How to Insulate the Foundation: A Comprehensive Guide for Effective Thermal Insulation

The foundation is the basis of any building, and its thermal insulation plays a crucial role in creating energy-efficient and comfortable housing. Insulating the foundation is not just a desirable option, but a necessity that affects the overall thermal balance of the building, the durability of the structures, and the microclimate in your home. In this article, we will take a detailed look at how to insulate the foundation, what to insulate the foundation of the house with, whether it is necessary to insulate the foundation at all, and how to effectively and correctly insulate the foundation yourself.

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Why is foundation insulation important?

Before delving into methods and materials, it is important to understand why to insulate the foundation. Ignoring this stage of construction can lead to a number of problems:

1. Significant heat loss: An uninsulated foundation is a major bridge for cold, through which up to 20% of the heat is lost from the house. This not only increases heating costs but also makes the home less comfortable.
2. Soil freezing and foundation deformation: Frost heave of the soil caused by freezing creates pressure on the foundation, which can lead to cracks, tilting, and destruction of the entire building. Insulating the foundation prevents soil freezing directly under the building.
3. Discomfort in the room and cold floors: Cold from the uninsulated foundation is transmitted to the floor, making it icy, especially on the first floors and in basements. This reduces overall living comfort.
4. Condensation and humidity: The temperature difference between the cold foundation and the warm air in the house can lead to condensation forming on the inner walls of the foundation. This promotes the development of mold, fungus, and the destruction of building materials.
5. Increased heating costs: Due to constant heat loss, your heating system will operate at full capacity, consuming more energy and increasing your utility bills.

Types of foundation insulation: choosing the optimal method

There are two main approaches to insulating the foundation of a house: external and internal. Each has its advantages, disadvantages, and areas of application.

1. External foundation insulation: the gold standard

External foundation insulation is considered the most effective and recommended method. The insulation material is installed on the outer surface of the foundation, protecting it from low temperatures and soil freezing.

Advantages of external insulation:

• Maximum energy efficiency: Creates a reliable thermal envelope around the foundation, minimizing heat loss and preventing freezing.
• Protection from external moisture: Many materials for external insulation, such as extruded polystyrene (XPS), have low water absorption, additionally protecting the foundation from ground moisture.
• Reduced risk of foundation deformation: Prevents frost heave of the soil, reducing the load on the foundation and the risk of deformation.
• Durability: With the correct choice of materials and installation technology, external insulation lasts for decades.

Disadvantages of external insulation:

• Higher cost: Typically, external insulation is more expensive than internal due to the larger volume of work and the cost of some materials.
• Difficulty of execution on already built buildings: External insulation is best planned at the construction stage. For already built houses, excavation work is needed to uncover the foundation.
• Need for protection from mechanical damage: The insulation layer on the foundation’s plinth requires protection from impacts, moisture, and ultraviolet light, for example, by using plinth tiles, plaster, or protective panels.

Materials for external foundation insulation:

• • Extruded polystyrene (XPS): What to insulate the foundation with? XPS is the leader among materials for external foundation insulation. It is characterized by:
    • Low thermal conductivity: Excellent at retaining heat.
    • Moisture resistance: Almost does not absorb water, ideal for contact with the ground.
    • High compressive strength: Withstands loads from the soil.
    • Durability: Lasts 50 years or more.
    • Easy installation: Easily cut and glued.
    • Recommended thickness: 50-100 mm, depending on the climatic zone and soil type.

• • Mineral wool: An eco-friendly and non-combustible material with good thermal insulation properties. Rigid high-density mineral wool boards are used for the foundation.
    • Advantages: Fire safety, vapor permeability (breathable material), eco-friendliness.
    • Disadvantages: Hygroscopicity (absorbs moisture), requires mandatory protection from moisture with a waterproofing membrane and protective cladding. External use of mineral wool for foundations is less common than XPS.

• • Polyurethane foam (PUF): A modern insulation method that involves spraying liquid polyurethane foam onto the foundation surface.
    • Advantages: High thermal insulation properties, absence of seams and cold bridges, waterproofing properties (closed-cell structure of PUF).
    • Disadvantages: Higher cost of equipment and work, requires specialized equipment and skills for application.

2. Internal foundation insulation: when external is impossible

Internal foundation insulation is used less frequently, mainly in cases where external insulation is impossible or impractical:

• • Already built house without excavating the foundation.
  • Technical limitations (for example, very close neighboring buildings).
  • The need to insulate only the basement or ground floor from the inside.

Advantages of internal insulation:

• Lower cost: Usually cheaper than external insulation, especially if the work is done by yourself.
• Ease of execution: Does not require excavation work.
• Possibility of insulating already existing buildings without significant reconstruction.

Disadvantages of internal insulation:

• Lower energy efficiency: Does not prevent soil freezing under the foundation, does not protect the foundation from temperature fluctuations outside.
• Risk of condensation and humidity: The dew point can shift inside the structure, creating conditions for condensation of moisture between the insulation and the foundation wall, leading to mold and fungus formation. Vapor barrier is absolutely necessary on the inside of the insulation.
• Reduction of usable space: The insulation “eats up” part of the internal space of the basement or ground floor.

Materials for internal foundation insulation:

• Mineral wool: Can be used for internal insulation, but must have a vapor barrier layer on the inside (from the room side) to prevent moisture from entering the insulation. It is recommended to use non-combustible mineral wool.
• Polystyrene (PSB-S): A cheaper option than XPS, but less resistant to moisture and mechanical damage. Also requires vapor barrier. It is recommended to choose self-extinguishing polystyrene (marked “C”).
• Polyurethane (PUF): Spraying PUF from the inside is also possible, but with the need for vapor barrier and ventilation of the room to avoid moisture accumulation.
• Polyisocyanurate (PIR): A modern material with high thermal insulation properties and low water absorption. Can be considered for internal insulation, but also requires attention to vapor barrier.

How to properly insulate the foundation yourself: step-by-step instructions for external insulation with XPS

Let’s consider the detailed process of insulating the foundation yourself using extruded polystyrene (XPS) for external insulation of a strip foundation.

Necessary materials and tools:

• Extruded polystyrene (XPS) boards of the required thickness (as calculated).
• Adhesive for XPS (specialized adhesive or mounting foam with a low expansion coefficient).
• Dowel umbrellas for mechanical fastening of the boards (if necessary).
• Waterproofing mastic or roll waterproofing.
• Reinforced mesh for plastering (if plastering the plinth is planned).
• Facade plaster (for finishing the plinth).
• Deep penetration primer.
• Construction knife or saw for cutting XPS.
• Spatula.
• Level.
• Tape measure.
• Screwdriver or drill (for dowels).
• Protective gloves and goggles.

Work stages:

1. Preparing the foundation surface:

   • Clean the foundation from dirt, dust, formwork remnants, moss, and mold (if necessary, treat with antiseptic).
   • Level the surface (if necessary, fill large irregularities or protrusions).
   • Treat the foundation with deep penetration primer to improve adhesion (bonding) of the adhesive.

2. Waterproofing the foundation:

   • Apply a layer of waterproofing mastic to the prepared surface of the foundation or stick roll waterproofing (especially important for foundations that are below the groundwater level). Waterproofing will protect the insulation from moisture from the ground.

3. Fastening XPS boards:

   • Mark the XPS boards and cut them to size with a construction knife or saw.
   • Apply adhesive for XPS around the perimeter of the board and in several spots in the center (according to the manufacturer’s instructions for the adhesive).
   • Press the board against the foundation and hold it. Start fastening from the bottom up, in a staggered order, so that the seams between the boards do not coincide.
   • Leave small gaps (1-2 mm) between the boards to compensate for thermal expansion. Fill the gaps with mounting foam with a low expansion coefficient after the adhesive has dried.
   • Mechanical fastening with dowel umbrellas (if necessary): For additional reliability, especially on high foundations or when using thick XPS boards, the boards can be additionally secured with dowel umbrellas (3-5 pcs. per board). Fastening with dowels is usually done 24-48 hours after gluing, when the adhesive is completely dry.

4. Protecting the insulation:

   • Above-ground part: To protect XPS from mechanical damage, ultraviolet light, and moisture, the above-ground part of the insulation is usually plastered over a reinforced mesh.
     • Glue the reinforced mesh onto the XPS with an overlap of 10-15 cm.
     • Apply a base layer of plaster, leveling the surface.
     • After the base layer has dried, apply a finishing decorative layer of plaster or cladding material (plinth tiles, stone, etc.).
   • Underground part: The underground part of the insulation is usually additionally protected from mechanical damage with a drainage membrane, geotextile, or covered with soil.

5. Backfilling soil:

   • After completing the insulation and protecting the insulation, perform backfilling of the soil, compacting it in layers.

Common mistakes when insulating the foundation

Is it necessary to insulate the foundation and how to do it correctly – these are important questions, and to avoid ineffective insulation and unnecessary costs, it is essential to be aware of typical mistakes:

• Insufficient thickness of the insulation: Saving on the thickness of the insulation leads to inadequate thermal insulation and does not solve the problem of heat loss. Choose a thickness of XPS of at least 50 mm, and for cold regions – 80-100 mm.
• Absence or incorrect waterproofing: Moisture is the main enemy of insulation. Lack of waterproofing leads to the insulation getting wet, losing its thermal insulation properties, and destruction.
• Poor quality materials: Using cheap, uncertified materials can lead to their rapid degradation and ineffective insulation. Choose quality materials from trusted manufacturers.
• Violation of installation technology: Incorrect fastening of the boards, large gaps between the boards, lack of protection for the insulation – all of this reduces the effectiveness of insulation. Follow the installation technology and the instructions of the material manufacturers.
• Insulating only the plinth without insulating the underground part of the foundation: For maximum effectiveness, the entire external surface of the foundation, including the underground part, should be insulated.

Conclusion: an investment in comfort and energy efficiency

Insulating the foundation is an important investment in your comfort, energy efficiency, and the durability of your home. Properly performed insulation of the foundation yourself or with the help of specialists will allow you to significantly reduce heating costs, create a cozy microclimate in the house, and protect the building from the destructive effects of frost and moisture. When choosing the method and materials for insulation, consider the type of foundation, climatic conditions, budget, and your own skills. We hope this article has helped you understand how to insulate the foundation correctly and effectively!