Getting to grips with U-values!

U-values of roof space

Around 25% of heat is lost through the roof, so insulating your loft is one of the best ways to improve the energy efficiency of your home. Luckily, it is a pretty cheap and easy DIY job!

    • An uninsulated roof space has a U-value of approximately 2.5 W/m2k
    • To achieve a U-value of less than 0.15 W/m2k you can use the following
      • 175mm of expanded polyisocyanurate / PIR (Celotex or Kingspan)
      • 195mm of Rockwool insulation
      • 225mm of Sheepwool insulation
      • 270mm of Fibreglass
      • 270mm of Knauf space blanket (fibreglass wrapped in space blanket)

To achieve the 0.15 W/m2k U-value in your loft you can expect to pay the following prices per square metre.

  • Celotex – £21 per square metre
  • Sheepwool – £20 per square metre
  • Fibreglass – £7 per square metre
  • Knaufs Space blanket (fibreglass in metallic space blanket) – £15 per square metre

U-values of windows

New build windows must achieve a maximum U-value of 1.6 W/m2k under Part L of the Government Building Regulations (for new builds).

    • A single glazed window will have a U-value of around 4.8 W/m2k
    • Double glazed window with a 6mm gap between panes filled with air – 3.1 W/m2k
    • Double glazed window with a 12mm gap between panes filled with air – 2.8 W/m2k
    • Double glazed window with a 16mm+ gap between panes filled with air – 2.7 W/m2k

Now, a new double glazed window will typically have a U-value that adheres to building regulations – i.e. 1.6 W/m2k.

Double-Glazing-600x450The issue with windows though is that for a long time these have been seen as the weak point in a property’s thermal envelope. No matter how low legislators set the required U-value for windows they are never going to get close to that of walls, since you can’t simply add insulation to them. This has resulted in house builders reducing the size of windows in properties –  to the detriment of the homeowner, since it means less light entering the home.

There is now a better measure for the efficiency of the window and it is simply known as the ‘Window Energy Rating’ which not only takes into account the U-value, it also focuses on solar gain benefits to the household. This rates a window based on these two things on a scale between A-G, with A being the best. To make things nice and easy for consumers, the window energy rating applies to the whole window, both the frame and the glass – so you need not worry about doing any complicated calculations.

If you’re looking for a cheaper way to increase and improve your windows U-values, Ecoease secondary glazing is an option that is simple to fit and costs only about 15% of double glazing. It has a calculated U value of 2.7 using a 20mm gap to the existing single glazing. Find out more Ecoease here.

So there you have it – a guide to U-values – we hope you find it useful when looking to insulate your home!

 


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