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What is R-value?

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What is R-value?
by: Joshua Luttmer

Insulation in your home provides resistance to heat flow and lowers your heating and cooling costs. Insulating your home properly will not only lower heating and cooling costs but also increase your comfort level.

How Does Insulation Work?

Understanding heat flow is essential to understanding how insulation works. It involves three main mechanisms: radiation, conduction, and convection. Radiant heat transfers its energy to any solid object that can absorb it and travels in a straight line. Radiant heat is generally transferred through electromagnetic waves. Think of the sun warming your skin, that is a great example of radiant heat.  Conduction describes how heat travels through materials. For example, a pan warming on a glass cooktop. As the cooktop warms up the heat is transferred via conduction into the pan. Convection is how heat circulates through liquids or gases. This is why warmer and lighter air rises, while cooler and denser air sinks.

The majority of insulation materials slow down conductive heat flow, and to a lesser degree convective heat flux. Radiant barriers and reflective insulation systems work by reducing radiant heat transfer. The reflective surface must be in direct contact with an air space to be effective though.

No matter what mechanism it is, heat moves from warmer areas to colder areas until both reach the same temperature. This means that heat from your home will flow directly to unheated attics, garages and basements in winter. Indirect heat flow can also occur through interior walls, ceilings, floors, and walls--wherever there's a temperature difference. Heat flows from the outside to the inside of a house during the summer.

Comfort means that heat as your home loses its heat in the winter, it must be replaced with heat from your heating system, and in the summer the heat intruding from outdoors must be removed through your cooling system. Insulating your home properly will reduce heat flow and provide heat resistance.

What is R-Value?

The thermal resistance of an insulating material that resists conductive heat flow can be measured or rated using its R-value (or thermal resistance). A higher R-value indicates greater insulation effectiveness. The type of insulation, the thickness and density of the insulation will all affect its R-value. The R-value of different types of insulation can be dependent on temperature, age, and moisture accumulation. To effectively calculate the R-value of multiple layers of insulation, you must add the R-vale for each layer together.

Adding more insulation in areas that are already insulated in your home will increase the R-value. The R-value will increase proportionally with an insulation thickness that is greater. Loose-fill insulation does not maintain its R-value once it settles because its density increases as it compresses under its own weight. The R-value of loose-fill insulation does not proportionately increase with its thickness. To determine how much insulation you need for your climate, ask your inspector or an insulation specialist.

How and where insulation is installed will also affect its resistance to heat flow. Insulation that has been compressed may not deliver its full R-value for example. Because heat flows through insulation more easily than through studs, beams and other building materials (a building science term called thermal bridging), the overall R-value for a ceiling or wall will differ from its insulation's R-value.

Unlike traditional insulation materials, radiant barriers are highly reflective materials that re-emit radiant heat rather than absorbing it, reducing cooling loads. Radiant barriers have no actual R-value but do increase the effectiveness of insulation materials.

The R-value or amount of insulation you need will depend on the specific climate in which you live, the type of heating and cooling systems you have, and the area of the house that you are trying to insulation.

Insulation Materials

Insulation materials vary greatly from bulky fiber materials such as fiberglass, rock wool, cellulose, and natural fibers to rigid foam boards to shiny foils. Bulky materials can resist convective and conductive heat flow in a wall or ceiling cavity. Rigid foam boards trap air and other gases to resist conductive heat flows. Radiant barrier systems use highly reflective foils that reflect radiant heat thereby reducing the amount of head that makes it into your home during the summer. There are other materials, such as perlite, vermiculite, phenolic and cementitious foams.

To calculate the approximate R-value of some of the most common insulations see below:

  • Loose Fiberglass: R = 2.5 x thickness
  • Batt Fiberglass: R = 3.2 x thickness
  • Rock wool: R = 2.8 x thickness
  • Cellulose (newspaper): R = 3.7 x thickness
  • Vermiculite (asbestos): R = 2.7 x thickness
  • Rigid Foam: R = 5 x thickness

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