SLAB-ON-GROUND FOUNDATION

Using the sturdy and waterproof Finnfoam can significantly improve the performance of a slab-on-ground foundation and increase its resilience. The resilience of a structure is one of the most important guarantees for its functionality and durability. If the thermal insulation is installed under the slab, the structure will be significantly more resilient towards water damage, for example. Today, as the thicknesses of base floor insulations are increasing, it is very important to also increase the strength of the insulation in proportion. If this is neglected, the compression of the insulation will increase, leading to expensive repairs, as baseboards have to be dropped down and torn waterproofing in wet rooms requires mending. More information about these issues is included below!

Keep your slabs dry!

In the ground under the base floor insulation, the relative humidity is always 95–100%. Thus, it is important that your thermal insulation has a high enough water vapor resistance and that it won't become waterlogged. A waterlogged insulation will no longer insulate as designed and will keep the poured floor moist, which may lead to indoor air issues. Finnfoam is waterproof!

With proper thermal insulation, you can also avoid overheating the subgrade in small houses, where the construction span of the building is quite small. With floor heating, the risk of overheating will always be higher. As the construction span of a building increases, the amount of thermal insulation required keeps growing. As Finnfoam has low permeability for water vapor, it provides time to the concrete slab to dry inside. Thus, the moisture content of the concrete slab remains lower and the moisture performance of the structure is improved.

During a hot and humid summer, water vapor flows toward the ground from inside. This is why you should never place separate plastic vapor barriers between any of the slab-on-ground structures, as they reduce the moisture performance of the structure! To curb the flow of water vapor, thermal insulation should be highly resistant toward water vapor, as Finnfoam is. (Source: VTT and TTY)

With leak-proof coating, Finnfoam improves the moisture performance of slab-ground foundations significantly. 
Source: TTY – Virpi Leivo ja Jukka Rantala.

Actual thermal conductivity is measured using the lambda U value

The declared thermal conductivity λD specified for thermal insulation and frost insulation products specifies the optimal thermal conductivity of the product. However, in designing and dimensioning thermal/frost insulation you should always use the actual thermal conductivity of the product, i.e. the lambda U (λU) value, which factors in the negative impact on thermal conductivity caused by moisture, for example. To be able to calculate the λU for an insulation used in the ground, for example, we must know the water absorptivity of the product when immersed and by diffusion, as well as after freeze-thaw-resistance testing.

Included below are lambda U values calculated for Finnfoam and FF-EPS insulation products when used on the ground, i.e. in slab-on-ground foundations, as vertical insulation inside or outside of footings or as frost insulation. In addition, we have also listed actual lambda U values of other EPS insulation products used in similar applications for comparison.

 

The Lambda U values in the enclosed table have been calculated using the following formula, based on the EN 10456 standard and the RIL 225 instructions, which are being finalized:

λ_U = λ_D x F_T x F_M x F_a 

λ_D = Lambda Declared, F_T = Temperature conversion factor, F_M = Moisture conversion factor, F_a = Aging conversion factor

Do not allow the slab to subside

As the thickness of thermal insulation is increased, the insulation must also be made stronger. Neglecting this will lead to higher subsidence. This is particularly highlighted today with growing base floor insulation thicknesses as we move increasingly toward zero-energy houses. The following can be used as a rough guide for the sufficient strength class of floor insulation in residential buildings: With 100 mm of thermal insulation, the sufficient short-term compressive strength is 100 kPa, 200 mm insulation = 200 kPa, 300 mm = 300 kPa, etc.

The sturdy Finnfoam has a very low subsidence, as it reaches its maximum strength (short-term compressive strength) at approximately 2% of compression, and thus its long-term compressive strength is also very high. The sturdy Finnfoam can also be used to produce solid thermal insulation under fireplaces and load-bearing partition walls. In addition, the Finnfoam insulation products are highly resistant to point loads during construction without crumbling occurring within the floor.

The long-term subsidence of Finnfoam (F-300, 32 kg/m³) is less than one tenth of the subsidence of EPS commonly used in floors (EPS 120 20 kg/m³ or EPS 100 18 kg/m³).

Cost effective panel size

Finnfoam insulation panels are very resistant to the strains resulting from casting while constructing a split footing. The sturdy Finnfoam simultaneously functions as footing insulation and casting mold panels, which ensures efficiency. The smooth surface of Finnfoam allows it to be removed intact after casting, as it does not stick to the concrete. If you do want the insulation to adhere to the concrete, the Finnfoam surface must be roughened or fluted slightly. Finnfoam insulation can be later installed on top of the footing using renovation mortar, for example. The Finnfoam panels may be coated with a thin coat of plaster, which should be done according to the instructions provided by Fesco, for example.

The standard Finnfoam panels are quite large at 2,500 x 600 mm, or 1.5 m². The panels are still appropriately sized and light enough for one person to handle, which means that their installation is cost-effective.

Ventilated slab-on-ground floor

Finnfoam can also be used to construct a ventilated slab-on-ground floor, where a network of small ventilation channels is created between two layers of insulation. Even minor ventilation is enough to remove the water vapor load coming from the ground through the lower layer of Finnfoam. The ventilation has no impact on the thermal insulation capacity of the structure. The ventilation channels must be set up in a continuous manner.

Natural convection will generate a small amount of ventilation, which means that when ventilation is not needed in the summertime, its amount is reduced. Most of the radon moving up from the ground will also be removed through the same ventilation channels.

A ventilated slab-on-ground floor structure constructed with Finnfoam will also work in larger buildings where the ground temperature is higher in the center of the building. In addition, the surface material of the slab may be highly vapor proof. The structure will also reduce the radon risk.