How to Insulate External Walls

Introduction: How to Insulate External Walls

About: We love home improvement and enjoy sharing tips on YouTube and Home Repair Tutor. Everything we do is self taught. Over the last 12 years we've bought and rehabbed several rental homes in Pittsburgh. Somet...

Learning how to insulate external walls can save you a boat load of money. And make your home more comfortable.

There are a ton of insulation options out there but Roxul insulation was my choice for our master bathroom remodel. I’ll explain why.

It’s made of Basalt rock and recycled slag. If you don’t know, slag is a by-product of steel production. I live in Pittsburgh, so it’s kinda second nature to know about slag. Little side note: I was a geology major in college for about 1 year…then switched to chemistry. So I geek out on cool things that make your home a better place to live. Today you’ll see why I chose Roxul for our wonky 100 year old bathroom remodel.

Why is mineral wool so awesome? Reason one: Roxul won’t catch on fire until about 2000F. I don’t know about you, I want a chance to run if my house catches fire. Reason two: Roxul is inorganic and therefore not a food source for mold. Mold in a house is BAD, unless it’s in the form of blue cheese. Reason three: it’s water repellent. If Roxul gets damp it won’t lose it’s R-rating, meaning it won’t lose it’s ability to insulate. Cool stuff.

Here are the supplies you need to install Roxul

  • Roxul ComfortBatt
  • Utility Knife
  • Drywall Knife
  • Long Sleeve Shirt (insulation is itchy!!)
  • Gloves
  • Respirator
  • Goggles or Safety Glasses
  • T-Square
  • Measuring Tape

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

Step 1: Start at the Top of the Stud Bay

Hopefully if you remodel your bathroom it’s a bit easier than my project. This situation is strange because the bathroom is over 100 years old and the framing is a bit wonky to say the least.

Fortunately the exterior wall had framing that’s 16 inches on-center. No matter what your situation you’ll have to cut insulation to size, unless of course you’re using blown-in insulation…which is messy. I like starting at the top of the stud bay and placing Roxul flush with the header.

Step 2: Inspect Roxul for Gaps

Roxul should sit flush with adjacent batts and studs, there shouldn’t be any gap.

Step 3: Cut Roxul With a Utility Knife or Drywall Knife

Roxul is super easy to cut. Seriously, it’s easier than cutting bread. Use either a utility knife or drywall saw to cut it. A drywall T-square will help you make clean, straight cuts.

Step 4: Making Room for Electrical Lines and Pipes

You can slice or groove out the Roxul and fish electrical lines or pipes into it.

Step 5: Cutting Roxul for Irregular Framing

Like I said, this house has strange framing…but that’s not a problem. You can cut Roxul to any size you need. For example, this little cheese slice.

Step 6: Insulate Around Windows

Windows are one of the worst culprits when it comes to losing heat or cool air. Roxul made it easy to insulate around our new window and make this bathroom even more energy efficient for our tenants.

Step 7: Test for Fire Resistance (Optional)

Remember how I said Roxul is fire resistant. I decided to try and light it on fire.

I was unsuccessful. Which was cool…don’t try this at home!!

Step 8: Watch the Video Tutorial

Watch my video for the complete step-by-step Roxul installation instructions.

Thanks as always for reading, watching, and making our DIY community super awesome. Without you DIY just isn’t that fun.

Down in the comments tell me what you think of Roxul and how you would use it in your house.

Hope you have a great day,


Be the First to Share


    • Backyard Contest

      Backyard Contest
    • Silly Hats Speed Challenge

      Silly Hats Speed Challenge
    • First Time Author Contest

      First Time Author Contest

    3 Discussions


    3 years ago

    An electric carving knife works wonders for cutting the Roxul. Zero tear. The Roxul safe and sound is also a great sound absorber.

    Nice instructable.


    4 years ago

    lots of great information. thank you.