You might think that all insulation can be effectively used to insulate an attic. But depending on the material you’re using, you might not be able to achieve a high enough R-value without installing so much insulation that your ceiling is in danger of sagging!
Weight impacts insulation in surprising ways, and even insulation that seems lightweight can pose risks if installed incorrectly. This is why there are industry guidelines for insulation weight.
Rigid foam board is the lightest insulation. Fiberglass and cellulose are common and effective for attics, but if insulating at R-49 to R-60, thicker ceiling boards might be needed. Mineral wool, natural fiber, and spray foam would be too heavy at R-49 to R-60.
Why Knowing the Weight Is Important
When homeowners take to homebuilding forums to ask about the weight of insulation, their main concern is ceiling sagging. If a heavy material is installed at too thick of a depth, it can put pressure on the drywall ceiling boards and cause them to sag.
Sagging can eventually cause the ceiling boards to fall, so it absolutely needs to be addressed and, ideally, prevented in the first place. You need to know the weight of your insulation to make sure that your drywall ceiling can handle it.
While this article is mostly about ceiling insulation, you should note that insulation weight is also pertinent to walls.
Insulation that is too heavy can sag inside wall cavities. It needs to stay vertical and snug inside the wall to live up to its R-value.
If it sags under its own weight, the insulation will leave large gaps towards the top of the wall. Air will move through these gaps and transfer heat, undermining the efficacy of the insulation.
Fiberglass
Loose-Fill
Batt
Explaining the Tables
Overall, fiberglass is one of the lightest insulating materials (the other is rigid foam board). Fiberglass is used in all sorts of products because it is extremely strong yet lightweight.
The fluffy pink fiberglass you see used as insulation is made by spinning glass-wool fibers. The raw materials are recycled glass and sand. Streams of glass are created through holes in a rotating spinner and chopped up into tiny strands.
In addition to the inherent light weight of fiberglass as a material, fiberglass insulation is particularly light because it is less dense than, say, cellulose. Think of the cotton candy-like structure of fiberglass compared to the denser, somewhat clunkier shredded newspaper that makes up cellulose.
It does not have the highest inherent R-value, so you need thicker layers of fiberglass batts to achieve the same R-value that foam board or mineral wool, for example, could achieve with thinner layers.
The density of loose-fill fiberglass depends on how much is packed into the space (although compression should be avoided).
Fiberglass is expected to settle, though not as much as cellulose insulation since cellulose is denser. A professional installer will take settling into account when determining the thickness.
Fiberglass batts come in different densities, with higher-density batts having higher R-values and higher prices.
Despite the lightweight nature of fiberglass, you could run into some issues with weight if you’re trying to achieve high R-values.
For example, the Department of Energy’s recommendation for attics is R-38 to R-60 for most parts of the US, with R-49 to R-60 recommended for states in the Northwest, Upper Midwest, Northern Plains, and Pacific Northwest.
Achieving R-49 with fiberglass batts could mean putting over 15 inches of insulation in your ceiling, which would have a weight of 1.47 lbs per square foot.
The U.S.G. Corporation, which is a leading drywall manufacturer, has set maximum weight guidelines for drywall:
| Installation of drywall | Maximum weight of insulation above drywall |
| 1/2 inches drywall on joists spaced 24 inches on center | 1.3 lbs/ft^2 |
| 5/8 inches drywall on joists spaced 24 inches on center | 2.2 lbs/ft^2 |
| 1/2 inches drywall on joists spaced 16 inches on center | 2.4 lbs/ft^2 |
To follow manufacturer recommendations when insulating to R-49 to R-60 with fiberglass batts, you would want to go above and beyond the basic drywall dimensions (½” drywall on 24” joists) to ensure that your ceiling doesn’t sag.
[Replace with licensed image; joist spacing example]
Loose-fill fiberglass would be heavier, requiring just over 17 inches and 1.92 lbs per square foot to reach R-49. Loose-fill needs to be thicker because it will settle over time, whereas batts are more structured and stable.
You don’t need to worry about the weight of fiberglass in walls. A standard 2×4 wall only needs to achieve R-13 to R-15, which would weigh less than 0.5 lbs per square foot.
Cellulose
Explaining the Table
Cellulose is paper-based, made from recycled wood, newspaper, and cardboard. These materials are less fine than fiberglass strands, so cellulose is denser.
Because cellulose is organic and combustible, it must be extensively treated with fire-retardant chemicals. These fire retardants add weight. By contrast, fiberglass does not need to be treated.
Cellulose also settles significantly, so it will end up denser than fiberglass. With loose-fill insulation (cellulose or blown-in fiberglass), you can pack a lot into a small space. Be careful, because overfilling will put pressure on the drywall.
Compared to fiberglass, cellulose carries a greater risk of drywall sagging. Let’s look back at the U.S.G. Corporation’s guidelines about maximum insulation weight:
| Installation of drywall | Maximum weight of insulation above drywall |
| 1/2 inches drywall on joists spaced 24 inches on center | 1.3 lbs/ft^2 |
| 5/8 inches drywall on joists spaced 24 inches on center | 2.2 lbs/ft^2 |
| 1/2 inches drywall on joists spaced 16 inches on center | 2.4 lbs/ft^2 |
To achieve R-49 to R-60, you’d typically need 13-17 inches of cellulose insulation. This would weigh 2.1-2.6 lbs per square foot. Manufacturers say that drywall can withstand 2.2-2.4 lbs per square foot under certain circumstances (thicker drywall, closer joists).
There is a lot of debate about the U.S.G. Corporation’s guidelines. Many green builders insulate attic floors at even higher R-values than required, going up to R-70 or even R-100. Cellulose at R-100 would mean more than 2 feet of cellulose, which by far exceeds manufacturer recommendations.
Yet, professional contractors on Green Building Advisor forums note that they routinely ignore these recommendations. But despite anecdotal success stories, remember that ignoring U.S.G.’s chart could constitute a code violation as you are required to follow the manufacturer’s instructions.
Tip: beware of moisture. Cellulose holds more moisture than fiberglass, as organic materials absorb water while fiberglass strands do not. Condensation or water leaks can make cellulose much heavier than expected. Furthermore, when the paper facing of drywall gets wet, it can start to sag.
Mineral Wool
Loose-Fill
Batt
Explaining the Tables
Mineral wool, also called stone wool or rockwool, is an insulation made from fibers of mineral materials. Materials such as slag and ceramics are spun into extremely thin fibers that are layered together to create a squishy fabric resembling fiberglass.
But while mineral wool might just look like brown/yellow fiberglass, it is very different.
First of all, mineral wool is much denser than fiberglass or cellulose. Mineral wool batts are so dense that you can mark a batt simply by pressing the edge of a tape measure into the surface to make an indentation.
[Replace with licensed image: example of texture/density]
Mineral wool is designed to be denser for several reasons:
- Firstly, that density allows mineral wool to achieve a higher R-value per inch than fiberglass.
- Secondly, the dense, rigid batts are easy to install. They “cut like bread” and do not fall apart or fall down, even if installed in between ceiling joists without support.
- Thirdly, mineral wool is marketed as a great soundproofing material because of its density.
But that extra density also has its downsides. Because mineral wool has more material packed into each square inch, it is one of the more expensive insulation options out there.
Most importantly, the density of mineral wool contributes to it weighing more than fiberglass or cellulose. The mineral materials weigh more than glass or newspaper in the first place, and the dense packing adds to that weight.
Mineral wool is less commonly used in attics because its weight can be prohibitive.
If installed to R-49 to R-60, loose-fill mineral wool would weigh 5.3-6.5 lbs per square foot and mineral wool batts would weigh 2.5-3.2 lbs per square foot—both way over U.S.G.’s upper limit of 2.4 lbs per square foot.
Mineral wool is most popular for use in exterior walls, as wall cavities have limited space and just 3.5 inches of some mineral wool batts can get you to R-15. The required R-values for walls are also lower.
Tip: When insulating walls, there’s a limit on thickness and not much of a limit on weight. When insulating unfinished attics, there’s a limit on weight and not much of a limit on thickness.
Natural Fiber
Loose-Fill
Batt
Explaining the Tables
Natural fiber insulation can include sheep’s wool, cotton fibers, and/or hemp fibers. These organic materials are heavier than fiberglass and even cellulose, and about as dense and heavy as mineral wool.
Now, it seems like wool and cotton would be lighter than rock-based materials, but think of your thickest wool coat or sweater. Those things can be heavy!
In addition, natural fiber insulation needs to be installed at a greater thickness than cellulose, fiberglass, or mineral wool to achieve the R-values necessary for attic insulation. Natural fiber batts need to be installed at 23-28 inches thick for R-49 to R-60, whereas cellulose needs to be 13-18 inches thick.
Because of the need for more layers as well as the difference in material, natural fiber rated at R-49 weighs more than cellulose rated at R-49.
As with fiberglass and mineral wool, natural fiber insulation in loose-fill form tends to weigh more (at a given rating) than natural fiber in batt form.
This is partly because it becomes denser: loose-fill is installed to account for settling, so 15 inches of loose-fill (settled) was probably thicker when first installed.
For both loose-fill and batts, natural fiber insulation will weigh more than the U.S.G’s maximum weight guidelines (1.3-2.4 lbs per square foot) for drywall ceilings. It’s best to use natural fiber insulation in walls rather than ceilings.
You could also look into combining natural fiber insulation with a layer of foam board, for example, to reduce the amount of natural fiber you need to use to reach attic R-values.
Plastic Fiber
Explaining the Table
Plastic fiber insulation is made out of recycled plastic milk bottles—a type of plastic called polyethylene terephthalate or PET.
It is difficult to generalize about the weight and thickness of plastic fiber insulation because it comes in so many different forms: thin blanket sheets, thick fluffy batts, and spray. Additionally, it’s the least common form of insulation on this list.
Plastic fiber insulation is fairly comparable to fiberglass in its weight and efficacy. When installed to an R-value of 3, plastic fiber insulation typically weighs 0.0947 lbs per square foot while fiberglass batts weigh 0.0899 lbs per square foot and cellulose weighs 0.1331 lbs per square foot.
It makes sense that plastic fiber insulation would weigh slightly more than fiberglass, because the plastic is flammable and needs to be treated with fire retardants which contributes to the weight.
While uncommon, most forms of plastic fiber insulation should not be too heavy for attic use, but it’s important to check your particular product and ask a professional contractor.
Spray Foam
Explaining the Tables
Spray foam is made of foam plastics and is much denser than batts or loose-fill insulation, reaching high R-values at low thicknesses. But even at low thicknesses, spray foam is heavy.
In particular, high-density and medium-density spray foams tend to be heavy because they are closed-cell foam (they have a denser structure). Low-density spray foam is open-cell. Only medium- and low-density spray foam are used for interior attic insulation.
These tables suggest that some spray foams can be prohibitively heavy for attic floors if layered more than 7 inches thick.
When installed to an R-value of 30, spray foam needs to be about 9 inches thick and will weigh over 3 lbs per square foot. This exceeds the U.S.G Corporation’s guidelines for what drywall can handle.
But insulation contractors still use spray form on attic floors, often in conjunction with other types of insulation to reach those higher R-values without too much weight. It’s very common to use spray foam to seal around fiberglass batts, for example.
Additionally, people often spray the rafters directly, rather than the floor. Spray foam allows you to do this because it is so adhesive. It also has some structural integrity, as it dries hard and won’t slip down the slanted rafters. That way, you don’t even have to worry about your ceiling drywall.
If you use spray foam to insulate walls, be sure that you scrape down excess foam to create a smooth surface before installing the drywall. Failing to trim the foam can put pressure on the drywall.
Rigid Foam Boards
Explaining the Tables
Rigid foam boards are the most lightweight insulation option available. Foam boards are the best way to achieve R-60 without exceeding U.S.G.’s weight limits for standard drywall ceilings (½ inch drywall with joists spaced 24 inches on center).
Foam boards pack a lot of R-value into a thin and light package. Compared to a foot and a half of cellulose or fiberglass, you can use just a foot of foam board to reach R-60.
So why is spray foam so much heavier than rigid foam boards, if they’re both made of foam?
Spray foam is designed to be a better insulator than foam boards, as it is used to seal cracks and create airtight bonds. Spray foam is more expensive than rigid foam board because it is denser and has a higher inherent R-value.
Additionally, spray foam and foam boards are made from different types of foam plastics.
Spray foam is polyurethane or polyisocyanurate. Rigid foam board is polystyrene, which is cheaper, lighter, and has less thermal resistance than polyurethane. Polyurethane has more structural rigidity.
But that doesn’t mean that spray foam is better than rigid foam boards!
As we have seen, spray foam is problematically heavy for use on attic floors. Rigid foam board is highly versatile because it is thin, light, and effective. If you’re using foam board, you really don’t need to worry about drywall sagging.
Sources
https://homeguides.sfgate.com/fix-sagging-drywall-ceilings-42297.html
https://cen.acs.org/materials/inorganic-chemistry/s-fiberglass-does-delicate-material/96/i38
https://www.forbes.com/home-improvement/insulation/what-is-fiberglass-insulation/
https://www.greenbuildingadvisor.com/article/will-thick-cellulose-cause-your-ceiling-to-sag
https://www.jm.com/en/blog/2021/march/head-to-head–mineral-wool-vs–fiberglass-/
https://www.dbsg.com/blog/this-vs-that-spray-foam-or-rigid-foam-insulation/