How to Insulate Concrete Garage Walls (DIY Project)


Now that I’ve framed my garage wall, the second step in my garage transformation is to insulate my garage walls. The entire garage insulation project was really easy. It can be done in about an hour as long as your studs are lined up correctly.

If you missed the first post in this series, you can check them out using the links below:

I highly recommend you check them out so you can see the entire project, from start to finish.


Should You Insulate Your Garage?

A lot of people skip the insulation step and just go straight to the drywall. Sure, that makes your garage walls pretty, but it doesn’t do anything to regulate the temperature.

One of my goals in this project was to make the garage more livable in the hot Florida summers.

Adding insulation will do two main things: help regulate the temperature and add some soundproofing as well.

You might not think about insulation keeping noise down in your garage, but it definitely helps.

Sound vibrations travel in waves, so any hard, smooth surface with a lot of mass will reflect the sound back. The insulation that we’re putting behind the walls will actually help to absorb some of those vibrations. It won’t be as good as soundproofing, but it’s a start.

But the big benefit is to regulate the temperature. Adding insulation won’t make your garage a perfect temperature by itself, but it will take the edge off the freezing winters and the sweltering summers. 

I wrote a big article discussing whether you should insulate your garage. Check it out for more details and specific recommendations!


What Type of Insulation to Use in the Garage

The most important thing about insulation is this.

The higher the R-Value of the insulation, the more effective it is at keeping the heat where you want it.

However, you don’t necessarily need to chase the highest R-value possible when you insulate your garage.

For example, an R-Value of 16 is only about 5% more efficient than insulation with an R-Value of 8.

Be sure to get exterior wall insulation. Garage walls are exposed to the elements, so you want to choose an insulation that was designed with that in mind.

I chose to use Johnson Manville TempControl mineral wool unfaced batt insulation with a rating of R-15.

What garage insulation I used

There’s a lot to that, so let me break it down.

Mineral Wool vs Fiberglass Insulation

Mineral wool insulation is similar to the fiberglass insulation that we know and…well…tolerate.

In general, you can get mineral wool with slightly higher R-values, but not enough to make that a major buying decision.

Faced vs. Unfaced Insulation

Unfaced means there’s no paper barrier on one side of the insulation. Basically, you want to use unfaced insulation when you don’t need a vapor barrier or have a separate one.

Matt Risinger, a builder in Texas on the cutting edge of building science (yes..that IS a thing), has a great video about whether or not you need a vapor barrier.

In Florida, I do not, so I opted for unfaced insulation.

Batt vs. Roll Insulation

Batt insulation is similar to a roll of insulation, except they’re cut in strips instead of one continuous length. In this case, the strips are sixteen inches wide x four feet long. Batts are easier to install than rolls because there’s less cuts to make, but they’re more prone to air leaks since it relies on the installer to get a good fit.

Finally, the insulation is rated at an R-15, which is pretty good, but not excellent. Since my walls both face other buildings and aren’t exposed to direct sunlight, I didn’t feel it was necessary to go crazy high on the R-Value.

Remember, at a certain point, there are diminishing returns. Get the insulation that works for your garage, and your area of the country. Don’t overspend.

John Manville R-21 Unfaced Fiberglass Insulation Batt 15 in. x 93 in. (10-Bags)
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How to Insulate a Garage (Step by Step)

Here’s my step by step process for how I added insulation over my concrete garage wall.

Step 0: Last Chance to Waterproof Your Wall

I wrote a tutorial on how to waterproof and seal your garage walls using Drylok. If you didn’t do that before you put the frame up, you can still do it now, although it’s going to be harder since the frame is in the way.

But this is your last chance to take care of that before the walls are covered.

Step 1: Seal Up Any Gaps

Even the smallest cracks can cause drafts. So we want to seal up any gaps before we insulate our garage wall.

Air sealing the walls is different than insulating them, but they go hand-in-hand.

You’re spending your hard-earned money to insulate your garage. Sealing the gaps behind the walls will set you up right from the start.

What Type of Air Sealant To Use

I used a combination of a couple of different sealants. Partly because I didn’t buy enough the first time and partly because I found one that also included an insect repellent. That’s a great added benefit for the areas along the floor and garage door to help keep bugs out of your garage.

It comes in both “big gap” and “small gap” varieties. The “big gap” is for any gaps larger than one inch that you want to fill. For everything else, use the “small gap” spray.

Depending on the gap you’re filling you may need a can of both. I definitely did.

Great Stuff 3-Pack Insulating Foam Sealant Big Gap Filler
$27.20
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10/18/2021 03:13 pm GMT

How to Use Air Sealant (Cleaning up the Mess)

The first thing you want to do is spray the sealant in every gap between the frame and the concrete.

I like to start on a small area and spray a little bit, just to see how the spray expands.

Ideally, you want the sealant to look like a solid bead that runs along the gap between the frame and the concrete, like the picture below.

As you can see, the sealant bubbled up and expanded over the edge of the frame.

Be sure to clean that up before you put the insulation on top.

In other areas, the spray sealant wrapped around the edges of the frame and I needed to trim it with an X-acto knife. This image was really sloppy, and it was a pain to trim and sand it once it dried. 

In this corner, I trimmed out the outside edges flush to the frame and I cut out space in the corner to allow for the insulation to fit flush.

You want the sealant to look as uniform and neat as possible, but don’t be shy about making a mess ether. We can trim off any excess and make it neat. It’s just a little more work we’ll have to do later.

Remember, we don’t get points for neatness. We get them for keeping drafts out of the house.

Step 2: Safety Precautions

Here’s my little safety warning.

Wear long sleeves and gloves when you install insulation. Fiberglass insulation is made up of small particles of glass which can irritate the skin.

Even if you’re using mineral wool insulation like I was, it’s still a good idea to keep exposed skin protected.

Step 3: Insert the First Piece of Insulation Near The Ceiling

The entire process to insulate my garage wall was one of the easiest things I’ve done around the house. It took less than an hour, and that includes cutting pieces to fit.

I recommend starting from the top corner and working your way down. Batt insulation is designed to fit snugly in between your wall studs, as long as your studs are a standard width (either 16″ or 24″).

The insulation needs space to breathe. Actually it needs to trap the cold air between the exterior wall and not let it through to your garage. Its the same thing in principal.

To do that, it’s critical not to compress the insulation in to fit a space. When it doesn’t fit snugly in the space you’re trying to put it, cut it to fit using your knife. Important note: Never compress the insulation!

Laying the first strip of insulation to my garage walls

You want to make sure that the edges of the insulation fit flush with the edges of the garage wall stud. That way, when the drywall is mounted on top, there’s a minimal gap between the edge of the drywall and the edge of the insulation.

In the image above, note how almost all of the edges are flush except for the top center. If you have a section that’s pressed in slightly like that is, just pinch the insulation and try to fluff it out a bit. You don’t want any space where large pockets of air can be trapped.

Step 4: Continue Down the Wall Stud Until You Reach The Floor

The top row of insulation is easy. The challenge comes when you’re putting the next piece of insulation in place underneath it.

You’ll use the same techniques to make sure there’s a clean fit with the piece above it.

Just slide the next piece of batt insulation about 3″ underneath the one above it. From there, gently slide the insulation up the stud until it rests against the batt above it.

I’m going to sound like a broken record, but your goal here is not to compress any of the batts. If you do, they’ll lose their ability to keep the outside air where it belongs – outside.

Adding insulation to my garage walls

In the image above, you can see one join that worked out perfectly (on the left) and one that had to be fluffed out a bit (on the right).

Obviously, you’re going to want everything to fit snugly like it does on the left. But even if it doesn’t you can still adjust. On the right side, I’d also pull out the right corners next to the stud so it’s as smooth as possible.

You won’t be able to get every join perfect on the first try. It’s worth going back and taking a second look once you’re all done.

Remember, we want to have as smooth a surface as possible for the drywall to lay on top of.

Step 5: Cut the Bottom Insulation Pieces

If you’re lucky, it’ll all work out perfectly so you can use full lengths of batt insulation for your garage walls.

That never happens by the way.

You’re always going to have to cut insulation to fit at some point. For me, it was a little 2′ section at the bottom.

If you need to cut insulation to fit, grab your box-cutter and a T-square or level and a tape measure. You want to measure our just enough so that it’s a snug fit and not leave any gaps. But you also don’t want to compress the insulation. I know, I sound like a broken record. But this is important. You can’t fix it later without ripping out the wall.

Cutting insulation is easy.

All you need to do is measure out the length that you want and then use your T-square or level as a guide. Then use your boxcutter or X-acto knife to cut the insulation to fit.

This is the one time that I’ll say not to worry about compressing the insulation. Before you mount it in the wall, just fluff out the edges and you’ll be fine.

Step 6: The Finished Product: An Insulated Garage Wall

Depending on how many pieces you have to cut, actually installing the insulation will only take an hour or so per wall of your garage.

How to insulate a garage wall over concrete (step by step)

How Much It Cost to Insulate my Garage

For one garage wall (20′ long x 10′ high) my total cost for sealant and insulation was under $200. Not too bad when you consider that finishing your garage will increase your home’s resale value. It’s a small investment now that will pay off when it’s time to sell the house.

Spray Sealant

Since this was my first time ever using spray sealant, I ended up using four cans. I didn’t understand just how much this stuff expands once it dries and I ended up being messy in some places.

I could probably have gotten away with three cans, but I’ll do the calculation with four just to be consistent.

Total cost: $24

Insulation

Each pack of insulation will cover just over 34 square feet so I’d need 6 packages. Accounting for a little waste from cutting, I bought 7 packages at $22 each from Lowe’s.

Total cost: $154

Grand total: $178


Conclusion

By itself, insulating one wall isn’t going to make that much of a difference. The wall on the other side of the garage is my project for next summer, so I’m looking at this as one step in my garage renovation.

Overall, I’m really happy with the way that the insulation turned out. Even though I only insulated one of my garage walls, I can definitely feel the difference in temperature during the summer.

Next step is to drywall the garage. Stay tuned!

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