Attic insulation should be deep enough or thick enough to provide the R-value recommended for your area of the country. Attics with the recommended R-value save on energy costs and provide a comfortable living environment. Ninety percent of homes in the US are underinsulated. Uninsulated or underinsulated attics can be responsible for over 25% of heat loss or heat gain in a home.
Deciding Attic Insulation Depth
All insulation has an R-value per inch–the measure of the thermal resistance of a product. Attic insulation depth is determined by four factors. Three of them are dependent on R-values.
R-value of New Insulation
The amount of insulation you need is the difference between your existing R-value and the recommended R-value. The R-value of the new product lets you figure out how much to add.
For instance: Ten inches of existing loose fill fiberglass equals R-25.
Zone 4 recommendation is R-38. A difference of R-13.
Add 5.5 inches of loose fill fiberglass to achieve R-39.
Total depth is 15.5 inches.
Cost of New Insulation
Contractor-installed loose fill blown-in attic insulation costs between $1.00 and $2.80 per square foot depending on R-value and depth installed. DIY insulation reduces the cost by about a third.
Finding asbestos insulation in an attic–which might include vermiculite–can be a serious health risk. Attic asbestos removal is expensive. Most states require professional removal and disposal.
Installation Considerations
Loose fill insulation or batt insulation can be added over most existing products. With two exceptions. Loose fill fiberglass cannot be compressed without losing some of its R-value. Use only loose fill fiberglass to top it off.
Adding more insulation over kraft paper-faced batts will trap moisture between the layers. The kraft paper is a vapor barrier.
Whether contractor-installed or as a DIY project, it is possible to add too much insulation. Not because of R-value but by cutting off ventilation to the attic. Soffit vents need to be left open. Install soffit vent baffles before adding insulation.
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