R-value (insulation)
The R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive[2] flow of heat, in the context of construction.
The measure is therefore equally relevant for lowering energy bills for heating in the winter, for cooling in the summer, and for general comfort.
R-values are used in describing the effectiveness of insulating material and in analysis of heat flow across assemblies (such as walls, roofs, and windows) under steady-state conditions.
In most countries the properties of specific materials (such as insulation) are indicated by the thermal conductivity, sometimes called a k-value or lambda-value (lowercase λ).
However, R-value is widely used in practice to describe the thermal resistance of insulation products, layers, and most other parts of the building enclosure (walls, floors, roofs).
Other areas of the world more commonly use U-value/U-factor for elements of the entire building enclosure including windows, doors, walls, roof, and ground slabs.
Another important factor to consider is that studs and windows provide a parallel heat conduction path that is unaffected by the insulation's R-value.
The practical implication of this is that one could double the R-value of insulation installed between framing members and realize substantially less than a 50 percent reduction in heat loss.
Insulation installed between the studs may reduce, but usually does not eliminate, heat losses due to air leakage through the building envelope.
Installing a continuous layer of rigid foam insulation on the exterior side of the wall sheathing will interrupt thermal bridging through the studs while also reducing the rate of air leakage.
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 heat loss across an uninsulated air-filled space is natural convection, which occurs because of changes in air density with temperature.
Porous insulations accomplish this by trapping air so that significant convective heat loss is eliminated, leaving only conduction and minor radiation transfer.
However this cannot be realized fully because the glass wool or foam needed to prevent convection increases the heat conduction compared to that of still air.
As such, the least well insulated section of a wall will play the largest role in heat transfer relative to its size, similar to the way most current flows through the lowest resistance resistor in a parallel array.
To find the average heat loss per unit area, simply divide the temperature difference by the R-value for the layer.
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[35] 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, and convective heating.
For instance, the compaction of loose-fill cellulose creates voids that reduce overall performance; this may be avoided by densely packing at initial installation.
Some types of foam insulation, such as polyurethane and polyisocyanurate are blown into form with heavy gases such as chlorofluorocarbons (CFC) or hydrochlorofluorocarbons (HFCs).
Results suggest an exponential decay law over time applies to R-values for closed-cell polyurethanes and aerogel blankets.
[39] Correct attention to air sealing measures and consideration of vapor transfer mechanisms are important for the optimal function of bulk insulators.
One of the primary values of spray-foam insulation is its ability to create an airtight (and in some cases, watertight) seal directly against the substrate to reduce the undesirable effects of air leakage.
By synchronizing these measurements and making some basic assumptions, it is possible to calculate the heat flux indirectly, and thus deriving the U-value of a building element.
[40] By measuring the heat that is flowing through a building element and combining this with the inside and outside temperature, it is possible to define the R-value precisely.
[64] In practice the above surface values are used for floors, ceilings, and walls in a building, but are not accurate for enclosed air cavities, such as between panes of glass.
The effective thermal resistance of an enclosed air cavity is strongly influenced by radiative heat transfer and distance between the two surfaces.
[67] The primary purpose of the rule is to ensure that the home insulation marketplace provides this essential pre-purchase information to the consumer.
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.
