> What is R-Value, U-Value and how do they relate to insulation?

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What is R-Value, U-Value and how do they relate to insulation?
Last updated 10:09 am, Wednesday 3rd May 2017

Insulation, what is it?

Insulation, simply put, is a quality of a substance to 'resist' transferring heat (or cold, basically energy) through it. In building construction high insulation is used to allow the internal environment temperature to maintain constant independently of external temperature changes. This not only makes the inside a more 'pleasant' place to live, it helps reduce heating and cooling costs by making it easier to keep at the same temperature; which in turn means your home is more ecologically friendly, as you are consuming less environmental resources.

So what is R-Value and what does it have to do with this?

The R-value of a substance is its direct measure of its resistance to transferring energy or heat; R Values are expressed using the metric units (m2.K/W). Basically the higher the figure the better it is at resisting energy transfer, so the easier it is to maintain a difference in temperatures across it for a longer time.

In the metric system, the R value measures per meter squared the amount of degrees kelvin temperature difference required to transfer one watt of energy. So an R value of 1 means per meter squared a single degree difference will transfer one watt of energy. So an R value of 2 will transfer half a watt of energy for a degree of difference.

Usually the R value is given for a certain type and thickness of material as installed (often known as the 'added R value'); i.e. a low density glasswool batt would need to be 130mm installed to achieve an R of 2.5, but only 100mm thick of medium density. Note: We say 'as installed', taking a low density batt that is designed to work in 130mm as installed and squashing it to fit in 100mm will not be the same as using a medium density batt in the first place.

So how do I use the R-Value?

All main building materials (be it wall, floor, ceiling, loft or roof components) have known R-Values. A table at the bottom of this article gives the R-values for many common materials. You can use this table to work out the 'ideal' R-values you want for your building in different seasons. Yes, the R-value of a material can vary depending on the 'mode' of heat transfer you are trying to block (radiant or conductive); so for different seasons it can be advantageous to use materials with different qualities to suit whether you want to stop heat getting out (Winter) or heat getting in (Summer).

Obviously the higher the R-value of a material the better an insulator it is, but this usually also implies higher costs. So there is also an economic driver to find the best suited R-value material to given situation. For instance ceiling insulation often comes in the form of insulation batts, this makes it relatively easier to handle and install compared to the older 'loose fill' variety of ceiling insulation. Its also somewhat safer for the installer as well.

Also of importance is the degree of external temperature range you need to deal with where you live. Basically its easier to suitably insulate a property in a mainly temperate climate than one in a desert or snow zone! You will often find your local building regulations have guidelines on what insulation is required in different climate zones.

R-Value Calculator - Work out the R-Value of a set of materials.

Passive Solar & R-values

R-value is quite important in passive solar building design, knowing the correct R-Values for the external walls, floors and ceilings is key in working out what is termed 'Skin Losses'; i.e. the amount of heat that gets lost from the passive solar building into the surrounding environment. See this article for more information on passive solar building design.

What is the U-value?

The U-value is the inverse of the R-Value; i.e. you divide 1 by either the R or U value to convert to the other unit. So as the R-Value goes up the U-Value goes down and as the R-Value goes down the U-Value goes up.  So the U-Value is a measure of how well a material transmits heat.

For instance a substance with an R-Value of 2 has a U-Value of 0.5 = (1 divided by 2).

An example R-Value calculation

Say for instance you wanted to maintain the temperature of a room at 20 degrees. you find out that the roof space in winter is on average around 10 degrees. This creates a 'temperature differential' of 10 degrees (which is 10 K). So if the rooms ceiling in the roof is insulated with the 130mm low density glass wool as mentioned before this will result in 4 watts (10K/2.5R) of energy 'lost' from your room per hour. To work out the exact heat loss, you need to know the room size. Now if a 140mm medium density  glass wool batt is used you loose instead 2.8 watts (10K/3.5R) - a 30% reduction in heat transfer.

Working out the overall R value of combined materials

This is actually quite easy to do, you just add together the R values of all the materials in the layers of construction as a group, remembering to take account of internal air spaces and the surface air on each side. Also take proper account of the thickness of materials as needed.

NEW We now have an online R-Value Calculator to assist with working out the R and U-Values of several materials combined together.

Working out the R value of a wall with windows in it..

First you need to work out the R value of the wall and the R value of each window. Then work out the area taken up by each window and what the total area of the wall is as if there was not any windows. Then work out the total area taken up by the windows, this is then subtracted from the total wall area to get 'pure' wall area (i.e. the area which is just wall). Then take the pure wall area R value multiple it by it area, then do the same for each windows R value and its area; then sum this up and divide it by the total wall area without windows - this will give you the average R value for the whole wall, with windows, as one unit.

For instance if you had a wall whose total area was 30 meters square. 10 meters square is taken up by windows. The wall has an R value of 3, the windows an R value of 1. 30-10 = 20 square meters of pure wall. 20 * 3 = 60 and 10*1 = 10 - so 70 in total - so 70/30 = 2.33 R value on average for the whole wall with windows.

Now, this is of course assuming you have no covers over your windows, such as curtains or blinds. If you have such covers this can improve the windows R value by up to around an additional 0.4 R. If you use a special thermal curtain it could add up to 1.15 R (better than double glazing).

NEW We have just implemented an online R-Value area calculator.

NEW We have just implemented an online ceiling insulation Batts coverage calculator.

How 'correct' are these calculations?

For purposes of working out how much R-Value you need in your walls / floors and ceilings - these should be sufficient. There are a few things you need to keep in mind though:
  • Stop that draught - any 'gaps' or means for the external air to directly interact with the internal air will adversely effect the benefits of the insulation. At least make sure external windows and doors are properly sealed all the way round. For doors this includes providing a bottom weather strip. You might also want to draught proof certain key internal doors that separate heated and heavily used areas from unheated and hardly used areas; for instance doors off the entrance hall. This of course assumes you will be closing the doors to get the benefit.
  • Some materials have different insulation properties depending upon whether they are used in Summer or Winter - in particular ceiling insulation where in Winter you need to stop heating getting out (known as the 'up' direction in the literature) and in Summer when you need to stop heat getting in from the roof (known as the 'down' direction in the literature).
  • Solar effects - you can use techniques like Thermal Mass and solar gain to make a suitably insulated wall perform two useful functions (the insulation and thermal mass), so further reducing your heating costs and being even more ecologically sound. See this article for tips on how to do this.
  • Surface air resistance - the material needs to interact with the air on both sides, this interaction has its own R-value, which can be anywhere from 0.11 to 0.80 depending on the orientation of material, whether the material 'easily' exchanges heat with the air and whether the air is still or not. 
  • R-values of a material are measured in precise well controlled conditions - the same conditions more than likely do not exist in their final place of installation. Therefore we suggest you err to slightly more insulation that you need rather than err under the amount you need.
  • Watch out for down lights! - most building codes require a clear space around a down light of around 200mm to provide adequate cooling (otherwise the down light or transformer can overheat and cause a fire!); thats about 0.25 m2 of ceiling uninsulated per down light (do the maths this can be quite a high percentage). You need to either: cut down on the down lights; use a down light surround in the loft; or up the R-Value of the remaining ceiling insulation to compensate (work out the % of ceiling without insulation and up the R-Value by that %). Our insulation coverage calculator will work this out for you and give you advice on what to do.

Related Articles and Links

Material R values table

 R-Value (up/out)
 R-Value (down/in)
 Weather board wall
 0.55  0.55
 Brick Veneer wall
 0.51  0.51
 Cavity double brick wall
 0.53  0.53
 Solid brick wall (230mm thick)
 0.44  0.44
 Solid concrete wall (100mm thick)
 0.23  0.23
 Solid concrete wall (200mm thick)
 0.30  0.30
 Aerated concrete wall (100mm block)
 0.78  0.78
 Aerated concrete wall (200mm block)
 1.54  1.54
 Mud brick wall (300mm block)
 0.40  0.40
 10mm plasterboard
 0.059  0.059
 50mm wall batt
 30mm wall batt
 1.157  1.046
 19mm T&G wood floor
 0.12  0.12
 90mm floor batt
 bubble foil reflective foil
 0.14  0.14
 foil-faced polystyrene board
 0.253  0.240
 150mm concrete slab
 0.104  0.104
 75mm AAC Floor Panel
 0.49  0.49
 R1.5 Foil Faced Glasswool Board
 1.559  1.466
 R3.0 Glasswool Batts
 3.145  2.894
Tiled Roof (just the tiles)
Metal Roof (just the metal)

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