What is a U-Value?

Without getting too technical, lets explain U-Values!

A U-Value is a measurement of how effective a material is as an insulator. Which is extremely important for Windows and Skylights, seeing as they are going to be part of your home.

Basically, when we install any Window or Skylight we want to make sure that we are not losing heat through this part of the house. Losing heat will cost us more money and use more energy, especially in the winter time.

The lower the U-Value figure, the better! Let’s explain why…

Calculating a U-Value is actually quite simple. The U-value can be calculated by finding the reciprocal of the sum of the thermal resistances of each material making up the building element in question. Note that, as well as the material resistances, the internal and external faces also have resistances, which must be added. These are fixed values.

U-Values are measured in watts per square metre per Kelvin or shortened to (W/m²K).

Nearly all manufacturers of Roof Windows and Rooflights will be able to give you U-Values of their products. One way a manufacture will test their product is in a “Hot Box”. This is a machine that can test how well an insulator their product is by exposing the product to hot and cold elements, like it would in your home. (Inside your home is warm, outside is cold.)

The image on the right here is an example of how this works.

So, lets take an example! Let’s say that a Double Skin Polycarbonate Roof Dome has a U-Value of 2.8W/m²K… for every degree difference in temperature between the inside and outside of the glazing, 2.8 watts will be transmitted every square metre.

But lets also take a Glass Rooflight. The Glazing in this kind of Rooflight may have a Gas Filled Cavity, usually Argon, which gives better insulation. So, let’s say for example the U-Value is 1.1W/m²K. For every degree difference in temperature between the inside and outside of the glazing, 1.1 watts will be transmitted every square metre. So less heat loss.

Lower the U-Value, less heat is transmitted, or lost, through the glazing.

Making Sense?

Getting a good U-Value for your Rooflight is not only important for making sure your home is well insulated, but it’s worth noting that Building Regulations will require a minimum U-Value for different parts of a new build. This will be anything from External Walls, Floors, Roofs, Glazing (Windows and Rooflights) and Doors.

This is covered in Part L (Conservation of fuel and power) of the building regaulations, which is listed on our website.

It is worth noting that the specification for the notional domestic building which is referred to in Part L1A, has considerably lower values.

For example:

External wall: 0.18 W/m²K.
Floor: 0.13 W/m²K.
Roofs: 0.13 W/m²K.
Windows, roof windows, glazed rooflights and glazed doors: 1.4 W/m²K.

Although Building Regulations has limits for U-Values, the overall thermal performance of buildings is now assessed is a much more complex procedure.

For example, non-domestic buildings, the Simplified Building Energy Model (SBEM) developed by the BRE for the Department for Communities and Local Government, determines the energy performance of a proposed building by comparing its annual energy use with that of a comparable notional building.

And for dwellings, energy performance is measured using the Government’s Standard Assessment Procedure which is also know as (SAP).

R-Value, which shouldn’t get confused with U-Value, is a measurement of thermal resistance rather than thermal transmission, are often described as being the reciprocal of U-values.

R-Value – Thermal Resistance
U-Value – Thermal Transmission

Note: R-Value Does not include surface heat transfers.

G-Value is another important figure that you should be looking at when it comes to purchasing a new Rooflight for your home.

G-Value is the solar transmittance through translucent and transparent materials such as glass and is very important for determining the solar heat gain into the space they enclose during sunny conditions.

Solar heat gain can be beneficial in the winter, as it reduces the need for heating, but in the summer can cause overheating.

Ever stood inside a greenhouse in the summer? Or inside a Polycarbonate conservatory that everyone had installed in the 90’s? This is a classic example of G-Value, high heat buildup, lots of solar heat gain.

If you are planning to install large areas of glazing into your home (Large Rooflights), you will certainly want to get in touch with us to ensure we offer a product that will balance the solar heat gain.

G-Value – Solar Heat Gain