What R-value, U-value and the dew point really mean in building physics — explained with no formulas, on real-life examples. Get to grips with building physics in 5 minutes.
What you'll learn
When you read about insulation, "R", "U", "λ" and "dew point" flash everywhere. It sounds scary, but behind these letters are simple ideas. In this article we explain them with everyday examples, so you can confidently understand what's being talked about and check a wall yourself.
λ (lambda) — how well a material "holds" heat
λ is a material's thermal conductivity: how easily heat passes through it. The lower the λ, the better the material insulates.
- Insulation materials (mineral wool, expanded polystyrene) have a low λ — heat passes through them with difficulty. That's good.
- Concrete or metal have a high λ — heat passes easily. That's why they make poor insulators.
The λ values for materials are in the materials library — you don't have to memorise them.
R-value — how warm the whole structure is
R is thermal resistance: how well the whole wall (all its layers together) resists heat escaping. The higher the R, the warmer the wall.
A simple rule: a thicker insulation layer → a higher R → warmer. The ДБН standard sets the minimum R the wall must reach.
Think of R as the wall's "warm clothing". A thin T-shirt is a low R. A winter jacket is a high R. The standard says: dress the house no lighter than a jacket of a certain thickness.
U-value — that's R the other way round
U is the heat-transfer coefficient, essentially the inverse of R. The lower the U, the better (less heat passes through).
- A high R = a low U = a warm structure.
- A low R = a high U = a cold structure.
U is handy when comparing windows or individual elements: a window's datasheet often quotes exactly the U.
Dew point — why a wall can get damp
The dew point is the temperature at which moisture in the air falls out as droplets. If a wall surface is colder than the dew point, condensation appears on it. A warm wall (a high R) stays dry. More on this in the article why a wall gets damp.
See it on a live example
The best way to understand these concepts is to see them on a real wall. The thermal conductivity calculator shows R, U, the heat flux and the dew point for the layers you enter — along with an explanation of how the number came about.
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Frequently asked questions
Which matters more — R or U? It's the same thing from different sides: U = 1/R. Look at R for walls, at U for windows.
Where do I get λ from? From the materials library — the λ for each material is already listed there.
I want to check my wall against the standard. That's the next step — the article does my wall meet the ДБН standard.
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