Ventilated base floor
In a ventilated base floor structure, which uses a hollow-core concrete slab, the Finnfoam insulation should be placed on top of the slab. In this case thermal bridging is minimized, and the fire safety of the structure is ensured. This will also ensure the energy efficiency of a potential floor heating, as the structure allows the floor heating to react quickly to changes in the outdoor temperature and energy is not wasted on heating the hollow-core slab.
When constructing a ventilated base floor structure with a hollow-core concrete slab or timber joists, it is always very important to install thermal insulation with high water vapor resistance throughout on the floor of the crawl space against the foundation soil. A 10–20 cm layer of sand is placed on top of the Finnfoam to keep the insulation in place and to facilitate moving around in the crawl space. This will significantly improve the moisture performance of the ventilated base floor, as the length of the condensation period in the spring is reduced due to the quickly heating crawl space. In addition, Finnfoam will prevent the moisture rising from the ground from reaching the base floor structures.
Finnfoam ventilated base floor insulation
Planching attached to batten strips can be installed directly on top of Finnfoam. A filter cloth or similar can be used as cushion between Finnfoam and the flooring. The FL-K600 210 mm insulation has a ready-made recess for the strips.
Wind-proofing, insulation, backing, vapor barrier, and casting mold with a single installation. The insulation used in the structure is FI-K600 210 mm.
Finnfoam Oy has designed an insulation panel specifically for ventilated base floors. The high compressive strength (200–700 kPa, or 20–70t/m2) and rigidity of Finnfoam make it possible to simplify the ventilated base floor structure. The ventilated base floor can be completed quickly and as it can be walked on immediately after installation, the need for moving unsafely on top of the supporting rails is reduced. Due to the strength and waterproofing of Finnfoam thermal insulation, the ventilated base floor can be insulated first, thus providing a solid foundation to start building the walls and roof. The joints are sealed with an elastic/flexible PU joint foam before casting or after the roof-covering sheeting has been installed.
The entire thermal insulation, wind-proofing, vapor barrier, and casting mold required for a ventilated base floor can be completed within a single stage in just few minutes. The low number of stages makes installation highly cost effective.
View an animation of ventilated base floor insulation...
A concrete floor can be constructed directly on top of the Finnfoam ventilated base floor insulation with hydronic underfloor heating and any other technical solutions required. Another option is to lay plank flooring on top of the ventilated base floor insulation, or use flooring panels as a distributive layer, on top of which you can lay laminate or parquet flooring.
The short-term bearing capacity of a Finnfoam ventilated base floor is little over 1,700 kg/m2, and the long-term capacity exceeds 800 kg/m2. The bearing capacity is similar to a ventilated base floor with crosswise supporting rails. The insulation is designed for 50 mm floor joists with an on-center spacing of 600 mm (K600) and has a tolerance of -10...+10 mm. If the floor joists are thicker than 50 mm, the space in between the joists must always be 550 mm regardless.
The Finnfoam ventilated base floor insulation is installed on the inside between the supporting rails. The insulation is designed for 50 mm floor joists and has a tolerance of -10...+10 mm for the support spacing. If the floor joists are thicker than 50 mm, the space in between the joists must always be 550 mm regardless. The impermeable Finnfoam insulation does not require separate wind-proofing or vapor barriers. The sturdy Finnfoam panels can be walked on immediately after installation. This eliminates unsafe moving over the supporting rails. As the Finnfoam thermal insulation products are waterproof, they can be installed even where roof covering does not yet exist.
In case of water damage, Finnfoam will not get waterlogged and thus dries quickly, as the closed cell structure does not allow moisture to permeate the insulation. The mold risk is also minimized when all surfaces dry quickly. The interior seams of the panels are sealed with a flexible polyurethane foam. Foam can also be applied to the exterior seams, but this is usually unnecessary.
The interior joints of the panels are sealed with a flexible polyurethane foam. Foam can also be applied to the exterior seams, but this is usually unnecessary.
Finnfoam's high compressive strength and rigidity enable quick construction of simple and sturdy ventilated base floor structures.
Theory of the moisture performance of a ventilated base floor
A fully ventilated base floor, which is not surrounded by footings, is the safest possible solution with regard to radon. This type of structure is also very functional in terms of moisture performance. The temperature and moisture content of the crawl space fully follow the outside air conditions. If footing is constructed around the crawl space, the situation changes significantly. The U value requirement for a pillar-based ventilated base floor (0.09) is nevertheless significantly higher than it is for closed footing structures (0.17). Thus, ventilated base floor foundations are usually built with closed footings.
The average ground temperature is very close to the yearly average temperature. This can be easily observed in a root cellar, where the temperature remains quite stable throughout the year. The relative humidity of the ground is always 100%. These two factors result in a moisture risk, which is at its highest in the spring when the crawl space is significantly colder than outside air. The warm air coming from the outside increases the humidity, which cools down and condensates on all surfaces in the crawl space where the ground remains cooler for a long time. This can be prevented, or at least reduced significantly, by insulating the subgrade surface (we recommend installing 30 or 50 mm of Finnfoam throughout). This will allow the temperature of the crawl space to closer follow the temperature of the outside air, and thus eliminate condensation. Finnfoam is also highly effective in preventing moisture from rising up from the ground (water vapor), which occurs when the ground temperature is higher than the temperature of the crawl space or the relative humidity in the crawl space is low. The goal here is to maintain the lowest possible humidity in the crawl space.
Example A – ventilated base floor in the spring and summer:
When the ground temperature is +5° C, the partial pressure of water vapor in the pore space of soil is approximately 8.7 mbar (100%). If we want the relative humidity of the crawl space to reach 70% at most, the temperature of the crawl space should be +10° C or higher (partial pressure of water vapor 8.7 mbar) to ensure that water vapor does not rise from the ground into the crawl space. Water vapor will flow toward lower pressure! A Finnfoam thermal insulation placed on the ground surface will prevent water vapor from rising up from the ground and heat in the crawl space from transferring to the ground.
To ensure that ventilation air from the outside reduces humidity in the crawl space instead of the opposite, the outside air should therefore be drier than the air in the crawl space. Thus, if the temperature of the crawl space is +10° C and relative humidity is 70% (partial pressure of water vapor 8.7 mbar), the RH of the outside air may at most be 80% and the temperature +8° C. In the spring and summer, ventilation air can thus easily introduce more humidity into the crawl space, as the outside air is warmer than the crawl space. Finnfoam thermal insulation will prevent heat from transferring to the ground, which allows the temperature in the crawl space to rise quickly with the temperature of the outside air, thus reducing relative humidity in the crawl space.
Example B – ventilated base floor in the fall and winter:
Cold air reduces moisture in the structures effectively. The partial pressure of water vapor in outside air whose relative humidity is 90% and temperature -10° C is only approximately 2.6 mbar and the moisture content is 1.8g/m3. When the air in the crawl space is heated above +1° C, its moisture content is already 3.5 g/m3 and the partial pressure is 4.6 mbar when relative humidity is 70%. If the ground temperature is +5° C, the partial pressure of water vapor in the soil is 8.7 mbar. In this case, water vapor will rise from the ground at 4.1 mbar (8.7-4.6). The impermeable Finnfoam thermal insulation will prevent water vapor from rising from the ground to the crawl space!