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Heat transfer vs. R-value graph


BlueSkyGuy's Avatar
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Join Date: Jun 2005
Posts: 27

10-22-05, 04:05 PM   #1  
Heat transfer vs. R-value graph

I am in the market to have a new home built. A contractor just showed me a graph of Heat transfer by percent against R-value. I maintained that R-38 was minimum for an attic or cathedral roof design. The contractor pulled out the 1977 graph which showed that R-20 stopped about 90 percent of the heat and R38 only gained 1 or 2 percent more. R-values above R-38 did nothing. The conclusion was that R20 is all that is needed. I would like to counter that conclusion with current data if it exists but I can't locate it on the internet. If anyone reading this knows where I can turn up good data please post. Thanks in advance.

 
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chicago111's Avatar
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Join Date: Oct 2004
Posts: 126
IN

10-22-05, 06:18 PM   #2  
"Contractor"
If your paying him tell him to put his chart away and give you R38.
Most new construction codes require this anyhow.

 
resercon's Avatar
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Join Date: Nov 2001
Posts: 1,873
NJ

10-22-05, 07:49 PM   #3  
Insulation does not stop heat flow, it slows it down.

http://www.ornl.gov/sci/roofs+walls/...on/ins_15.html

This is a Fact Sheet on Insulating. The following is the math the contractor used for the graph;

1/20 - 1/38 = 19/380 - 10/380 = 9/380 = 2%

R-20 means 1/20th of a BTU traverses one square foot of this insulation per hour. R-38 means 1/38th of a BTU...

This is referred to as the "Law of Diminishing Returns". As you increase the R-value, the amount of savings lessens. However, this contractor is misusing this principle. Grant it, the savings between R-20 and R-38 is only 2%. But the cost difference of initially installing R-38 instead of R-20 justifies the installation of the R-38. In other words, you compare the savings to the cost difference between installing R-20 to R-38.

 
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