How to insulate the foundation of a house outside: step-by-step instructions for thermal insulation of the base

Thermal insulation of a foundation is as important as thermal insulation of walls, especially in areas with harsh climates. Frozen soil cannot protect the base of the house from moisture and cold, and the foundation itself, after repeated freezing, begins to collapse faster. To get rid of these problems and reduce the cost of heating the home, you need to know how to insulate the foundation of the house from the outside, and how you can do it on your own.

Requirements for insulation

The foundation of the house is regularly exposed to moisture, increased stress, temperature extremes. After the onset of frost, the accumulated moisture freezes, destroying the material, and through cracks in the concrete the cold penetrates into the dwelling.

As a result, there are always cold floors in the house, dampness pulls from under the skirting boards, condensation or frost appears on the basement walls (in severe frosts). For thermal insulation to be effective, the insulation must have the following characteristics:

• low thermal conductivity;
• water resistance;
• good mechanical strength;
• temperature resistance drops.

There are no strict requirements for vapor permeability of the material, but heaters with low steam permeability, like that of the concrete foundation itself. The flammability of the material does not matter much, because the underground part of the building is least susceptible to fire risks.

Which insulation to choose

Mineral wool heaters for foundations not the most suitable option: they tend to accumulate moisture, while losing their thermal insulation properties, and are not sufficiently resistant to mechanical stress.

The soil pressure will cause the material to shrink and the insulation efficiency will be reduced to zero.

Prices for mineral wool

Mineral wool

But polymer heaters are devoid of such disadvantages and fully meet the necessary requirements.

Let's consider their characteristics in more detail.

Expanded polystyrene

Expanded polystyrene, or foam, very widely used in the construction of thermal insulation. It is lightweight, its sheets have the correct shape, and therefore coping with the installation is not a problem even for a novice master.

The material is produced in various densities and thicknesses, and, depending on this, has certain areas of application. Only the densest sheets with a thickness of at least 50 mm are suitable for foundations, the rest are best used for insulating walls and partitions.

In terms of thermal conductivity, foam plastic is much more effective than wood, expanded clay, mineral wool insulation. It almost does not absorb water, is not prone to shrinkage and deformation, and serves as an excellent sound insulator. In addition, expanded polystyrene plates fully retain their properties when exposed to salt and chlorinated water, soap solutions, and weak acids.

The insulation can come in contact with bituminous mastics, lime, adhesive water-soluble solutions and cement plaster.

For slab foundations, as well as deeply buried bases, it is necessary to use expanded polystyrene with the marking PSB-S- 50, which is able to withstand high mechanical loads and provide reliable protection against soil swelling. For columnar and standard strip foundations, PSB-S-35 foam is used.

All other varieties are not suitable for foundations due to the low density and fragility of the sheets.

Extruded polystyrene foam

Expanded polystyrene made by extrusion, in all respects, significantly exceeds the foam plastic.

It is many times stronger, absolutely moisture resistant, has the lowest vapor permeability and thermal conductivity. Microorganisms also do not develop in it, even under conditions of prolonged exposure to dampness.

EPS boards with a smaller thickness have a much greater effect. For thermal insulation of the foundation in a temperate climate, a 40 mm insulation thickness is sufficient, in northern conditions - about 60 mm (two-layer laying of plates with a thickness of 30 mm is recommended). For ease of installation and greater packing density, the slabs are equipped with a tongue-and-groove connection.

They are attached to the base using adhesive mortar and disc dowels.

EPS boards can be laid vertically (when insulating columnar and strip foundations) and horizontally (when arranging slab foundation).

Due to its increased strength, the material perfectly transfers the load from the concrete layer, does not flatten or shrinks, in contrast to polystyrene and polyurethane foam. The most popular brands of extruded polystyrene foam are Penoplex and Technonikol.

Main characteristics

Polyurethane foam

Sprayed thermal insulation, or polyurethane foam, is increasingly used in the insulation of foundations.

Polyurethane foam forms a very durable, seamless coating that is resistant to adverse effects. At the same time, it simultaneously performs the functions of steam and waterproofing, which greatly simplifies the workflow. Another useful property of this insulation is high adhesion to the base. It adheres equally firmly to stone, concrete, brickwork, wooden structures, is easily applied to hard-to-reach areas, fills in the slightest gaps.

Polyurethane foam is applied in two ways - pouring and spraying.

The first option is more laborious and costly, therefore it is rarely used when insulating private houses. The second method is actively used to insulate the entire building - from the roof to the foundation. Before application, the components are mixed with air in a foam generator and the finished mass is sprayed under pressure onto the work surface. Within a few seconds, the composition hardens, and upon completion of the work, you can immediately proceed to the finishing. The only disadvantage of such insulation is that it is impossible to do without installation, and these are additional costs.

In addition to the described heaters, bulk materials can be used for foundations, in particular expanded clay. It is environmentally friendly, inexpensive, has good thermal insulation characteristics, and is suitable for both vertical and horizontal insulation. But since expanded clay can absorb moisture, while losing some of its properties, and the working process requires more effort and time, this insulation is used less and less, yielding to more modern and efficient materials.

Prices for expanded polystyrene

Expanded polystyrene

Foundation insulation technology

Insulation of pile, columnar and strip foundations can be performed both during construction and during house operation. Before installing the insulation, the base must be prepared, which includes cleaning, sealing cracks, and arranging waterproofing. The better the preparation, the more effective and durable the result. Slab foundations are insulated directly during construction, because heat-insulating plates are located horizontally under a layer of reinforced concrete.

Strip foundation

The most labor-consuming stage in warming such a foundation is earthworks.

The foundation must be excavated to its base or to the level of soil freezing. If possible, it is better to hire special equipment, because digging by hand will take a lot of time and require a lot of physical effort. The foundation is dug around the entire perimeter to a width of at least 1 m, then the bottom of the trench is tamped and covered with a layer of sand and gravel.

The walls of the foundation are cleaned from the ground with a stiff brush and left open for 10 days for moisture to evaporate. For this reason, work should be done during warm and dry seasons.

Step 1. The dried walls are carefully examined for damage, and the identified defects are eliminated. Cracks before sealing are embroidered and cleaned of dust, if necessary, the surface is leveled with cement-sand plaster. If the irregularities exceed 20 mm, the plaster is additionally reinforced with a metal mesh. After plastering, you must wait until the surface is completely dry.

Step 2. Waterproofing the foundation. It is most convenient to use a coating waterproofing, for example, polymer, rubber or bitumen water-soluble mastics. Compositions based on organic solvents cannot be used, since when in contact with them, polystyrene foam plates are destroyed.

To improve adhesion, the surface is pre-treated with a bituminous primer, applying it in a thin continuous layer.

It takes about a day to dry the primer, after which you can start applying the mastic.

Step 3. Stir the mastic, if the mass is too thick, add a little water.

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Bituminous mastic

For application, use a wide brush with dense bristles or a roller. The composition is evenly distributed over the walls in a continuous layer, very carefully smearing the corners.

The layer should not show through, but it should not be made too thick, so that streaks do not form. The mixture is rubbed well over the surface, filling the smallest pores of the recess. When the first layer hardens, the second is applied in the same way.Usually two coats are sufficient, but if the area is often flooded by groundwater, it is recommended to apply 3 coats of mastic.

Step 4.

After the waterproofing has dried (after 5-7 days), you can proceed for the installation of foam or EPS. For fixing the boards, a special adhesive is used, for example, ALLFIX, Ceresit CT 83, Titanium.

The insulation is installed from the bottom up, in horizontal rows, with obligatory bandaging of vertical seams. Apply glue, press the slab to the surface and check its location with a level. When installing the next slab, it is necessary to align the mounting slots as tightly as possible so that the joints are barely visible.

Tip. When installing two-layer insulation boards, the joints of the lower layer must be completely covered by the boards of the upper layer. Combining the joints will lead to the formation of cold bridges, which means that the effect of insulation will be lower than planned.

Step 5. As a rule, the underground part of the insulation does not need additional fixation, because after filling the material is firmly pressed by the soil.

But in the above-ground part of the slab, it is recommended to strengthen it with dowel-nails with wide heads. Fasteners are performed after the adhesive solution has completely dried. To do this, through holes are drilled in the slabs (in the center of the slab and in the corners) and buried in the base by 40-50 mm. Next, insert the dowels and hammer them all the way.

Step 6.

To protect the surface of the boards from accidental damage, plastering is performed with a reinforcing fiberglass mesh. Apply an adhesive solution to the insulation, distribute it in an even layer 40-50 mm thick, lay the mesh on top and smooth it with a spatula with effort, deepening into the solution. For convenience, it is recommended to cut the mesh into pieces and lay them with an overlap of 10 cm.

Step 7. After a day, a fine leveling is performed, and when this layer is dry, the surface is treated with a float with an emery cloth.

Step 8. Fill the trench with soil, leaving a distance of about 30 cm to the top. The soil is tamped well to reduce shrinkage. A layer of sand 10-15 cm thick is poured on top, which is also carefully rammed, and then a waterproofing membrane and a layer of insulation are laid under a slight slope from the wall. The final stage is the arrangement of the blind area by pouring a concrete screed.

If it is decided to insulate the foundation with polyurethane foam, there is no need for leveling and waterproofing. After digging a trench and cleaning the base from the ground, it is only necessary to eliminate the cracks, as well as remove the delamination, if any.When the walls of the foundation are dry from moisture, you can start applying PPU. A spraying unit can be rented from a construction company or bought, but if you do not have the skills to work with such equipment, it is better to contact a specialist. After the material has hardened, the trench is covered with soil, and a blind area is made on top, as described above.

Column foundation

Column and pile foundation thermal insulation is performed a little differently. In order to insulate the space between the grillage and the ground, it is necessary to build a fence that serves as a basement. This design does not carry loads, therefore there are no special requirements for the mechanical strength of materials.

Step 1. A narrow trench is dug between the pillars of the foundation, up to half a meter deep, and a third of it is covered with a layer of sand and fine gravel.

Step 2. A frame made of metal rods is laid on top and a layer of concrete mortar is poured.

Step 3. After the concrete hardens, the space is laid with bricks around the entire perimeter, leaving small air vents in the opposite walls for ventilation.

Step 4.

When the masonry is dry, insulation plates are glued on the outside, and then they are plastered using a reinforcing mesh. After the plaster has dried, the trench is covered with soil and compacted.

In the end, decorative finishing of the basement is carried out - it can be painting, applying decorative plaster, facing with artificial stone.

Instead of brick filling, you can install a frame made of a bar or a metal profile between the foundation supports. The timber before installation must be treated with antiseptic impregnation and dried well.

The bars are fastened with metal corners, bolts 65-80 mm long and self-tapping screws; it is preferable to use welding to assemble the metal frame. After installation, the frame is sheathed with EPS or foam plates, and corrugated board or basement panels are attached on top.

Slab foundation

For horizontal insulation, the slabs can also be laid in one or two layers. Most often, for slab foundations, insulation with a thickness of 50 or 100 mm is chosen. Thermal insulation is started after the pit has been prepared and the sand cushion is backfilled.

Step 1. The sand layer is well compacted over the entire area and leveled horizontally. The smoother the base, the denser the insulation will lie.

Step 2. The film and heat-insulating plates are laid on the sand cushion, joining the mounting grooves.

In each subsequent row, the joints are shifted by the width of half of the sheet.

Tip. With a two-layer installation of insulation, the lower plates are placed in rows along the long side of the foundation, and the upper ones along the short side. This will not only exclude cold bridges, but also additionally strengthen the strength of the heat-insulating layer.

Step 3.

A formwork is installed along the perimeter of the foundation close to the slabs, inside which a reinforcing frame made of steel rods is mounted. Mix and pour concrete.

Step 4. After the concrete has set, a blind area is made. The sand is carefully leveled and tamped with a slight slope from the foundation.

Plates are laid tightly, and they are covered with a waterproofing membrane on top. The membrane sheets are laid with an overlap of 10-15 cm. Next, backfilling with sand is performed, again everything is leveled and tamped.

Prices for thermal insulation materials

Thermal insulation materials

Video - How to insulate the foundation of a house outside

Video - Insulation of the foundation with polyurethane foam

Video - Insulation of the slab foundation