Lack of the floor attic insulation (house top floor ceiling), or inadequate amount of the insulation are definitely some of the most important reasons responsible for migration of conditioned (hot or warm) air into the attic.
With sufficient attic ventilation, conditioned air that has penetrated ceiling surface into the attic would be discharged from the attic into the house exterior, sometimes without even causing any problems.
However, your heating and / or air conditioning system will operate much longer and more often in order to keep up with the thermostat settings – that’s more cost to you.
The amount and R-value of your attic floor insulation depends on your geographical location / zone.
The R value or R-value is a measure of thermal resistance (ability to resit heat traveling through it) used in the building and construction industry. The higher R-value – the better insulation performance. (Full Wikipedia definition).
Some states, do not have building codes regulating energy efficiency at all, and each existing municipal code might be different. Therefore, you may want to check with your local building department how much attic insulation is required in your particular area before you start a project.
Use the US map and the chart below to determine recommended attic insulation R-values in your area and how much attic insulation you need (source – Energy Star website ).
|Zone||Recommended insulation values based on your current attic condition|
|There’s no attic floor insulation – install following:||If you have 3″ – 4″ of the attic floor insulation, install additional:|
|1||R30 to R49||R25 to R30|
|2||R30 to R60||R25 to R38|
|3||R30 to R60||R25 to R38|
|4||R38 to R60||R38|
|5-8||R49 to R60||R38 to R49|
How much attic insulation on the floor you need (its thickness) will depend on the insulation type / each one has a different R-value, so you’ll need more or less of the particular insulation type to achieve the same thermal protection.
The average R-values of two most popular attic floor insulation types are (you’ll find the exact R-values on the insulation packaging material):
To find out how much attic insulation you need for a home located in #4 zone, without any existing attic floor insulation (according to the table recommended minimum R-value is R38), wee need to perform this simple calculation:
R38 divided by R3.2 (cellulose) = 11.875″ loose fill insulation thickness
R38 divided by R2.2 (fiberglass) = 17.27″ loose fill insulation thickness
I hope that the above map and tables will help you determine how much attic insulation you should have to prevent many serious problems from happening, and to keep your monthly energy bills as low as possible.
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This article was written by Dariusz Rudnicki
I'm a retired Illinois home inspector, founder and editor of checkthishouse.com, a blog which attracts around 2 thousand readers daily and is dedicated to answering the many questions of home owners and home buyers. Connect with me on Google+ Find me on Google+ Local