Thursday, January 6, 2011

Understanding R-Value

R-Value has long been part of the building and construction industry as the measure of thermal performance of a structure. It has been a long-held belief that higher R-Value is the same as more efficient building insulation. Spray Polyurethane Foam (SPF) has changed some of the most common beliefs associated with R-Value. There are many places online that discuss the R-Value “Myth” or “Fairy Tale”. Before talking about the “Myth” and “Fairy Tale” argument given by some, Thermosaver would like to give brief definitions of two important terms:

Conduction: transfer of thermal energy between neighboring molecules in a solid substance such as insulation.

Convection: transfer of air molecules through insulation and ultimately the building envelope.

R-Value is a measure of a material’s resistance to heat transfer. The R-Value test performed under the guidelines of ASTM C518 takes into account the resistance of all three heat transfer mechanisms: conduction, convection and radiation; however, convection is given only minor consideration. Unwanted convection through the building envelope is the biggest contributor to energy loss.

Conduction typically accounts for 20% of a building’s heat loss, while convection accounts for up to 80% of the energy loss depending on the quality of construction. Any convective loops inside the insulation material are taken into consideration by the R-value test method, but air leakage through the entire composite building envelope is not considered at all. Closed cell spray foam at ¼” can stop 99% of heat loss by convection and at 3″ will prevent 95% of heat loss by conduction. Obviously, the same holds true for an air-conditioned structure.

Commonly Asked Questions and Concerns

Questions about R-Value are some of the most common concerns of home owners and builders. To help contractors and home owners alike, Thermosaver has compiled a short collection of Common Questions and Answers:
  1. “The higher the R-Value the better, correct?”
    • Yes, higher R-Value in your home or building is somewhat better; however, it should not be overrated at all.
    • SPF’ ( spray foam insulation) ability to create an air-tight seal for your home or building is the most important factor in creating an energy-efficient, comfortable, and healthy space. About 80% of the energy loss of your home or building is caused by unwanted and uncontrolled air flow through the walls, ceilings and floors.
  2. “What is the R-Value of your product?”
    • R-Values vary by manufacturer.
    • True Closed cell SPF offers higher R-Values per inch than conventional insulating alternatives.
    • Most importantly, only spray foam provides total air tightness. As a matter of fact, just a little more than 2 inches (5 cm) of closed-cell foam provides a vapor barrier. No other insulation system can provide that! When considering foam, the air-tightness alone should be a compelling reason to insulate with SPF.
Besides home owners, even professionals like builders and architects often have not thought through R-value vs. air leakage.

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