Internal thermal insulation of the balcony

A cold balcony is not merely a matter of comfort. If the wall facing the balcony, the floor or the ceiling remain unprotected, warm air from the dwelling meets cold surfaces, and the result may be condensation, mould in the corners and higher heating or cooling costs. A well-designed interior balcony thermal insulation simultaneously addresses heat loss, vapour movement and airtightness - not merely the thickness of a material bonded to the wall.

For owners renovating a flat or converting a balcony into usable space, the correct choice depends above all on one thing: does the balcony remain unheated, is it enclosed but separated from the room, or does it become part of the thermal envelope of the dwelling? Each situation requires a different build-up.

When interior thermal insulation of the balcony is necessary

Interior insulation is generally chosen when the exterior façade cannot be modified, when the building regulations of the condominium do not permit visible interventions or when the work must be carried out from within the flat. It is a practical solution, but more sensitive to installation details than exterior thermal insulation.

If the balcony is enclosed with glazing but is not permanently heated, it remains a colder area than the room. The wall between the room and the balcony, the concrete slab and the window surround may become thermal bridges. If, by contrast, the balcony is integrated into the living space, all surfaces in contact with the exterior must be analysed: the parapet, the side walls, the ceiling beneath the neighbour's balcony and the floor above an unheated area.

In many homes in the US, the local energy code sets minimum thermal resistance values, expressed as R-value. The material and thickness are chosen according to the climate zone, the wall type and the available space. A thin solution can be very useful where every inch counts, but it should not be presented as a universal equivalent of a bulky insulation with a high R-value. Actual performance comes from the complete system.

Why condensation and mould appear on the balcony

Water vapour in the interior air migrates towards colder areas. When the temperature of a surface falls below the dew point, the water condenses. Behind an interior lining, the phenomenon may remain invisible for months, until stains, a persistent odour or deterioration of the finish appear.

The most frequent mistake is installing an insulating layer without a vapour strategy and without airtightness. Warm air can penetrate through joints, sockets, edges or perforations, carrying far more moisture than slow diffusion through materials. For this reason, joints, overlaps and connections to the window frames are as important as the board or wool selected.

A reflective membrane with a vapour barrier function can contribute to air and moisture control when correctly installed, with sealed joints and with an air cavity where the product requires one for the reflective effect. Not all membranes have the same vapour permeability, and not every membrane should be placed on the warm side of the wall. In mixed climates or those with intensive summer cooling, the position of the vapour control layer must be verified in order to avoid trapping moisture in the wall.

Do not insulate only the main wall

The balcony slab is often the element that transmits the most cold towards the interior. If you insulate exclusively the vertical wall, the edge of the floor and the perimeter of the ceiling may remain cold. This temperature difference explains why mould frequently appears in corners, behind furniture or at the base of the wall, even after an apparently correct renovation.

An intelligent approach seeks continuity of the thermal insulating layer. Where the geometry permits, the insulation must be carried around returns, into the window reveal area and to the junction between the wall and the slab. If an area cannot be fully covered, careful sealing and the elimination of gaps become all the more important.

Suitable materials for an interior system

The choice is not limited to “the thickest insulation”. The available space, existing moisture, the wall structure and the desired finish determine the build-up. For a lining on a framework, ecological thermal insulation wool can provide thermal and sound insulation, especially when a reduction of traffic noise is desired. It requires correct protection against air currents and a vapour control layer adapted to the project.

Reflective thermal insulation membranes are useful when the available thickness is limited and when it is desired to combine the functions of thermal reflection, vapour control and airtightness. Their benefit depends on the installation detail: the reflective surface generally requires a functional air cavity, and the joints must be taped with compatible tape. Installed directly and repeatedly perforated, they may lose a significant part of their role.

Rigid boards can provide high thermal resistance within a controlled thickness, but they must be checked in terms of fire behaviour, joint sealing and compatibility with the finish. In a balcony with cold masonry or concrete, direct installation on a damp substrate is a recipe for problems. First, infiltration, cracks and water sources from the exterior must be eliminated.

For the floor, a thin layer of insulation and cushioning can reduce the sensation of cold and impact noise, especially beneath floating parquet or other compatible finishes. Nevertheless, for a floor above the exterior, the limited thickness cannot by itself compensate for an uninsulated concrete slab. It is a useful improvement, not an unrealistic promise.

The correct sequence of works for interior balcony thermal insulation

Before selecting materials, inspect the balcony. The window frames must be airtight, the sills must drain water towards the exterior, and infiltration from the parapet, ceiling or slab joints must be remedied. Interior insulation does not repair water entering from the exterior - it can only temporarily conceal its effects.

The substrate is then assessed. Concrete and masonry must be clean, stable and dry. Mould stains are treated after the cause has been eliminated, not by covering them. If there is persistent moisture, a specialist must determine whether it originates from condensation, infiltration, services or a structural crack.

Next comes the creation of the thermal insulating layer and the airtightness plane. In a lightweight framework build-up, the cavities are filled uniformly with insulation, without excessive compression and without gaps. The vapour control or airtightness membrane is installed continuously, and the joints are sealed with professional accessories. The connections to windows, ceiling, floor and adjacent walls are the points at which performance is gained or lost.

Finally, the finish is installed – for example, plasterboard suited to the space, decorative cladding or a system compatible with local fire protection requirements. Sockets and light fittings must be treated with care, since every cut-out may interrupt the seal. A service cavity between the membrane and the finish reduces the number of penetrations and allows services to be routed in a more controlled manner.

Mistakes that cost more than the material

Do not enclose the balcony with insulated cladding before resolving water ingress. Do not leave joints unsealed on the assumption that the finish will cover them. Do not combine vapour barriers at random with highly impermeable layers on both sides of the wall. And do not assume that a reflective membrane, mineral wool or a rigid board performs identically in every wall.

Another risk is the occasional heating of a poorly insulated balcony. Warm, moist air quickly reaches cold surfaces, and condensation becomes more likely. If the space is to be used as an office, play area or extension of the living room, design it as a conditioned space, with continuous insulation, suitable windows and adequate ventilation.

How to choose a system that genuinely works

Ask for clear values: thickness, Lambda, R-value where available, vapour permeability, area covered and the recommended installation method. Check whether the product is intended for walls, floors or roofs and whether it requires an air cavity. A complete package, with membrane, sealing tape and compatible accessories, reduces the improvisation that compromises the work.

Isoflect can assist in configuring a system according to the dimensions of the balcony, the type of substrate and the planned finish. For a predictable recommendation, prepare a few simple pieces of information: photographs of the area, the surface area, the maximum available thickness, any presence of damp and the way in which you will use the balcony.

The balcony becomes comfortable when the materials work together, not when layers are added at random. A technical check before installation is far more affordable than removing a new finish to remedy hidden condensation.

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Not sure which Isoflect product suits you? See Reflectiva Multistrat or open the package configurator to find out exactly what you need and in what quantity.