The feasibility of insulating a foundation slab, criteria for choosing insulation, tips for carrying out work

A slab foundation is a flat or ribbed slab of reinforced concrete formed under the entire area of ​​the building.

A slab foundation is suitable for any type of soil; it provides sufficient reliability and stability to the entire structure. To ensure a cozy atmosphere in the building, prevent dampness, and the possibility of using basements as gyms and laundries, it is impossible not to create insulation for the slab foundation.

The concept of a slab foundation

Slab foundations are characterized by the maximum supporting area among all foundations. Because of this feature, the design ensures the stability and reliability of the entire structure on any soil, even if its bearing capacity is low. These types of foundations are resistant to soil heaving forces, under the influence of which the entire structure rises and falls. That is why the slab foundation is called floating; the monolithic structure is protected from local deformation and functions on the principle of a single platform.

Diagram of a slab foundation.

Buildings erected on such foundations move entirely along with their foundation, which prevents distortions. Slab structures have proven themselves to be strong and reliable; they are mainly used in the construction of private houses on soils with a low load-bearing coefficient. The foundation slab is used for the construction of buildings of any type on soils with any bearing capacity.

The construction of a slab foundation has disadvantages: it is characterized by material consumption, high consumption of concrete and reinforcement, high material costs, the need to create a subfloor to prevent freezing of premises in the cold season, and there is no possibility of creating hidden communications and utility networks.

Its advantages are: reliability, strength, high load-bearing capacity due to the large supporting area, ease of installation, the possibility of construction on difficult soils while reducing the volume of excavation work in places where the soil is deeply frozen, uniform distribution of the load of the building over the entire building area, lack of moisture in the premises, there is no need to create a pit and a complex formwork system, slab foundations can be erected by people with little work skills, concreting is done from the trays of concrete mixer trucks, without expensive equipment, which includes concrete pumps.

Expanded polystyrene: what glue is suitable for it? What is tow needed for?

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Why insulate?

A columnar foundation has a number of advantages over others: it is easy to create, the price is many times less than a monolith, it can be easily repaired and lasts up to 100 years. It must be insulated for the same purpose as the monolith - to minimize heat loss.

1. Creating a reinforced concrete grillage greatly increases heat loss, because in cold months the concrete cools very quickly and literally begins to draw heat from the walls and floor of the house.
2. If a metal or wooden grillage is chosen as the base, an air cushion is formed between the floor and the ground and constant air circulation occurs, and therefore in winter the floor and base of the walls of the first floor will be cold, even if you use the best thermal insulation material for interior decoration.
3. All communications of the house pass just in the space under the house. By insulating the foundation, you will thereby protect communications from freezing and possible mechanical damage.

Formation of the foundation slab

The construction of a foundation involves the following stages: site preparation, digging a pit, creating a cushion, drainage, installing formwork, pouring screed, laying waterproofing, reinforcement, pouring concrete.

Slab foundations can be shallow or deep. Shallow foundations are those laid at a depth that is less than the depth of soil freezing. During construction, the fertile layer from 200 to 500 mm is removed from the surface, and a pit is dug according to the dimensions approved in the building construction project.

After this technology work, work is underway to create a cushion of sand and gravel; it is pre-compacted. Layers of waterproofing are placed on top of it, and a reinforcement cage is placed on top of the waterproofing.

Slab foundation reinforcement diagram.

The foundation slab experiences transverse and longitudinal bending and torsion loads, so the requirements for the frame are severe. Ribbed reinforcement is used for reinforcement, which ensures maximum adhesion to concrete. The diameter of the reinforcement should be 12-16 mm.

The reinforcement is connected to each other with wire, since this method of fastening has advantages over welding: the welding points are fragile, the steel experiences overheating, losing its performance characteristics. The reinforcement frame consists of upper and lower mesh, rigidly fastened together with reinforcing bars. Metal consumption for a slab foundation is higher than for other types of foundations.

Proper reinforcement guarantees the durability and reliability of the structure. In this case, preliminary calculations are carried out taking into account the load of the building on the slab base. When the reinforcement frame has been laid, they begin pouring concrete in the pit. As a result, a large monolith slab is erected, serving as the foundation and floor.

The upper part of the slab turns out to be the floor of the basement or basement floor, without which the construction of a building on a slab foundation is unprofitable. The construction of a deep foundation involves digging a pit with a large volume of excavation work, which amounts to large material costs. These structures are being erected for the construction of very heavy structures and multi-story reinforced concrete buildings; they are not suitable for light houses.

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How to establish communications

To lay the pipelines of life support systems inside the cottage, it is necessary to make an input at a certain level, in accordance with the standards SP 31.13330 and SP 32.13330. The structural features of foundations should be taken into account to ensure the highest possible maintainability of engineering systems.

Requirements SP 31.13330 (water supply)

The sleeves are placed in the foundation when installing the formwork; it is not recommended to make holes after the concrete structure has gained strength by impact. When designing nodes for passing a water supply through a foundation slab or strip, it is necessary to take into account:

• the minimum permissible diameter of the communication pipe (sleeve) is 50 mm;
• to accommodate linear expansions during seasonal temperature changes, compensators should be used;
• the laying depth must exceed the soil freezing mark by 0.5 m, or polystyrene shells or heating cables are used to heat pipes;
• it is necessary to take into account the load on the soil from transport traffic;
• the entry hole in the supporting structures must be at least 0.2 mm larger than the diameter of the pipe; tightness is ensured by an oil seal at high groundwater levels or elastic materials in dry soils.

Sealing of communications inputs at high groundwater levels.

Water supply can only be installed vertically in floating slabs. In the underground, basement floor of a recessed strip foundation, the pipes enter horizontally. When using MZLF or NZLF, the main line passes under the tape, rises vertically inside and passes through the ceiling or floor along the ground.

Standards SP 32.13330 (sewerage)

Scheme of sewerage supply under the foundation

The laying of a sewer pipe through the enclosing structures of the cottage must meet the following requirements:

• minimum depth of 0.7 m from the blind area to avoid mechanical damage from vehicle traffic, people, soil cultivation;
• insulation with a heating cable, polystyrene foam shells when laid above the freezing mark;
• the outlet should be 1.5 - 5 m from the water supply with a sewer diameter of up to 20 cm, larger than this size, respectively;
• the slope of the sleeve or hole in the tape is 4 - 7 degrees to ensure gravity flow.

Recessed strip foundation

When installing the formwork for a deep strip foundation, sleeves for communications are laid taking into account the above requirements:

Insulation of slab base

Insulation is carried out after the construction of the basement floor. For insulation work, the necessary tools, equipment, and materials are purchased:

• hammer drill,
• knife,
• swing hand,
• hammer,
• level,
• industrial dryer,
• nail puller,
• pliers,
• heat insulators (extruded polystyrene foam, foam glass, polystyrene foam).

With the help of a special type of insulation of the soil around the foundation, resistance to frost heaving is created. The thermal insulation is laid to a width of 1.5 m around the supporting structure along the entire perimeter of the building. Then thermal insulation is created for the outer part of the base (base).

The foundation is insulated using heat insulators. Extruded polystyrene foam is used for external thermal insulation on the outer sides of the floor.

Scheme of insulation of a slab foundation.

A layer of expanded polystyrene is placed on a sand and gravel bed during concrete preparation, when insulation is laid and concreting is performed. This represents the internal insulation of the slab base. A layer of insulation is applied to the walls and floor of the structure.

The thermal insulation layer prevents freezing of the soil under the building, the penetration of cold into the room, eliminates heat loss through the floor, and contributes to the formation of a favorable microclimate during the cold season of the year. The passage for cars should be moved away from the building by the width of the external insulation to avoid deformation of the insulation.

The insulation of the foundation is done at the construction stage of the building, after which the creation of external insulation is problematic: there is work to be done to clean up crumbled soil and clean the foundation from cement mortar after installation work. The walls are cleaned of all kinds of inclusions, and shells are sealed with cement-sand mortar. The foundation is left to dry.

After drying, it should be insulated with polystyrene foam, and the side walls should be treated with waterproofing. Next, work is underway on facing the foundation and constructing a blind area.

Creating proper insulation of the slab foundation in conjunction with other work will give the entire building reliability and safety; it will be comfortable to live and work in under any climatic conditions.

When building a house, special attention should be paid to the thermal insulation of the foundation. Regardless of the insulation method, a blind area must be made along the perimeter of the foundation.

This is especially true if the building is located on ground that freezes very much in winter. Russians should think about this first. The thing is that in Russia almost the entire territory consists of heaving soil.

During the period of freezing, such soil greatly increases in volume and is capable of destroying or deforming even the strongest foundation. Insulation helps prevent excessive soil freezing. The horizontal thermal insulation of the slab is especially good.

Insulation also reduces heat loss through the floor. After all, it has already been proven that about 20% of heat loss occurs in this way. Insulation helps avoid air condensation, protects the foundation, prevents mold, and reduces heat loss. If a waterproofing system is installed in the house, then it also needs to be insulated.

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general information

The design of the slab base consists of layers:

• geotextile is covered with overlapping strips on the sand layer, the joints are taped;
• pour crushed stone in a layer of 15-20 cm;
• pour a leveling layer of cement mortar, 5-10 cm thick;
• be sure to isolate the structure from moisture using rolled or coating materials;
• arrange a heat-saving layer;
• cover with plastic film in stripes overlapping by 20 cm;
• lay the reinforcing mesh;
• poured with concrete.

Installation and insulation of a slab monolithic foundation is expensive due to the high consumption of building materials. When the soil freezes to a great depth and a significant deepening of the strip foundation is required, installing a slab will be cheaper and less excavation work will be required.

Types of insulation

The process of spraying polyurethane foam lasts much less than installing slabs.
Insulation work must be handled responsibly. The first step is to choose an insulation material. It should not deform under pressure and should not absorb moisture.

For example, mineral wool boards are not suitable because they compress too much and can absorb water. You can use foam glass or expanded polystyrene. But insulation with expanded polystyrene will cost several times less.

To insulate a foundation located in an area of ​​high humidity, it is recommended to use extruded polystyrene foam. Plates made from this material meet all the requirements for thermal insulation of an object. A big advantage of such slabs is also their significant service life, so they can be used to insulate any structure.

You can purchase special thermal insulation boards on the construction market. They have practically zero liquid absorption, that is, in a month they absorb no more than 0.5% of the moisture accumulating in the soil. Therefore, the volume of the material does not change at all. It withstands any temperature changes, so it does not collapse.

It is strictly forbidden to use polystyrene foam to insulate the foundation. This is a good insulation, but only for walls and ceilings. If you put foam plastic slabs under the foundation, then after a couple of years they will simply crumble into small balls.

The fact is that polystyrene foam absorbs moisture very strongly. In winter, this moisture will freeze, increasing the pressure inside the material. Very often, such slabs crumble after the first winter.

If vertical thermal insulation is assumed, then it is necessary to use slabs with very high compressive strength. In this case, you can also use extruded polystyrene foam. It does not lose its qualities throughout its service life, has high compressive strength, and rodents or insects cannot settle in it.

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Is it possible to carry out all the work yourself?

Thermal insulation of the base of the house usually occurs at the laying stage. But sometimes it happens that the insulation was done incorrectly or turned out to be insufficient. As a rule, this happens when making a strip foundation.

To improve the situation, you will have to do the following:

1. Dig out the foundation of the house to the full depth (it will be 50-60 cm).
2. Expand the ditch by 30-50 cm.
3. Clean and dry the surface.
4. If you plan to use polystyrene foam and its analogues, you will need to remove bitumen waterproofing, as well as oil stains (foam plastic is destroyed by contact with them).
5. If mineral wool is chosen as insulation, no additional cleaning is required.
6. Apply waterproofing to the dried surface - roll or using polymer mastics.
7. Apply special glue to the waterproofing layer and apply foam boards.
8. Once again treat the resulting surface with glue and put a reinforcing mesh on top to protect it from rodents.
9. After the glue has dried, fill the underground part with soil and cover the outer part with facing material.
10. For mineral wool, you will need to build a frame from a metal profile, lay the material and cover it with a waterproof film, build a brick wall, and fill the underground part with soil.

Calculation of thermal insulation thickness

If the house has a basement, then it is better to use slabs with a thickness of 100 mm.

The first step is to calculate the thickness of the insulation layers. It is calculated depending on the type of building that needs to be insulated. There are several formulas:

1. The thickness of the insulation of a basement wall, which is located above ground level, is calculated as follows: subtract 0.16 from the heat transfer resistance, subtract the wall thickness divided by the thermal conductivity of the material, and multiply the resulting result by the thermal conductivity coefficient of the insulation.
2. The thickness of the insulation of a basement wall, which is located below ground level, is calculated as follows: subtract 1.05 from the heat transfer resistance, subtract the wall thickness divided by the thermal conductivity of the material, and multiply the resulting result by the thermal conductivity coefficient of the insulation.

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Types of waterproofing

If you nevertheless come to the conclusion that there is a need to provide waterproofing, you should take into account the following points at the initial stage of constructing the base and using moisture-proofing materials:

1. Before constructing the slab itself, the fertile layer is first destroyed on the site.
2. For a more stable position of the structure, a cushion is made under the base from a layer of sand and crushed stone.
3. Large crushed stone can easily damage moisture-proofing film or rolled materials, so it is better to use its small fractions. If the crushed stone is not placed evenly, it will simply be impossible to ensure the tightness of the overlaps or joints of the material.

Due to these reasons, builders often first fill the surface of the site with concrete to get an even layer, and then make a cushion. To ensure waterproofing of the foundation, the following materials are used:

• Roll materials are the most common option. This group also often includes various moisture-proofing films. The most popular representative is roofing felt.
• Penetrating compounds. After being applied to concrete, the substances create a potential difference, thereby changing its structure.
• Additives to concrete. Today, there are many substances that are introduced into the concrete solution, and then the base is poured.
• Coating materials. They are mainly used for waterproofing strip foundations; they are not particularly suitable for slabs, since it is not possible to coat the base of the base.

For waterproofing monolithic slabs, the most suitable option is rolled materials, additives and compositions. The last two options significantly reduce costs and time losses. Rolled materials may be more expensive, depending on regional prices. In addition, laying roofing felt or other materials of this group can take quite a lot of time. However, at the same time, roll materials are considered one of the most reliable.

Insulation methods

A layer of extruded polystyrene foam can also be placed on a concrete base.

Foundation insulation can be external or internal. The first option is much more common, because...

he is the most rational. External insulation is done so that the ground along the walls and under the foundation freezes less. If extruded polystyrene is used for external protection, it must be laid very evenly, the deviation angle should not exceed 2%.

Internal insulation involves fastening the material indoors. Usually only walls are protected in this way, although sometimes you can also find internal insulation of the foundation.

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Polyurethane foam and its application

The main feature of this heat-insulating material is its dense closed cellular structure, filled with inert gases by 85-90% and ensuring its low thermal conductivity. To insulate foundations, the material can be used both in the form of finished sheets and in the form of liquid self-foaming two-component compositions, inflated by spraying.

The application of a liquid polyurethane foam composition to a concrete screed under the foundation slab being prepared compares favorably with the use of similar sheet materials.

High adhesion ensures strong adhesion to the surface without leaving gaps or cracks. But slab polyurethane foam requires pre-treatment of concrete with special compounds for reliable bonding. When polymerized, the material forms a seamless coating that does not allow moisture to pass through. When using sheet polyurethane foam, additional waterproofing is required. The composition is sprayed in 2-3 layers, which makes it possible to form any thickness of thermal insulation.

In addition, the environmental friendliness of the insulating material allows it to be used to insulate a finished foundation even indoors. But the main disadvantage of using polyurethane foam is the high cost of the components of the sprayed insulation and the unavailability of special equipment for doing work at home.

Foundation insulation

Insulation is laid under the slabs during the construction stage. This is a fairly easy job that one person can do. To work you will need:

• insulation (for example extruded polystyrene);
• Buy stretch film, packaging inexpensively and use it for insulation. The material is resistant to low temperatures;
• glue;
• double sided tape;
• dowels

The insulation must lie on the waterproofing layer. The insulation is laid in several layers and secured with a special adhesive. To make the fastening as reliable as possible, dowels are used; they will prevent any movement of the material.

To prevent liquid concrete from getting on the material, the heat-insulating layer is covered with a film on top. If welding work is planned, then a low-grade concrete screed is made on top of the film. It is recommended to use double-sided tape to secure the plastic film. If all work is carried out correctly, then during the operation of the facility there should not be any problems with the foundation.

The monolithic slab foundation has proven itself well when used in weak and heaving soils. The annual freezing of soils in winter leads to uneven rise and settlement of the slab base, which contributes to the occurrence of mechanical deformations that can cause destruction of the slab itself and the building built on it.

A layer of horizontal insulation can reliably isolate the foundation slab from the zone of frost heaving, contact with soil moisture and prevent freezing.

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Features of the work performed

A shallow strip foundation rests on layers of soil that can freeze in winter.
This creates the risk of heaving loads that can deform the supporting structure or even destroy it.

Under the house there is a section of heated soil, which reduces the intensity of heaving loads. To obtain greater efficiency in the insulation of the tape, insulation of the blind area is used.

A layer of heat insulation is laid under it with a slope section towards the drainage pipes located around the perimeter of the house at a certain depth.

The blind area is poured over the heat insulator, which forms a protective screen that cuts off cold and moisture from the side walls of the tape.

As a result, the temperature of the soil layers adjacent to the foundation increases, and heaving loads are significantly reduced.

Advantages of insulating a slab base

Horizontal slab insulation

High-quality insulation of a monolithic foundation slab guarantees the durability of the building and long-term operation without the need for unplanned repair work. Particularly relevant is the insulation of the foundation slab under residential buildings, when it is possible to avoid significant heat loss on the first floors of the house.

Insulation of the foundation slab must be performed for the following reasons:

• Providing increased waterproofing of the foundation.
• Significant reduction in heat loss.
• Saving money on heating a residential building, real heat saving mode.
• Preventing the formation of condensation that can destroy the building structures.
• Improving living comfort.
• Stabilization of temperature in the interior of a residential building in use.

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Raw material selection criteria

The slab base, especially in conditions of high humidity of the external environment, needs high-quality insulation.
Thermal insulation allows:

1. Protect concrete from temperature changes.
2. Save on heating your home.
3. Reduce heaving of the soil under the base.

Penoplex - extruded polystyrene foam - can perform these functions. The material is characterized by low thermal conductivity, high strength and service life.

Despite the fact that penoplex can be easily cut with a knife, this material is not brittle, so balls should not be visible in places where it is broken, like foam plastic. If you press your finger on the surface of the sheet, the material will immediately return to its previous state.

When choosing a material, you need to pay attention to storage conditions. Penoplex is not left in the open air, but covered with cloth or packaging. Raw materials are selected depending on the purpose: for foundations, the material with the highest specific gravity is suitable - from 33 to 45 kg/m3

Raw materials are selected depending on the purpose: for foundations, the material with the highest specific gravity is suitable - from 33 to 45 kg/m3.

Material characteristics

Technological properties of penoplex:

• thermal conductivity – 0.03 W/mK;
• density – from 29 to 45 kg/m3;
• compressive strength – 0.27 MPa;
• water absorption – 0.5% by volume for 8 days;
• fire resistance category – G4;
• operating temperature range – from –50 to +70 ͦС;
• size – 600x1200 mm;
• sheet thickness – from 20 to 100 mm.

Advantages and disadvantages

Advantages of the material:

1. Minimum water permeability. The liquid penetrates only into the outer structure of the penoplex, which was damaged when cutting the sheets. The closed cells of the material remain waterproof throughout its entire service life.
2. Low coefficient of thermal conductivity, which allows the use of penoplex for insulation of foundations with a basement, saving on waterproofing.
3. Low vapor permeability. Extrusion-type raw materials are resistant to moisture evaporation: sheets with a thickness of only 20 mm have the same vapor permeability as rolled roofing felt.
4. Long service life (more than 50 years), which is not affected by repeated freezing and thawing, as well as constant loads.
5. Easy to install and process. You can cut foam sheets with a regular knife. At the same time, the properties of the material do not change if sedimentary waters fall on its surface.
6. Environmental Safety. The raw material is not toxic and does not harm the environment when decomposed.
7. Low chemical activity. Only a small amount of chemical reagents contained in soil and groundwater can interact with penoplex, softening its structure. In most cases, the material maintains its integrity throughout its entire service life.

At the same time, in order for the material to retain its qualities and shape during operation, it is necessary to take into account the permissible ambient temperature.

Materials for insulating a monolithic slab foundation

The choice of materials for insulating a slab base depends on a number of certain conditions, first of all, on the possibility of using special equipment (polyurethane insulation must be performed using a special polyurethane foam installation for spraying insulation), and the availability of sufficient free funds to purchase insulating materials.

Typically, insulation of foundation structures is carried out using the following materials:

One of the best insulation materials is polystyrene foam.

• Polyurethane foam is a type of foamed plastic that is riddled with pores containing air bubbles. The composition for polyurethane foam insulation is made directly at the construction site, for which two compositions are mixed, which, as a result of a chemical reaction, form a durable hardened foam. Different ratios when mixing the starting materials make it possible to obtain insulating compositions with different qualities, suitable for insulating foundations made of different materials (including reinforced concrete). Polyurethane foam has unique qualities: it helps retain heat in the room; reduces noise; improves sound insulation; does not decompose under the influence of excessive moisture; Resistant to a number of chemical liquids. The material is highly resistant to ignition (some brands of polyurethane foam are not flammable and belong to the group of low-combustible materials).
• Foam plastic - the structure of the material consists of a foamed substance containing pores filled with air. Foam plastic is used to insulate building structures, including foundations, but the surface of such insulation requires additional surface protection due to the low mechanical strength of the material.
• Extruded polystyrene foam - produced in the form of rectangular sheets with a fine-cell structure. The material has unique properties - the ability to withstand high compressive loads without changing the geometric dimensions and internal structure. Extruded polystyrene foam is widely used to insulate foundation slabs without additional protection. When using material to insulate foundation slabs, it is very important to provide special openings for the free exit of soil moisture.

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Well-known brands of thermal insulation products

TechnoNIKOL is a well-known domestic brand of insulation for concrete.
URSA is a well-known brand in Europe. The manufacturer specializes in the production of polystyrene foam, expanded polystyrene and extrusion versions of the product. The products are of high quality in terms of uniformity and thermal insulation characteristics.

TechnoNikol is an industrial giant of insulation in the domestic market. The plant produces expanded polystyrene and a wide range of other insulation products. The production facilities have their own laboratories where new types of insulators are developed.

Elit Plast is a manufacturer of Ukrainian origin. It produces decent polymer insulation materials that meet all environmental global standards.

What about foam?

When deciding how to properly insulate a foundation, the choice often falls on the foam insulation method. This is a universal and affordable material.

Polystyrene foam is sold in sheets that are easy to install. Therefore, its use allows you to do all the work yourself.

Waterproofing

Before fixing the insulation sheets, it is important to waterproof the surface. There are many methods of waterproofing:

• applying several layers of bitumen mastic;
• waterproofing with roofing felt;
• plastering the surface;
• application of special penetrating compounds.

Laying slabs

After ensuring waterproofing, sheets of expanded polystyrene are mounted on the insulated surface. The sheets are laid from the bottom of the foundation to the level of the future floor. The insulation is secured using special adhesives that are applied pointwise to its surface. Expanded polystyrene sheets must be laid close to each other to create a monolithic surface. The seams between the insulation boards are sealed with polyurethane foam.

Expanded polystyrene is destroyed over time when exposed to sunlight, so it should be securely covered with cladding panels on top.

Video description

Details about the heat loss of the building and the purposes of insulating the foundation are described in the video:

But there is another function of thermal insulation of the foundation of a house - increasing the service life of the foundation. Building materials have such a criterion as frost resistance. Its numerical expression means how many freezing cycles (assuming water saturation) must pass before signs of destruction appear. Therefore, for regions with cold and long winters, insulation and waterproofing of the foundation is a prerequisite to ensure the strength and durability of the structure.