Mineral Wool vs. Fiberglass Insulation

By Scott Sidler January 15, 2018

mineral wool vs fiberglass insulationYou 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.

//youtu.be/clN-wB8Vl_k

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.

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14 thoughts on “Mineral Wool vs. Fiberglass Insulation”

  1. Fiberglass comes in R-38 but Roxul tops out at R-30. I live in the Northeast and the energy Dept recommends R-38 or higher for attics in zone. Which should I use?

    1. the information which is missing is, how thick is the insulation you’re looking at, Mineral Wool can easily reach R-Values over 40 but the thickness for both it and Fiberglass are the defining component.

      Mineral Wool, by every measurement I’ve seen, beats Fiberglass. whether in blatt or blown form.

  2. Rockwool has been around for decades. We have it in the attic of a 1932 house. It left the residential market but I am so happy to see it’s comeback!

  3. We have a 1930’s Cape Cod with an uninsulated attic. So far my plan is to install radiant barrier across the rafters. Would you recommend supplementing that with insulation installed between the attic floor joists? You’ve recommended blown-in insulation in the past for attic floor insulation, are batts of mineral wool a viable alternative? My main concern is the ease and cleanliness of installation, mineral wool seems like it would be much cleaner to work with and not require blower rental or operation.

    1. Mark, both are good options for an attic floor. Blown-in insulation would be much easier to install in your situation. Mineral wool has the benefit of working in other situation that blown in won’t. I would stick with blown in for the attic floor because the installation with likely be 1/3 of the time!

  4. What do you think about just adding the sheets of Styrofoam-type insulation in our old homes, provided you’re already tearing off damaged clapboards to replace/repair?

  5. One other thing you forgot is supposedly mice don’t like to nest in mineral wool, whereas they will gladly line a nest with fiberglass. Haven’t seen for myself if this is true, but I’m hoping

  6. I have a 95 year old Sears Craftsman. The crawl space is damp. I’ve put down a moisture barrier. How high should I go on the foundation with the moisture barrier? Also, what insulation should I use to improve the R value. It is original save the kitchen and bathrooms. Thank you for your knowledge and help!

    1. If it’s damp I would first check to see you have enough ventilation in the crawl space. Then bring the moisture barrier at least 6-12” above the ground and seal the joint between the moisture barrier and stem wall. I like mineral wool for this application. Installed between the joists.

      1. David,
        Something else you may want to explore is actually sealing all of the vents in your crawlspace and carrying the vapor barrier within 6 to 8 inches of the sill plate. This gap will provide you the ability to inspect for termites. Also in this scenario you can actually control the moisture content in the crawlspace one of two ways. First you can create a supply and return from your duct work, provided it is located in the crawlspace. Or the method I prefer is to install a dedicated dehumidifier. I have done this in two separate crawlspaces and have kept a data logger for temperature and humidity and I can keep the relative humidity below 60% all year included in hot humid summers we get in Alabama. Also this would allow you to insulate the walls of the crawlspace with rigid foam versus. The joist bays. Anyways just my two cents. I don’t think I will ever own a home with a vented crawlspace again. Look up “crawlspace encapsulation” to see examples. YourCrawpsace.com has a great system which requires very few specialty tools. Also check out buildingscience.com to get a really good studies and white papers on crawlspaces. I think you might consider encapsulation.

  7. We installed Roxul in our 1925 Bungalow, and couldn’t be happier. We did it ourselves since it is a much easier and safer product than fiberglass, which we had priced out too to be installed professionally. Although Rockwool is more expensive than fiberglass, by installing it ourselves it came to less than what was quoted for fiberglass, so we ended up with a far superior product for the same price. An added benefit we discovered is noise reduction, the house felt a lot quieter after the installation, and since we were down to the studs, we also installed the soundproof rockwool in the walls separating the bathrooms from the bedrooms. All in all, a GREAT product.

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