Is it worth converting my portable AC to a dual-hose setup to fix the negative pressure?

You’ll run into a major efficiency hit if the intake and exhaust are right next to each other on the window. It causes a loop where the intake just sucks in the 50-degree exhaust air you just pumped out, making the compressor work twice as hard for zero gain. Try to keep them at least 40cm apart, or better yet, mount the exhaust at the very top of your window board and the intake at the bottom. Since heat rises, that physical gap helps ensure the intake is pulling in the relatively 'cooler' outside air rather than its own waste heat. Hitting 21 or 22 degrees in an 18m² room is a totally realistic goal once you eliminate the **negative pressure** problem. Single-hose units are basically glorified exhaust fans; they spend all day pulling hot air from the rest of your house under the bedroom door to replace the air they blow outside. Once you convert to a dual-hose setup, the machine is just recirculating and scrubbing the heat from the air already in the room. I've seen setups go from a 25-degree floor down to 20 degrees just by fixing the airflow physics.

To handle the filter issue, don't tape the shroud directly to the unit. Instead, build a small plenum box out of your polystyrene that fits over the intake fins and use adhesive-backed foam weather stripping to create a seal against the AC chassis. You can hold the whole assembly in place with a couple of long bungee cords or heavy-duty **Velcro strips**. That way, you just pop the box off, slide the filter out for its bi-weekly cleaning, and slap it back on without having to re-tape everything.

And definitely wrap that exhaust hose. Those thin plastic tubes essentially act as a 1000-watt radiator inside your cool room. Using reflective bubble insulation or even a dedicated insulated sleeve keeps that heat contained til it’s actually out the window. If you seal the window board with foil tape and keep the hoses short, you'll notice the compressor cycling off much more frequently because the room actually stays cold.

I’m not so sure the hose setup is the only culprit here. Have you ruled out that the unit might just be undersized for 18m², especally if you’ve got sun-facing windows or high ceilings? Even with a dual-hose mod, these portables dump alot of heat directly from the compressor casing back into the room. Before you go through the hassle of building a shroud, have you checked your actual vent temps? If the air coming out the front isn't hitting 10-12 degrees, no amount of window sealing will get you down to 21. You might just be hitting the thermal limit of the machine itself.

I used a plastic storage bin for my intake shroud instead of foam. Just cut a hole in the bottom for the hose and lined the edge with adhesive foam. It stays put better and I can just pop the whole bin off when I need to get to the filter. One thing I'll say: keep that exhaust hose as short as possible. I cut mine down so it’s basically a straight shot to the window with no slack. Getting rid of those extra coils made a massive difference in how fast the room cooled down.