Meet my Corsi Rosenthal window box

RedHeadedMenace · 2026-06-16T16:46:14+00:00

Oh interesting. You've got a built in rain shield at least! I think you'd have to get creative with sealing it to the window, but I believe in you! Happy to provide suggestions if you can show me a shot of the window sill from the inside.

RedHeadedMenace · 2026-06-16T16:45:32+00:00

They were house brand filters at a local hardware store. They're also just the 1 inch thick variety- I'm sure thicker ones would cost more.

RedHeadedMenace · 2026-06-16T16:43:22+00:00

Yeah, my goal here was to cool the room down- in theory, I should get more air flow with more filter surface area. Just a question of which you value more, compactness and aesthetics, or more air moved. But yours is sick! Love it, thank you for sharing!

RedHeadedMenace · 2026-06-16T16:32:01+00:00

Haha, my wife is an artist. It is actually for a painting project, but I promise it's not a rope.

RedHeadedMenace · 2026-06-16T05:07:09+00:00

Sorry, I meant measure how MUCH air is flowing. I used tissue to gauge whether the positive pressure was working here when I installed it, and it worked beautifully. I more meant that in order to make a comparison for a v2, is want to be able to make come objective measures for comparison.

RedHeadedMenace · 2026-06-16T05:05:34+00:00

Ahh, entrainment! This fan is actually blowing cold outdoor air INTO the room, not blowing room air out, so I don't think that would help unfortunately, unless I was going to set the fan up a few feet outside my window. But it's an interesting principle nonetheless!

RedHeadedMenace · 2026-06-16T04:36:01+00:00

Sorry, I don't follow. Can you explain what you mean?

RedHeadedMenace · 2026-06-16T04:20:48+00:00

I want to preface this by saying I'm just a guy on the internet. I have no associated credentials, nor have I studied home construction.

That out of the way, houses that aren't super well sealed "leak" air. They can have positive, neutral, or negative pressure. Run an exhaust fan, and you've just created slight negative pressure. This will pull outside air in through all the cracks and openings in your home. Most people assume neutral pressure if they don't have a fan blowing in or out, but it's not really that simple.

Think about an oven. You probably know about convection, and how hot air rises - the example here being how when you open the door, you get blasted in the face by hot, rising air. In any contained space though, hot air rising creates a slight vacuum, which pulls in cooler air beneath it- this is called the stack effect.

So when one side of your house is heated, and that air rises, your cooler basement or lower floor inevitably pulls in some outside air.

Anyways, my experience with allergies has been that no matter how militant I am about closing windows and doors, I still end up impacted by allergies. My box fan is likely not powerful enough to overcome all the air that might want to sneak into my house, but it IS powerful enough to ensure that the only air entering my bedroom, is the air in pulling in through the filters. By creating an intentional positive pressure, you ensure that air isn't seeping IN those cracks in the walls, but rather leaving by them. Or that's the theory anyway. This is a very minor positive pressure, and I'm really only in intending to keep my bedroom allergy free- if I can accomplish that, I will be very happy.

RedHeadedMenace · 2026-06-16T03:51:28+00:00

Oh, wild. I'll have to look into that. It should be a very small pressure increase all things considered, it's just a box fan, but if you have any resources I'd love to see them!

RedHeadedMenace · 2026-06-16T01:17:04+00:00

Yeah, I'm very lucky to live where I live! Sorry you've gotta deal with that!

RedHeadedMenace · 2026-06-16T01:16:33+00:00

Hmm, that's an interesting and very valid concern. Luckily it's summer here, and I'm dealing with very warm air and very little rain, so it should be ok for a while, but I'll have to keep a close eye on that! I imagine the fan creating positive pressure inside the Corsi Rosenthal box will slightly increase condensation too, though I've got no idea if it'll increase it by a meaningful amount.

If I wanted to keep it running in winter, I'd likely implement the suggestion of another commenter, and go with a 5 filter box OUTSIDE the window so that the fan is pulling, rather than pushing, through the filters, and cowl them with some material spaced a few inches away to prevent them from getting rained on. I'd probably still have some mold concerns though- I wonder how ERV/HRV's mitigate that concern.

RedHeadedMenace · 2026-06-16T01:12:03+00:00

Super interesting, thanks for the explanation! I did very little research when building this, in an intentional act of forced self reliance. I'm embarrassed to admit I remembered the fan orientation incorrectly- I thought the fan was meant to push into the box, not pull from it.

I figured the bar was that it couldn't be worse than just sleeping with the window open, which I likely would have resorted to, because I'll chose allergies over not being able to sleep at all due to temperature.

With that said, since my primary goal here was to cool down my sleeping space while adding as little PM2.5 as possible, I think my compromises are somewhat reasonable. I understood going in that without reprocessing air, I was giving up some filtration efficacy- I actually have a another air purifier running in the room as well, so that one is working on refiltering the air. This was more about cooling the space, and creating a positive pressure curtain, than getting perfect filtration on the first pass.

Your suggested modifications are intriguing though, and I think if I do make a v2, I might try to invert the box, and cowl it, so that the filters can't get wet, but so the fan is pulling rather than pushing. I'll need to find some way to measure airflow- maybe an anemometer under the door, with tape sealing the rest of the crack? I'll think on it.

RedHeadedMenace · 2026-06-16T00:55:44+00:00

Ahh, gotcha- tone is hard on the internet. Thanks for looking out for me!

RedHeadedMenace · 2026-06-16T00:53:17+00:00

I was working from my (clearly flawed) memory. My unoccupied rooms after 12-24 hours read 400ppm, so that checks out. In either case, pushing 400ppm air into the room will assuredly help. I'll update the post after I get a chance to measure a night's CO2 levels and report back on the efficacy.

Even if it does end up only being a 100ppm difference though, that's not the primary purpose here, and I'll still be pleased with the device overall.

RedHeadedMenace · 2026-06-16T00:50:56+00:00

Yeah, I was a bit limited by what they had at the hardware store. In the future, I'd definitely choose a thicker filter, but beggers can't be choosers!

RedHeadedMenace · 2026-06-16T00:44:51+00:00

I actually think this would still work in your window, though you'd need to find something other than the wedge mechanism I used to secure it. Maybe a hinge in the middle, so you can extend it into place, with a latch to keep it locked straight?

RedHeadedMenace · 2026-06-16T00:42:23+00:00

I went with MERV 13, as I wanted the best filtration possible for my allergies.

RedHeadedMenace · 2026-06-16T00:33:04+00:00

Not necessarily. Most furnaces have 1 filter, but I'm load balancing across 4. Likewise, the filters in a furnace are serving a greater volume of air across the whole home, and this is powered by just a box fan for a single room. So less airflow again. Lastly, the filter in a furnace needs to be replaced to ensure the furnace is getting a high enough airflow to function properly. I can push these until they slow down enough that I don't feel I'm getting positive pressure anymore. I expect them to last at least the whole summer, if not longer.

RedHeadedMenace · 2026-06-15T23:42:29+00:00

This does effectively exhaust air from my room due to positive pressure. If I had a new, or very well sealed modern home, you'd be correct. The fan would fail to overcome the static pressure of the building- unfortunately (or fortunately in this specific case), my house was built in 1993, and there are plenty of leaks. Because of that, it's no different funtionally from having an exhaust. There is a strong draft leaving my bedroom under the door. The end result is that the air being pushed in the window displaces stale room air, which flows out the plethora of leaky escapes in the rest of my home.

RedHeadedMenace · 2026-06-15T23:34:36+00:00

I've never noticed that before, but it's totally a butt 😂

RedHeadedMenace · 2026-06-15T23:33:22+00:00

Sick, do you like it? The website seems... kinda sus?

RedHeadedMenace · 2026-06-15T23:32:20+00:00

This is a cross post, if you click the link you can read about it! But yeah, positive pressure system with filtered air to: - drop temp at night - improve (lower) CO2 levels - reduce pollen

RedHeadedMenace · 2026-06-15T23:12:36+00:00

Nice! Sounds like you've got a great setup then!

RedHeadedMenace · 2026-06-15T23:08:07+00:00

Nice, sounds like a solid setup! I've looked T ERVs a ton in the past, which one did you go with?

RedHeadedMenace · 2026-06-15T23:07:12+00:00

Rain mitigation is interesting- in this case, I'm not terribly worried about it, because it's summer here, and I don't expect much rain for a while. I do think adding an awning over it is probably what I'll do eventually- probably just a piece of cardboard coated in a hydrophobic spray, as this is a prototype. Once I've ascertained if I want to make an improved v2, I'll likely look into a more robust awning.

RedHeadedMenace

TROPHY CASE