Five MERV-13 filters, measuring 20 inches by 20 inches and 1 inch thick. $100 if you’re in a rush, like me. ($50 if you buy in bulk).
20-inch window fan, $19. (Any will do — Use an old one to cut the project cost.)
A roll of adhesive weather-stripping, $4.
Duct tape, $3.
Hold two of the filters at right angles, butting the edge of one up against the face of the other while your partner applies a 20-inch length of duct tape. Be sure to make a tight seal.
Add the other three filters, one by one, until you have a five-sided box, leaving an open side to accommodate the fan. The arrows indicating the direction of airflow (printed on the filter edges) should point inward, toward the fan.
Note: With five filters that measure 20 inches square, a couple of the edges in your box won’t line up quite right. You have to play with it a little, sealing off gaps with duct tape.
Rosenthal had a better design, combining two different sizes of filters. All of them were two inches thick, but two measured 16-by-20 on a side, and three were 20-by-20. Those dimensions allow for a perfect snug fit.
Home Depot didn’t have either of those sizes, so Meyer and I made the best of it.
Lay the fan face down on a table, so the back (the side through which air gets “sucked”) is facing up.
Line the outside edge with adhesive weather-stripping, creating a sort of gasket.
When you attach the box of filters in the next step and turn on the fan, the air flow will pull the box against the gasket, helping to maintain a tight seal. Airborne particles are captured in the filters as air is sucked through.
Another option is to attach the gasket and filters to the front of the fan — in other words, capturing particles by blowing the air through the filters. But in that design, the airflow pushes the filters away from the fan. Over time, the seal made by the duct tape will weaken.
Place the box on the gasket.
Seal the edges with more duct tape.
Double-check to make sure there are no gaps.
Stand the fan upright, place it in the center of the room, and …
We first tried the fan on “low” setting, then bumped it up to “medium” as Rosenthal recommended. It was fairly quiet, though it vibrated a bit.
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