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