U value 0.18 explained.
What 0.18 means in real‑world temperature terms.
A U‑value of 0.18 means:
• For every 1°C difference between inside and outside,
0.18 watts of heat escapes through each square metre of wall.
So if:
• Inside is 20°C
• Outside is 0°C
• Temperature difference = 20°C
Then heat loss per m² is: 3.6watts per m2
That’s very low — which is why 0.18 is considered a high‑performance wall.
What 0.18 typically gives you inside
With a U‑value around 0.18, and a normal Irish winter outside temperature (0–5°C), the internal surface temperature of the wall usually sits around 17–18°C when the room is heated to 20°C
That’s warm enough to avoid:
• Condensation
• Cold‑surface discomfort
• Mould risk
And it keeps the wall performing well in BER/Part L terms.
How to think about it intuitively
If the U‑value is low:
• The inside surface of the wall stays warm.
• You feel less cold radiation.
• The room feels more stable and comfortable.
• Heating bills drop because heat isn’t leaking out.
If the U‑value is high:
• The inside surface gets cold.
• You feel draughty or chilly even without air movement.
• Condensation risk increases.
• Heating system works harder.
More Detail ?
With a U‑value of 0.18, the internal wall surface sits at ~19.5°C when the room is 20°C.
That is excellent — the wall feels almost the same temperature as the room.
Why this matters
• No cold‑surface discomfort.
• No condensation risk (dew point is far below 19.5°C).
• Very stable indoor comfort.
• Lower heating bills.
• Better BER performance.