I spent a long time thinking about whether I’m going to use Rockwool or fiberglass batt insulation in the new house that I’m building. After lots of research, I decided on fiberglass, and in this article I’m going to explain exactly why.
Many consumers assume Rockwool is far superior to fiberglass, but what they don’t realize is that both insulators are mineral wool! They actually have remarkably similar properties. Continue reading to learn more…
Comparison of Fiberglass to Rockwool
Before we get into the nitty, gritty details, I should just go ahead and clear something up: Rockwool is just a name brand maker of mineral wool – stone wool, specifically – made from igneous rocks – basalt, to be exact. There are other makers of mineral wool, and like I said at the top of this post, even fiberglass is a type of mineral wool.
However, everyone calls stone wool Rockwool these days, so I’m just gonna stick with it for the rest of this article. It’s exactly like how everyone calls polystyrene Styrofoam – another name brand product. Say “polystyrene” to the average person, and they’ll look at you like you’re talking gibberish; but say “Styrofoam,” and they’ll know exactly what you’re talking about.
Now, without any further ado, let’s get into the specifics…
Both Insulators Are Made from Minerals
I first embarked down this building-science rabbit hole when I asked myself, “If fiberglass is made from glass, and glass is a mineral, then how can it be that different from other mineral wools?” So I began doing research… Color me not shocked when I realized that Rockwool and fiberglass are virtually identical except for a few small differences.
Cost
Perhaps the biggest difference between these two types of insulation is cost. At the time of writing this, Rockwool is about double the cost of fiberglass. That’s a tremendous difference for a product that really isn’t any better than the alternative we’re comparing it to.
R-Value
Fiberglass batts and Rockwool basically have the same R-value per inch. You can find varying ranges for both all over the Internet, but if you average all the numbers out, they both end up at about 3.2 per inch. This actually makes a lot of sense because, with the exception of foam insulation, the material an insulator is made of has almost no insulation properties – it’s the air it traps that’s doing the insulating. So when comparing fibrous batts to fibrous batts, we’ll always find comparable R-values per inch.
Fire Resistance
One of the strongest selling points you’ll hear from advocates of Rockwool is that it’s “fire proof,” as in it can’t be lit on fire. Well, the same is true of fiberglass. Hold a torch up to a batt of unfaced fiberglass – no kraft paper – and you’ll singe it a bit but it won’t combust. It’s glass after all. Try to light anything made out of glass on fire and see how well that works.
That being said, both materials will begin to melt at temperatures around 2,000º F.
Soundproofing
No amount of insulation can literally soundproof a room, since it’s nearly impossible to truly soundproof just about any space, but insulation can reduce sound transmission and reverberations. If by any chance you’ve ever been in a building with hollow walls, you may have noticed the acoustics are pretty lousy.
Even though Rockwool and fiberglass have similar STC ratings – sound transmission class – in the real life experiments I’ve seen – or more accurately, heard – fiberglass is the clear winner. Most audio experts seem to agree with me on this, as well. I’m also a recording artist, and I used to work as an assistant engineer in a professional studio, so I feel somewhat qualified to have this opinion.
However, according to many decibel meters, it’s not anyone’s opinion that fiberglass is a superior sound dampening material, it’s just a fact. Now, to be fair, Rockwool does make a batt specifically for acoustic treatment. It’s called Safe’n’Sound. It has no R-value assigned to it, though, because it is not intended for exterior walls, and I’ve never seen a direct comparison between it and fiberglass, so I can’t speak to it’s effectiveness.
Density
Stone wool is much denser than glass wool, so a batt of Rockwool weighs about twice as much as a batt of fiberglass – 6 lb compared to 3 lb. I don’t really see this as a pro or a con either way, but perhaps it matters to you.
Stiffness and Stability
Rockwool is very rigid and difficult to compress. If you squish it, it immediately returns to its normal shape. This is good if the batt fits perfectly inside of the cavity. Fiberglass, on the other hand, is soft and easy to compress. This can be a plus or a minus depending on the circumstance. If you need to compress the insulation so that it fits into wherever you’re trying to put it, then its squishiness is a plus; however, if the cavity is just he right size or oversized, the fiberglass will gradually sag over time, reducing its R-value.
Ease of Use
Both materials are easy to handle and easy to cut. Rockwool is, however, a bit easier to install, especially in ceiling cavities because it is more rigid.
Health Risks
Neither material poses a health risk once it is behind wall sheathing; however, you should wear a respirator, gloves, and long sleeves/pants when handling either. Tiny fibers can irritate your skin, or even worse, get in your lungs. I would also recommend eye protection, just to be extra safe.
Eco Friendliness
I think the eco friendliness of each material is a toss up. Both materials can be recycled, so long as they are clean, and both are made from a certain percentage of recycled materials. Fiberglass consists of about 30% recycled glass, and Rockwool is partially made from recycled stone wool, though I’m not sure what the percentage is. Neither material off gasses.
There is some concern about the environmental impact of creating both materials. Both go through a similar manufacturing process that involves heating the ingredients to a molten state, and then spinning them into fibers.
However, I think it’s worth mentioning that the manufacturing of virtually all building materials has some negative impact on the environment. So, it is often more important to think about the greenness of a material after it has been created, not before.
If environmental friendliness is your biggest concern, then you may want to consider denim or cellulose instead – arguably the greenest types of insulation, though not without their flaws; but that’s a conversation for another article.
Vapor Permeance
Both types of insulation are vapor open except for fiberglass that is faced with kraft paper or foil. Personally, I would not rely on insulation, especially batts, as my vapor retarder. If you live in a climate zone that requires one, I think there are better options – preferably a continuous layer that covers the studs. However, if you want an all-in-one solution, then faced-fiberglass is the way to go.
Water Resistance
This is the one area where Rockwool is the undisputed champion. Rockwool is hydrophobic. It does not absorb water at all. You could spray with a hose all day, but it won’t ever sponge up the water. Fiberglass, on the other hand, has some water resistance, but if it gets soaked by bulk water, then it will become saturated and sag.
I don’t see this as a major issue, though, unless you live in an unusually damp climate or an area prone to flooding. If I were in such an area, I would use stone wool in my exterior wall cavities, and fiberglass in my interior walls for acoustic treatment and cost savings.
Conclusion
For the purpose of insulating assembly cavities with batts, I find it hard to justify the cost difference between fiberglass and Rockwool. Unless excessive moisture is a major concern of yours, I see little to no benefit spending nearly twice as much on Rockwool.
