Getting to grips with U-values!

Getting to grips with U-values!

U-values might seem a pretty dull subject, however it is absolutely key that you understand them so that you can insulate your home with the most appropriate material. You can learn more about what a U-value is by clicking here.

The U-value signifies the heat lost through a given thickness of a particular material. You don’t really need to understand the mechanics of how it is calculated – instead it is useful to be able to compare different substances by their U-values.

The best insulating materials have a U-value of close to zero – the lower the better. Building regulations currently stipulate that for a new building, the elements must have maximum U-values as follows:

  • Wall – 0.3 W/m2k
  • Roof – 0.15 W/m2k
  • Windows – 1.6 W/m2k

So in each section below we are going to examine each of the different elements and their typical U-values, we will then show you how to achieve the best possible U-values as stipulated in the building regulations (Part L).

U-values of SOLID WALLS

Solid walls were the norm in most properties built prior to the 1930s – unfortunately at this time energy efficiency wasn’t really on the radar, since fuel was very cheap. However nowadays trying to keep this sort of home warm is very costly, so insulating them can considerably lower the heating bill.

    • An uninsulated solid brick wall with a thickness of 225mm value will have a U-value of 2.70 W/m2k
    • To insulate a solid wall you can either insulate internally or externally.
    • 100mm of EPS insulation located internally or externally should bring the U-value of the wall down to 0.29 W/m2k in line with building regulations (in this example we have use EWI-PRO 100mm graphite EPS with thermal conductivity λ=0.032W/mk)
    • To achieve this with rockwool insulation you will need to add about 110mm to either the inside or outside wall (in this example we have use rockwool 100mm with thermal conductivity λ=0.036W/mk) – obviously if this is done externally you will need to attach it firmly to the wall and it will need to be rendered. If you want to do this internally it is going to steal a large area of floor space.
    • ThermaLine Plus is internal wall insulation (often referred to as dry-lining) attached to plasterboard and if you opt for the 40mm thick ThermaLine Plus you should achieve a U-value of about 0.65 W/m2k. This isn’t bad, but if you need to adhere to building regulations (i.e. it is a new extension for example) you are going to have to opt for something else.

U-values of CAVITY WALLS

Cavity walls became the norm in the 1930s. However until 1995 we assume that they were built but left unfilled (with insulation).

 Unfilled cavity walls

    • Unfilled cavity wall (built prior to 1900) will have a U-value of 2.0 W/m2k
    • Unfilled cavity wall (built 1900 – 1975) will have a U-value of 1.6 W/m2k
    • Unfilled cavity wall (built 1976 – 1982) will have a U-value of 1.0 W/m2k
    • Unfilled cavity wall (built 1983 – 1995) will have a U-value of 0.6 W/m2k

Filled cavity walls – assumed post-1996

All cavity walls built after 1996 are assumed to have filled cavities as part of more regimented building regulations.

U-value of cavity walls

    • Filled cavity wall (built 1996 – 2002) will have a U-value of 0.45 W/m2k
    • Filled cavity wall (built 2003 – 2006) will have a U-value of 0.35 W/m2k
    • Filled cavity wall (built 2006 – 2010) will have a U-value of 0.3 W/m2k
    • Filled cavity wall (built 2010 +) will have a U-value of 0.2 W/m2k

Mechanisms for improving your cavity wall U-rating

    • For a new build cavity wall to achieve a U-value of  0.2 W/m2k you would need to install 150mm of wool but this obviously means extremely thick walls – but this is the cheapest option available to builders.
    • Using Celotex to fill the cavities is more expensive, but you will only need to use 100mm celotex to achieve a U-value of  0.2 W/m2k . The only thing to bear in mind here is that you still need to keep a residual cavity on the outside – but this means the cavity can be marginally thinner.
    • The maximum U-value you can achieve by retrofitting cavity wall insulation in any property older that 1975 is 0.5 W/m2k since you are limited by the thickness of the cavity. In this instance to achieve a U-value of less than 0.2 W/m2k, you need to apply either internal or external wall insulation too, although you will only need to apply 50mm of celotex either internally or externally to achieve this.
    • If you retrofit insulation in cavity walls built between 1970 and 1995, you should achieve a U-rating of 0.5 W/m2k.
    • Any cavity wall after that date should already have cavity wall insulation as part of building regulations – the U-values of these types of walls can be seen above in the Filled Cavity Walls section.

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