FXSTD – Alternative Energy Center

By: John Mahoney
U- Values of Materials

In its simplest form a U-Value gives you the rate of heat loss through a material. In nature heat, like all energy types, has the tendency to flow from denser to less dense medium through any material it can find, i.e. glass, wall bricks and even air. All it needs to is to transfer itself to less energized molecules of the material.

If you ask “Why would a home-owner care for the U-values?” here is the answer:

During winter months you heat your house and during summer months you cool your house. In winter the hot air inside and in summer the hot air outside the house will try to transfer the energy it has to the less dense medium, i.e. the colder air. Thus, in winter you will lose heat and fuel as a result of this and in summer you will spend more electricity to cool your house. This means that you should seek for the lowest possible U values in your house while having any kind of construction, insulation etc. If we need to give figures for these, single layer glass windows have a typical U-value of 5.6 while double glazed windows have a typical U-value of 2.8.

The following table is taken from the Sustainable Energy Ireland website which is the official sustainable energy agency of Ireland.

Typical U-Values

Note: These U-values are provided as a guide. U-values may vary depending on the particular construction e.g. block or brick walls.

WALLS (Building Regulations: 0.37) ROOF, ATTIC AND ATTIC ROOM (Building Regulations: 0.25)

Cavity Wall Insulation U-Value Pitched Roof with Attic Space U-Value

Note: To achieve these U-values, insulation must be between and over ceiling joists

U-Value

100mm Blown Polystyrene 0.30 150mm Glass Wool 0.25

100mm Blown Cellulose Fibre 0.30 150mm Rock Wool 0.23

100mm Blown Mineral Wool 0.30 150mm Sheep's Wool 0.23

60mm Extruded Polystyrene Insulation 0.40 200mm Glass Wool 0.19

35mm Polyurethane Foam Board 0.39 200mm Cellulose Fibre 0.16

35mm Phenolic Foam Board 0.37

Timber Frame 150mm, Mineral Quilt 0.25 Attic Room U-Value

Timber Frame 140mm, Cellulose Fibre 0.19 70 mm Polyurethane 0.21

Safewarm Home Construction 0.31 130 mm Expanded Polystyrene 0.25

External Wall Insulation U-Value

60mm Moulded Polystyrene 0.44 WINDOWS (Building Regulations: 2.2) U-Value

Internal Wall Insulation U-Value Single Glazing 5.6

50mm Expanded Polystyrene 0.48 Double Glazing 2.8

38mm Polyurethane 0.45 Double Glazing, with Argon 2.6

Double Glazing, Low-E 1.8

Double Glazing, Low-E with Argon 1.5

FLOORS (Building Regulations: 0.37) Note: U-values are based on concrete slab construction

60mm Polystyrene 0.45

38mm Polyurethane 0.45



As seen from the table above U-values which should be one of the most important parameters you should care if you are trying to make your house more energy efficient, changes from material to material and from thickness to thickness. However, it is obvious that insulation application significantly reduces the heat transfer rate of the surfaces of your house, i.e. walls, roofs, windows, doors etc. To choose the best material and best application is not a job that you can do on your own. Materials have many advantages and disadvantages depending on the usage area. So in order to choose the best material suitable for your house you need to consult to an expert. Also, you should keep in mind that insulation investment may not be as high as you think they are. Many insulation methods are eligible for SEI grants on condition that they provide heat efficiency. From design to installation the assistance of a trained and certified specialist will result in high gains in the long run.