What is a U-Value & Condensation Calculator?
The U-value calculator computes the thermal transmittance (U-value) of a wall or window assembly from the thickness and thermal conductivity of each material layer, and checks for condensation risk under given indoor/outdoor conditions.
How it's calculated
U = 1 / (Rsi + Σ(d/λ) + Rse), where d is material thickness (m) and λ is thermal conductivity (W/m·K). Units are W/m²·K — lower values mean better insulation.
Variable definitions
| Variable | Meaning | Unit |
|---|---|---|
| U | Thermal transmittance | W/m²·K |
| d | Material thickness | m |
| λ (lambda) | Thermal conductivity | W/m·K |
| Rsi, Rse | Interior/exterior surface resistance | m²·K/W |
Assumptions
This assumes the wall is made of flat, fully-contacting material layers with no air gaps between them. In real construction, a small gap between insulation boards creates a localized thermal bridge, making the actual U-value worse than calculated at that specific point.
Worked example
A wall built from 200mm concrete (λ=1.6) plus 100mm insulation (λ=0.035)
- Concrete resistance: 0.2÷1.6 = 0.125
- Insulation resistance: 0.1÷0.035 ≈ 2.857
- Assuming Rsi+Rse ≈ 0.17, U = 1÷(0.125+2.857+0.17) ≈ 0.31 W/m²·K
Why does condensation happen?
Air can only hold a certain amount of moisture at a given temperature. When warm air touches a cold surface and cools down, it can no longer hold onto that moisture, and water condenses out. So condensation is fundamentally a temperature problem, not a humidity problem — it happens whenever a wall's surface temperature drops below the indoor air's dew point.
Good to know
- Mold appears before visible condensation. Mold can start growing at around 80 percent relative humidity, well before water droplets are visible.
- Doubling insulation thickness does not double performance. U-value is the inverse of thermal resistance, so returns diminish past a certain point.
- Even with excellent wall insulation, a thermal bridge (a point where heat escapes more easily, like a balcony slab or window frame) will cause localized condensation there.
- Windows always have a worse U-value than walls. Walls might reach 0.2–0.3, while even high-performance windows are typically 1.0–1.5.
- The cheapest way to prevent condensation is ventilation. Better-insulated homes are also more airtight, which traps humidity indoors — making ventilation more important, not less.
Frequently asked questions
Q. Is a lower U-value better?
Yes. U-value measures how easily heat passes through, so a smaller number means better insulation.
Q. Why does condensation happen even with good insulation?
Thermal bridges, or excessive indoor humidity from increased airtightness, are common causes.
Q. What is the difference between R-value and U-value?
R-value is thermal resistance; U-value is its inverse (thermal transmittance). Higher R-value and lower U-value both mean better insulation.