-
Home - |
- Roof Insulation
- |
- Insulation value
Rd and Rc
Insulation value is expressed as Rd and Rc. Rd represents the insulation value of the insulation material, i.e. the effect of added insulation. Rc represents the insulation value of the structure. This includes all structural elements - roof tiles, roof framework, plastering, etc. If you add (extra) insulation, in most cases you can calculate the effect by adding the Rd of the added insulation material to the Rc of the existing structure.
Difference between summer and winter
TONZON insulation uses layers of motionless air, trapped between layers of TONZON foil. The foil stops almost all heat radiation, while the motionless air limits heat conduction. However, air also warms up, and therefore does rise. This means the heat also rises more easily, while it has difficulty in flowing downwards. This effect can be prevented if the thickness of the air layers is such that the air can no longer circulate, this is the 'optimum depth' of an air layer. Optimum depth depends on the slope of the roof. For a flat roof: 13 mm depth; for a 30º slope: 15.5 mm depth; 60º slope: 18.5 mm depth.
Rd value of TONZON Thermos Cushion
Tonzon Thermos Cushions are often used for system ceilings and in warehouses. These are 'air cushions' made of a thin, but strong, foil. They are easy to mount and work best with flat roofs, to keep out the summer heat. In winter, when the warm air rises, the effect of the Thermos Cushion will be somewhat less, as shown in the table below. The insulation value also depends on the thickness or depth of the insulation layer. Wider Thermos Cushions hang lower and have a higher insulation value:
| Rd value of Thermos Cushion (m²K/W) | 3 chambers (summer) | 3 chambers (winter) | 4 chambers (summer) | 4 chambers (winter) |
| Flat roof | 4.8 | 2 | 7 | 2.5 |
| 30º slope | 4.1 | 2.2 | 5.2 | 2.7 |
| 60º slope | 3.3 | 2.5 | 4.2 | 3.1 |
Rd value of dry liner with TONZON
When it comes to roof insulation, there are many different options available, which is why we offer a calculator to determine the exact insulation value for your situation. When insulating a roof with Thermosheets, as in the case of a dry liner wall, the insulation value is determined by the layers of air trapped between the Thermosheets. This method is a bit more labour-intensive than installing Thermos Cushions, but provides higher insulation values in winter, while taking up less space.
In winter
Flat roof in winter (air layer depth = 12 mm):
Depth (cm) structure Rd (m²K/W) 3.9 3 air layers, 3 Thermosheets 1.4 5.2 4 air layers, 4 Thermosheets 1.8 6.5 5 air layers, 5 Thermosheets 2.3 7.8 6 air layers, 6 Thermosheets 2.7
Roof with 30º slope (depth of air layer = 15 mm):
Depth (cm) structure Rd (m²K/W) 6.5 3 air layers, 3 Thermosheets 1.6 6 4 air layers, 4 Thermosheets 2.1 7.5 5 air layers, 5 Thermosheets 2.5 9 6 air layers, 6 Thermosheets 3.0
Roof with 60º slope (depth of air layer = 18 mm):
Depth (cm) structure Rd (m²K/W) 5.4 3 air layers, 3 Thermosheets 1.7 7.2 4 air layers, 4 Thermosheets 2.3 9 5 air layers, 5 Thermosheets 2.8 10.8 6 air layers, 6 Thermosheets 3.4
In summer
Summer insulation values are similar to those in the tables above. Air layers thicker than the optimum depth will slightly reduce insulation value in summer, but the insulation value in winter will reduce significantly. This is therefore an approach that will save material and costs if an attic room is not heated and if insulation in winter is not important.
Example:
Roof with 30º slope (depth of air layer = 20 mm):
Rc value
The added insulation value assumes that only foil and air insulation are used, but this is not always the case. This is because there must be a structure that can support the dry liner boards. Such a structure, in turn, creates cold bridges.
If such a structure is already present, for example in the case of wooden beams, the Rd value may be used and the Rc value can be calculated on that basis.
In most cases, however, this structure is included in the construction of the dry liner, and consists of battens or metal studs.
Metal studs, however, are strongly discouraged, as metal is a very good conductor and will therefore greatly detract from the insulation value. Timber battens are preferable, as wood is a relatively good insulator.
However, timber battening still has quite an impact on the final Rc value. However, in the case of a dry liner used in combination with TONZON, cold bridges within the structure will be very limited. This is due to the use of thermal bridge breakers. This is one of the reasons that dry lining in combination with TONZON provides very good insulation value per cm of depth when compared to other insulation materials.
The Rc values achieved by dry lining with TONZON will correspond closely with the Rd values mentioned earlier, due to limiting the effect of cold bridges. With other materials, there can be considerable differences between Rd value and Rc value, especially if metal studs are used. On average, Rc values are about 0.1 m²K/W higher than Rd values.