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My cabin is in north Texas (aka Hell’s Furnace) and is 16x24. I have R19 walls, R25 ceiling, and R20 floors. The walls are 2x6 and I have 7/16 OSB sheathing and Tyvek wrap between sheathing and siding. There are two high quality fiberglass doors, and 4 high quality windows, 2@ 36x48, 1@24x36, and 1@12x24. There is one bathroom 6x8 ft where the 12x24 window is. The attic is vented with a ridge vent and vents are in place under the overhangs. Interior walls are 1/2 in drywall. Ceilings are 8 feet.

So having said all that, I am probably going to buy a window unit rather than a portable. From what I hear and research, they’re just better all around.

As far as sizing the unit, the boxes say 10k BTU is appropriate, and a calculator I found said 13k, but that’s seems right for normal places, not the sweltering sauna that is the Lone Star State in the summer. I figured I’d go to 14K. Does this make sense?

Also how is the quality of the big-box window units? Especially the GE models?

  • Are the units in the range of 12 kBTU/h powered by 120 V or by 240 V? Do you have both available in the cabin? – Jim Stewart Mar 16 '18 at 7:36
  • @JimStewart I can install either receptacle – Kerry Thomas Mar 16 '18 at 8:57
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    With your insulation and a not so large size a window unit would be fine, I would go with 14k in my opinion it is better to be a little oversized for the really hot days, with that said I would not go huge as this would cause the unit to cycle more often and waste power and be really cold while it is running. – Ed Beal Mar 16 '18 at 14:43
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    Another nice feature of modern window units is a wireless remote control. This allows mounting of the unit at a height which would be better for cooling, but would be inconvenient for making adjustments if one had to operate controls on the unit. The remotes presumably come with little wall mountable cradles which allow convenient switching off the a/c when the last person is leaving the room or the cabin. This saves a lot on electric power consumption. I remember when living in Austin TX in the mid 1960s some people making the false claim that an a/c left on all day used less power! – Jim Stewart Mar 16 '18 at 18:31
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    Windows are what make a cabin pleasant and it is a shame to obscure one with a window a/c unit. But putting a unit in a window is a relatively easy and quick job. Making an opening in a wall for a window unit (or slide-in) is non-trivial carpentry. If you look at the detailed specifications for a given window unit that you are considering, then it will give details of the width of opening required. Don't assume that a given window unit with a nominal width of 24" will fit in a given window described as 24" wide. The reason is that the installation reqts might require more side clearance. – Jim Stewart Mar 16 '18 at 19:18
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I would think that 12 kBTU/h would be fine even in north Texas. One interesting feature of window units is that the chilled condensed water from inside is used to cool the bottom coil of the condenser before the condensate is conducted out to the ground. (At least this was the arrangement in the early 1970s when we had a window unit in north Florida.) Central units just dump the 40 deg F condensate to a drain and so waste the cooling capacity of this chilled water.

If there is a separate bedroom, you might consider a mini-split ductless with one condenser unit supplying two air handlers. This would cost a lot more, and require professional installation, but would almost certainly use less power and cool better. They are advertised as employing "inverter technology"; some of these minisplits have a claimed SEER of 20 to 30 and they are available as heat pumps which would heat the cabin in winter. But personally given the price difference I would first try the window unit.

If you put in a window unit, you could cut a hole in a wall rather than use a window and there are even "window type" units (I think called slide-in) which are designed for this type of installation. Some window units can't be installed in a hole in a wall because the wall is so thick that it would cover the intake vents for the exterior part of the unit.

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    I have not seen a unit that floods the condensate over the condenser coils, this will increase the cooling but shorten the life of the condenser fins. – Ed Beal Mar 16 '18 at 14:45
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    The old window units did not trickle the condensate over the coils, but the condensate drained from the evaporator coils into a tray and from there was conducted to and filled a second narrow shallow tray at the rear, immersing the bottom coil of the condenser. This tray had a pouring lip on one end and the condensate overflowed there and came out a tube at the back on one side. I saw this design on one unit which I had to pull out of its case. I don't know if this was a widely used feature and I should have explained that when I reported this design detail above. – Jim Stewart Mar 16 '18 at 16:43
  • A mini-split ductless would allow putting an air handler on a wall between windows without opening the wall and redoing studs. All one has to do is cut a circular 3" or 4" dia hole for the freon and condensate lines. The condensing unit can either be hung on the outside wall or put on the ground. There are 12 kBTU/h mini-split a/c - heat pumps available acwholesalers.com/LG-Air-Conditioners/… – Jim Stewart Mar 18 '18 at 11:00
  • most split systems have way smaller line sets, a 5 ton unit with a 1-1/4 suction line and a 1/2" liquid line would be way smaller even including the insulation on the suction line , not even a 10 ton commercial ice maker would require a 4" hole. – Ed Beal Mar 18 '18 at 22:27
  • I have seen videos of the installation of these minisplits and IIRC the hole was 3" or so, but I didn't check when I wrote that and so I could be way off. The hole I described as "3" or 4" dia" is required for power and control cables, both freon lines, and the condensate drain line. I wasn't thinking that the freon suction line would be 3". youtube.com/watch?v=ozaw9QhxkQU – Jim Stewart Mar 18 '18 at 23:36

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