Water-based underfloor heating in basements

Laying underfloor heating in older buildings, whose foundations are based on the principle of having a basement and a slab on the ground, may pose risks of moisture problems. Moisture problems may occur due to the ground structure (in both cases there is a concrete slab on the ground) being defective or not having:

  • A capillary break and drainage layer, which consists of washed gravel, macadam
  • Insulation – generally made of cellular plastic (EPS or XPS, or else mineral wool)

Capillary break and drainage layer

Capillary moisture transportation involves moisture being moved in liquid form in cavities and pores with small radii. An example of this is when you put a thin straw in a glass of water. The water level in the straw rises to a higher level than the water in the glass. Another example of this is a dishcloth or sponge – it absorbs the water through its pores when you use it to wipe down wet surfaces. The “suction” which occurs is merely due to capillary moisture transportation, and this property also applies to building materials.

To ensure that concrete in the ground structure (which has the ability to absorb water if it is in contact with water) does not make contact with the wet ground, there should be a porous layer underneath the structure capable of preventing water from being absorbed – the capillary break and drainage layer – which also has the task of discharging the water away from the building’s foundation. This is done via the drainage system. The water is usually what is termed surface water, which comes from precipitation, but also from below in the form of groundwater.

In older ground structures this layer is missing and, even if present, may consist of sand or otherwise be ineffective in its function. This means that the concrete slab is damp and, at worst, wet with visible damp areas primarily where the slab meets the walls. Subsequently inserting this kind of layer will be labor-intensive and financially burdensome, although technically feasible.


Ground insulation has two functions, the main one being to provide heat insulation in order to reduce heat loss. As the underfloor heating increases the slab’s temperature, the downward heat loss increases, which will increase heating costs. This heat loss may also increase the risk of future problems with damp in the form of vapor diffusion. Diffusion is a form of moisture movement where individual water molecules migrate, in a random manner, from a higher concentration to a lower concentration. An example of diffusion is when a drop of ink falls into a glass of water. The ink’s molecules will spread in the water from an area with a higher concentration to one of lower concentration, until the concentration is equal after a certain period of time.

Vapor diffuses via air in material pores. In the case of structures on the ground, the vapor migrates from a higher concentration to a lower one. The damp ground, which comprises up to approx. 30% air containing moisture, may have a higher vapor concentration than the air in the building. In this case, the moisture migrates from the ground, through the structure, to the ambient air inside. The vapor concentration of the air in the ground is highly dependent on temperature, which means that the hotter the air the more moisture it may contain. The link between the air temperature and the amount of moisture the air can contain is known as the vapor concentration at saturation point.

Vapor concentration at saturation point

Vapor concentration at saturation point

In an underfloor heating context, this means that heat loss is heating the ground. When the ground’s temperature rises, the vapor concentration increases in the ground’s pores – setting water molecules in motion. With a larger vapor concentration in the ground (in other words, an increase in concentration), there is greater momentum for moisture to diffuse through the ground structure and into the building. Based on moisture diffusion, ground insulation has a technical moisture function. By reducing heat loss, the ground will remain cool, which keeps the ground’s vapor concentration to levels preventing an upward vapor flow or at least maintaining it at a low level.

The vapor flow can be small for uninsulated slabs or slabs with little insulation. As long as the vapor is allowed to diffuse through the structure and be released into the air, the structure will cope.

The risk of moisture problems may occur when and if:

  • The structure is covered with a sealed material, such as plastic flooring. This prevents the vapor from permeating through, which means that it is accumulated under the membrane to the extent that the vapor concentration will be the same under this layer as in the ground;
  • The temperature in the room space/structure drops in the long term, which may cause condensation to form in the structure;
  • The temperature in the room space/structure rises in the long term, which may cause an increase in vapor flow through/in the structure;
  • A combination of the above points – which may be considered applicable in an underfloor heating context.

System solutions

The above descriptions of the processes involved contain constant reference to risks. It is even more difficult to express in terms of figures exactly what state the basic structure is in and what it will be like with or without underfloor heating installed – many aspects are directly dependent on a number of local conditions and variables, such as:

  • The capillary break and drainage layer’s properties and status;
  • Local ground conditions, for example, groundwater table, ground properties, gradients;
  • This structure’s design, constituent materials, and thicknesses;
  • Choice of flooring material;
  • The level of ventilation, temperature, moisture sources in the room space.

However, you can think about strategies for making structures more damp-proof, where (unfortunately) the more damp-proof the structure is, the more expensive it will be. However, what is clear is that you should:

  • Make sure that as much insulation as possible is laid under the underfloor heating structure;
  • Make sure that the structure has, as far as possible, an effective capillary break and drainage layer.

Otherwise, there are basically two strategies available:

  • Prevent penetration of moisture by using a damp-proofing layer at the surface;
  • Help remove any excess ground moisture via the structure, usually by using a ventilated air gap.

In cases where the foundations are found to be dry and there is little risk of moisture migrating, use of the Easy system, for instance, may be the preferred option. NB: The floor panels should not be attached to the concrete with flooring adhesive – a pourable adhesive (adhesives for larger tiles), such as Mapei Adesilex P4, should be used, as this product is moisture-resistant. As the Easy system’s material is relatively impermeable to moisture (EPS and, above all, aluminum foil), increased vapor concentration can be expected in the concrete slab. This can cause increased moisture flow around basement walls – you should use moisture-permeable paint on the walls.

There are a number of different systems on the market for the second strategy:

  • System Isola Platon (www.isola.se)
  • Nivell-systemet (www.nivell.se)
  • Floor Board System (www.fbs.se)

These systems form an air gap between the concrete slab and the overlying systems. Moisture can either diffuse via slotted skirting boards around the edge of the room or be extracted using mechanical ventilation (fan). The disadvantage with these systems is that they add to the height but, on the other hand, the structure will be more damp-proof. The Flooréwa and Easy system work well when laid directly on top of the Nivell or FBS system. In the case of the Platon system, a load-bearing board material must be laid first in order to produce a more rigid surface on which the Flooréwa or Easy system can be laid.