Episode 105

How to cut foam for custom campervan or RV mattresses, beds and cushions


June 14th, 2021

13 mins 6 secs

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About this Episode

👉 New snap knives on Amazon
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In this episode of The Campervan Podcast from Vansage.com, we answer the questions:
• How do you cut a foam mattress?

• What’s the best DIY way to cut a foam mattress?
• What tools are need to cut foam?
• How do you measure and cut foam?

And this applies to us because so many of us are creating custom campervan mattresses and cushions and bed sections. There are a lot of people asking this question online.

I've cut many pieces of foam myself for custom mattresses, bed sections and cushions. And it's not difficult. You can do it.

The best DIY approach to cutting a foam mattress is with a snap blade knife because the long sharp, thin blade will easily cut through the foam while allowing you to maintain a straight accurate cut.

Conversely, the best professional approach to cutting a foam mattress is with a foam cutting tool. Well, here's the thing with foam cutting tools: They cost several hundred dollars. The only advantages to using a foam cutting tool are that the cut will be slightly more straight or clean, and you can make the cut quickly.

So just real briefly, first of all, there's no reason to spend several hundred dollars on a foam cutting tool, unless you're going to get into the foam cutting business.

Don't spend the several hundred dollars for a tool that you'll use once every several years.

And as far as the speed and accuracy of the cuts go, here's what you're going to find:

When you use your snap blade knife, or some of the other tools I'm about to talk about here, you'll find that the cut will be a little bit imperfect.

You'll find that cut is somewhat imperfect, but you're going to cover it up with a mattress cover or a cushion cover. And no one will be the wiser.

The only people who know that my foam mattress cuts are a little bit jagged are me and you. So don't worry about that. It's not a big deal.

Again, the, the characteristics you're looking for in a cutting tool for foam are
• The knife should be thin and long and very sharp.
• So it is possible to use something other than a $2 snap blade knife, such as a chef's knife or a bread knife, but that knife has to be very, very sharp.

Snap blade knives: The nice thing about them is that they provide a long, straight, very sharp, almost razor-sharp edge. And of course you would want your snap blade knife to be new.

So you extend it all the way. If you're not familiar with snap blade knives, the way that they work is the reason that they are called snap blade knives is because you come out maybe it's an inch and you use this section of blade for a while until it's dull and then you snap it off and then can slide the knife out, another click and there's another section of blade there and it's new and sharp.
Well, when you're cutting foam, you just extend the snap blade knife all the way, and it comes out four or six inches or so.

When cutting foam, the length of the knife doesn't have to be the same as the thickness of your foam.

In other words, if you have a six inch piece of foam, six inch thick piece of foam, your knife, even if it's only three inches long, that's fine because no matter what, you'll need to make several passes to make one cut.

So before we get into that, let's talk about how to measure and mark your foam before you cut it.

There are two best ways to mark the foam:

The, the best way to measure it, of course, is just with a measuring tape. Now it's nice to have an upholsters tape, or a sewing tape, just a flexible cloth tape.

I keep one of those around. Sometimes they’re very handy when you don't want a carpenters measuring tape, that's stiff and kind of unwieldy when you're working with soft materials, such as foam or the cover that you're going to be making.

So it measure accurately, first of all, for length and width, however, you're cutting the foam.

And then the two ways to mark: one is with a straight edge. Now, if you've got limited space, you may not want to store a four-foot straight edge or a six foot straight edge in your garage or whatever your workspace is.

You can just use a section of a two by two or you could use a length of two by four. You just want to site down it and make sure that it's very straight before you use that as a guide.

My favorite way to mark foam for cutting is with a chalk line, actually snap a chalk line. It creates a beautiful straight straight line, every time.

You'll probably need someone to hold one end because normally with wood, if you're using a snap line on a sheet of plywood, you can use a small nail or just hook the snap line at the edge of the wood.

And then one person can hold the hold the line tight and reach out and snap… this may not work so great with foam, so an extra set of hands is a good idea.

If you do use a straight edge, then use a Sharpie to mark your phone that way. Sharpie shows up really clear on foam. I keep two or three different colors of the smaller fine pointed Sharpies around.

Now getting back to blades real quickly. Okay. You know, there's a lot of talk online about using electric meat carving knife and some claim that that's the only way and the best way and use the power saw power knife.

If you have an electric meat carving knife, go ahead and experiment with it. See if it works, but really don't go buy one for this job. It's not necessary. For example, my everyday carry pocket knives are so sharp that I can cut foam with them.

Right. So there's an example of just just how much special gear you don't need to cut foam. It's not complicated. It doesn't require special equipment, again, unless you're a professional cutting mountains of foam.

Just use a thin sharp knife and I've tried the electric meat carving knife. It works fine. I wouldn't say that it creates a cut that's that much cleaner than a snap blade knife.

But your mileage may vary. Give a meat carving knife a try if you've got one, or maybe your neighbor has one and won't mind if you use it to cut foam.

So the real trick with cutting a foam, the secret to cutting foam, is that you're going to go slow and easy. So don't expect this to be like a hot knife through butter.

That's not how it's going to go. Uh, let's just picture that you've got a slab of foam laid out. And by the way, this could be normal foam or it could be memory foam. There's also the question all over the place of how do you cut memory foam? Well, memory foam can be kind of gooey and oddly soft and it behaves weird under the knife when you're pressing into it.

But this tip of how to cut foam in general will resolve all of those issues. And that is again, slow and easy.

Imagine you've got your slab of foam out. You've marked it very straight and clean with your straight edge or your chalk line and your Sharpie. Now what you're going to do is, open your snap blade knife up, or whatever knife you're using, and just make a kind of gentle pressure, even motion cut into the foam, perpendicular to the foam and along your line.

Move the blade very consistently and smooth from one end of the piece of foam to the other. And then what you'll see is that you probably didn't go very deep into the foam.

Even if you went in with a four-inch snap blade knife, you're probably only going to cut in on that first pass an inch, or maybe if you're lucky, an inch and a half or two inches.

Now, what you don't want to do is press really hard and try to cut all at once, because what will happen is the foam will bunch up under your knife and it will be really jagged and it'll wander off in some direction. Again, the secret to cutting foam is to go slow and easy.

So once you've made that first. Uh, kind of a gentle test cut then you want to kind of spread that cut open a bit and get the knife right down in the lowest point of the cut, where you've already cut through and do that same cut again. You can kind of spread the foam with your fingers as you're cutting, and you can see how deep it's going and you can make sure that you're staying along your straight line and that you're keeping the knife perpendicular as you cut.

And what will happen is on that second cut, you might end up cutting a little less deep. You don't want to push too hard. Make this nice and slow and easy.

You might find that your foam is cutting really nicely and a little more pressure is okay, and it doesn't bunch up and your knife is sliding through beautifully and you can get your cut done in two or three passes.

But here's the thing: If it takes six or 10 passes to get that cut clean and straight and perpendicular, then take your time. You know, you're only going to do this once and it's for a mattress that you're going to sleep on many, many times for years and years.

So take your time and get it right. And don't be in a hurry. Now all of those rules apply at a much higher level to cutting memory foam. Really take your time and really pay attention to how it behaves under the knife. You know, you might find that your power knife, your meat, carving knife, gums up really quickly with memory foam.

So just skip that if it doesn't work and do it in this slow, gradual fashion, as I've described, with a very sharp snap blade knife.

Now snap blade knives are cheap. So if yours is old and worn out, go to the hardware store or get on Amazon and buy a new one. It is worth it to have a very sharp knife to do this. Especially if you wind up doing a bunch of cushions and bed sections, you'll want a really sharp knife.

So maybe even grab a couple of them while you're at the store, just so you're sure you've got a good sharp knife while you're working. Okay. I've said enough about how to cut a foam mattress. I hope that works for you.

If you have questions about cutting foam or anything else related to van life. Shoot me an email at [email protected]. I'd love to chat with you, answer your questions, whatever.

Maybe I'll even answer your question on the show. All right. Thanks for listening to my foam cutting how-to on The Campervan Podcast from Vansage.com.