When someone is talking about the “R-value” of an insulation material, what are they talking about exactly? What does the term refer to specifically? And how does one determine what it is?
All good questions. To oversimplify, you could say that the “R-value” is a measurement of the thermal resistance of a material — that is to say, a measurement of the ability of thermal energy (heat) to move through a material or structure.
So, the “R-value” of an insulation material essentially just gives you an idea of how effective the material is as an insulator — how effective it is at preventing heat transfer, to word it yet another way.
This ability to prevent heat transfer depends to a degree upon 1) the specific qualities of the material in question, 2) its thickness, and also 3) the way that it is integrated into or with other materials as part of an overall structure or assembly (fittings/studs can act as a thermal bridge and reduce effectiveness, etc.). Higher R-values are thus used commonly to signify greater insulation.
Generally speaking, most effective and affordable insulators work by trapping/slowing air, and thus limiting conductive and convective heat loss somewhat. Notably, though, “R-values” don’t give you anything to work with as regards the properties of the material’s surface — so regardless of R-value ratings, that is something to consider and keep in mind.
Something else to be kept in mind is that R-values will inevitably vary with temperature changes — with different materials responding differently, another factor not accounted for in the rating figure. The R-values of materials can also degrade significantly over time due to problems with moisture and compaction, amongst other things.
And one other thing to keep in mind on that count … the R-value ratings used in the US are slightly different than those used/notated in metric units, with the so-called “US customary units” around 5.678263 times higher than metric units. With the wide discrepancy between the two, it’s not too easy to confuse them unknowingly, but still something to keep in mind.
As far as what the numbers themselves are supposed to represent, the R-value figure is an expression of the thickness of the material in question as “normalized” around a common thermal conductivity. In other words, materials are evaluated so as to determine the thickness necessary to meet a generalized baseline thermal conductivity. There are other ways to understand the numbers as well though.
For the more mathematically inclined: R-values are essentially (in the US) understood as units of ft2•°F•h/Btu; and in Europe as units of m2•K/W or m2•°C/W — “C” refers to Celsius and “K” refers to Kelvin.
Which brings us to…
Calculating Expected Heat Loss Rates With R-Values
First, what you’ll need to do is determine the R-value for your total insulating layer — in other words, you’ll need to determine the sum R-value of your insulating layers, sheathing, etc. (not just the R-value per inch of the insulation being used).
To account for materials that can act as thermal bridges (framing, fittings, etc.), you’ll need to average the R-values of these materials out with those of the insulation — by an area-weighted average. You could express this semi-mathematically by saying: ?%/R-Value average of frame and fittings + ?%/R-Value of total insulation layer = total R-value average (with the two unstated percentages adding up to 100% in total).
Once this has been done, you can then determine expected heat loss rates per square meter by dividing the temperature difference between the two bodies of air in question (inside/outside; porch/indoors; etc.) by the total R-value of the insulation layer.
To translate that as a mathematical equation: Temperature Difference K/R-Value • m2•K/W = Watts lost for every square meter of insulation material. (Those in the US can substitute the equation given earlier in the article in the appropriate spot.)
R-Values By Insulation Type
R-values vary widely based on the materials in question, but it could reasonably be said that the better a material is at trapping air, the higher the R-value will be. Thus, vacuum-sealed insulation panels possess some of the highest R-values of any materials out there.
Something that needs to be considered when selecting materials for insulation, though, is the intended thickness of use — in other words, if walls/insulation will be made very thick, then there’s no reason for the use of space-age insulation materials based around petrochemicals despite the very high R-values possible.
As an example here, we’ll discuss igloos. The R-value of snow can vary quite a bit depending upon different factors, but could be considered to have an average of around 1.0. As igloo walls are traditionally made to be fairly thick, despite the fairly low R-value, they are regardless very effective insulators — and can easily provide for indoor temperatures far, far above outdoor temperatures even when heated with nothing but body heat and a single seal-oil candle.
Something else to consider are the effects of associated thermal mass. Thermal mass will absorb some of the heat that it’s exposed to, and radiate it back out over a long period of time. Thus, for example, stone walls are considered a poor choice from the perspective of R-values but not from the perspective of heat retention and release over long periods of time. Something to keep in mind — so that one doesn’t focus on R-values too much and lose sight of the wider purpose.
A basic overview of generalized R-values for various types of insulation materials can be found on Wikipedia.