R-value is a measure of thermal resistance that is especially useful in energy management strategies. More specifically, they provide a rating for a material's ability to resist heat flow.
Understanding R-values is useful for knowing how to discuss issues of energy efficiency. The energy required to keep a multi-tenant building warm in the winter and cool in the summer is staggering, as is the accompanying bill. Often, changing how a building is insulated is an effective energy management measure that pays off over a relatively short time frame.
Traditional insulation materials work by slowing conductive heat flow. Conduction is the direct flow of heat through materials: When you hold a hot cup of coffee to warm your hands, conduction is keeping you warm.
For reference, the other two types of heat flow are convection and radiation. Convection describes how heat circulates through a liquid or gas, like when you add just a little bit of very hot water to a warm up your bath; and radiation describes heat that escapes an object in a straight line, like how energy from the sun heats the Earth.
Most insulation works by trapping small pockets of air. Because still air is a poor conductor of heat, it takes a much longer time for heat to travel through all those pockets than it would through a solid object, like brick. This means that the rate of heat loss (or gain) is much lower, so the building’s HVAC system doesn’t have to work as hard.
A high R-value indicates a high level of insulation, so from an energy management perspective, the higher the better.
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The most obvious factors that affect thermal resistance are the type and thickness of material. Generally speaking - though not always - doubling the thickness of an insulator will double its R-value. This makes sense; it should take about twice as long for heat to pass through twice as much material.
Materials can vary wildly in R-value. A batt of standard fiberglass insulation might have an R-value of 3.14 to 4.3 per inch, and some more sophisticated types of insulation may have R-values above 5 or 6 per inch of thickness.
Silica aerogel, an ultralight synthetic material, can have R-values as high as 30 per inch in a moderate vacuum. That makes it one of the best insulators in existence.
By contrast, carbon steel has an R-value of about 0.003 per inch of thickness. That shouldn’t be surprising to anyone who has ever burned their hand on a hot spoon warmed by a cup of tea - steel is an excellent conductor.
In some foam insulators, a gas other than air is used to form the bubbles that are trapped throughout. These gasses are even worse conductors of heat than air, meaning that they contribute to a higher R-value. However, because such insulators generally leak (very slowly) over time, the age of an insulator can also be a factor.
Radiation is a constant, but usually slow cause of heat loss or gain. To account for this, many insulators are covered with a reflective shield that serves to reduce radiation. This shield is also a factor in determining R-value.
Because most insulators rely on trapped air, compression can be an important factor. A compressed insulator has smaller pockets of trapped, still air, and so is not as good at reducing heat transfer. For this reason, even though doubling the amount of insulation in a fixed space will still generally increase that space’s R-value, it won’t double the value.
Finally, we need to consider the parts of a surface that are not insulated when calculating that surface’s total R-value. Windows and the wood studs in a wall naturally conduct more heat than insulation, and some amount of heat energy will pass through them, “bypassing” the insulation. This process is called thermal bridging and is a factor in correctly determining R-value.
Understanding the vocabulary of energy efficiency can be helpful in better assessing short- and long-term investments for a building and in communicating with engineering staff. R-values are a small, but important factor in realizing successful energy management.
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