Interior finishing of aerated concrete: 7 rules that will save your walls from condensation and cracks
Interior finishing of aerated concrete requires a special approach because of the material's high vapour permeability — 0.16–0.23 mg/(m·h·Pa). The main rule: each subsequent layer must let vapour through better than the previous one. Gypsum plaster is the optimal choice for dry rooms, cement plaster risks condensation. A vapour barrier on the inside is a serious mistake.
Why aerated concrete dictates its own finishing rules
Finishing aerated concrete walls inside is a frequent question, so let us examine it in detail. Aerated concrete differs fundamentally from brick or monolith in its porous structure. This determines three key factors that directly influence the choice of materials and finishing technology.
Vapour permeability of aerated concrete: the key characteristic
The vapour permeability of aerated concrete is 0.16–0.23 mg/(m·h·Pa), which is 5–7 times higher than that of ordinary concrete and 2–3 times higher than that of ceramic brick. The wall works as a membrane: water vapour from the room passes through the blocks and is removed outside through the facade. This property is the main argument against using vapour-tight materials (dense cement-sand mortar, oil paint, polythene film) on the interior surface. In the dry climate of Almaty, with a winter temperature difference of up to 30–40°C between the house and the street, the dew point shifts into the wall. If the finish lets vapour through less readily than the aerated concrete, moisture is retained in the body of the block — condensation and mould begin, and after freeze-thaw cycles, peeling and destruction of the cellular structure follow.
Shrinkage and moisture: what you need to know before finishing
Aerated concrete shrinks by 0.3–0.5 mm/m in the first 1–2 years after masonry, with 60–70% of the deformation occurring in the first month. If plastering begins before 28 days of curing of the blocks, or when the moisture content of the blocks is above 27% (the standard for supplied autoclaved blocks is 25–30%), shrinkage cracks in the finish are practically inevitable. In Almaty projects, where masonry is laid in September-October and finishing is planned for the New Year, this is a typical mistake: the blocks do not have time to dry to the working moisture content of 5–10%. Even when curing times are observed, reinforcement with fibreglass mesh at joints and corners is essential — an alkali-resistant mesh with a 5×5 mm cell compensates for micro-deformations and prevents cracks at points of stress concentration.
How humidity in a room affects aerated concrete
Aerated concrete is hygroscopic — it absorbs moisture from the air, and when indoor humidity exceeds 60%, the blocks begin to become saturated with vapour to a depth of up to 10–15 mm. If the finish (for example, cement plaster with a dense grout) blocks the escape of vapour, the wall's thermal conductivity increases by 20–30%, and mould appears on the surface. In Almaty winter conditions, when residents often block natural ventilation to keep in heat, indoor humidity can rise to 70–80%. Ventilation of the rooms is not an optional extra but a mandatory condition for an aerated concrete house: without proper air exchange (an air change rate of 0.5–1 volumes per hour), no “breathing” plaster will save you from condensation on the walls.
Key principle: the rule of increasing vapour permeability
Plastering aerated concrete indoors is a common question, so let us examine it in detail. We look at the key principle that determines the choice of all materials when finishing aerated concrete walls indoors — from primer to topcoat. Without understanding it, any interior finishing of aerated concrete risks becoming a moisture trap.
What is the rule of increasing vapour permeability
The rule of increasing vapour permeability states: each successive finishing layer from the interior towards the street must transmit water vapour better than the previous one. For aerated concrete this means that the vapour permeability of the interior plaster must be lower than that of the blocks (0.16–0.23 mg/(m·h·Pa)), but higher than that of the final coating. In practice the pyramid is built as follows: the most “breathable” layer is the aerated concrete wall, then the primer and plaster with reduced vapour permeability, and on top paint or wallpaper with minimal transmission capacity. If the layers are swapped — applying vapour-tight cement plaster (0.03–0.05) to porous aerated concrete — the vapour hits the barrier and condenses within the block. In Almaty houses with frequent temperature swings this shows up already in the first heating season as damp spots on the wallpaper, and mould in the corners develops within 2–3 months rather than over years.
Vapour permeability figures: a visual comparison of materials
The vapour permeability of materials when finishing aerated concrete is measured in mg/(m·h·Pa). The higher the figure, the better the material lets vapour through.
| Material | Vapour permeability, mg/(m·h·Pa) | Compatibility with aerated concrete | Recommendation |
|---|---|---|---|
| Aerated concrete (base) | 0.16–0.23 | — | Initial characteristic |
| Gypsum plaster | 0.07–0.12 | ✅ Optimal for dry rooms | Recommended |
| Specialised plaster for aerated concrete | 0.08–0.12 | ✅ Ideal | Recommended |
| Cement-lime plaster | 0.05–0.08 | ⚠️ Suitable for wet rooms | Use with caution |
| Cement-sand plaster | 0.03–0.05 | ❌ Blocks the escape of moisture | Not recommended |
| Vapour barrier film | 0.001–0.005 | ❌ Complete vapour blocking | Prohibited |
How external insulation affects interior finishing
If the aerated concrete is insulated on the outside with a vapour-permeable material (mineral wool, stone wool with a value of 0.3–0.6 mg/(m·h·Pa)), the interior finish may be less vapour-permeable — the outer layer partly takes on the load of removing moisture, but the plaster on the inside must still remain “breathable”. The worst-case scenario is insulation with expanded polystyrene (0.02–0.05), which hardly lets vapour through; with it, the interior finish must be as vapour-permeable as possible — gypsum plaster without dense finishing coats. If there is no external insulation, the interior plaster is likewise chosen for maximum vapour permeability, otherwise condensation is inevitable — in houses across Almaty with a humid climate this problem is aggravated by frequent temperature swings between day and night.
Wall preparation: primer, reinforcement, moisture control
Preparing aerated concrete walls is no less important a stage than the plastering itself. We break down how to finish aerated concrete indoors correctly, taking into account all the nuances of the substrate.
Humidity control and curing of aerated concrete
Before finishing begins, aerated concrete must cure after laying for at least 28 days. The moisture content of the blocks must not exceed 27% by mass — this is checked with a moisture meter or indirectly by the absence of dark spots on the surface. Freshly laid blocks contain up to 40–50% moisture from the masonry mortar, and if plastering starts too early, the water will begin to escape through the plaster layer, causing it to blister and peel. In Almaty, with sharp temperature swings and damp autumn periods, the drying process is especially unpredictable — in an unheated house the blocks can take up to 45 days or longer to dry. If finishing begins on damp blocks, moisture from the aerated concrete will escape through the plaster layer, causing it to peel and crack. In the cold season the drying process slows down — the curing period may increase to 40–45 days.
Deep-penetrating primer: why and how
A deep-penetrating primer for aerated concrete is essential — it strengthens the surface layer of the blocks, reduces absorbency and provides adhesion for the plaster. It is applied in 1–2 coats, with each coat drying for 3–4 hours. Unlike an ordinary acrylic primer, which remains on the surface as a film, primers for aerated concrete penetrate the pores to a depth of 5–10 mm, binding dust and fine particles into a solid mass. The Almaty market offers Knauf Grundiermittel concentrates, Ceresit CT 17 and domestic equivalents — they are all diluted with water in a ratio of 1:3–1:5. It is important to apply the primer without gaps, especially on the ends of the blocks and the areas around window openings, where absorbency is at its highest. Do not confuse a deep-penetrating primer with an ordinary acrylic one — aerated concrete needs a mix that penetrates the pores to a depth of 5–10 mm. Saving on primer leads to the plaster “tearing away” from the wall together with the top layer of aerated concrete.
Fibreglass mesh reinforcement: where and how
Reinforcing fibreglass mesh is mandatory at all corners, at the joints of dissimilar materials (aerated concrete-concrete, aerated concrete-brick) and where walls adjoin door and window openings. The mesh is embedded into the first layer of plaster to 1/3 of its thickness — this secures it and prevents cracks from appearing as the house settles. In Almaty, where the soils are clayey and seasonal foundation movement is inevitable, reinforcing the corners becomes critical — a crack at the joint between aerated concrete and a concrete floor slab can open up to 2–3 mm during the first year of use. Use an alkali-resistant fibreglass mesh with a 5×5 mm cell — ordinary facade mesh breaks down in the alkaline environment of cement plaster within 2–3 months. Any fibreglass mesh is suitable for gypsum plaster — gypsum is not aggressive towards glass fibre, but the mesh density must be at least 160 g/m² so that it holds its shape when the layer is applied.
Temperature conditions and working environment
Finishing work inside an aerated concrete house is carried out at a temperature of +5 to +30°C and an air humidity of no more than 70%. Draughts are unacceptable — they cause uneven drying of the plaster: the edges dry faster than the centre, which leads to tensile stresses and cracks. In the Almaty climate with sharp temperature swings, it is especially important not to start finishing in an unheated house in autumn or winter — night frosts down to -10°C in October are not uncommon here. If the house is not connected to heating, wait for sustained warm weather — otherwise the moisture in the plaster will freeze and destroy the layer. A practical tip: before starting work, tape over the windows with polythene film to eliminate draughts, and set a heat gun to 15–18°C for 3–5 days before applying the plaster — this equalises the temperature of the walls and the air, preventing localised over-drying.
What to plaster aerated concrete with indoors: gypsum, cement or special mixes
Choosing plaster for aerated concrete is the key decision that determines the indoor climate of the house. We compare three options and work out which one suits your conditions.
Gypsum plaster: the optimal choice for dry rooms
Gypsum plaster is the best option for finishing aerated concrete in dry rooms (bedrooms, living rooms, corridors). Its vapour permeability of 0.07–0.12 mg/(m·h·Pa) combines well with aerated concrete, and its thermal conductivity coefficient of 0.25–0.3 W/(m·K) is lower than that of cement-based mixes — this provides additional thermal insulation for the walls. It is applied in a layer of 5–20 mm without mandatory reinforcement of the entire surface — it is enough to strengthen the corners and the joints of the floor slabs with joint tape. In the Almaty climate with sharp humidity swings, gypsum acts as a natural regulator: when indoor humidity rises it absorbs the excess, and when it falls it releases it. This is especially valuable for aerated concrete houses, where it is important to maintain a stable humidity regime without additional humidifiers.
Cement plaster: when it is acceptable and when it is not
Cement-sand plaster with a vapour permeability of 0.03–0.05 mg/(m·h·Pa) is not recommended for aerated concrete — it blocks moisture from escaping, which leads to condensation inside the wall and subsequent destruction of the blocks. The exception is cement-lime plaster (vapour permeability 0.05–0.08), which is acceptable in wet rooms provided there is good ventilation: in a kitchen or bathroom it performs more reliably than gypsum. But even a cement-lime mix is applied to aerated concrete only with mandatory fibreglass mesh reinforcement and only after a high-quality deep-penetrating primer. If you have already applied cement-sand plaster to aerated concrete, the only way to fix the situation is to remove it completely and replace it with gypsum or a specialised plaster. “Adding something to the mix” does not work: vapour permeability is determined by the structure of the hardened cement stone, which cannot be changed with additives.
Specialised plasters for aerated concrete
Specialised plasters for aerated concrete — for example, Knauf Rotband, Ceresit CT 24 and Volma Layer — have a vapour permeability of 0.08–0.12 mg/(m·h·Pa), optimal for cellular concrete. They contain polymer additives that improve adhesion to the porous surface and do not require additional reinforcement of the entire wall — only the corners and the joints between dissimilar materials. On the Almaty market these mixes cost 20–30% more than ordinary gypsum (around 2500–3500 KZT per 30 kg bag versus 1800–2200), but they guarantee the absence of cracks. The main difference of the special compounds is balanced shrinkage: ordinary plasters shrink by up to 1–1.5 mm/m when drying, which on aerated concrete leads to cracks, whereas specialised ones shrink by 0.3–0.5 mm/m, comparable to the shrinkage of the blocks themselves.
Comparison of plasters for aerated concrete: selection table
| Type of plaster | Vapour permeability, mg/(m·h·Pa) | For which rooms | Application specifics | Risks |
|---|---|---|---|---|
| Gypsum | 0.07–0.12 | Dry rooms (bedrooms, living rooms, corridors) | Layer thickness 5–20 mm, reinforcement only at corners | Not suitable for wet rooms |
| Cement-lime | 0.05–0.08 | Wet areas (bathrooms, kitchens) | Requires mandatory reinforcement with mesh | Higher risk of cracks during shrinkage |
| Specialised for aerated concrete | 0.08–0.12 | Any (universal) | Minimal shrinkage, good adhesion without primer | Higher price |
| Cement-sand | 0.03–0.05 | ❌ Not recommended | — | Condensation, mould, destruction of the blocks |
Vapour barrier when finishing aerated concrete: needed or not
One of the most frequent questions when finishing a gas block inside is whether a vapour barrier is needed. We explain why for aerated concrete the answer is unequivocal.
Why an internal vapour barrier is a mistake for aerated concrete
A vapour barrier on the inside of an aerated concrete house blocks the escape of water vapour from the room, shifting the dew point into the wall — this guarantees condensation, mould and destruction of the blocks. A family of three releases up to 2-3 litres of water a day as vapour from breathing, cooking and showering; if polythene film or foil-faced insulation is placed between the wall and the finish, the moisture is locked inside the aerated concrete. With sub-zero temperatures outside and humid Almaty winters, the blocks in the northern districts (Alatau, Zhetysu) do not have time to dry out over the summer — condensation forms at the junction of the cold block and the vapour barrier. Many people confuse a vapour barrier with waterproofing: waterproofing protects against liquid water (foundation, blind area), while a vapour barrier protects against vapour; for aerated concrete a vapour barrier is needed only on the outside, to protect the insulation, but not on the inside.
When a vapour barrier may still be needed
A vapour barrier on the inside is justified in only one case: if the external insulation is made of vapour-tight expanded polystyrene or XPS without a ventilation gap — but that is a design error, not the norm. Local developers do encounter such situations when a facade is insulated with 100 mm extruded polystyrene and plastered without a gap: the vapour meets the vapour-tight layer on the outside, gets trapped in the block, and the wall becomes damp. The only way to save the situation is to install a vapour barrier on the inside so that vapour does not enter the aerated concrete, but this turns the house into a thermos with stuffiness and condensation on the windows. The right solution is not to install a vapour barrier but to ensure the vapour permeability of all layers of the finish; if the external insulation has been done incorrectly, redo it — that is cheaper than sealing the walls and fighting mould.
What to use instead of a vapour barrier: the right solutions
Instead of a vapour barrier, use the following sequence: deep-penetrating primer → gypsum plaster → final finish (water-based paint, paper or non-woven wallpaper). Every layer must be vapour-permeable — for the Almaty climate with humidity swings from 40% in winter to 70% in autumn, gypsum plaster acts as a buffer: it absorbs excess moisture when it is damp and releases it when it is dry. If you plan to lay tiles in a bathroom on aerated concrete, do not put a vapour barrier underneath — waterproofing of the joints and grout is enough: porcelain stoneware (0,01 mg/m·h·Pa) and ceramic tiles (0,02) are themselves vapour-permeable. The only case for a membrane is a shower without a tray with direct water contact, but it is laid under the tiles on the floor screed, not on the walls.
5 common mistakes in interior finishing of aerated concrete
Even experienced builders make mistakes when working with aerated concrete. We have gathered the five most common ones — check whether you are repeating them.
Use of cement-sand plaster
The most common mistake is plastering aerated concrete with a cement-sand mix. Its vapour permeability of 0.03–0.05 mg/(m·h·Pa) is 3–5 times lower than that of aerated concrete, which is guaranteed to cause condensation inside the wall during temperature swings. Almaty winters with frequent thaws create ideal conditions for this effect: moisture accumulates at the boundary between the block and the plaster, freezes in winter and destroys the coating from within. If you have already applied such a finish, the only solution is to knock off the plaster completely and replace it with gypsum or a specialised product — “skimming over the top” will not help, as the cement layer will still remain a vapour-tight barrier.
Lack of reinforcement at corners and joints
Cracks in aerated concrete plaster after a year are almost always the result of the absence of fibreglass mesh reinforcement at corners, wall and ceiling joints, and where walls meet door openings. The shrinkage of aerated concrete of 0.3–0.5 mm/m creates stresses that tear the plaster apart without mesh. In Almaty houses with seasonal humidity swings this process runs faster — cracks appear as early as 6–8 months after finishing. You do not need to reinforce the entire wall, only the problem areas: corners (internal and external), joints between dissimilar materials and junctions with openings — the mesh is embedded in the first plaster layer to a depth of 1/3 of the coating thickness.
Ignoring the curing period of aerated concrete after masonry
Fresh aerated concrete contains up to 30–40 % moisture from production, and starting finishing earlier than 3–6 months after laying is a gross mistake. Moisture trapped by the plaster escapes through microcracks, peels the coating and creates an environment for mould. In the Almaty climate with high humidity in spring and autumn this process accelerates — dark spots appear on the walls within the first winter. Accelerated drying with heat guns does not replace natural curing — it creates a moisture gradient between the centre of the block and the surface, which leads to uneven shrinkage and cracks in the finish.
Conclusion
We have gone through the main principles of finishing aerated concrete in Almaty. Here are the key takeaways that will help you avoid typical mistakes and achieve a durable result.
Key takeaways
- Vapour permeability is the main principle: every layer of the finish must let moisture through no less readily than the aerated concrete. Breaking this rule leads to condensation, mould and destruction of the walls.
- Priming is an essential stage: without a deep-penetrating primer, the adhesion of the plaster to the aerated concrete drops by a factor of 2–3, which guarantees detachment in the first year.
- Plaster — gypsum or specialised only: cement-sand mixes block moisture from escaping and create a vapour-tight barrier. Gypsum plasters with a vapour permeability of at least 0.12 mg/(m·h·Pa) are suitable for aerated concrete.
- A vapour barrier is almost never needed: inside a heated house, aerated concrete regulates humidity by itself. A vapour barrier is justified only in wet areas or when external insulation is made of polystyrene.
- Curing and reinforcement are protection against cracks: begin finishing no earlier than 3 months after masonry, and be sure to reinforce corners and joints with fibreglass mesh — this prevents shrinkage cracks.