One of my favorite insulations on the market today is mineral wool otherwise known as Rock Wool. Even though it lags in use compared to the more popular fiberglass insulation that most people are familiar with it is, in my humble opinion one of the best insulations available today.
You may be wondering why you should consider this new insulation compared to the standard fiberglass you’ve known for years. I’ve given a much more detailed write up on mineral wool’s advantages in this previous post, but I’ll give you the main points below.
Mineral Wool Advantages
- Higher R-value than fiberglass (R-15 vs R-13 in a 2×4 wall)
- Hydrophobic (won’t absorb water)
- Less sagging than fiberglass
- Easier installation for DIYers
- Itch-free product
- Less palatable to critters
With all the advantages above it was a no brainer when I was remodeling my bathroom this year to install mineral wool on the exterior walls. It was also the perfect time to show you howe to install it since the installation, while easier in my opinion is considerably different than fiberglass insulation. Let me show you what I’m talking about.
How To Install Mineral Wool
Mineral wool does not come in faced batts like fiberglass. There is not a faced version so if you need a vapor barrier then you’ll have to install one separately.
The main difference when installing mineral wool is the cutting process. To cut fiberglass insulation you just need a razor knife and a piece of plywood or drywall to compress it and slice it to size. For mineral wool you need to use a stone wool knife or a serrated bread knife will work just as well.
Mineral wool is much thicker and dense than fiberglass and can’t be compressed in the same ways which, I feel, makes the installation more foolproof. The biggest issue with DIYers installing fiberglass is that they compress it around obstructions or it sags in the cavity. This is not something that happens with mineral wool since it won’t compress the same way.
Step #1 Cut to Size
The mineral wool needs to be cut to length for the stud bay using the stone wool knife making sure to cut it tight. Don’t leave any gaps. Lay the mineral wool on the floor and with a sawing action cut the insulation to size both length and width if necessary due to unusual stud spacing.
Step #2 Cut Out Obstructions
Next, place the batt in the bay and mark any obstructions like electrical boxes, plumbing, wire, etc. You’ll need to cut each of these out like a puzzle piece using that same knife.
When cutting for wire simply cut a slit in the mineral wool and the wire will slot right in. For pipes a reverse shaped V cut works best to give the pipe room to bed properly within the insulation. Make sure to cut everything tight and not leave any gaps to have the most effective insulation coverage.
Be especially mindful on exterior walls around plumbing lines to ensure that the insulation goes to the exterior of the pipes to prevent freezing in winter. Don’t insulate just the interior side of the pipe as a shortcut.
Step #3 Press into Place
Unlike fiberglass, this is designed to be simply pressed into the wall with a tight fit. There is no stapling needed. If you are installing on a ceiling then installing strapping to keep the insulation in place until drywall is installed works best. Typically these straps are unnecessary because it’s designed to fit standard stud widths very tightly.
That’s it! You’ve now got a hydrophobic, R-value dense insulation installed and ready for drywalling or plastering. No worries about sagging or even wetting since mineral wool is dense, and inert. You’ve successfully avoided mold and critters too.
If you haven’t considered mineral wool insulation for your house it may be time to give it a look.
Founder & Editor-in-Chief
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.
Any advice on how to know when a vapor barrier is necessary? Been thinking about adding mineral wool to an uninsulated cabin, as well as a few spots in my older home, and every time I research vapor barriers, my head starts spinning a bit (or google routes me to random builder forums, which are not remotely authoritative).
I bought a bunch of this and I’m so excited to install it, I just have to finish changing out the old wiring to make it safe. As someone who is hyper allergic to mold this is the best!
Scott, thanks for all you do to educate all us novice remodelers. I’ve learned so muchfrom your blog and IG page! I was wondering if Rockwool would work well under a raised house? Also if it transmits more or less noise than fiberglass? This might be just what I need, but I need more info. Thanks
Sounds great! Would this work well for under floors in a raised house? What about sound transmission – quieter or louder than fiberglass? Thanks for all you do to educate us novice remodeled. I’ve learned so much from you!
Hello Scott, my terrific handyman, Josh installed wool insulation yesterday all around the basements floor sill. The sill is only about 3″ deep in my 100 yr old home. Therefore we got double the coverage from each package.
Josh used an electric bread knife to cut from the strip then gently separated the piece down thru the thickness. I was amazed at how fast he completed the job. One hour to do an 800 sq foot house. The results where immediate.
I did a lot of homework reading on which insulation to use, with mold, the R rating and adhesion being my concerns. Keep on recommending wool insulation Scott as it’s a great natural product. I’ll wager my heating bill will really go down this year.
What about relative cost? I assume it costs more than fiberglass since you did not list cost among the advantages.
Two more reasons to use the mineral wool are 1) it appears to be much more fire retardant than fiberglass, giving you more time to get out if there is a fire and 2) it creates more of a soundproof barrier than fiberglass. I think the higher cost makes it worth it.
Scott a few years back when I was remodeling my split level home I had a lot of issues with the layout so decided to relocate a bathroom, 2 utility rooms A) (my water heater and water softener with a salt tank) B) My HVAC Setup with ductwork as well and a storage closet. I also relocated my suite bathroom bedroom. While I was at it I then redid my family entertainment room across from it as the fiberglass insulation was garbage. I elected to go with Mineral wool vs foam or fiberglass. I paid someone to come in and redo the new insulation with mineral wool as I was busy doing 12 other things and wanted to get it done. I ripped out all the old drywall and fiberglass which was a joke glad I did. (Thus reinsulated the walls and ceiling using the mineral wool) worked great for soundproofing also and I think better then fiberglass. I was pondering on foam but thought what a mess it would be not to mention the health issues as well? You could tell the difference as far as temps in the rooms. Glad I did this lowered my utility bill and would recommend it to anyone redoing remodeling in a home or business.
Love this insulation!! I did the kitchen remodel with this. Keeps it cool/warm in the right seasons. Lowered my energy bill.
Oh I am looking forward to the day when I can use mineral wool. I live in a 98-yr-old house and sometime in the next few years, it’ll be time to replace the shingles. It’s a Cape Cod shingle house and there is no insulation. I figured when I pull down the shingles to replace them, that’ll be the time to install the modern convenience of insulation. 😉 And no, the shingles on the sun facing side cannot be saved. Curled and deteriorating. Perhaps on the other sides, the shingles could just be repainted but without taking them off, can’t insulate.
I’ve looked at the rock wool before and look forward to using it. Seems that it must be purchased in a big lot which I wish was different but oh well! Thanks for all you do for old houses and for us. I enjoy reading your blog.
Pam
in sweltering Texas