In some countries that encounter both SI and local imperial measurements for R-value; the nomenclature RSI maybe used to denote the standard SI form of the value.
A conversion factor can be applied to turn a US R-value into a non-US R-value. This conversion factor is 1 ft²·°F·h/Btu ≈ 0.1761 K·m²/W, or 1 K·m²/W ≈ 5.67446 ft²·°F·h/Btu.
U is the inverse of R with an SI definition of W/(m² K).
For example, if the interior of your home is at 20 °C, and the roof cavity is at 10 °C, the temperature difference is 10 K. Assuming a ceiling insulated to R–2, energy will be lost at a rate of 10 K / 2 K·m²/W = 5 watts for every square metre of ceiling.
It is reasonable to sum the R-values of bulk insulators e.g., R-value(brick) + R-value(fibreglass batt) + R-value(plasterboard) = R value(total).
Thermal conductivity is conventionally defined as the rate of thermal conduction that occurs through a material. That is, for a layer of material of known area and thickness, the rate of thermal energy transferred can be calculated based on the surface temperature differential between sides. It is not specifically related to the difference in air temperature or heating energy.
Experimentally, thermal conduction is measured by placing the material in contact between two conducting plates and measuring the energy fluxes required to maintain a certain temperature gradient.
A definition of R-value based on apparent thermal conductivity has been proposed in document C168 published by the American Society for Testing and Materials. This describes heat being transferred by all three mechanisms -- conduction, radiation, and convection.
Debate remains among representatives from different segments of the U.S. insulation industry during revision of the U.S. FTC's regulations about advertising R-values illustrating the complexity of the issues.
There are weaknesses to using a single laboratory model to simultaneously assess the properties of a material to resist conducted, radiated or convective heating. Surface temperature varies depending on the mode of heat transfer.
In the absence of radiation or convection, the surface temperature of the insulator should equal the air temperature on each sides.
In response to thermal radiation, surface temperature depends on the thermal emissivity of the material. Light, reflective or metallic surfaces exposed to radiation tend to maintain lower temperatures than dark, non-metallic ones
Convection will markedly alter the rate of heat transfer (and surface temperature) of a insulator depending on the flow characteristics of the gas or fluid in contact with it.
With multiple modes of heat transfer, the final surface temperature (and hence observed energy flux and calculated R-value) will be dependent on the relative contributions of radiation, conduction and convection even though the total energy contributions remains the same.
This is an important consideration in building construction because heat energy arrives in different forms and proportions. The contribution of radiative and conductive heat sources also varies throughout the year and both are important contributors to thermal comfort
In the hot season, solar radiation predominates as the source of heat gain. On the other hand, conductive and convective heat losses play a more significant role during the cooler months.
Unlike bulk insulators, radiant barriers resist conducted heat poorly. Materials such as reflective foil have a high thermal conductivity and would function poorly as a conductive insulator. Radiant barriers retard heat flow by two means - by reflecting radiant energy away from its surface or by reducing the emission of radiation from its opposite side.
The question of how to quantify performance of other systems such as radiant barriers has resulted in controversy and confusion in the building industry with the use of R-values or 'equivalent R-values' for products which have entirely different systems of inhibiting heat transfer. According to current standards, R-values are most reliably stated for bulk insulation materials. All of the products quoted at the end are examples of these.
Calculating the performance of radiant barriers is more complex. The tests and procedures to evaluate bulk insulators are not applicable to radiant barriers. Although radiant barriers have high reflectivity (and low emissivity) over a range of electromagnetic spectra (including visible and UV light), its thermal advantages are mainly related to its emissivity in the infra-red range. Emissivity values are the appropriate metric for radiant barriers. Their effectiveness when employed to resist solar radiation is established, even though R-value do not adequately describe them.
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, while a more fair assessment would be its settled value. The foam industry has now adopted the LTTR method which rates the R-value based on a 15 year weighted average. While more realistic, the LTTR effectively provides only 8 year aged R-value, short in the scale of a building which may have a lifespan of 50-100 years.
One of the primary values of spray-foam insulation is its ability to create an water-tight and air-tight seal directly against the substrate to reduce this effect.
Absolutely still air has an R-value of about 5 but this has little practical use: Spaces of one centimeter or greater will allow air to circulate, convecting heat and greatly reducing the insulating value to roughly R–1.
|Material||Value per inch (Min)||Value per inch (Max)||Reference|
|Air with no external wind||R-1 (0.18) or less (with convective currents)||R-5 (0.88) (Still)|
|Wood chips and other loose-fill wood products||R-1 (0.18)|
|Straw bale||R-1.45 (0.26)|
|Wood panels, such as sheathing||R-2.5 (0.44)|
|Vermiculite loose-fill||R-2.13 (0.38)||R-2.4 (0.42)|
|Perlite loose-fill||R-2.7 (0.48)|
|Rock and slag wool loose-fill||R-2.5 (0.44)||R-3.7 (0.65)|
|Rock and slag wool batts||R-3 (0.52)||R-3.85 (0.68)|
|Fiberglass loose-fill||R-2.5 (0.44)||R-3.7 (0.65)|
|Fiberglass rigid panel||R-2.5 (0.44)|
|Fiberglass batts||R-3.1 (0.55)||R-4.3 (0.76)|
|High-density fiberglass batts||R-3.6 (0.63)||R-5 (0.88)|
|Cementitious foam||R-2 (0.35)||R-3.9 (0.69)|
|Cellulose loose-fill||R-3 (0.52)||R-3.8 (0.67)|
|Cellulose wet-spray||R-3 (0.52)||R-3.8 (0.67)|
|Icynene spray||R-3.6 (0.63)|
|Cotton batts (Blue Jean Insulation)||R-3.7 (0.65)|
|Icynene loose-fill (pour fill)||R-4 (0.70)|
|Urea-formaldehyde foam||R-4 (0.70)||R-4.6 (0.81)|
|Urea-formaldehyde panels||R-5 (0.88)||R-6 (1.06)|
|Polyethylene foam||R-3 (0.52)|
|Phenolic spray foam||R-4.8 (0.85)||R-7 (1.23)|
|Phenolic rigid panel||R-4 (0.70)||R-5 (0.88)|
|Molded expanded polystyrene (EPS)||R-3.7 (0.65) (low-density)||R-4 (0.70) (high-density)|
|Extruded expanded polystyrene (XPS)||R-3.6 (0.63) to R-4.7 (0.82) (for low-density)||R-5 (0.88) to R-5.4 (0.95) (for high-density)|
|Open-cell polyurethane spray foam||R-3.6 (0.63)|
|Closed-cell polyurethane spray foam||R-5.5 (0.97)||R-6.5 (1.14)|
|Polyurethane rigid panel (Pentane expanded )||R-6.8 (1.20) initial||R-5.5 (0.97) aged (5-10 years)|
|Polyurethane rigid panel (CFC/HCFC expanded)||R-7 (1.23) to R-8 (1.41) initial||R-6.25 (1.10) aged (5-10 years)|
|Polyisocyanurate spray foam||R-4.3 (0.76)||R-8.3 (1.46)|
|Foil-faced polyisocyanurate rigid panel (Pentane expanded )||R-6.8 (1.20) initial||R-5.5 (0.97) aged (5-10 years)|
|Silica aerogel||R-10 (1.76)|
|Vacuum insulated panel||as high as R-30 (5.28)|
|Cardboard||R-3 (0.52)||R-4 (0.70)|
|Thinsulate clothing insulation||R-5.75 (1.01)|
Values for a specified unit (not per inch)
|Material||Value not per inch (Min)||Value not per inch (Max)||Reference|
|Reflective insulation||R-2||R-17 (highly dependant on method of installation)|
|Single pane glass window||R-1 (0.18)|
|Double pane glass window||R-2 (0.35)|
|Double pane glass window with low emissivity coating||R-3 (0.52)|
|Triple pane glass window||R-3 (0.52)|
Materials such as natural rock, dirt, sod, adobe, and concrete have poor thermal resistance (R-value typically less than R-1 (0.17)), but work well for thermal mass applications because of their high specific heat.
The primary purpose of the Rule, therefore, is to correct the failure of the home insulation marketplace to provide this essential pre-purchase information to the consumer. The information will give consumers an opportunity to compare relative insulating efficiencies, to select the product with the greatest efficiency and potential for energy savings, to make a cost-effective purchase and to consider the main variables limiting insulation effectiveness and realization of claimed energy savings.
The Rule mandates that specific R-value information for home insulation products be disclosed in certain ads and at the point of sale. The purpose of the R-value disclosure requirement for advertising is to prevent consumers from being misled by certain claims which have a bearing on insulating value. At the point of transaction, some consumers will be able to get the requisite R-value information from the label on the insulation package. However, since the evidence shows that packages are often unavailable for inspection prior to purchase, no labeled information would be available to consumers in many instances. As a result, the Rule requires that a fact sheet be available to consumers for inspection before they make their purchase.
Furthermore, comparisons per inch of thickness are mostly relevant for conductive and convective heat transfer—not radiant heat transfer—but some of the materials listed below are designed to prevent radiant heat transfer.