An Explanation of lambda K-Values, R-Values and U-Values for Insulation and how to get best value for money.
When building an extension or alteration to your property, or a new build house, the home owner will need to gain an understanding of the differences between lambda Values, U-Values and R-Values when referring to the performance of materials, and how they then relate to the SAP Ratings that you are aiming for on your new build property or towards meeting your building regulations requirements on any build. 
What is a lambda value?
The lambda value, also portrayed as ‘K-value’ or ‘λ-value’, measures a product’s thermal conductivity in units of W/m·K.
A good insulation will have as low a lambda value as possible to reduce heat loss. For example, some of the newer Kingspan Kooltherm lower lambda products each have a lambda value of 0.018 W/m·K across all thicknesses.
What about an R-value?
A product’s R-value measures its thermal resistance in units of m²·K/W.
By dividing a material’s thickness (in metres) by its lambda value, you can discover how well it resists heat transfer at a specific thickness.
The best insulation will have a higher R-value at lower thicknesses, indicating that it is just as good at reducing heat loss as some of its slightly thicker but lower insulating alternatives.
So, what is a U-value?
A U-value is a sum of the thermal resistances of the layers that make up an entire building element – for example, a roof, wall or floor. It also includes adjustments for any fixings or air gaps, along with the internal and external surface resistances.
A U-value value shows, in units of W/m²·K, the ability of an element to transmit heat from a warm space to a cold space in a building, and vice versa. The lower the U-value, the better insulated the building element.
A building element’s U-value is extremely important as there are certain standards that should be reached according to Building Regulations / Building Standards.
It is possible to have a play with one of the many free U-value Calculators on manufacturer websites. These demonstrate how you need information on each aspect of a building element to get an accurate U-value across that part of structure.
Understanding U-values
Understanding U-values more fully will help you to create a comfortable home with lower running costs.
A measure of thermal transmittance, U-values express the rate of heat transfer through any element of a building, such as a wall, roof or window. Because the construction of these elements can vary so much with design and the choice of materials available. Most importantly the U-values can vary too – hence they need to be calculated specifically for each element.
In practice, the manufacturers of insulation (for instance) will give guides to the U-values achieved when their products are used in a variety of constructions and some have U-value calculators on their websites.
The units used to express U-values are watts per m² Kelvin (W/m²K). This means that if a wall, for example, had a U-value of 1.0 W/m²K, for every degree of temperature difference between the air on the surface inside the wall and the air on the surface outside, 1 watt of heat would pass through any m² of walling.
How are U-values calculated?
Most materials have known and published values for the rate at which heat is conducted through them under specified conditions. These values are known as k-values – they are a measure of thermal conductivity. In any element of a building (such as a wall, floor or roof), the K-values for each material’s layer are divided into the thickness of that material to obtain the R-value for that layer or the thermal resistance.
When all of the R Values are then added together, the total resistance for the complete section of the wall is almost found. Then even the surface air resistances for the inner and outer face and any fixings such as wall ties then have also to be added in to the calculation. The total resistance is then divided into one to obtain the reciprocal of resistance and this figure is the U-value for the whole wall for example.
What are the consequences of getting U-values wrong?
Since materials may vary in their k-value even though they look the same selecting the wrong one may have a bad effect on the U-value. For example, a mistake frequently made on site is to choose an inner-leaf block for a cavity wall that may appear the same as the one specified. Which in fact has a higher crushing strength and a worse (higher) k-value. The resultant U-value may mean that the wall has to then be lined internally with more insulation. Possibly beneath dry lining plasterboard to recover the U-value back to where it needed to be!
Do the Building Regulations require maximum U-values to be achieved?
Part L (Requirement L1) of the Building Regulations covers ‘Conservation of Fuel and Power’. U-value maximums have traditionally been set by these regulations. They still are in the case of extensions, but for new buildings a more holistic approach has to be taken to prove reasonable energy efficiency. Whole building calculations that assess the carbon emissions from a building have to be carried out now. And although U-values are included in these assessments, they form only a part. For new homes up to 450m² – SAP (Standard Assessment Procedure) programmes are used to perform these calculations. For larger projects you would use the SBEM (Simplified Building Energy Model).
Limiting U-values
Because whole-building energy calculations can take account of many factors. U-values in some elements exceed the values shown in the list above and can be compensated for elsewhere in the design. But not to an unreasonable extent – there are limits set out in the Building Regulations.
- The limiting U-values are shown below and should never be exceeded:
- External walls: 0.30
- Roofs: 0.20
- Floors: 0.25
- Party walls: 0.20
- Windows: 2.0 (whole window value)
- You must also achieve a maximum air permeability of 10m3/h.m2 at 50Pa
These restricting parameters exist to ensure that some future-proofing is built into the home. Ensuring that you haven’t completely designed out insulation by using sustainable sources like solar panels or wind-turbines. Its great to generate your own fuel and power but future owners might replace them with mains gas.
So what is a good U-value?
Remember, the higher the figure, the worse the thermal insulation quality. If you want a comfortable, modern home that has relatively small heating requirements, aim to keep your U-values low. Ideal figures would be less than the figures below.
- External walls: 0.20
- Windows: 1.40 (whole window value)
- Floors: 0.20
- Roofs: 0.13
How Could Your Insulation Specification Save You Money?
From a merchant point of view we often find ourselves explaining to customers that if saving thickness is an issue or a need, then it can often be best to look at using something like a Kingspan Kooltherm board which uses a phenolic foam rather than a standard PIR insulation foam like most insulation boards are made with. The Kooltherm boards at a comparable thickness would generally give a 10-15% improvement in the insulation (R) value when compared to PIR. Importantly it will be likely to cost a much larger percentage more on the price per sheet (often 50% to 200% more!).
Interestingly its worth considering if it is possible to accommodate a thicker standard PIR insulation board in its place. If this is possible then this can bring a huge saving when planned for in advance. Importantly the additional space for the extra thickness must have been allowed for at design stage.
*Here are some examples of a few thicknesses of
Kooltherm Phenolic Insulation board (Thermal Conductivity 0.018 W/mK) versus a standard
PIR insulation board (Thermal Conductivity 0.022 W/mK)
which performs as well or better R-Value wise when used at a slightly thicker thickness:
• 25mm Kooltherm – R Value = 1.35
o 25mm Celotex PIR – R Value = 1.10 (Too Low)
o So use 30mm Celotex PIR – R Value = 1.35 (OK as the same)
• 50mm Kooltherm – R Value = 2.75
o 50mm Celotex PIR – R Value = 2.25 (Too Low)
o So use 70mm Celotex PIR – R Value = 3.15 (Higher so better!)
• 75mm Kooltherm – R Value = 4.15
o 75mm Celotex PIR – R Value = 3.40 (Too Low)
o So use 100mm Celotex PIR – R Value = 4.50 (Higher so better!)
• 100mm Kooltherm – R Value = 5.55
o 100mm Celotex PIR – R Value = 4.50 (Too Low)
o So use 130mm Celotex PIR – R Value = 5.90 (Higher so better!)
*Obviously the above are just examples to illustrate the example that savings can be made if you think ahead. Allowing more space for a slightly thicker insulation product in some instances can save you money. We advise you consult your Architect, Project Manager, Builder and Building Control Officer to ensure an acceptable solution for your project.
For any insulation advice or to place an order for your insulation requirements on your project, please call the BMD National Sales Office on 01793-791010 or email us on insulation@buildingmaterialsdirect.com
