A strip foundation is not just the base of a structure, supporting its construction, but can also perform quite utilitarian functions, serving as the walls of habitable basement, cellar, or semi-basement rooms. For such rooms to be warm and cozy, the foundation should not be a simple concrete casting, but a structure that provides reliable thermal insulation for the interior spaces. For this purpose, not only the horizontal, but also the vertical surfaces of the concrete strip foundation need to be insulated.
Concrete itself, despite its high strength and great resistance to loads, is a rather poor insulator. It easily absorbs heat and just as easily releases it. A bare concrete wall will not be a good barrier between the cold winter ground and the warmth of a habitable room.
It should be noted that the depth of the foundation’s placement also affects its cooling. If a strip foundation is not buried below the soil’s freezing depth, areas of frozen ground can form under the house, which will negatively impact the climate in your home.
What are the ways to insulate a foundation?
There are two main ways to insulate foundations. They differ depending on the stage at which the work is carried out. One option is to insulate the foundation during its pouring phase, and the second is subsequent insulation performed after the concrete casting has cured.
Insulating the foundation during the construction phase is the most preferable method. This comprehensive approach yields good results. To achieve reliable thermal insulation for the foundation, even in the harsh winter conditions of central Russia, the insulation process must be carried out on both sides.
The most logical approach to insulating the foundation during construction is using permanent formwork.
Foundation Insulation with Permanent Formwork
Permanent formwork is a spatial structure into which concrete mixture is poured. During the concrete setting phase, it acts as regular wooden formwork, meaning it restricts the spread of the concrete mixture. However, unlike wooden formwork, permanent formwork is not removed after the concrete cures; instead, it remains on the concrete mass as an insulation layer.
Permanent formwork can be made from various materials. Building permanent formwork from expanded polystyrene (EPS) boards is particularly popular. These are panels made of foamed polymer material, containing many tiny air bubbles.
At first glance, a drawback of such formwork is its high cost. The expenses for its purchase and installation are many times higher than those for classic wooden formwork. However, such a foundation will not need to be insulated later, so the total costs will be quite comparable.
Insulating the Foundation After Pouring
Work on insulating a poured foundation is best started directly during its curing process. If you neglect this stage of house construction and return to it later, you will not be able to perform the insulation work effectively.
Classic methods for insulating a foundation after its construction include pouring a layer of expanded clay aggregate on the basement floor, which is then covered with a leveling screed. Expanded clay aggregate is clay baked in special kilns, and its mass is an excellent thermal insulator. Also, to increase the level of thermal insulation between the ground, the basement, and the living rooms, the floor of the first floor of the house is insulated.
Insulating the Foundation with Earth
This method of foundation insulation is extremely economical on one hand, but quite labor-intensive and extensive on the other.
The essence of this insulation is very simple: all foundation walls up to the first floor level are covered with earth. Thus, all basement rooms end up beneath a layer of earth, inside a kind of mound. Earth or sand themselves are not bad insulators, but for this insulation to be sufficiently effective even for central Russia, a substantial volume of sand will be needed to backfill the foundation. For instance, backfilling a foundation with modest dimensions of 10 by 10 meters will require about one hundred cubic meters of sand.
A significant advantage of this approach is the exclusion of the impact of heaving soils on your foundation. In some cases, backfilling the exterior foundation wall with sand is recommended not even for thermal insulation purposes, but to eliminate the impact of soil movements on the foundation.
When backfilling the foundation with soil or sand, it is necessary to provide channels for air ducts that will ensure ventilation of the basement rooms. Naturally, work on this insulation method should begin immediately after dismantling the wooden formwork, even before the walls of your future house are erected.
Insulating the Foundation with a Layer of Expanded Clay Aggregate
As already mentioned, this is also one of the classic ways to insulate a foundation. In principle, insulation with expanded clay aggregate can be combined with insulation using earth.
In addition to simple backfilling, expanded clay aggregate can also be used as a filler for concrete castings. Expanded clay aggregate consists of small lumps of clay baked in a special rotary kiln. Inside each lump, there are many air cavities, which accounts for the excellent thermal insulation properties of this material. Thus, expanded clay aggregate can certainly be used as a filler in concrete mixtures instead of crushed stone.
For example, expanded clay aggregate filler can be used in the construction of shallow strip foundations. Moreover, expanded clay aggregate can be added not only to the concrete mixture for the foundation walls but also during floor screeding, thereby providing a fully enclosed thermal insulation contour for the basement area.
Insulating the Foundation with Expanded Polystyrene
The development of chemical production has led to the widespread use of synthetic insulating materials. One such material is expanded polystyrene, which is a board containing many air bubbles inside.
Expanded polystyrene sheets can be glued to both concrete castings and brick walls. When installing such panels on a foundation made of concrete blocks, meaning it has joints between individual parts, they should be positioned so that the panel joints are not directly over the concrete block joints. This will prevent the formation of ‘cold bridges’.
Expanded polystyrene boards can be glued from the very lowest level of the foundation up to the roof – covering the entire exterior surface of the wall.
Foundation Insulation with Mastics
Certain insulation of the foundation can also be achieved during waterproofing work. For instance, it is recommended to cover the vertical walls of the foundation with a layer of bitumen mastic to prevent moisture penetration from the soil. Besides moisture protection, several layers of such mastic will help you insulate cracks and tiny openings, as well as joints in foundation slabs, through which heat loss from the premises can occur.
After applying several layers of bitumen mastic, a layer of roll waterproofing material can also be applied to the side surfaces of the foundation. Like mastic, it will serve as an additional layer of thermal insulation.