Apartment Insulation Inside and Out: Materials, Methods and Mistakes
Volodymyr SydorenkoJuly 16, 2026
How to insulate an apartment and cut heating bills: external vs internal insulation, dew point, mineral wool, EPS and PIR, insulating balconies, floors and corner apartments, fire safety, OSBB approval and state support.
Apartment Insulation Inside and Out: Materials, Methods and Mistakes
Cold walls, freezing corners, mold behind wardrobes and huge heating bills are typical problems in apartments in buildings built decades ago. Insulation solves them — but only with the right technique. Poorly done insulation, especially from the inside, can make things worse by moving the dew point into the wall and "growing" mold. Here's how to insulate an apartment properly.
Where Heat Escapes
In a typical apartment building, heat is lost through:
- external walls — the largest area in contact with the cold;
- windows and balcony doors — especially old or poorly installed ones;
- thermal bridges — panel joints, lintels above windows, balcony slabs, corners;
- the ground-floor floor above an unheated basement;
- the top-floor ceiling under a cold roof or technical floor;
- ventilation and leaks — uncontrolled drafts.
To find weak points, order a thermal imaging survey in the cold season — the images clearly show areas of heat loss.
External or Internal Insulation
External — the Right Choice
Insulation on the facade protects the wall from freezing, moves the dew point into the insulation layer, eliminates most thermal bridges and doesn't reduce floor area. The wall becomes a warm thermal mass that holds heat longer.
But the facade is common property of the co-owners of an apartment building. Insulating a single apartment from the outside requires the co-owners' consent (an OSBB or general meeting decision) under the Law on Specifics of Exercising Ownership Rights in an Apartment Building, compliance with facade requirements and industrial climbers. Facade "patches" are a compromise; the best result comes from a comprehensive retrofit of the whole building.
Internal — When There's No Other Option
Internal insulation is used when external insulation is impossible: a historic facade or no consent from neighbors. Its downsides:
- it reduces the room's area;
- the wall stays cold and freezes;
- the dew point may end up between the insulation and the wall — condensation collects there and mold appears;
- thermal bridges at junctions with the ceiling, floor and internal walls remain.
If you insulate from the inside:
- choose low vapor-permeability materials or systems with controlled capillary activity;
- always install a continuous vapor barrier on the warm side with taped joints;
- carry out a thermal calculation for your wall;
- ensure effective ventilation of the room.
Materials
- Mineral (stone) wool — non-combustible, vapor-permeable, good sound insulation. The standard for facade systems and house insulation.
- Expanded polystyrene (EPS) — cheap and light, with low vapor permeability. Combustible, so on facades it's used with firebreaks as required by codes.
- Extruded polystyrene (XPS) — strong and moisture-resistant. For foundations, plinths, floors and balconies.
- PIR boards — the lowest thermal conductivity at small thickness, often foil-faced. Useful where every centimeter counts: balconies and internal insulation.
- Spray polyurethane foam — a seamless coating for complex shapes; requires professional application.
- Cellulose insulation — for filling cavities in frame structures and floors.
The key indicator is thermal conductivity (λ): the lower it is, the thinner the layer needed for the same effect. Minimum insulation requirements for the building envelope are set by state building codes on thermal insulation and energy efficiency.
Fire Safety
Combustible insulation on facades is a serious fire-spread risk. Use certified facade systems, mineral wool firebreaks, and non-combustible materials around windows and on escape routes. General fire safety requirements are set by the Civil Protection Code of Ukraine.
Insulating Specific Elements
A Corner Apartment
Corner rooms have two external walls and freeze the most. Options: external insulation of the corner with co-owners' consent, internal insulation with a vapor barrier, underfloor heating or an extra radiator along the cold wall, and careful humidity control.
Balcony and Loggia
If the balcony is joined to a room or used as living space:
- Warm glazing — see window replacement.
- Insulate the floor with XPS or PIR, plus the ceiling and walls.
- A vapor barrier on the warm side.
- Electric underfloor heating if heating is needed. Moving central heating radiators onto the balcony is prohibited.
The Floor Above a Basement
The most effective option is insulating the basement ceiling from below (with co-owners' consent). If that's impossible — insulation under the screed with XPS or a dry screed with insulation.
The Top-Floor Ceiling
Insulating the attic or technical floor is a matter for the whole building. If the roof leaks, repair it first, since wet insulation doesn't work.
Don't Forget Ventilation
Insulation together with airtight windows drastically reduces uncontrolled air exchange. Without planned ventilation, humidity rises, condensation and mold appear and air quality worsens. The World Health Organization's Housing and health guidelines stress that homes should be both warm and dry, with adequate ventilation. Solutions include trickle vents, extractor fans or ventilation systems with heat recovery. More in mold and damp prevention.
Building Retrofits Through an OSBB
It's most effective to insulate the whole building rather than one apartment: the facade, roof, basement, windows in entrances and a modernized heat substation with weather compensation. An OSBB can get partial reimbursement through Energy Efficiency Fund programs. The legal basis is the Law on Energy Efficiency of Buildings, which introduces European approaches to energy efficiency — see the EU Energy Performance of Buildings Directive. How to create an OSBB and organize the process is covered in how to create a condominium association.
How Much You Can Save
The effect depends on the building's initial condition. A comprehensive retrofit of an old building can substantially reduce heat consumption, especially combined with an individual heat substation and meters. The International Energy Agency sees improving building efficiency as one of the key tools for reducing energy use. The ENERGY STAR program notes that air sealing and insulation are among the most cost-effective measures for homeowners.
What the Dew Point Is and Why It Matters
The dew point is the temperature at which water vapor in the air turns into condensation. In winter, warm moist air from the apartment penetrates the wall and gradually cools. If at some depth the temperature drops below the dew point, water forms there.
- With external insulation, the wall stays warm and the dew point moves into the insulation layer, from which moisture escapes freely through a vapor-permeable facade. The wall stays dry.
- With internal insulation, the wall becomes cold, and the dew point often ends up at the boundary between wall and insulation. Without a vapor barrier, water accumulates there, causing mold and deterioration.
That's why a continuous vapor barrier, calculations and ventilation are critical for internal insulation. A simple test for an already insulated wall: a musty smell, dark patches on wallpaper near the floor and in corners, peeling finishes.
Insulation and Heating Work Together
After insulation, the heating system may become "oversized" for the apartment — rooms get too hot and you end up opening windows. To get real savings:
- install thermostatic valves on radiators — they maintain the set temperature in each room;
- with individual heating, set the boiler to a lower flow temperature;
- in private houses, insulation makes it possible to switch to low-temperature heating — underfloor heating and a heat pump;
- in apartment buildings with heat meters, savings show up in bills, especially after the heat substation is modernized.
Order of Works in an Apartment
- An energy audit or thermal imaging survey.
- Window replacement, if needed.
- Eliminating drafts and sealing joints.
- Insulating the balcony, corners and floor.
- Setting up ventilation.
- Balancing the heating system and fitting radiator thermostats.
Common Mistakes
- Internal insulation with polystyrene and no vapor barrier.
- Insulating a damp wall without fixing the cause of the damp.
- Ignoring thermal bridges — an insulated wall with cold reveals.
- Combustible materials without firebreaks.
- Insulating without ventilation — mold and stuffiness.
- Insulating one apartment's facade without co-owners' consent.
Frequently Asked Questions
What insulation thickness do I need? It depends on the wall material, climate zone and chosen insulation. Facades of apartment buildings often use 100–150 mm of mineral wool, but the exact value comes from a thermal calculation.
Can I insulate walls from the inside with plasterboard and mineral wool? Yes, but only with a continuous vapor barrier and a dew point calculation. Otherwise the wool gets wet and the wall grows mold.
Will insulation help with noise? Mineral wool improves sound insulation; polystyrene hardly does.
What if the walls freeze but external insulation isn't possible? Minimize thermal bridges, tune heating and ventilation, don't place furniture tight against cold walls and consider internal insulation with a calculation.
Conclusion
The best insulation is external and comprehensive, at the level of the whole building. Internal insulation is a forced compromise requiring a vapor barrier, calculation and ventilation. Either way, start with a survey, eliminate the causes of damp, choose certified materials and don't forget fire safety. For private houses, combine insulation with efficient heating such as a heat pump.
Sources
- Law of Ukraine on Energy Efficiency of Buildings.
- Law of Ukraine on Specifics of Exercising Ownership Rights in an Apartment Building.
- Civil Protection Code of Ukraine.
- WHO Housing and health guidelines.
- IEA: buildings, ENERGY STAR: seal and insulate.
- EU Energy Performance of Buildings Directive, Energy Efficiency Fund of Ukraine.