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Batt Insulation Laid Over Blown Cellulose

marennorge's Avatar

Join Date: Mar 2001
Posts: 31

11-15-01, 07:40 AM   #1  
Batt Insulation Laid Over Blown Cellulose

I have about 10 inches of blown cellulose insulation in my attic. Would there be any value added in placing batts of the plastic sleeved fiberclass insulation over it? I live in Massachusetts and the house is very economical to heat as is.


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resercon's Avatar

Join Date: Nov 2001
Posts: 1,873

11-15-01, 04:43 PM   #2  
There would be very little added value in putting more insulation over the top you already have. This is referred to as the law of diminishing return. Furthermore it is not advisable to mix different kinds of insulation. "R" Values with insulation work very similar to half lives. This is why they say more than 70% of the thermal effect with insulation comes from the first R-3. Without insulation you have a R-0 and 100% loss. With a R-1 you have a 50% loss. With a R-2 you have a 25% loss and a 75% savings. With a R-3 you have a 12.5% loss and a 87.5% savings and so on.

With 10 inches of cellulose you have approximately a R-30. Any insulation you added to it would start at R-31. So if you added R-19 to the R-30, the batts would be rated from R-31 to R-49. Based on the aforementioned statements, the savings per year might come out to a few dollars a year.

rbisys's Avatar
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11-19-01, 04:46 PM   #3  

The previous info was correct. Also, there is data suggesting that too much bulk insulation in ant attic can INCREASE a/c costs. This is because the fiberglass material abosrbs heat energy and later releases it.

You can install a radiant barrier over the existing material and that could save you up to 22% or more on your a/c costs. Depending on your stucture it could reduce the a/c run time up to 65%. It must be perforated and a two layer system is recommended with cellulose because of the high moisture retention for that material.

Regarding plastic encased fiberglass, that would be putting a vapor material on top of your existing material, a sure fire way to have condensation problems.

Since you are interested in saving utilty costs I would suggest, www.radiancecomfort,com. This is an insulating paint that can save you considerably on energy costs.

Thank you for considering my opinion.

marennorge's Avatar

Join Date: Mar 2001
Posts: 31

11-21-01, 02:31 AM   #4  
Thanks for the information. A perforated radiant barrier might be of interest. I will look into that.

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