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How to Insulate Walls

Are you afraid your house might burn to the ground?

It scares the bejesus out of me, especially since we have kids.

Of course we take all the precautions: smoke detectors, GFCIs, fire extinguisher, trained guinea pigs….

Yah, did you know you can train guinea pigs to squeak when they smell smoke?

Just kidding but that would be really cool.

If you want an ironclad way to save energy and make your house more fire resistant check out Roxul insulation.

I’ve wanted to use it for years and finally got the opportunity this past weekend. And of course I’m going to share my experience with you.

Plus, there are not 1 but 3 videos you need to watch that prove how awesome Roxul is for DIYers like you and me 🙂

What’s Roxul (Now Rockwool!)

A lot of shows on HGTV show contractors using Roxul. So I decided to research it and found out why they love it so much.


Roxul is made from stone fibers. As a result, it can withstand 2000F temperatures without burning.

That’s pretty hot!!!

I’m not saying Roxul with prevent your home from catching on fire. But it gives you extra time to escape a bad situation.

When I forked over $300 for 6 bags I knew it was money well spent.

Plus, Roxul has soundproofing qualities that help keep homes a bit more quiet.

This is super important for me since I get up at 4:25 am every weekday. Let’s just say my kids don’t share my enthusiasm for early mornings, haha.

Roxul is totally a DIY project.

Here are all the supplies you need

  • Long Sleeve Shirt
  • Gloves
  • Respirator
  • Drywall Knife or Bread Knife
  • Measuring Tape
  • Roxul Insulation (I used R15 ComfortBatt)

Here’s what you’ll learn

  • How to insulate walls with Roxul

I can’t wait to show you the videos, so here we go.

Using Rockwool to Insulate Walls

If you can cut a loaf of bread you can use Roxul.

You’ll see why a bit later on.

Roxul fits into standard stud bays that are 16 inches on center. Each batt is

  • 3 1/2 inches thick (standard stud bay depth)
  • 15 1/4 inches wide (standard stud bay width)
  • 47 inches long
  • fire and water resistant

One bag has 12 batts and covers 59.7 square feet.

I needed about 6 bags to insulate a 12 x 12 bathroom (7 foot ceilings) and 10 x 5 closet. Hence the 6 bags for $300.

The bags were $47 each at Lowe’s. Other kinds of insulation were about the same price.

Oh that’s the other thing, you shouldn’t have to visit any specialty store to buy Roxul. It’s carried at most local box stores.

Start the installation by butting a batt flush with the stud header, aka top wood piece in stud bay.

Butt roxul to top of stud bay

Measure the distance between the bottom of the top batt and the bottom of the stud bay.

I cut off about 2 inches of Roxul using a drywall or bread knife.

Trim Roxul

Cutting Roxul is literally as easy as cutting through a loaf of Italian bread. But it doesn’t taste nearly as good (just kidding, please don’t try eating Roxul).

When you push the Roxul into the stud bay don’t compress it. It should be fluffy and flush with the wood studs. This helps the Roxul retain it’s energy efficiency.

Make roxul flush with studs

When you cut Roxul make the new piece 1/2 to 1 inch wider or longer than you need. This allows the Roxul to fit nice and tight.

What do you do if you’ve got pipes, wires, or electrical boxes?

How to Use Rockwool Around Pipes and Wires

Not all stud bays are empty.

Some bays will have pipes, wires, and electrical boxes to work around.

No problem.

You can cut out a portion of the Roxul and slide it around vent or waste pipes. Check out my video for specific instructions.

Slide Roxul around pipes

Here’s a little tip, add the Roxul to the middle of the stud bay to work around a pipe because the pipe will have some flexibility.

This makes it way easier to push the Roxul behind the pipe.

If you’ve got wires that run horizontally across stud bays just cut a slit in the Roxul. The wires can fit snuggly in that slit within the Roxul.

And if you’ve got an electrical box cut out a portion of the Roxul. It’s okay for the Roxul to fit snug around the box, after all, this is where many drafts come from.

Check out my video to see how easy it is to work with Roxul – just one reason among many why I shelled out the dinero for it!!

What’s Next

Roxul changed it’s name to Rockwool after we made this tutorial. It’s still our #1 insulation and we use it to insulate old homes.

If you’re doing a bathroom remodel and need help, join one of our online courses – they’ll make your bathroom renovation much easier!

Let me know if you have any questions and I’ll do my best to help.

Thanks as always for reading, watching, and being part of our awesome community.



Roxul Insulation for Bathrooms

  1. mary says:

    I was wondering if this could be used in the attic floor as well. I would like to insulate and then put plywood on top.

    Thank you!

    1. You could totally do that Mary. In fact, that’s what we did for our bathroom. We had to tear out the ceilings and while they were down we took out the old insulation.

      Once the new ceilings went up we added Roxul to the attic.

  2. Denis H. says:

    Thanks for the post! Been thinking about getting Roxul as I’m about to build an 8×10 cold room (totally DIY) in the barn and need to insulate it pretty well. I don’t like how fibreglass batts react to humidity so Roxul is my best choice so far. How much square footage does each bag cover exactly (should be marked on the bag)?

    1. Thanks Dennis for your question, each bag covers about 59.7 square feet.

      I’ve used fiberglass before but after seeing the fire test with Roxul decided it was a awesome stuff. And the price isn’t all that different between Roxul and fiberglass.

      So not a big issue to switch. Let me know how your room addition goes. Sounds like a cool project.

    2. Steve says:

      Excellent info! In an effort to save money, I am using 2X4 16″oc for the interior of my home (2X4 24″oc). Would you suggest cutting down the length of the rockwool or through the width to accommodate the stud cavity of 23″? Thank you for your help I’m sure many of the honey don’ts such as myself appreciate your expertise.

  3. Just a little weekend project, tear the bathroom down to the bare bones and insulate it! Nice! The problem is, now I want to go slice myself a nice big piece of Italian bread.

    I know you covered up pretty good, but how does Roxul compare on the Home Repair Tutor Itch-o-meter when compared to standard fiberglass insulation?

    1. Haha, I know right? Nothing like demoing all weekend. Fun stuff, well sort of!!

      Roxul is a 7 on the itch-o-meter out of 10.

      The worst, as you know, is fiberglass. I think Darth Vader should have tortured Han Solo with that stuff.

  4. sheila says:

    this is what our contractor is telling us about the roxul insulation. Can we believe him??
    We are in a process of insulating a new home in Wisconsin. We want to use roxul insulation to exterior walls and talked to our contractor. And he said this.
    “I would strongly recommend NOT using Roxsul insulation for your home. What I’ve been using Roxsul for is interior walls and ceilings as a sound barrier. It is awesome for that. However, in my experience, the thermal performance just isn’t the same as fiberglass. Also, the cost will be astronomically higher as the material costs are way higher. (3″ fiberglass = .50/sq ft. 3″ Roxsul = .95/sq. ft.) The thicker you go, the greater the cost increase due to the higher cost of material”

    1. I’m not sure I follow his reasoning Sheila. Roxul isn’t that much more expensive than fiberglass.

      Once of the big reasons I like Roxul is that it doesn’t burn like fiberglass. So in my twisted mind it’s safer than fiberglass.

      If you want to compare the insulation value you’ll need to compare the R values. It sounds like he might shy away from it because he doesn’t have the experience is just doesn’t want to find it. Although most home stores carry it.

  5. Tony says:

    This might be a dumb question and scenario however let me go ahead with it. I purchased a bunch of fiberglass batt without knowing about roxul. I installed it, found out about roxul and then I ripped it all out. I purchased a bunch of roxul and installed it very easily and I love it. But now I have all this brand new fiberglass that I am not sure what to do with. So my question..I framed up on the upper portion of the basement wall because when the builder built the home they left a shelf about halfway up the wall which I didnt want. I wanted a straight smooth flat surface allt he way up to the ceiling. So I have about 3 feet of framed in space to fill. Does it make sense to install fiberglass batt over roxul on the upper portion of my basement walls? I have R-19, but I just don’t think its a good idea to put this over roxul. Seems like it would defeat the purpose. Please advise! Thanks!!

    1. Yah, I’d stick with Roxul over Roxul Tony because that way you have the fire retardant feature intact. Thanks for asking 😀

  6. Mike says:

    Recently remodeled my 1958 ranch house and insulated all exterior walls- and a few interior ones- with Roxul. It’s great to work with, easy to install, and offers great performance. There are no empty spots to allow cold in with Roxul- it fits snug and tight. I also sprayed my studs with fireproofing before insulating. It also appears to offer superior rodent/insect resistance. A/C bills average $80 and gas and electric heat bills in cold weather ran about $150. We are very pleased with Roxul Insulation.

  7. Michel says:

    Is this formeldahyde free? Also do you suggest using this up against basement brick… we want to finish the basement and trying to figure out what’s the best way to go. We don’t want moisture issues. Thanks!

  8. Carol says:

    I want to use Roxul to finish a cement basement ,what do I use on the walls first and the walls are uneven I don’t want to be taken advantage of cause I am a woman

  9. Daniel Buchanan says:

    So I am doing a basement Reno as well and have a few questions. So my basement walls are cinder block halfway up then 2×8 framing above it. So I installed comfort board over the block R6. Now I am doing the 2×8 cavities they are 16oc and 40″ long I have been using 2 batts per cavity of the r14 for 2×4 framing to fill them as my local hardware store does not carry the r28 for 2×8 framing. So is this ok or should I be locating the r28 for the 2×8 framing I feel I am getting the same r value out of the 2 batt install? Also if I cut the batt exactly 40″ it leaves me just enough to fill my rim joist cavity with the left over piece it is actually shy on the rim joist by 1/4 inch but I am not seeing any gaps anywhere I don’t wanna cut the roxul into multiple pieces to fill my rim joists at the end of i don’t have to it does seem to be fitting pretty snug the way I am doing it. So I guess my question is how critical is the extra 1/2 to an 1″ extra length top to bottom?

  10. Virginia D Hall says:

    I want to insulate my attic with Roxul. Right now I have vaulted ceilings, aka the underside of the roof. I’m not sure where to put the vapor barrier. Is it against the wood of the roof (115 years old, lots of nails sticking through) or do I put the Roxul in between the rafters and cover it with the vapor barrier. And what type of vapor barrier is best? We are in Ohio so lots of repeated snowing and melting in the winter and humidity to rival a sauna in summer. Once the roof is insulated, I’m going to use tongue and groove boards to make the ceiling. I want to do this right!

    1. Totally understand your concerns Virginia and in that case I’d contact Rockwool. They will be able to provide you with a precise answer and should have a good tech support line. Sorry I couldn’t be more helpful, but didn’t want to give you bad advice.

  11. Jeff says:

    Can you double up Batts of regular fiber glass isolation on a out side wall in other wards put 2 in one cavity

  12. Erika says:

    I took all the insulation out of my 1978 house and I’m using rockroom to re- insulate. Do I need a vapor barrier with R15 rockwool in my exterior home walls, and soundproof in my interior walls. I am also wondering if my attic needs a vapor barrier. Thank you

  13. Laura says:

    I am reinsulating my cellar. There is already plastic covering the concrete walls and then the studs. Can i put the rock wool bats directly against the plastic or do i need to do something different? Also, can i put the Sheetrock right over the insulation or do i need another layer of plastic separating them. I originally had fiberglass, but it was getting wet from condensation so i want to change to the rock wool. Thanks,

  14. Steve says:

    Excellent info! In an effort to save money, I am using 2X4 16″oc for the interior of my home (2X4 24″oc). Would you suggest cutting down the length of the rockwool or through the width to accommodate the stud cavity of 23″? Thank you for your help I’m sure many of the honey don’ts such as myself appreciate your expertise.
    **Sorry originally posted in the wrong place.

  15. Paddy says:

    I’m looking to insulate the knees walls in an attic space. I’ll be replacing faced fiberglass batt insulation. Will I need to install a facing such as kraft paper? Or can I use the unfazed rock wool?

  16. Travis says:

    Is the comfortbatt ridgid enough to be installed horizontally over fiberglass? This would be for a knee wall in a bonus room attic over a garage.

  17. Mike Kiessling says:

    Hi Jeff,

    I live in Vermont, do you normally see vapor barriers installed on Rockwool in New England? TIA


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