Floor on a strip foundation with backfill: insulation and installation of the coating

Recommendation points

• Soil preparation and excavation
• Warming options
• Floor carrier system
• Rough coating
• Finishing and adjoining

One of the easiest and cheapest ways to install a floor in a private house is on the ground. However, at the same time, certain difficulties arise with insulation, the solution of which we will propose on a particular example of a floor arranged on a strip foundation with backfilling..

Soil preparation and excavation

After removing the formwork from the strip foundation and backfilling the external sinuses, all efforts should be focused on the central part of the plan of the future building. The main task is to remove the upper layers of the soil and fill the inner volume with material with certain qualities: hygroscopicity, sterility and low thermal conductivity. This task must be completed taking into account a number of features..

First of all, you need to understand that the floor area is not the most pronounced thermal bridge in the thermal protection of a building. Not only because of the relatively small area, but also because of the slight temperature difference. However, the heat flux through the floor is enhanced by high density, and hence the heat capacity of the external environment. In addition, when installing the floor on the ground, the zero mark of the house is located above the foundation and much higher than the ground level, therefore, preparation should be carried out taking into account the need to install not only a horizontal, but also a vertical heat protection belt.

Option of a concrete floor on the ground with backfilling of the foundation of expanded clay or gravel: 1 – parent soil; 2 – clay lock; 3 – backfilling of gravel or expanded clay; 4 – sand pillow; 5 – rough screed; 6 – a layer of insulation; 7 – finishing reinforced screed; 8 – finishing floor

The preparation process begins with excavation inside the foundation to the exposure of a layer of dense clay without extraneous inclusions, or to the level of occurrence. This is done in order to eliminate the migration of moisture into the backfill from the outer layers of the soil. Therefore, if during the opening to the depth of the foundation, no dense waterproof layer was found, it is created manually using the clay-based method. After that, the foundation is backfilled. With the subsequent installation of a screed on the ground, for this purpose, a sand-gravel mixture can be used, which is poured to the top of the foundation in layers of 3-5 cm with periodic dry compaction. If the floor is wooden, it is better to give preference to expanded clay, which is poured up to a level 15 cm below the foundation mark.

Warming options

Effective floor insulation on a strip foundation is possible only if the interior space is isolated from the ground and the concrete strip at the same time. The latter is extremely important: even in the presence of external insulation of the basement, concrete will be a pronounced bridge of cold. And even if freezing of the foundation is excluded, the temperature of its inner surface at a certain moment may be low enough for condensation to fall.

For high-quality floor insulation, a device of three thermal protection belts is required.

The vertical inner one is made of low density EPPS up to 20 mm thick. The foundation should be insulated from the inside before backfilling is performed. Insulation plates must be glued to concrete.

The vertical outer belt is arranged simultaneously with the insulation of the blind area and provides protection not so much from heat leaks from the house, but from frost heaving of the soil caused by freezing of concrete. It is important that the basement thermal protection does not end at the level of the foundation, but continues at least until the zero mark.

The main horizontal belt of thermal protection limits the outflow of heat from the space under the floor to the ground. Its device is possible in two ways:

1. If the depth of soil freezing in the region is insignificant, or high-quality insulation of the blind area has been performed, an expanded clay embankment can be used as thermal insulation. This option is most preferable when installing a wooden floor..
2. If the subfloor is formed by a screed, and overcooling of the soil at the level of the foundation is not excluded, it is required to place EPS boards with a thickness of up to 100 mm under the supporting layer.

Floor carrier system

A support system or sub-floor on a strip foundation can be arranged in two ways. Boarding on joists is considered preferred due to its ability to self-regulate indoor humidity, however, in terms of energy efficiency, this is the least profitable option. In addition, additional difficulties arise with the preparation of the floor in the premises where the tiled flooring should be laid..

The reinforced screed is characterized by a high service life, while an almost unlimited amount of insulation can be placed under it. The disadvantages of this method are expressed in the greater cost and complexity of the work, however, such a design is considered the most optimal for houses on a strip foundation, especially if a floor heating device is planned.

For a wooden floor, a lag system is required – massive beams supported by support posts made of bricks. The pillars are placed directly on the layer of material used to backfill the foundation. The height of the pillars is calculated in such a way that their tops are below the zero mark for the total thickness of the floor cake, taking into account the lag plus 20-30 mm. After laying on the pillars of roll waterproofing in 2-3 layers, they are covered with caps made of thick cement mortar of grade M400. After laying the logs on the mortar cap, they are recessed to the desired height, thereby ensuring preliminary alignment with a tolerance of up to 3 mm / m. The pitch of the beams can be easily calculated using a construction calculator for the thickness and allowable deflection of the boards. The section of the lag is determined taking into account the own weight of the floor and the payload (150 kg / m² constant and 300 kg / m² temporary) according to the method of calculating the deflection of a wooden beam on fixed hinged supports. For example, with a lag step of 1 m when installing a flooring of pine boards with a thickness of 32 mm, their sufficient section is considered to be 150×100 mm (per edge), supported by support posts every 120 cm.

When installing a screed, the foundation is pre-filled with layer-by-layer compaction, which subsequently provides the main bearing function. If large crushed stone or expanded clay is used as a sealant, the backfill is separated from the soil and screed by layers of quarry sand 40-50 mm thick. After the bedding is thoroughly compacted, insulation is laid on it, as a rule, in two layers with displaced joints. This is followed by rolling and gluing the hydro-barrier made of polyethylene film and fiberglass as temporary protection. Further, on 20 mm distance plugs, a reinforcing mesh 10x140x140 mm and a heating system are laid, after which a screed with a thickness of 60 mm or more is poured.

Both types of floor support systems have one key feature: they are located entirely above the upper plane of the foundation, that is, they cover it from above. This is required both for better protection of the ends of the cake, and for providing additional bearing capacity. For these purposes, the width of the tape must be chosen so that after the erection of the box, a ledge of 60–80 mm forms on the inside.

Rough coating

There are several ways to ensure high rigidity of wood flooring and good load distribution. This is important for such finishing materials as linoleum, during the operation of which deflection between adjacent boards is not allowed. Therefore, you should either use a grooved board for flooring, or additionally cover the floor with sheets of 5 mm plywood or 6 mm laminated fiberboard. When laminate flooring is used as a floor covering, a fixed joining of the boards is not required, but the direction of the flooring must be predetermined so that it is crosswise to the laminate boards..

The screed is, in fact, a ready-made sub-floor that practically does not require additional processing. When laying lightweight floor coverings such as linoleum or laminate flooring, all that is required is to eliminate residual dust, either by sanding the weakened upper layer or by priming for highly absorbent surfaces. If the floor requires leveling with a self-leveling screed, the rough surface is pre-coated with a primer such as “Betonkontakt”. If you plan to lay tiles, it will not be superfluous to wipe the screed with a cement-polymer adhesive mixture.

Finishing and adjoining

Almost any type of coating can be laid on the floor along the strip foundation, including a continuous contour, it all depends on the features of the preparation of the rough surface described above. You just need to observe a number of technological nuances.

When making a boardwalk, it is imperative to leave an expansion gap of 8–12 mm from the walls, depending on the length of the subfloor. In places of longitudinal joining of the boards, gaps should also be left to compensate for wood shrinkage of the order of 2-3 mm. The same rule applies when laying floor coverings: the gaps from the walls should not be less than 5–8 mm, subsequently they are covered with a plinth. When installing the screed, it should also be separated from the walls with a damper tape made of polyethylene foam. Its thickness is selected based on the calculation of 1 mm for each meter of floor length between parallel walls.