Wall U-value and R′ calculator
For the typical Romanian wall: a reinforced-concrete frame with ceramic-block or AAC masonry, ETICS with polystyrene or stone wool, base coat and decorative render. The calculation shows heat loss including the columns, beams and anchors, the temperature through the wall, condensation month by month and summer behaviour. Drag the outer face of the insulation in the drawing, or move the sliders.
—
PHI: Passive House Institute, cool-temperate zone · nZEB: Mc 001-2022, new residential buildings (R′ 4.00) · renovation: Mc 001-2022, major renovation · Order 2017: Order 2641/2017, the previous value. Recommended values, shown for comparison.
—
Moisture and heat through the wall
Condensation is calculated month by month with the chosen city's average climate. Summer behaviour is calculated for a day with a sinusoidal outdoor temperature.
The calculation describes the plain wall: linear thermal bridges at windows, balconies, the plinth or the parapet are not included and are calculated separately. Condensation is estimated from vapour diffusion (ISO 13788), without capillary transport or driving rain. It is not a design calculation.
Common Romanian walls, compared
A Porotherm 25 N+F wall with 10 cm of white EPS, very common in Romania, has U′ 0.304 W/(m²K), i.e. R′ 3.29 m²K/W. The same kind of masonry, but Porotherm 30 with 15 cm of graphite EPS, reaches U′ 0.174. The reinforced-concrete columns and beams raise U′ by 3–9 % over the masonry field. Tap a wall to open it in the calculator.
| Wall | U masonry W/(m²K) | U frame W/(m²K) | U′ wall W/(m²K) | R′ m²K/W | Time shift | Condensation, Bucharest |
|---|---|---|---|---|---|---|
| 0.278 | 0.348 | 0.304 | 3.29 | 11.3 h | no | |
| 0.170 | 0.194 | 0.187 | 5.35 | 11.9 h | no | |
| 0.154 | 0.193 | 0.174 | 5.76 | 16.0 h | no | |
| 0.168 | 0.217 | 0.190 | 5.28 | 19.1 h | no | |
| 0.146 | 0.215 | 0.172 | 5.83 | 24.8 h | no | |
| 0.133 | 0.193 | 0.157 | 6.38 | 16.1 h | no | |
| 0.109 | 0.147 | 0.129 | 7.76 | 16.7 h | no | |
| 0.113 | 0.165 | 0.136 | 7.38 | 23.7 h | no |
Inside: lime-cement plaster 1.5 cm. Outside: 5 mm adhesive, 5 mm mesh-reinforced base coat, silicone decorative render. Surface-mounted anchors, χ 0.002 W/K. Indoor humidity class 3 (ISO 13788).
How much insulation for the reference values
The thickness, rounded up to the centimetre, at which the whole wall (with a 20 % frame and 6 anchors per m²) reaches U′ 0.25 W/(m²K), the Mc 001-2022 nZEB reference value, and 0.15 W/(m²K), the Passive House value for the cool-temperate zone.
| Masonry | White expanded polystyrene (EPS 80) λ 0.039 | Graphite expanded polystyrene λ 0.031 | Stone wool, facade boards λ 0.035 |
|---|---|---|---|
| Porotherm 25 N+F | 13 / 25 cm | 11 / 20 cm | 12 / 23 cm |
| Porotherm 30 N+F | 12 / 23 cm | 9 / 19 cm | 11 / 21 cm |
| Porotherm 38 N+F | 10 / 21 cm | 8 / 17 cm | 9 / 19 cm |
| AAC 25 cm, 450 kg/m³ | 10 / 22 cm | 8 / 17 cm | 9 / 20 cm |
| AAC 30 cm, 450 kg/m³ | 9 / 21 cm | 8 / 17 cm | 8 / 19 cm |
| AAC 30 cm, 550 kg/m³ | 10 / 22 cm | 8 / 17 cm | 9 / 20 cm |
| AAC 35 cm, 450 kg/m³ | 8 / 20 cm | 7 / 16 cm | 8 / 18 cm |
First number: thickness for U′ 0.25; second: for U′ 0.15. The reference values are recommendations, not requirements.
The wall, the insulation and moisture
How much polystyrene does an nZEB wall in Romania need?
Mc 001-2022 recommends R′ ≥ 4.00 m²K/W for the walls of new residential nZEB buildings, i.e. U′ ≤ 0.25 W/(m²K). With a 20 % reinforced-concrete frame and 6 anchors per m², that means at least 13 cm of white EPS on Porotherm 25 N+F, 9 cm of graphite EPS on Porotherm 30 N+F or 8 cm of graphite EPS on 30 cm AAC. For the Passive House value (U ≤ 0.15) the thicknesses rise to 25, 19 and 17 cm.
How much do the reinforced-concrete columns and beams matter?
Reinforced concrete (λ 2.3) conducts heat 8 to 19 times better than ceramic-block or AAC masonry. For a Porotherm 30 N+F wall with 15 cm of graphite EPS, U at the frame is 0.193 against 0.154 in the masonry field; with a 20 % frame the wall's U′ rises by 5 %. For the walls in the table the rise is between 3 and 9 %, largest for well-insulating masonry (Porotherm 38, AAC), where the gap to concrete is widest.
Polystyrene or stone wool: what difference does it make to heat and vapour?
On Porotherm 30 N+F, 15 cm of graphite EPS (λ 0.031) gives U′ 0.174, stone-wool boards (λ 0.035) 0.190 and white EPS (λ 0.039) 0.205 W/(m²K). For vapour the difference is large: 15 cm of EPS has an equivalent air-layer thickness sd of about 9.0 m, stone wool only 0.15 m, so a wall with stone wool stays open to diffusion towards the outside.
Does the decorative render matter for moisture in the wall?
Over polystyrene, hardly at all: acrylic render has sd ≈ 0.30 m, while 15 cm of EPS has ≈ 9.0 m. Over stone wool the render becomes the most vapour-tight layer. For 30 cm AAC with 15 cm of stone wool in Brașov, with high indoor humidity (class 4), the calculation gives 159 g/m² of condensation with acrylic render, which evaporates by April; with mineral render there is none.
What is the time shift, and how much does an ETICS wall have?
The time shift is the delay with which a summer day's heat wave travels from the outer to the inner surface. A Porotherm 30 N+F wall with 15 cm of graphite EPS has a time shift of 16.0 hours and a decrement factor of 0.048; a 30 cm AAC wall with the same insulation, 16.1 hours and 0.055.
How much heat does a house wall lose in a year?
For 150 m² of exterior walls in Bucharest (2005–2023 typical meteorological year, 172 days averaging 12 °C or less, 2,579 degree-days), a Porotherm 25 N+F wall with 10 cm of white EPS loses about 2,820 kWh in the cold season, a Porotherm 30 N+F wall with 15 cm of graphite EPS 1,611 kWh, and Porotherm 25 N+F without insulation 13,066 kWh. In summer the gain through insulated walls is small, 119 kWh, because the wall's mass smooths out the facade's daily heating. SR 4839:2014, with 1981–2010 climate, gives 3,145 degree-days for Bucharest; recent years are warmer.
Where do the blocks' λ values come from, and why do shops differ?
The calculator uses the λ10,dry values declared by the manufacturer. For Porotherm 30 N+F, Wienerberger declares 0.204 W/(mK), while some distributors list 0.160 or 0.139. With 0.139, the U′ of the wall with 15 cm of graphite EPS would come out at 0.162 instead of 0.174. The value can be changed in the calculator's λ field.
How the calculation works
The calculator follows three European standards and one Passive House rule: EN ISO 6946 for U, EN ISO 13788 for condensation and the inner surface, EN ISO 13786 for summer, and the Passive House Institute rule for anchors. The summer calculation reproduces the published EN ISO 13786 reference case exactly.
Layers and U
Each layer's resistance is its thickness divided by λ. The surface resistances Rsi 0.13 and Rse 0.04 m²K/W are added, and U is the inverse of the sum.
Source: EN ISO 6946:2017
Frame and anchors
Columns and beams have the same layers, but reinforced concrete (λ 2.3) instead of masonry. The two zones are area-weighted. Anchors add n·χ, as the Passive House Institute requires for elements spread over the whole area.
Temperature through the wall
Temperature falls in proportion to each layer's resistance, from 20 °C inside to the design outdoor temperature of the climate zone.
Source: SR 1907-1 design temperatures
Condensation month by month
For each month the vapour pressure is compared with the saturation pressure at every interface between layers. Condensate builds up and evaporates over time. The outdoor climate is the city's monthly average, the inside is 20 °C, and the humidity depends on the chosen class (with the 1.10 safety factor).
Sources: EN ISO 13788:2012 · PVGIS
Inner surface
The temperature factor fRsi, with Rsi 0.25, is compared with the monthly minimum that keeps surface humidity below 80 % and with the Passive House value 0.70.
Summer
The EN ISO 13786 matrix method for a 24-hour period gives the wall's time shift, decrement factor, periodic thermal transmittance and internal heat capacity.
Sources: EN ISO 13786:2017 · reference case
Checks
| Check | Reference | Calculator |
|---|---|---|
| EN ISO 13786, 20 cm concrete: |Y₁₂| | 1.83 W/(m²K) | 1.827 |
| EN ISO 13786: Y₁₂ time shift | −5.68 h | −5.68 h |
| EN ISO 13786: Y₁₁ / Y₂₂ | 5.70 / 11.59 | 5.70 / 11.59 |
Limitations
- Linear thermal bridges (windows, balconies, plinth, parapet) are not included.
- λ is the declared dry value; in service, damp materials conduct heat somewhat better.
- The μ and χ values are typical and vary by product.
- Condensation is calculated by diffusion only, without capillary transport or driving rain.
- For Timișoara and Prahova the climate zone is not confirmed in the sources checked.
- Mc 001-2022 and Passive House values are shown as references, not as a verdict on the wall.
Sources
- EN ISO 6946:2017, thermal resistance and transmittance
- EN ISO 13788:2012, condensation and inner surface
- EN ISO 13786:2017, dynamic thermal characteristics
- Passive House Institute, criteria for opaque construction systems
- Mc 001-2022, minimum thermal resistances (summary, Romanian)
- Wienerberger Romania, Porotherm N+F data
- PVGIS, monthly climate of the cities
Calculation and text: SEMPER SUPRA INITIATIVE S.R.L. · Climate data: PVGIS © European Union, 2001–2026.