We are dealing with extremely high temperatures this year:100°-119° already just since the last part of June. I live in a 16'×40' Park Model trailer and with a ceiling fan & a box fan combined with a 5,000 BTU window fan, my 256 sq ft living room still reached temperatures of 89°-92° during the heat of the day, and not lowering to even high 70°'s until well after midnight. This is with all 3 on their highest settings 24/7. Will adding a 2nd 5,000 BTU unit make any type of significant difference? I keep my bedroom door shut almost always and would also do the same with the bathroom door, but it has a domed skylight in it so it's already too hot in there. I am a renter, so I can't do any type of modification, etc. to it. Also, I have no roof shelter above or any real shade protection from trees. It also faces East with full sun exposure. I hope this gives sufficient enough details. Thank you in advance for any and all serious assistance provided.

  • 1
    From experience and all the details you've shared I think a 20 degree differential is pretty good for a 5000 BTU unit and a second one should help a lot. Does the trailer, and the second window, have an appropriate electric supply? You'll need a 15 or 20 amp circuit that does not (ever) have any large load on it. You can't use the same circuit as the existing unit. Hopefully you are keeping the shades/curtains shut during the day?
    – jay613
    Jul 27, 2021 at 18:21
  • Blocking most of the sun coming into windows will help some, using drapes. Sunlight coming in will add 20 or more degrees inside.
    – crip659
    Jul 27, 2021 at 18:22
  • Block the bathroom skylight with something, which you can remove in the winter. A foil-backed shade or if you want to hack something quickly, just use aluminum foil. And you're right to keep the bedroom door closed ... it's not a tiny trailer! You'd. need two more units I think to stand a chance at cooling the whole thing given that much heat outside.
    – jay613
    Jul 27, 2021 at 18:24
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    I'm looking at some 5000 BTU A/Cs and they all run around 4 amps, some even a bit below. Although I wrote the opposite above, you could probably have two of these on the same circuit. Even the combined start-up peak shouldn't be a problem if they are both so small.
    – jay613
    Jul 27, 2021 at 19:16
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    Well, a ceiling fan and a box fan does absolutely no cooling whatsoever. I don't know what a 5000 BTU "fan" is but I think you meant to type "air conditioner". Those cool, that one to the tune of 5000 BTU/hr. Jul 28, 2021 at 3:29

3 Answers 3


Yes, it will help tremendously.

You need about 12,000 BTU per 400 sq. feet of living space to feel sufficiently cool.

640 sq. feet (16'x40') needs almost 20,000 BTU but you're only giving it 5,000.

Keep the window shades closed during the day and block that domed skylight with something, anything! A shirt, taped cardboard, whatever.

Worth noting that during heat waves all of the A/C units generally get sold out almost instantly and you are only left with those portable units. Models with a single hose can actually make things worse so don't jump on the first one you see and preferably get 8,000 BTU or more especially if you miraculously find a window unit.

  • NB: 400-500ft2/ton is only a good rule of thumb for poorly insulated/sealed shacks like the OP's park model. A properly sealed/insulated house (<1-1.5ach, foam sheathing + batts at a minimum) will do better than that, and it's possible for Passivhaus-type construction to get into the 1000-1500ft2/ton range Jul 27, 2021 at 23:21
  • @ThreePhaseEel Thanks! Yes, my answer was aimed at OP's situation specifically. Only getting down to 90° even with the bedroom and bathroom doors shut definitely tells me that OP is in the worst situation possible short of sitting outside in a lawn chair. Heck, a lawn chair, extension cord, and a box fan in the shade will probably feel better that the hot-box they're living in.
    – MonkeyZeus
    Jul 28, 2021 at 13:01

BEWARE the 1-hose portable A/C!

Humidity defeats air conditioning. When humid air comes into your home, the A/C unit must labor to condense that water before it can cool the air. It takes 1000 BTU to condense 1 pound of water. So humid air leaking into your home is a really big deal, and can suck up most of your A/C capacity!

So DO NOT USE 1-hose "portable air conditioners". Here's why. They are ejecting hot air, but to replace it, warm, humid outside air is being sucked in every leak in your house! Unforgiveable!

A 2-hose portable A/C solves that problem so they're alright if you just can't use a window unit.

So-called "personal air conditioners" which run on water and tiny amounts of power do not work at all and are a complete ripoff. They pretend to use "swamp cooler" techniques, but are far too small to be useful. Even proper swamp coolers only work in extremely dry climates.

Redundant arrays of inexpensive air conditioners

However, your run-of-the-mill $169 air conditioner will do the job, though at such a low price they may have issues with long-term reliability. Aim for a high SEER number; that's their efficiency and it's important if you are putting several on one circuit.

Note that if you have casement windows, the smallest "window A/C's" on the market are only 16" wide. They won't say that on the package because they expect you to use it with the provided accordion window seals, which take a lot of horizontal room. But you can toss those out, and use a little carpentry skill to make your own horizontal platform to sit in the window opening. Tilt it a couple degrees outward so water runs away from the window. Then build your own window seals. That's what I did. Just remember a window A/C will drool water, so make sure to paint or seal the platform or put a tin or plastic cover over it so the wood doesn't rot out.

Another nice thing about our "platform on casement" setup is we can stack two window A/C's right above each other. Haven't done it yet, but there's room.

Paint your roof white. PLEASE.

The #1 source of humidity is sucked-in air... but the #1 source of heat into your house is solar gain. The #1 place that comes from is a dark colored roof. Dark colored south and west facing walls, also.

Be VERY selective if painting a rubber roof. Consult with a competent roofer about any product you use. However, you want white -- not silver -- white paint, of 90% "albedo/reflectivity" or higher. You want a "snow white", not any kind of off-white.

Note that this paint will be at least $20-30/gal and will NOT be $12/gal like the cheap silver roofing tar "paints" you can buy (which will destroy rubber roofs). That stuff helps only a little - you'll get far more mileage out of white paint.

For the sides of the building, feel free to use a style and color that suits you, but ask the paint supplier the albedo or reflectivity - 90% is as good as paint gets, but try to find the highest albedo that you can live with aesthetically.

Let's crunch the numbers on that. Sunlight puts 1000 watts per square meter square-on to the sun. (90 watts per square foot). Now you convert that to BTUs, that's 3400 BTUs/hour per square meter (300 BTUs per square foot). YIKES!!!! The building structure holds the heat and insulation slows it down, which is why it continues to leak heat into the building after dark, which is why you need to run the A/C on into the night. Anyway, if you have 200 square feet square-on to the sun, and 300 BTUs/sqft, that's 60,000 BTUs/hr of solar gain.

  • If your darker roof has 50% albedo that's 30,000 BTU/hr reflected and 30,000 BTU/hr absorbed into the building structure.
  • If your white roof has 90% albedo that's 54,000 BTU/hr reflected, and 6,000 BTU/hr to contend with. Way more better!

And those BTUs are never absorbed into the building structure in the first place, so they don't come back to haunt you after dark.


Only real issue is power. Figure out what circuits you have available and what usage they currently have. It looks like a modern 5,000 BTU air conditioner uses just over 4 Amp. Probably a bit more when starting up. My original statement of "separate circuit" was based on a larger unit. Just remember that while a breaker won't trip (usually) until something > 100% of the rating (15A or 20A), you should only run continuously at 80%. For a 15A circuit, that's 12A. 12A would be enough for 2 x 5,000 BTU, but not much else. The ideal setup is a separate circuit just for air conditioning, but you probably won't be that lucky.

Larger units tend to be a little more efficient, but the sweet spot on purchase price seems to be 5,000 BTU at around $160 in the US right now. Long-term, larger is better. Short-term, with one functioning 5,000 BTU, adding a second 5,000 is going to be cheaper (and easier to move) than replacing with a 10,000 or 12,000.

  • 1
    If they're getting a high SEER unit, a 5000 BTU A/C doesn't draw so much that you couldn't have 2 with some other stuff on a 15A circuit. Jul 28, 2021 at 4:14

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