THE TRUTH ABOUT R VALUE
Around most of the world, R-values are given in SI units, typically square-metre kelvin per watt or m2·K/W (or equally, m2·°C/W). In the United States customary units, R-values are given in units of ft2·°F·hr/Btu. It is particularly easy to confuse SI and US R-values, because R-values both in the US and elsewhere are often cited without their units, e.g., R-3.5. Usually, however, the correct units can be inferred from the context and from the magnitudes of the values. United States R-values are approximately six times SI R-values. The conversion between SI and US units of R-value is 1 h·ft2·°F/Btu = 0.176110 K·m2/W, or 1 K·m2/W = 5.678263 h·ft2·°F/Btu. As an example an R11 in the United States is approximately R2 in SI units in Europe, Australia and the rest of the world. An R value advertised by a United States merchant may appear more appealing as the US R value is 6 times higher than SI values used in the rest of the world. It does not mean that a US R value is more effective than An R value used in Europe, Australia or the rest of the world.
Do not rely on the R value provided by the supplier as the R value may increase or decrease depending on the application.
Furthermore, the actual R value of the insulation depends entirely on the application and how you apply the insulation. An R 2 value may increase or decrease depending on how the insulation is applied. With the Silver Foil bubble Insulation you will need an air pocket between the surface of the structure you are insulating and the insulation material. The air pocket has its own r value depending on the width of the air pocket thus increasing the R value of the insulation material. Conversely, if you apply the insulation material directly on the surface of the structure will defeat the purpose of insulation because the insulation material will act as a heat conductor and will end up heating the premises in the hot summer and will also be counterproductive in winter.
Radiation is minimized by low emissivity (highly reflective) exterior surfaces such as aluminum foil. Lower thermal conductivity, or higher R-values, can be achieved by allowing an air gap between the surface of the structure and the aluminium foil. The air gap will act an heat pocket where heat will accumulate instead of being circulation inside your structure. Trapped air has its own R value. The R value of trapped air depends on how much gap is between the insulation material an the structure.
A wider gap will produce a greater R value and a narrower gap will produce a smaller R value. Air by itself has no R value; only trapped air will produce an R value. However, if you allow a very wide gap, the air will not be trapped and may reduce your R value and cause an opposite result.
The R-value is a measure of an insulation sample's ability to reduce the rate of heat flow under specified test conditions. The primary mode of heat transfer impeded by insulation is conduction, but insulation also reduces heat loss by all three heat transfer modes: conduction, convection, and radiation. The primary means of heat loss across an uninsulated air-filled space is natural convection, which occurs because of changes in air density with temperature. Insulation greatly retards natural convection making the primary mode of heat transfer conduction
To find the heat loss per square meter, simply divide the temperature difference by the R value.
If the interior of a home is at 20 °C, and the roof cavity is at 10 °C, the temperature difference is 10 C° (= 10 K difference). Assuming a ceiling insulated to R–2 (R = 2.0 m2K/W), energy will be lost at a rate of 10 K/2 K·m2/W = 5 watts for every square meter of ceiling.
Doubling the thickness of the insulation material or even doubling the thickness of the trapped air layer doubles the R value. This does not hold for compressed bats of glass wool because compressing the bats does not proportionally increase the r value. Furthermore, doubling the thickness of the Silver Foil Bubble Insulation does not double the R value of the insulation material. The main feature of the Silver Backed Bubble Insulation is its reflective nature and not its thickness. To Double the R value of the Foil Bubble Insulation you will need to create a heat pocket which hads it s own R value that can be added to the R value of the Foil Insulation. As an example, the if Foil Bubble Insulation has an R value of r2 and the heat pocket has an R value of R 1,5, both R value are added together to achieve an R value of R3.5 in SI units or R19 in United States standards.
As discussed earlier porous material such as foam and bats does not allow the full potential of trapped air to be established and will not give you the proportional R value relative to thickness
In calculating the R-value of a multi-layered installation, the R-values of the individual layers are added:[10]
R-value(outside air film) + R-value(brick) + R-value(sheathing) + R-value(insulation) + R-value(plasterboard) + R-value(inside air film) = R-value(total).
To account for other components in a wall such as framing, an area-weighted average R-value of the whole wall may be calculated.
For the most part, testing the R-value of insulation is done at a steady temperature, usually about 70 °F (21 °C) with no surrounding air movement. Since these are ideal conditions, the listed R-value for insulation will almost certainly be higher than it would be in actual use, because most situations with insulation are under different conditions
Some types of foam insulation, such as polyurethane and polyisocyanurate are blown with heavy gases such as chlorofluorocarbons (CFC) or hydrochlorofluorocarbons (HFCs). However, over time a small amount of these gases diffuse out of the foam and are replaced by air, thus reducing the effective R-value of the product.
This has led to controversy as how to rate the insulation of these products. Many manufacturers will rate the R-value at the time of manufacture; critics argue that a more fair assessment would be its settled value.[citation needed] The foam industry[when?] adopted the LTTR (Long-Term Thermal Resistance) method,[14] which rates the R-value based on a 15 year weighted average. However, the LTTR effectively provides only an eight-year aged R-value, short in the scale of a building that may have a lifespan of 50 to 100 years.
Note that these examples use the non-SI definition and/or given for a 1 inch (25.4 mm) thick sample.
Vacuum insulated panels have the highest R-value (approximately R–45 per inch in American customary units); aerogel has the next highest R-value (about R–10-30 per inch), followed by isocyanurate and phenolic foam insulations with, R–8.3 and R–7 per inch, respectively. They are followed closely by polyurethane and polystyrene insulation at roughly R–6 and R–5 per inch. Loose cellulose, fiberglass (both blown and in batts), and rock wool (both blown and in batts) all possess an R-value of roughly R–-2.5 to R–-4 per inch. Straw bales perform at about R–1.5. However, typical straw bale houses have very thick walls and thus are well insulated. Snow is roughly R–1.
The effective thermal resistance of an enclosed air cavity is strongly influenced by radiative heat transfer and distance between the two surfaces.
