You probably know fiberglass insulation well. It’s been a staple of residential insulation for decades in the United States. You know, the pink stuff! The insulation that makes you itchy when you install it. Wait, they removed the itchy making stuff from it now, didn’t they?
Fiberglass, while it’s very simple to install, has some definite shortcomings in the insulation game that might make you want to give mineral wool a look. The most common name you can find mineral wool going by in the states is Rockwool (formerly Roxul).
Of all the different types of insulation, mineral wool and fiberglass are probably the most similar since they both come in batts that are trimmed to fit inside stud bays, but that is really where the similarities end. They are completely different materials, and after a little studying and some real world experience, I have come to be quite fond of mineral wool and I’ll tell you why below.
What is Mineral Wool?
Mineral wool is a stone-based mineral fiber insulation comprised of Basalt rock and recycled steel slag. It costs about about 25% to 45% more than fiberglass, but I feel its benefits more than justify the additional costs.
The manufacturing process involves taking molten rock and spinning it while blasting it with cool air. It’s actually surprisingly similar to making cotton candy except instead of melted sugar you’re using melted rocks.
This makeup gives mineral wool some of its fantastic benefits that we’ll talk about below. Check out the video below if you’re a dork like me and want to see how things are made.
Benefits of Mineral Wool
They are plentiful and wide ranging, which is why I have become a fan of this stuff. Not all of these benefits may matter to you, but definitely keep them in mind when and if you’re looking at how to insulate and old house.
1. Fire Resistant
Unlike fiberglass which melts around 1,200°F, mineral wool has an extremely high melting point and can withstand fires up to 2,000°F making it one of the safest forms of insulation when it comes to house fires. It will not melt or off-gases any dangerous fumes in case of fire and functions as a fireblock, which delays the spread of the fire buying you valuable time to escape.
2. Water/Mold Repellent
Mineral wool is manufactured with a small amount of oil in the mix which helps give it a hydrophobic property. This characteristic keeps mineral wool performing effectively and does not lower it’s R-value when exposed to water.
Any water that does end up on mineral wool rides down the surface rather than absorbing into the body of the insulation. This awesome feature and the fact that it is comprised of rock makes it virtually impossible for mold to grow on or in mineral wool.
Compare that to fiberglass, which readily absorbs and holds water which greatly decreases it’s effectiveness and lowers its R-value when wet.
3. Higher R-value
R-value is a big deal in insulation, so lets see how they stack up. Fiberglass has an R-value of between 2.2 to 2.7 compared to mineral wool weighing in at 3.0 to 3.3. So that means, for standard 2×4 wall mineral wool comes in R-15 batts, while fiberglass comes in R-11 or R-13. For 2×6 walls, mineral wool comes in R-23. Fiberglass? R-19 with special order of up to R-21
Another bonus is that mineral wool is available in batts that fit 2×8 framed walls at R-30. Fiberglass? Not available in that size.
The biggest advantage is the consistent R-value of mineral wool as opposed to fiberglass that comes with poor installation. Fiberglass is easy to accidentally compress which greatly diminishes its R-value. With mineral wool that isn’t an issue since it is already compressed.
4. Easier to Install
Installation of mineral wool is different from fiberglass entirely in that it comes in thick batts almost like a huge loaf of bread that must be cut by what unsurprisingly looks like a bread knife. Unlike fiberglass, you don’t have to compress it and then cut it with a razor knife before stapling a kraft paper face onto the stud.
There is no paper facing because mineral wool does not come with a vapor barrier- you have to install your own vapor barrier if it is necessary in your situation. In my opinion, this usually results in a better installation because the vapor barrier is one solid piece, rather than a bunch of joints that have a greater likelihood of not being properly sealed.
For mineral wool, you cut the piece to size but leave it a little tight so that it compression fits into place in the stud bay. You can install straps for installation on a ceiling to make sure it stays in place. A tight installation is best and I find that this is easier for most of us to accomplish than trying to ascertain if a piece of fiberglass is too fluffy or too compressed to perform properly.
5. More Versatile
Mineral wool is not just for inside the house either. There are versions that can be installed on the exterior of a building in place of rigid foam. Installation on the exterior is an excellent use of mineral wool because of its hydrophobic properties.
Combine that with its versatility of coming in configurations for 2×4, 2×6, and 2×8 walls, and you have an easy option for builders and remodelers to turn to. Especially for those of us in old homes where stud size is not always a standard dimension.
With its dense composition, mineral wool is also easier to ensure proper installation around cutouts like electrical boxes and plumbing lines. I find that fiberglass is usually just compressed in these sections, whereas mineral cannot be compressed to fit around them. It forces the installer to do it correctly or not at all.
It’s always a good idea to keep up with new products on the market that may work well for us old house owners. Mineral wool is something that should definitely be on your radar if it’s not already. While it’s not a new invention, simply a newer product, mineral wool has a place in insulating your old home and might be just what the winter ordered.
Founder & Senior Editor
I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.