● Wall energy performance
Our wall loses half the heat
that the law allows.
A warm house in winter, a cool one in summer, lower electricity bills. Below we explain in plain words what this means. At the end of the page is the full calculation for engineers.
The smaller the number, the less energy is lost through the wall. Timber-frame wall, calculated to EN ISO 6946.
In plain words
What 0.20 means for you.
U shows how much heat passes through one square metre of wall for every degree of difference between inside and outside. Cyprus allows up to 0.40 for new houses.
Winter
The warmth stays in
The wall lets out half the heat the law allows. The heating works less and the house holds its temperature all night.
Summer
The heatwave stays out
The same insulation works the other way too. When it is 40 °C outside, the heat takes a long time to get through and the air conditioning runs for fewer hours.
Bills
Less energy
Less heat lost means fewer kilowatt-hours for heating and cooling, every month. Every turnkey house of ours is handed over with a class A Energy Performance Certificate, the highest in Cyprus.
Comparison
Our wall next to brick.
The bar shows how much heat passes through. The shorter, the better. The dashed line is the legal limit.
A cold night, a hot midday
Drag the outside temperature. Inside, the house is at 22 °C. See how much heat passes through the external walls of a 130 m² house.
Example: about 125 m² of external walls, excluding windows, roof and floor. The brick values are approximate.
Where it comes from
Two layers make the difference.
The wall has seven layers, but the insulation comes mainly from the mineral wool inside the frame and the external insulation outside.
For engineers
The full calculation.
Calculation of the thermal transmittance U of a timber-frame external wall, using the combined method of EN ISO 6946.
Layers, from outside to inside
| # | Layer | Material | d (mm) | λ (W/mK) | R (m²K/W) |
|---|---|---|---|---|---|
| — | External surface | Rse | — | — | 0.040 |
| 1 | ETICS render | Knauf, reinforced base coat + silicone graffiato render | 6 | 0.70 | 0.009 |
| 2 | External insulation | Knauf EPS 100, λD ≤ 0.036 | 50 | 0.036 | 1.389 |
| 3 | Sheathing | OSB/3 | 11 | 0.13 | 0.085 |
| 4a | Frame insulation (85%) | Knauf Insulation Unifit 035, glass wool | 150 | 0.035 | 4.286 |
| 4b | Studs (15%) | Softwood | 150 | 0.13 | 1.154 |
| 5 | Membrane | Breathable waterproof membrane | — | — | 0 |
| 6 | Sheathing | OSB/3 | 11 | 0.13 | 0.085 |
| 7 | Internal lining | Knauf DFH2 moisture- and fire-resistant plasterboard | 12.5 | 0.25 | 0.050 |
| — | Internal surface | Rsi | — | — | 0.130 |
EN ISO 6946 combined method
| Step | R (m²K/W) |
|---|---|
| Insulation section, fa = 0.85 | 6.07 |
| Stud section, fb = 0.15 | 2.94 |
| Upper limit R′ (parallel paths) | 5.24 |
| Lower limit R″ (isothermal planes) | 4.83 |
| RT = (R′ + R″) / 2 | 5.03 |
| U = 1 / RT | 0.20 W/m²K |
1/R′ = fa/Ra + fb/Rb
R″ = Rsi + Σ Ri + 1/(fa/R4a + fb/R4b) + Rse
Assumptions
- Studs at 600 mm centres. Timber fraction 15% including horizontal members, a typical value from BRE BR 443.
- Rsi = 0.13 and Rse = 0.04 m²K/W, horizontal heat flow.
- Conductivities: timber 0.13 · OSB 0.13 · high-density plasterboard 0.25 W/mK (BRE BR 443). EPS and glass wool from Knauf data.
- The membrane adds no thermal resistance. Corrections for screws and gaps are less than 0.01 W/m²K and are not taken into account.
- Without studs the wall would have U = 0.16. The timber adds 0.03 W/m²K.
- Steel frame: below the 0.40 limit. The detailed calculation is available on request.
Comparison
| Walls | U (W/m²K) | Note |
|---|---|---|
| Bulma, timber frame | 0.20 | This calculation |
| Maximum permitted, Cyprus | 0.40 | New buildings from 1/1/2017, Cyprus law K.D.P. 119/2016 |
| 20 cm brick + 5 cm EPS | ≈ 0.49 | Approximate, brick λ 0.45 |
| 20 cm brick, no insulation | ≈ 1.53 | Approximate, brick λ 0.45 |
This calculation is a technical analysis of our standard wall. It is not an official energy performance study or an Energy Performance Certificate. These are issued for each building by a licensed expert. Standards: EN ISO 6946, EN ISO 10456, BRE BR 443.
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