How Much Attic Insulation is Enough?
Everyone needs insulation in their attic, but did you know there are a lot of other things to consider than just blowing it in (filling loosely) or using batt insulation (blanket)? There are several recommended levels depending on where in the US you live. That determines how much attic insulation is enough.
The recommended R-value varies depending on the state
When figuring out how much loft insulation you need, it’s time to look at where you live. The following table will help you understand where your state is in relation to the recommendations for R-Value.
For example, when you find out how much loft insulation is enough for those in the southernmost part of the United States, things get interesting. Here, where temperatures get pretty hot for much of the year, attics should be between R30 and R60 (average is R38).
Those who live in the north want anything from R49 to R60. This helps insulate against extremely cold weather. The more insulation you use, the more insulated your home will be.
Attic insulation is more important than walls or floors
While there are also insulation recommendations for walls and floors, this article mainly looks at the often overlooked attics. If you know the R-value and the region, you will know how much attic insulation is enough. Here is a map of the different zones and corresponding R-value recommendations from the U.S. Department of Energy:
You find it hard to have too much isolation, although it is possible to simply overpay and reach a point where returns go down. Although the table above breaks down the isolation areas per climate zone, it is important to note that each zone has a range of possible R values. This is partly due to the different heating systems in the United States.
Blown or slatted loft insulation
Attic insulation can be blown in or laid down on rolls. The loose insulation is blown into the attic via a long pipe and system that is quite proprietary and is often best left to the professionals. The ceiling-style insulation can be purchased as either a bat (8 foot long rectangular pieces) or a roll (16 or 24 inches wide).
The insulation is usually made of fiberglass or mineral fibers. With all of the options, you should measure the distance between the bars to make sure you have found the best variety for your needs. For example, uncoated insulation is a great way to complement existing attic insulation. However, paper-backed insulation is best for new rooms.
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Calculation of the required amount of insulation (batts)
Calculating the amount of insulation required is not that difficult. You just need to know the correct formula. First, measure the length of each row between the roof joists and add everything up. Then if you divide by the individual length of bat or roller insulation you purchase, you have the amount you need to purchase. Where it gets difficult, the required R-value is taken into account.
Calculation of the required amount of insulation (blown in)
For blown insulation, you need to understand the R-value per inch. It is also very important to understand how the insulation “settles” so that you don’t under-insulate your attic. If you have someone to do this job for you, they will do it. Do it yourself, however, and you want to be twice as sure that you are buying the right amount of blown insulation.
Know your starting and ending R-values
If you’re starting fresh and don’t have attic insulation, start with an R-value of at least 38. You can check local regulations and the Department of Energy table above for specific recommendations. If you already have insulation, just subtract what you have from the value you want. R38 works well as a basic starting point.
To find out if there is enough insulation in your attic, measure the thickness of the insulation from top to bottom with a ruler. If it’s less than R-30 (i.e. 11 inches of fiberglass / rock wool or 8 inches of cellulose) you may want to add more. Check the following table as a guide to your existing R-value:
|Type of insulation||Form||colour||R value
|Fiberglass||blown||pink or yellow||2.2|
|Rock wool||blown||dark gray or brown||2.8|
|Fiberglass||Ceiling / batt||pink or yellow||3.1|
|Rock wool||Ceiling / batt||dark gray or brown||3.2|
|Cellulose (paper)||blown||dark gray||3.7|
|Polystyrene||shaped||pink or white||4th|
|Polystyrene||extruded||pink or white||5|
|Polyurethane||without face||beige or white||6th|
|Polyisocyanurate||without face||light brown or beige||6th|
|Polyurethane||faced||beige or white||7.1|
|Polyisocyanurate||faced||light brown or beige||7.1|
For those blowing insulation (our preferred choice), consider installing some makeshift reclaimed wood “depth gauges” in the attic. Cheap scales are also very suitable for this. If you are using reclaimed wood, you can mark it to indicate the thickness of insulation you want. You know for sure that your contractors actually blown the right amount of insulation.