It is believed that the UVF design was developed and first put into practice by engineers of the Finnish construction company Omatalo. The Finnish foundation is a constructive combination of four elements:
- shallow tape monolith in a section of 200 x 600 mm with a heel widened to 800-1000 mm;
- 200 mm thick concrete blocks mounted on top to form the basement of a building;
- a layer of extruded polystyrene foam (EPS) with a thickness of 120-150 mm laid on a packed rammed pillow and on the entire inner surface of the tape;
- reinforced concrete slab 60-80 mm thick, cast over a layer of insulation, with pipes of a warm water floor laid inside it.
Heat-insulating plates are laid on a two-layer rammed pillow made of sand and small-gravel rubble, covered with geotextiles. The exact thickness of the layers of insulation and concrete in the insulated Finnish foundation is determined by the climatic characteristics of the region, the type of soil on the site and the types of materials used.
For the cold regions of Russia, it is recommended to cast the tape and basement of the foundation into a single reinforced monolith, using EPPS as a permanent formwork.
To more effectively reduce heat loss around the entire perimeter of the building, a heated blind area with a width of at least 500 mm is required.
Differences from the insulated Swedish plate
The main difference between the Finnish and Swedish designs is that the Finns suggested using a strip foundation as an additional support around the perimeter.
USP is just a concrete slab cast in a kind of expanded polystyrene tray, which is what distinguishes it from UVF. As a result, the preparation of an insulated Finnish plate does not require the preparation of a strictly calibrated surface horizon, which means that construction can be carried out on sites with a slight slope. We already wrote about this kind of foundation here.
The absence of a rigid connection between the strip foundation and the floor platform makes the structure more resistant to fracture and increases resistance to loads on problematic unstable soils. Besides, UVF foundation technology allows you to perform the device and pouring the slab after the erection of walls and installation of the roof. This means that these works can be carried out in the cold season, without stopping construction for the winter.
Pros and Cons of UVF
Like most building structures, the insulated Finnish foundation has advantages and disadvantages. Among the main the benefits experts call:
- the possibility of mounting the base on any type of soil, except for weak peatlands with a high level of groundwater;
- economical consumption of materials in comparison with other types of substrates, except for the Swedish plate, where the cost is about the same;
- the floor screed is not bearing and the entire weight load from the building envelopes is transferred to the strip part of the foundation;
- the presence of a built-in heated floor reduces the cost of installing heating;
- filling the insulated floor is possible at any time of the year;
- the ability to mount a bearing base on small slopes and at elevation elevations.
TO disadvantages foundation technology finnish stove include:
- the need for excavation with digging trenches and pits, which may require the use of special equipment;
- performing backfilling with non-metallic materials after mounting the tape part followed by mechanical tamping;
- low bearing capacity of a shallow tape limiting the number of storeys of buildings.
As you can see, for light one-story individual construction, the insulated Finnish foundation is a technically and economically feasible option.
In what cases is applied
Due to a number of technical and operational advantages, UVF will be an excellent solution for the construction of light one-story buildings in case of:
- inclined topography of a building plot;
- high water table;
- unstable and heaving soils;
- lateral mobility of the upper layers of the soil.
Installation of a warmed foundation using Finnish technology is simple, which makes it possible to do it yourself without hiring brigades, as a result of which the foundation of the house will be much cheaper.
Construction technology
Installation of the foundation in the form of an insulated Finnish plate is carried out in several successive stages:
- selection of the construction site and marking of the contours of the building with a margin on the outside of 500-700 mm;
- removal and storage of the fertile soil layer;
- development of a foundation pit, trenches for a drainage system and external utilities;
- connection to external engineering networks;
- backfilling and tamping of sand and gravel pillows;
- laying and fixing the hydraulic insulation layer;
- installation of formwork and installation of reinforcing cages;
- installation of pipes or heating cable for underfloor heating;
- concrete pouring.
The latter type of work and reinforcement may be carried out in two stages. First, the tape part is concreted, and then, at a convenient time, a warm plate is arranged.
Pit and trench marking
The designation of the contours of the pit is similar to marking the usual shallow foundation, but here, using the cord and pegs, only the external contour is marked. It should be noted the exact line of the outer wall of the concrete strip, step back from it 500-700 mm and pull another cord. This will be the edge of the pit, but not the foundation.
Depending on the total weight of the building, the width of the strip foundation can be from 600 to 800 mm. In the lower part, a support heel with a width of 800-1000 mm is arranged. The width of the insulated blind area along the poured non-metallic materials should be at least 700 mm. The depth of laying of the tape part of the foundation is at least 600 mm from the lowest point of the soil surface.
Laying drainage and bedding
Drain pipes should be laid around the entire perimeter of the tape. To do this, at the edges of the pit, you need to dig a recess 500 mm below the supporting heel and remove the pipes outside the construction site. Laying of drainage pipes should provide for a slope of 4-5 degrees.
Backfilling of the first layer of sand should be carried out on geotextiles laid on the ground. This will stop the flow of moisture and prevent the germination of weeds.
Sand is carefully rammed and spilled with water. After that, fine-grained gravel is poured on it. The thickness of both layers is 80-120 mm.
Waterproofing device
On top of crushed stone, two layers of rolled hydraulic insulation are lined with a coating of overlapping joints with bituminous mastic and the establishment of edges above the ground. Adjacent strips of material should be facing each other for 100 mm. The position of the strips in different layers is perpendicular, which will completely eliminate the coincidence of the seams.
Protection of the outer surface of the strip of the foundation is done after pouring and hardening the concrete mixture. For this, roll insulating materials with bitumen impregnation are used. Before gluing the surface, it is primed with a primer in 2-3 layers. Gluing of strips is carried out on hot bitumen with simultaneous heating of the wall and hydroisol.
Concreting tapes
This stage of work begins with the assembly of the reinforcing frame. Its design consists of two pairs of longitudinal strings with a diameter of 8 mm, located one above the other. The distance from the reinforcement to the edge of the concrete layer on all sides must be at least 50 mm.
To fix the longitudinal rods, it is convenient to use prepared square or rectangular frames with a diameter of 6 mm. The connection of the individual elements is carried out using a knitting wire or self-tightening polymer clamps.
Next, you need to install the formwork from the boards or durable sheet material.
A good solution may be the use of subsequently removable EPS sheets. Thus, the outer wall of the foundation will be insulated and there is no need to look for material for a removable shuttering structure. A reinforcing cage is installed inside the formwork, after which it is possible to fill in the concrete mixture.
Insulation, reinforcement and filling of the slab
Polystyrene foam boards should be laid in 2 or 3 layers, depending on the accepted thickness of the thermal insulation. The location of the plates must be performed in such a way as to exclude the coincidence of joints in different layers. The inner wall of the tape is also sutured with an EPSS layer 30-50 mm thick to the bottom of the structure.
The small thickness of the plate and the absence of large bearing loads makes it possible to use a reinforcing cage in the form of a metal mesh with a mesh of not more than 150 x 150 mm. You can assemble it yourself from a rod with a diameter of 6 mm or buy factory-made welded billets (read more about reinforcing in this article). It is convenient to use special plastic supports or clamps to raise the reinforcement above the level of the underlying pillow.
Extruded polystyrene foam is not afraid of moisture and retains its characteristics even when immersed in water.
Pouring concrete
Before laying the concrete mixture directly on the reinforcing mesh, you need to fix the pipes of the warm floor or a heating electric cable. They can be tied with wire or plastic clamps. After that, proceed to pouring concrete.
The concrete mix is laid from one of the corners with a strip of 0.8-1.0 meters, moving along the wall. After laying one strip completely, compact the mortar with a vibrator and level the surface. Then, similarly fill the second strip and so on until the end of the site.
Blind device
The full efficiency of the construction of the Finnish slab foundation will be achieved only if there is an insulated blind area around the perimeter of the building (read about it here and here). As part of it, the presence of:
- supporting pillows made of sand and gravel covered with geotextiles;
- roll waterproofing;
- polystyrene thermal insulation layer; v
- cement or concrete screed.
Properly made blind area will provide additional thermal protection for the foundation of the building, eliminate water wash-off of structures and increase the service life of building structures.