What does U value mean?

Loosely, U values reflect the amount of heat transference across a structure. It is a way to quantify how much heat energy is lost through the fabric of a building across different materials and thicknesses.


With U values the lower the value achieved the better in terms of saving energy. This is why whenever new building regulations and amendments are brought out the U value needed to be met is invariably lowered. Indeed, the goal post on U values has been moved greatly over the last decade and is on a path to be even more stringent in the future which will require fundamental changes in building design to accommodate these tough new U value regulations. The lower the U value achieved the lower the energy bill will be to heat the building.

A U value is a measurement of the overall heat energy transfer rate (under standard conditions) through a particular section of construction material, e.g. roof, walls, glazing etc.
It is defined as the rate of heat flow in watts (W) through an area of 1 square metre (m) for a temperature difference across the structure of 1 C degree centigrade or Kelvin (K).
The U value measurement units are written as W/m2K.. It includes conduction through solids, and convection and radiation through air gaps in the construction and at the surfaces. Standard conditions enable comparisons of products of different materials and manufacturers. They include particular moisture contents and surface resistances.

Typical U values in W/m2K
Solid brickwork, no insulation 2.5
Cavity wall, no insulation 1.6
Cavity wall with insulation 0.5
Single glazing 5.7
Double glazing 2.8
Double glazing with low-Iμ coating 1.8
Interstitial Condensation Analysis
Interstitial condensation occurs when warm, moist air from inside a building penetrates into a wall, roof or floor construction and meets a cold surface. This causes the air to cool, lowering its capacity to carry moisture, and resulting in condensation on the cold surface.

Cold bridging
When installing internal insulation, the junctions of internal walls and floors with external walls may 'short-circuit' the insulation, allowing heat to escape. The presence of such 'thermal bridges' will reduce the effectiveness of internal insulation. In so far as is practical, insulation should be installed so as to minimise thermal bridging.
External insulation involves fixing insulation materials such as mineral wool or expanded polystyrene slabs to the outer surface of the wall. This insulation is covered with a special cement-based render to provide weather resistance. A steel or fibreglass mesh is embedded in this render to provide strength and impact resistance. It may be necessary to extend eaves and sills and relocate downpipes. A thin layer of insulation may be applied around the edges of window and door openings to minimise thermal bridging. The technology is well-established in northern Europe, where it has been in use for many years, but is not yet widely used in Ireland.

Power Predictor

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