Everyone needs insulation in their attic, but did you know there's more to it than simply blowing in (loose fill) or batting (blanket) insulation? There are different recommendations based on where you live in the US. This determines how much attic insulation you need.
Recommended R-values vary by state
Looking at your home can be helpful when determining how much attic insulation you need. The table below can help you understand where your state stands with regard to R-value recommendations.
Things get interesting, for example, when determining how much attic insulation is enough for the southernmost man in the United States. What you need is determined as much as where you live. Here, the temperature is very high for most of the year and the temperature in the attic should be between 30 rand and 60 rand (average 38 rand).
People who live in the north want anything from R49 to R60. This helps protect against extreme cold weather. The more insulation you use, the better your home will be insulated.
Check out our article on how to install a radiant barrier for another effective way to reduce energy costs.
Attic insulation is more important than walls or floors
While there are also insulation recommendations for walls and floors, this article mainly deals with attics, which are often overlooked. Knowing the R-value and zone will help you understand how much attic insulation you need. Below are DOE's maps of the different regions and the corresponding R-value recommendations:
You'll find it hard to have too much insulation, although it's possible to just overpay and hit a point of diminishing returns. Although the table above breaks down the insulation range for each 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 used across the United States.
Blown in or battened attic insulation
Attic insulation can be blown in or laid from a roll. Loose-fill insulation is blown into the attic through a long pipe and system, which is fairly proprietary and usually best left to the professionals. "Blanket"-style insulation can be purchased in batts (8-foot rectangular pieces) or in rolls (16 or 24 inches wide).
Insulation is usually made of fiberglass or mineral fiber, and with all options, be sure to measure the spacing between joists to make sure you find the best one for your needs. For example, unfaced insulation is great for adding to existing attic insulation, but paper-faced insulation is best for new spaces.
Calculating the amount of battery insulation required
It is not difficult to calculate the amount of insulation you need. You just need to know the correct formula. First, measure the length of each row between the attic joists, then add it all up. If you then divide by the individual lengths of bat or insulation roll you are buying, you will have the quantity you need to buy. The difficulty comes in considering the required R value.
Calculate the amount of blown-in insulation you need
For blown-in insulation, you need to know the R-value per inch. Also, it's important to understand how the insulation "stabilizes" so you don't end up with an underinsulated attic. If someone does the work for you, they'll take care of it. However, you do it yourself, and again, you want to make sure you buy the right amount of blown-in insulation.
Know Your Starting and Ending R Values
If you're just starting out and don't have attic insulation yet, start with an R-value of at least 38. You can check local regulations and the DOE chart above for specific recommendations). If you already have some insulation, just subtract what you have from the desired value. The R38 works well as a basic starting point.
To determine if your attic has adequate insulation, use a ruler to measure the thickness of the insulation from top to bottom. If it's less than R-30 (that is, 11 inches of fiberglass/rock wool or 8 inches of cellulose), then you may need to add more. Check out the table below as a rough guide to existing R-values:
| Insulation type | form | color | R value (per inch) |
|---|---|---|---|
| glass fiber | blow | pink or yellow | 2.2 |
| Perlite | loose fill | white | 2.4 |
| glass fiber | blow | white | 2.8 |
| rock wool | blow | dark gray or brown | 2.8 |
| Vermiculite | loose fill | white | 2.8 |
| glass fiber | Blanket/Cotton Batting | pink or yellow | 3.1 |
| rock wool | Blanket/Cotton Batting | dark gray or brown | 3.2 |
| Cellulose (Paper) | blow | dark gray | 3.7 |
| polystyrene | shaped | pink or white | 4 |
| polystyrene | extrusion | pink or white | 5 |
| Polyurethane | Faceless | beige or white | 6 |
| polyisocyanurate | Faceless | tan or beige | 6 |
| Polyurethane | face | beige or white | 7.1 |
| polyisocyanurate | face | tan or beige | 7.1 |
For those blowing insulation (our favorite option), be sure to install some makeshift "depth gauges" in the attic from scrap lumber. Inexpensive standards are also very effective for this. If you are using scrap wood, you can mark it to indicate the desired thickness of the finished insulation. It allows you to be sure that your contractor has indeed blown in the correct amount of insulation.