Attic and wall insulation is made from the same materials and look similar. However, the packaging of insulation will often specify where it should be used.
Attic and wall insulation differ in R-values and dimensions and using loft/attic insulation in the walls can cause installation issues.
Attic insulation is required to have a much higher R-value than wall insulation. This means that attic insulation is too thick to install in most wall cavities. It is also more expensive.
Differences Between Ceiling and Wall Insulation
The materials used in ceiling and wall insulation do not differ. For example, fiberglass batts or rolls are a common type of “blanket” insulation that comes both in ceiling/attic format and in wall format.
The difference is that they are designed to have different densities and sizes.
R-value refers to the insulation’s thermal resistance; basically, its ability to prevent the transfer of heat. The higher the R-value, the more effective the insulation.
Attic insulation is designed to have a higher R-value. The U.S. Department of Energy recommends R-13 to R-23 for exterior walls. But the lowest R-value they recommend for attics is R-30, and most states fall into the range of R-49 to R-60 for attics.
There is little to no overlap in the R-values of wall and attic insulation; they are simply designed for different purposes.
However, when you have space (such as in an attic), you can layer insulation to increase the R-value. For example, an R-19 batt plus an R-30 batt would have an R-value of 49.
High R-values make a bigger difference in the attic, where heat rises and escapes. High R-value insulation in attics can prevent up to 90% of heat transfer, while high R-value insulation in walls prevents up to 60% of heat transfer.
R-value depends on both the material from which the insulation is made and the thickness and density of that material.
Attics need higher R-values and tend to have more space for insulation, so attic insulation is always thicker than wall insulation.
Pre-set sizes do not exist for loose-fill or blown-in insulation. Instead, they are installed in different thicknesses for walls and attics.
For batts or rolls, the size of the product is based on the dimensions of the standard wall and R-value of the insulation.
2×4 walls hold R-13 or R-15 batts while 2×6 walls hold R-19 or R-21 batts. They come in standard widths of 15 inches and 23 inches. 16-inch and 24-inch options exist for steel stud construction.
R-30 and R-38 batts are most common for attic insulation. They tend to come in widths of 16 inches or 24 inches, which allow them to fit snugly between standard joist spacing.
Width is standardized, but length is more variable, especially when it comes to rolls, which you are expected to cut as needed.
In addition to size, wall and attic insulation come in different formats. Let’s compare what you might think would be the same format of insulation: fiberglass batts of attic insulation versus fiberglass batts of wall insulation.
Attic insulation batts will be thicker but less rigid, as they need to fit in around attic rafters.
Wall cavities have fewer obstructions, so wall batts do not need to be as flexible. In fact, wall batts need to be somewhat rigid to prevent them from slumping as they stand vertically in the wall cavity.
Similar principles apply to roll insulation: the wall version will be slightly more rigid than the attic counterpart.
3 Reasons Why It’s a Bad Idea
1. Insulation May Need to Be Compressed
When you try to fit thick attic insulation into a narrow wall cavity, you risk compressing the insulation, which compromises its thermal resistance capabilities.
The impact of compression on R-value can sometimes be confusing. You may have read that compressing fiberglass batts can increase the R-value per inch. This makes sense in theory because fewer air pockets mean less heat transfer.
However, compressing insulation will reduce the total R-value. For example, a batt that is rated as R-19 at 6 inches thick will, if compressed into a 5.5-inch wall, drop to R-18. If compressed into a 3.5-inch wall, it will drop to R-14.
Compressing too much insulation into a wall cavity can even put pressure on the wood and drywall.
Even so, there is some nuance here. Installing the wrong type of insulation will lead to large-scale, avoidable compression. But some minor compression in a few small areas is basically unavoidable.
When installing fiberglass batts around junction boxes in a wall, the batts will get packed down and compressed in these areas. As long as you fill in the area as completely as possible while avoiding further compression, you do not need to worry about these minor instances of compression.
What if you really want to raise the R-value in your walls but have limited depth to work with?
While you typically don’t need to worry about increasing the R-value of your walls (as we’ll talk about later), there are some options to consider:
- Foam board insulation can give you almost double the R-value that you would achieve with the same depth of most other insulating materials. Unlike thick attic insulation, you won’t have issues with slumping or with fitting it into the wall cavity.
- Make sure that windows, doors, and any other gaps are sealed with spray foam insulation.
2. Installation Will Be Difficult
Attic Insulation Is Too Thick
In theory, you could put attic insulation in your walls without negatively impacting your home’s energy efficiency. However, it tends to be physically difficult to effectively install attic insulation in wall cavities as it is too thick.
On the flip side, wall insulation could be installed in an attic with far less physical difficulty. But while it is physically feasible, it would have a huge negative impact on your home’s energy efficiency and might not adhere to energy codes.
Attic Insulation Might Be Too Wide
Attic insulation typically comes in widths of 16 inches and 24 inches, which matches the dimensions of batts for walls with steel stud construction. Still, the depth of the wall and thickness of the insulation would be an issue.
If you do not have steel stud construction, attic insulation will be slightly wider than the standard 15-inch and 23-inch wall batts.
Installing insulation batts that are too wide is difficult and can be especially frustrating if you are undertaking a DIY project. It will also lead to the edges being compressed.
You might need to cut and adjust the batts or rolls. This takes extra time and sometimes leads to waste, especially if you’re only one inch off from the correct width. It is easier to buy the appropriate dimensions in the first place.
Difference in Stiffness
In addition to issues in size, the slightly different textures and rigidity of wall and attic batts will make installation more frustrating and potentially less successful over time.
As we discussed previously, wall insulation is designed to be more rigid so that it can stand up in the wall cavity without slumping. Attic insulation is somewhat softer and more flexible, so you might have a harder time getting it to stay in place, both in the short- and long-term.
3. Wasted Money
The main reason not to use attic insulation in walls is that you just don’t need to. Walls do not need the high R-values that are required in ceilings.
Hot air rises, so the attic will be exposed to hotter air on the exterior in the summer and will need to contain hot air in the winter. Given limited funds and space—most people don’t want 24-inch-thick walls—the ceiling is the most important area to insulate.
The Department of Energy’s R-value guidelines do not require walls to have even half the R-value of ceilings, even if you live in the coldest states.
Insulation with greater R-value costs more.
Installing high R-value attic insulation in walls will likely not make enough of a difference in your home’s energy efficiency to warrant the higher cost.
Furthermore, the extra R-value might not make much of a difference at all—you might have to compress it which could cancel out the increased insulation capability.