Post #648: Instructions for making a Filtrete™ mask liner

Posted on April 15, 2020

EDIT:  See Post #807 for a vastly simpler approach to making a mask liner for a typical cloth mask.  Even if you decide to pleat it (as below), the methods in that later post are much easier than what I show below.

EDIT:  By contrast, If you want to make a stand-alone Filtrete mask (not a mask liner) See Post #780 for a better way of creating a high-quality aerosol-filtering mask.  What you see below is an OK retrofit for existing cloth masks.  But if you can find a mask that makes a good seal against your face (e.g., a 3M dust mask),  you’d be better off following the instructions in Post #780 and upgrading that tight-fitting mask so that it filters aerosols.

To see all posts on masks, use the drop-down search menu and pick “masks” as the category.

The white object above is what you are going to make.  The blue “single-use” disposable mask is there to show the size, and to clue you in that the you’re going to make a piece of Filtrete™ into something resembling the body of a pleated surgical mask.


This project takes a piece of 3M Filtrete™ filter medium and makes it into a thin, breathable liner for use with a cloth mask.  The point is to allow anyone who has any reasonable cloth mask to “upgrade” that with a piece of Filtrete™.  Plausibly, Filtrete™’s ability to capture small particles improve a cloth mask’s filtering performance.  But, as I will say several times on this page, I am making no guarantees or warranties.  If you do this, you do it at your own risk.

FWIW, many people have thought to use Filtrete™ as the basis for an expedient mask filter.  You can now find such designs in many places on the internet.

This product is intended as a one-size-fits-all upgrade for those using cloth masks.   Those who merely want a piece of Filtrete™ to put in a filter pocket in a mask can just take one of these and pull the staples out.

That said, I make no warranties or guarantees that this will, in fact, filter anything.  If you do this, you do it at your own risk.

Why don’t I simply use a piece of Filtrete™ material, all by itself?  For Filtrete™ 2500 Once you cut it up, you can see that it has visible, somewhat loose fibers on the back side of the material.  And the pleating of the filter makes it awkward to handle.  So, even though the Filtrete™ fibers are inert, and appear attached to the base material, I wanted to encapsulate it to reduce chance of inhaling a fiber, and to make it easier to handle.

If you still worry about inhaling a fiber, substitute a piece of thin cotton cloth for one side of this, as you make it.  That will reduce breathability somewhat.  As noted at the end, you can’t easily retro-fit a flat piece of cloth to this after-the-fact.  It needs to be worked into the pleating step to have the correct shape.

Encapsulating it this way also gives you a way to fit it to your mask, and attach it to your mask, without cutting or piercing the Filtrete™ itself.  So the broad edge of lightweight material give you something you can cut to shape.

You can find the filtration standards for Filtrete™ material on the 3M website, at this location (.pdf).  To read that and compare to other common standards, you’d need to know that a 0.3 micron particle is an E1 particle.  You can find an overview of many filtration standards in Post #593.

Again, I make no guarantees or warranties that this works.  I am merely providing a set of instructions.  If you do this, you do this at your own risk.

In particular, I am only providing plans for a mask liner because I am uncertain as to the extent that Filtrete™ effectively captures larger particles.  The 3M literature only addresses particles up to 10 microns (about a tenth the width of a thick human hair).  I don’t want to assume that it captures larger particles.  So, you’ll want something else in your assembly — presumably a piece of tightly-woven cloth — to provide some guarantee of filtration of larger particles as well.

A big advantage of this design is that this add-on liner is very breathable.  Chances are the end user will notice no increase in back-pressure when adding this to an existing cloth mask.

For what it’s worth, you could probably substitute a MERV-13 or higher filter for the Filtrete™ filter.  But I don’t think it’s as breathable.  Separately, every MERV-13 filter I’ve found had the metal mesh heat-welded to the filter fabric.  That makes them a pain to disassemble, and tears up the filter medium.

Finally, the only reason you would ever even consider doing this is if you can’t or won’t wear a proper commercially-made N95 mask, or a proper filtration-rates surgical mask.  At present, those are in short supply in the US.  Hence, this is an expedient substitute.  As soon as that mask shortage eases, you would be well advised to chuck this and buy a real mask.


  • Filtrete™ 1900 or higher air filter.  You only need a tiny piece of material for this.  Even a medium-sized filter will give you enough for 40 of these.  Please share.
  • Floating row cover material, or equivalent.  Any thin material that offers minimal air resistance will do.  The material I used is spun-bonded polyester sold in garden and hardware stores as “floating row cover”.  You don’t need to use that, but that’s what I’m going to call it here.
  • Fusible interfacing or Dritz Stitch Witchery or similar.  This is used to “glue” the floating row cover material together by ironing it.
  • Staples
  • Two sheets of thin cardboard.
  • A few feet of aluminum foil.


  • Tape measure or similar.
  • Sharpie (or pen or pencil)
  • Scissors.
  • Office stapler.
  • Iron and ironing board.
  • Thin cotton cloth (used damp, as you iron).
  • Metal ruler, wooden rule, or something like that to use as a weight.
  • A square is helpful for cutting out the pattern, but not necessary.

It goes without saying that all materials and tools should be clean before you start.  And that you will wash your hands thoroughly before doing this.



In a nutshell:  You’re going to seal the Filtrete™ between two layers of floating row cover, by ironing that together using strips of fusible interfacing.  Then use your hands to pleat that like a surgical mask, and staple the pleats in place.

Everything else below is just details.

If someone wants this to use in a mask pocket, just tell them to take the staples out.  Or send them a piece of this prior to the pleat-and-staple step.


1 Strip the filter material out of the 3M Filtrete™ air filter.

Put a mark on the filter material itself, on the fresh are (air intake) side.

Peel off as much cardboard as you can.  Get the metal mesh entirely free from the cardboard.   (You might have to take a knife to the cardboard.)

Once that is done, gently peel the the metal mesh from the white filter material and/or unzip the white material off the metal mesh.  You will mess up some spots on the filter cloth due to (e.g.) heat-welding at the edges, and a few spots of glue elsewhere.  Just avoid those as you cut this up for mask liners.

As far as I can tell, there is obvious way to do this.  Just try to do as little damage as possible to the filter medium.

Why mark the fresh air side?  I’m not sure this matters or not.  By eye, the Filtrete™ 2500 seems to have two distinct sides, one of which is more paper-like, one of which is is more fiber-like.  This stuff filters particles in a truly science-fiction fashion, and I don’t know if the orientation with respect to air flow matters.  But on the off chance that it does, you might want to note which side is the air intake (fresh air) side, then keep the fresh air side up in step 8.

2  Make two cardboard patterns.  I patterned mine after some cheap single-use non-medical masks.  (The blue mask, pictured above).  Measuring those, I found that they were 6.75″ wide, and (unpleated) 6.5″ tall.  So that’s the standard I’m copying.

Cut one piece of cardboard, 6.75″ x 6.5″, as the pattern for the Filtrete™.

I am not sure it matters, given how close those dimensions are, but I did pay attention to length versus width, due to the orientation of the pleats in the Filtrete™ material.  E.g., put an arrow across it, in the longer (side-to-side) dimension, and write “pleats” on it, or similar.

Cut a second piece of cardboard 10.75″ x 10.5″ (or thereabouts) for the thin cover material.  Exact dimension and exact cutting of this material does not matter.

3  Cut your Filtrete™ and your cover material.

Put the pattern on your Filtrete™ so that the long dimension lines up with the direction of the pleats.  Cut that out.  And cut out two pieces of the thin cover material.

4  Cut thin strips of fusible interface, or cut appropriate-length pieces of Dritz Stitch Witchery or equivalent.  An inch or so in width is adequate.  The only mistake you can make is making them too wide.  If they extend beyond the edge of the floating row cover fabric, you’ll end up gluing your damp ironing cloth to the Filtrete™ package you are assembling.

5 Create a heat shield and assembly aid by covering your Filtrete™ cardboard pattern in tin foil.

6  Arrange the whole sandwich of materials on your ironing board.

This is, frankly, a pain.  This is the step where you pay the penalty for using a bunch of lightweight materials.

There has to be a better way.  But this is what I finally settled on.

Put down one piece of the thin, floating-row-cover material.  Center your Filtrete™ on that.

Roughly arrange your fusible interfacing strips around (not on) the Filtrete™.  As if you were framing a picture.

Weight it down with something thin and flat, running perpendicular to the pleats in the Filtrete™ to keep that flat.  I used a steel ruler, but something along those lines

Fine-tune the location of the fusible interfacing.  It’s perfectly fine if it overlaps, because it’s going to melt at the next step.  You are looking to get a good seal all the way around the Filtrete™.  A continuous line of fusible interfacing is the goal. Snug it up next to, but not overlapping, the Filtrete™.

Place the other sheet of floating row cover over that.  Put your hand on it, and withdraw the steel ruler or equivalent.

Carefully place your heat shield on it, in place of your hand, centering the heat shield so that it neatly covers the Filtrete™.

7  Iron it.

Get a thin damp cloth (thin, so that you can see where the edges of the heat shield are, damp because you want to steam the fusible interface.  Something on order of a handkerchief or bandana.  Not a towel. Cotton preferred.

Cover that “sandwich” with the thin, damp cloth.  Using an iron set to “wool”, hold the heat shield down and iron around the edges of the heat shield.  Iron as directed by the fusible interfacing manufacturer.  A few seconds of hissing steam in every spot seems to do the trick for me.

Near as I can tell, this step is bulletproof.  With cloth this thin, I had no trouble getting the fusible interfacing to seal.

8  Pleat it and staple it.

I can belabor this step, but by far the best thing you can do is get a picture of a surgical mask or single-use mask, and study it a bit.  You will find that, for the great majority of them.

  •  they have three pleats (some have more)
  • the pleats lie flat when the mask is flat
  • the mask expands toward the side that you pleat from (front vs. back)
  • the pleats are all oriented in the same direction
  • the openings of the pleats face downward when worn (bottom vs. top).

That’s what you are trying to replicate.

As noted, this stop determines what the front of your mask is, and what the bottom (chin) is.  So you need to pay attention to a couple of details.

Mark the target width you are trying to achieve on a piece of cardboard.  Mine was 3 and 5/8ths inches.  You are trying to pleat this so that it matches the flat dimensions of whatever mask you are using as your pattern. Draw a couple of parallel lines, that far apart, on the back of one of the cardboard patterns.

The outside of this mask is the side that you are looking at, as you pleat it. Because of this, you must flip your stapler upside-down when you staple the pleats in place.  Otherwise, you end up with the raw ends of the staple against your face.  (Alternatively, flip the pleated assembly before you staple.)

Optional:  Refer back to Step 1. If you think it matters, now is the time to orient this so that the fresh air side of the filter material is facing upThis is the step where “up” becomes “outside”.  Decide for yourself whether that matters or not.  I didn’t bother.

Pleat it.  With the filter-and-fabric “sandwich” sitting on the cardboard that shows the final size, take both hands, and put in some pleats. In my prototype, I just put in two large pleats, following the pleating from the filter.  This is quick, easy, gave me about the right size, and it’s certainly what the filter medium “wants to do”. It seems to work.

If you have the dexterity to put in three smaller pleats, that’s probably better.  I didn’t.

Staple it. When you have your pleats in place, so that the pleated cloth assembly matches the width (in my case, 3 and 5/8ths or so), keep the assembly flat with your hand, push one edge over the edge of a table, turn the stapler upside-down, and staple.

Flip it around and staple the other edge. Again, with the stapler upside down.

Mark the outside and top.  Do your end user a favor at this point.  Take a sharpie, on the edge material, mark what is the outside, and what is the top.    Outside is the side you pleated from.  Top is the side that the pleats fall away from.

9 Final pressing.  Put the damp cloth over it, and press the pleated portion of the floating row cover flat.  Do not iron over the Filtrete™ portion of the mask liner.

Use, care, longevity.

Use:  These can be mailed flat.  The user will unfold it, just like a surgical mask.  Flex it a bit, and fit it inside their cloth mask.  Cut away the excess to achieve best fit.   And wear it underneath their cloth mask.

Ideally, fit this into the cloth mask so that the metal nosepiece of the cloth mask sits atop the Filtrete™ itself, so that if seals the Filtrete™ to the bridge of nose and cheeks.  If your cloth mask does not have a metal nosepiece, you should make one and use it.  There are plans for that all over the internet.  The filtration of surgical-style masks is compromised by air leaks.  If you don’t seal the mask tightly to your face around the bridge of the nose, you’ll end up breathing around the mask, instead of through the mask.

If the end user looks at it, and then won’t use it because they are afraid they might inhale a fiber from this, there is no good solution.  The end-user can’t just add a flat piece of cloth behind this and expect it to work well. Any such cloth needs to be worked into into the pleating process, so that it will conform to the 3D shape when the mask liner is unfolded.  And, to be clear, this mask liner, built this way, is so breathable that it would be a shame to ruin it by inserting some less-breathable cloth into the process.

Care.  My opinion is, do not wash this material.  I know that actual N95 masks deteriorate from washing.  I have no idea what washing does to Filtrete™. I wouldn’t do it.  If you are worried, set it in an open paper bag for a couple of days between uses.  That’s what hospital workers are doing to maintain N95 respirators.

Longevity.  I have no idea about longevity at this time, but my guess is these should be good for hundreds of hours of normal breathing.  I will do that calculation, based on the rated three-month filter life, and report that out here, at the end of this page, when I am done.  If the thin “floating row cover” rips, and exposes the raw filter material, please toss it and make a new one.

Option for mass manufacture

Cottage industry.  At this point, I have no clue whether there’s any demand for these or not.  But if so, this practically begs for a cottage industry approach, taking advantage of our home-bound idle work force.

Cutters:  Drop off a bolt of floating row cover, and a template, at the home of Person A.  Pick up a bag of cut pieces at the end of the day.  Ditto, Filtrete™.  Ditto, fusible interfacing.

Assemblers/ironers.  Shuffle supplies from the cutters into lots adequate for N masks.  Drop bags of all three materials, at home of Person B.  Pick up bags of encapsulated Filtrete at the end of the day.

Staplers/mailers:  Drop bags of encapsulated filtrete assemblies with envelopes and stamps at the home of person C, and they take the finished results to the US Post Office.

True bit of economic history.  The person who shuffles the materials around is the key player in cottage-industry manufacture.  That person organizes everybody else’s work, but at root, that person is just a glorified go-between.  They don’t actually produce anything.   They just take stuff between the cottages, keeping everybody else working.  In French, between is entre, to take is prendre.  That “bewteen-taker” person, that’s the entre-preneur.

If you had enough Filtrete™, or maybe even just MERV-13 or better filter material, some willing cottage workers, and the entrepreneur, seems like a group of people could crank out quite a few of these in a relatively short time.  Let’s hope the mask shortage is over before anybody actually has to do that.