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Building Science

Improving Indoor Air With a DIY Portable Air Cleaner

You will need a box fan, MERV-13+ filters, cardboard, and tape

The assembled DIY high-MERV portable air cleaner

Twitter is an utter hellscape in some ways, but it does have a few redeeming qualities. I’ve seen a lot of good information about the COVID-19 pandemic there, including ways to filter the indoor air for coronavirus and the emerging science about the spread of SARS-COV-2 via aerosols in addition to droplets. On the filtration front, one thing I started seeing on Twitter over the past few months was designs for portable air cleaners you could make with a box fan. Of course, the easiest way to do it is just tape one filter to the back of the fan. To reduce the pressure drop, though, you can attach two or more filters to the fan, and Shawna Henderson of Blue House Energy in Nova Scotia discussed the ones I’ve seen in this nice article: 3 Principles for Good IAQ: Eliminate, Ventilate, & Filtrate.

The one with four filters is the one I like best, so I made a couple of them, one for my home and one for the Energy Vanguard office. Last month, John Semmelhack sent out the tweet below of this four-filter design by Neil Comparetto, a top-notch home performance contractor in Virginia.

Portable air cleaner with MERV-13 filters - the Comparetto Cube
Portable air cleaner with MERV-13 filters – the Comparetto Cube

It’s a modification of a design that I think Dr. Richard Corsi (of Corsi Code fame) promoted. Corsi’s version uses five filters, which means you have to have a base or some other way to support it (or else have one filter that doesn’t get any air). Comparetto used four filters, with the bottom side covered by cardboard. Since it has six equal sides, we can call it the Comparetto Cube.

The cardboard base allows it to sit on the floor without any other support. It also blows the air straight upward, and that makes it ideal for when you have company in your home. Just set the Comparetto Cube in the middle of the room, and if anyone does happen to be sick, a lot of the droplets and aerosols should get pulled into the fan and filtered out. The photo below shows it in use at my house on a recent visit by my mother-in-law.

A high-MERV portable air cleaner can reduce your risk of getting COVID-19
A high-MERV portable air cleaner can reduce your risk of getting COVID-19

How to make a Comparetto Cube

I probably don’t need to give any instructions here because it should be obvious what to do, but here you go. First, get the materials you need. I ordered the 20″ box fan and 20-in. by 20-in. MERV-13 filters from Amazon for a total cost of $90.82. The box fan (Comfort Zone CZ200A) was $25.20, the filters (FilterBuy) $65.62. Once you have those, the only other things you need are tape and scissors or a box cutter. I used the box the fan came in for the piece of cardboard that goes on the bottom.

All you need for this high-MERV portable air cleaner are a box fan, filters, tape, and cardboard
All you need for this high-MERV portable air cleaner are a box fan, filters, tape, and cardboard

The first step is to tape the filters together. I used little pieces of tape at the corners to hold them together first and then covered the seams with long strips. Then I went back and double-taped the corners for a little extra reinforcement.

Two notes: First, make sure you tape the filters together with the arrows pointing into the center of the square. The fan will sit on top, pulling air in through the filters and sending it up and out through the top. Second, arrange the filters in a square, as you see below, not a rectangle.

Arrange the filters in a square, not a rectangle
Arrange the filters in a square, not a rectangle

Next, cut a piece of cardboard the size of the filter square and tape it to one end. Energy Vanguard’s Luke Bertram is taping the cardboard bottom onto the filters in the photo below.

Luke assembling a high-MERV portable air cleaner for the Energy Vanguard office
Luke assembling a high-MERV portable air cleaner for the Energy Vanguard office

Finally, turn the filters over and tape the box fan to the other end. I used blue painters’ tape, but you can use duct tape if you’d like. (Just don’t use duct tape on ducts!) The photo at the top of this article shows the assembled DIY MERV-13 portable air cleaner.

How to stay safe from COVID-19 in your home

The No. 1 thing you can do to prevent the spread of COVID-19 in your own home is not to have other people in it. My wife and I have done that since the middle of March now, with only a few exceptions. We’ve had a few people come into our house, though, and we’ve worn masks, run the ventilation more, and opened windows when the weather is nice enough. And now that we have a Comparetto Cube, we run it whenever others are in the house (with masks on) and for a while afterward. As mentioned earlier, keeping the portable air cleaner in the middle of the room should greatly reduce the risk of transmission, if someone happens to be infected.

Filtration, whether with a Comparetto Cube, masks, or HVAC filters, isn’t the whole answer, of course. Social distancing and ventilation are also really important. Now that we have nice fall weather in Atlanta, we occasionally have people over in the backyard, spaced about 10 ft. apart.

So yeah, Twitter is a hellscape. But in addition to the good building-science community there, I’ve found other gems. Paul Bronks (@SlenderSherbet) puts the best captions on animal videos. The Auschwitz Memorial account (@AuschwitzMuseum) provides sobering reminders of how bad things can get when we lose sight of the humanity of others. Stewart Brand (@stewartbrand) of Whole Earth Catalog fame puts out some of the most intelligent social media posts anywhere. From him I discovered the novel Stoner by John Williams. That tip alone was worth all the pain.

Who knows? Maybe you found this article on Twitter and have built your own Comparetto Cube. Maybe you even use it when you’re reading Stoner.


-Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard. Images courtesy of the author.



  1. lilingyoung | | #1

    Good stuff, Allison. I'm sending this to my peeps in California and Washington who are coping with wildfire smoke. I hear you can't find an air cleaner for any amount of money. Thank you.

  2. JC72 | | #2

    Smoke test this !!!!

    I wonder how well this cube would work at filtering out cooking odors in a kitchen. hmmmmmm.

  3. pjpfeiff | | #3

    Any guesses as to how wide of an area this would treat? I'm picturing a column of convective airflow, say, 8 ft wide that gets continuously recirculated (meaning that the rest of the air in the room is basically unfiltered). Note that I have positively nothing to base that on other than gut feel.

    1. JC72 | | #9

      I was thinking the same thing.

  4. Jon_R | | #4

    I predict that the fan (specifically the bearings) will last longer if the fan is vertical (as it was designed to be).

    I've found that a negative ion generator increases filter effectiveness. Causes particles to want to stick. But no measurement data.

  5. jkonst | | #5

    This is essentially the same thing as the air purifiers I have in my apartment (2 x BlueAir 211+, which have helped a lot with my wife's asthma), but for way less money upfront and ongoing. Thanks for the tutorial!

  6. exeric | | #6

    This is a fantastic DIY project and so useful at this time. It gives me the hope that small family gatherings during the upcoming holidays will actually be possible as long as common sense is used and you aren't overly reliant on this filtering device. Thanks for this!

    1. capecodhaus | | #7

      Hima mica muda tuda Did a mduda fucuda.

  7. exeric | | #8

    Just wanted to add that I've put my money where my mouth is and ordered the filters and fan described in this article. I look forward to putting them together in the near future. Thanks for the idea, Allison.

  8. steve_smith | | #10

    While previously looking for MERV-13 filters on Amazon nearly all the non-3M branded filters had reviews questioning the MERV-13 rating. Is there a way to be reasonably confident in the rating on these (or other filters)?

    1. exeric | | #11

      I think I may owe you one for your observation. When I ordered my filters on amazon I just looked at the proportion of 5 stars and didn't look farther. When I did look at the comments I saw the exact same thing you did. I am returning them and have ordered replacements at I ordered these: 20x24x2 (19.5 x 23.5 x 1.75) Accumulair Diamond 2-Inch Filter (MERV 13). I can't vouch for the accuracy of the merv rating but I've had good luck ordering from that company previously.

  9. mr_K_Green | | #12

    Use 5 filters... put a lightbulb inside... suspend it as a ceiling fan/light. You could even incorporate some sort of air redirection that sends the filtered air out to the edges of the room creating a mushroom-shaped convection cycle. Red Green would be proud!

  10. BeckyCR | | #13

    How often do you change the filters?

  11. michaelbluejay | | #14

    ConsumerLab notes that a four-filter setup improves CADR by only about 50% or less, so it's more efficient to run two single-filter fans than one four-filter fan.

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