R-Value

What is an R-value? Heat flow can be called heat loss or heat gain. An R-value is the measure of a systems resistance to heat flow by conduction.The greater the R-value, the greater the insulation power of the insulation.

Insulation works by holding some medium still. The medium is air in most systems. This still air lowers the heat transfer from hot to cold. 

In a closed cell Spray Foam system the insulating medium is a refrigerant gas trapped in countless little bubbles. Still air has an insulating value of R-5 per inch of thickness. Gasses used in closed cell Spray Foam Insulation have an R value of 7 per inch of thickness. 

Is that all?  No.

Heat Loss

How do we prevent heat flow? There are three components of heat loss. They are conduction, convection, and radiation. 

Conduction is heat transfer through a material as thermal energy moves from molecule to molecule through a substance; or from one object to an adjoining object. If you pick up the handle of a cast iron frying pan from a hot stove you'll experience conduction! The heat reaches your hand via conduction from the burner to the bottom of the pan through the metal handle to your hand. Heat is conducted through the ceilings, walls and floors of homes. Effective insulation slows conduction by keeping heat out during summer and in during winter.

Convection is the transfer of heat by a liquid or gas (such as air). Circulatory air motion due to warmer air rising and cooler air falling is a common mechanism by which thermal energy is transferred. An open chimney flue provides a good example of convective heat loss during the winter. Warm air will rise up the chimney and cold air will fall down into the home. The energy used to warm the air that escapes is lost. The cold air must now be heated. The greater the temperature difference between the inside and outside of your home, and the larger the openings in the home, the easier it is for air to move and the greater losses you will have due to convection. Convective heat loss occurs through cracks and holes in the home and gaps and voids in ceilings, walls, and floors-and in the insulation. Convection also occurs if air can circulate through the insulation. If insulation is to be effective, it must prevent air from flowing easily through it.

Radiant heat transfer occurs between objects that are not touching. The sun heating the earth is an example of radiant heat transfer. The sun warms the earth without warming the space between the sun and the earth. An example of radiant heat transfer is found in a typical attic during the summer. The sun radiates heat to the roof, which in turn radiates heat down toward the ceiling and into your home. If the insulation covering the ceiling does not effectively resist radiant heat transfer, then the ceiling will become increasingly warm. Radiating heat down into your home.


Until recently, mainstream building science completely ignored heat loss by convection and radiation.  Since all the emphasis was on conduction, R-values dominate the knowledge of builders, suppliers, and consumers when insulation systems are discussed.  Unfortunately, the best insulation systems in terms of price verses performance are often overlooked because the R-value is relatively low when compared to the cost. Worse yet, several “Super Insulated” home projects have been undertaken where framing and construction details were changed to increase R-Values but convection and radiation were still ignored.  This resulted in expensive houses with 12 inch thick walls that were not all that efficient or comfortable.

So what are the properties of a good insulation system?

A good insulation system will control heat loss by all three processes.  Conductive heat loss is in the codes, it is quantified, and we must follow the code. Convective heat loss is more elusive, but it can be successfully restrained using blown and foam products.  Radiant is usually not addressed. However, since this is the movement of heat as energy from one surface to another, forcing the surfaces in question to be at the same temperature as the rest of the surfaces in a room will effectively stop heat loss by radiation. 

Back to insulation. 

Are there insulation products that will stop all three components of heat loss?  Yes, there are. They are Fiberglass, Cellulose, and Spray Foam.  

Are all of these equally effective?  Absolutely not.  The differences are in the installation process. 

    Fiberglass Batt Insulation alone is only effective against conductive heat loss. Fiberglass Batt Insulation will fail miserably if it is asked to stop even the slightest air movement. That is why the building codes today provide for exterior air barriers like Tyvek, and interior vapor retarders like polyethylene. Imagine the practicality of installing a 3 dimensional barrier that will stop all air movement and vapor migration in a home. It is frequently a practical impossibility to provide these air barriers and is rarely done right. Batt insulation will eventually sag with time leaving a completely uninsulated area near the top of your walls.

    Blown Fiberglass and Blown Cellulose slightly better at stopping all components of heat loss.  By design these products will fill most voids in insulated spaces. They are also typically installed at higher densities than fiberglass batt insulation. These two properties cause them to be much more suitable to stopping convective heat loss.  The R value of these products will be more stable in adverse weather conditions than batt products because of their higher density. The limitations on these systems is they will not work in areas where they will be unsupported, and they cannot effectively be installed in wall cavities thinner than 4 inches. This insulation also tends to deteriorate and settle over time drastically lowering it's insulation properties.

    Spray Foam Insulation is the most complete system. This system fills all voids and it has a stable R-value in adverse weather. Spray Foam Insulation makes a perfect air seal and is not susceptible to failure due to convective heat loss. Spray Foam Insulation is self supporting, so it will perform equally in closed wall cavities and unsupported areas. Spray Foam Insulation is easily installed in narrow and closed cavities, and will maintain it's insulation properties for the life of your home. The only real objection to this is the price when compared to traditional insulation. Truthfully, if a consumer understood the difficulty and cost involved to bring a fiberglass batt insulation job up to the performance standards of Spray Foam Insulation, he or she would understand the price.

The R-Value Myth

The R-value given to each type of insulation is assigned by laboratories that test under optimal conditions, strictly controlling all three types of heat transfers. In real world applications, traditional insulation usually falls far short of their advertised R-value. Because of the expanding, adhesion, and closed cell properties of Spray Foam Insulation you get exactly as advertised for the life of your home. 





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