordination of connections, joints and installation passages and penetrations

It is always necessary to have a detailed plan for connecting the bottom panel insulation to the vertical insulation and to the insulation under the walls, for insulating the plinth area, and for sealing pipe penetrations, expansion joints and similar details. Their correct execution contributes to the long-term protection of the building against moisture and damage, which translates into its durability and safety of use for decades.

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Bibliography

[1] DIN 18195 Bauwerksabdichtung

[2] DIN 18533 Abdichtung von erdberührten Bauteilen

[3] Rokiel M. - Poradnik. Hydroizolacje w budownictwie. Projektowanie. Wykonawstwo", wyd. III, Grupa MEDIUM, Warszawa 2019.

[4] Rokiel M.– Hydroizolacje podziemnych części budynków i budowli. Projektowanie i warunki techniczne wykonania i odbioru robót, wyd. IV, Grupa MEDIUM, Warszawa 2019

[5] Specyfikacja techniczna wykonania i odbioru robót budowlanych. Roboty hydroizolacyjne. Izolacje przeciwwilgociowe i wodochronne części podziemnych i przyziemi budynków. wyd. II, OWEOB Promocja – 2017

[6] Rozporządzenie Ministra Rozwoju i Technologii z dnia 9 maja 2024 r. zmieniające rozporządzenie w sprawie warunków technicznych, jakim powinny odpowiadać budynki i ich usytuowanie, Dz.U. 2024 poz. 726

[7] DIN 4108-10 Wärmeschutz und Energie-Einsparung in Gebäuden – Teil 10: Anwendungsbezogene Anforderungen an Wärmedämmstoffe – Werkmäßig hergestellte Wärmedämmstoffe

mgr inż. Maciej Rokiel [MSc, Eng.] is a renowned mycological and construction expert and engineer with over 20 years of experience in protecting buildings against water and biological corrosion. He is the author of acclaimed publications, such as ‘Hydroizolacje w budownictwie’ (‘Waterproofing in Construction’) and ‘Renowacje obiektów budowlanych’ (‘Renovation of Buildings’), which are compendiums of knowledge for designers, contractors, and experts. His achievements include numerous expert opinions, articles and e-books published in prestigious industry titles such as ‘Izolacje’ (Insulation) and ‘Inżynier Budownictwa’ (Construction Engineer). He specialises in the diagnosis of dampness and salinity in walls, the design of waterproofing and the renovation of problematic buildings, and his works are widely cited in technical literature.