To answer your question, when sizing is on the border line, it is preferable to drop back the half ton rather than use the higher. Now come the caveats..That is provided all heat load and manual J calculations have been done properly, you don't intend to do any building additions any time soon. This is especially true in humid locations. As phillipeh mentions, you should look into better insulation also. As mentioned, you'll get a rebate there also and can be assured you squeeze more efficiency out of your unit.

I guess I'm still confused on your layout. Based on the above I believe you are saying... Lowest/1st Level - Basement - underground - 700 sq ft. 2nd Level - Ground level - 700 sq ft. 3rd Level - 1,400 sq ft. 4th Level - 1,400 sq ft. Is the above correct? If so, several questions... 1. Which levels are finished? I'm assuming the basement will not be finished or the assumption is it does not need cooling since there will only be a return & no supply. Regardless, that is a bad idea. If you have only a return & no supply & if you have a tight house, theoretically the basement will turn into a vacuum. 2. Adding up 2 levels of 1,400 sq ft & 1 level of 700 sq ft equals 3,500 sq ft, not 3,300 sq ft. Why the difference? 3. It must be a very unique house design to have a 700 sq ft ground level footprint but then supporting an area twice that large above. You can't cantilever (actually I guess you could but it would be a very expensive solution) that much so you must have additional footings & piers around the 1,400 sq ft perimeter. Can you post pictures of the house or design? 4. The "cost" of a 2 1/2 ton versus that of a 3 ton is actually quite small. A contractor may elect to "price" it much more but in a competitive market you typically would not see that much of a difference in the "price". I think the primary reason people oversize is to be sure it can cool the house to the desired temps regardless of the outside temps. He is right regarding the mold & mildew issues so you would be better off with a slightly undersized unit & live w/ a few days a year when it might not cool to 68 degrees when it is 100 degrees outside. You can play with that number with the Manual J variables to see how that changes the tonnage specifications. Personally, I used 94 degrees in my area even though we do have several days a year when it hits 100 degrees. Part of that reasoning is at night we like it around 68 degrees inside but obviously the outside temps are much lower at night. So be sure to use the desired inside temperature @ the time the outside temps are hitting their highs. In our case 75 degrees inside is fine during the day. What area of the country do you live in? 5. How much window coverage do you have? What is the U-factor of those windows? I'm actually surprised that is the Manual J was done that it does not suggest the need for larger unit for the top level. You would not input any heat gains/losses for the ceilings of the basement, the ground level, or the 3rd level (1st 1,400 sq ft level) since the areas above would be "conditioned". However, you would input the ceiling/roof heat gains/losses for the 4th level thus it should have a higher BTU gain/loss than the 3rd level. Maybe it just was not quite enough to trigger to the next size up. Ask to see the numbers & if the 3rd & 4th level are the same then there is something wrong, unless you have a lot more windows on the 3rd level than the 4th level.

david_cary, thanks for the suggestions! The canned foam that you mentioned, is it something that I may be able to buy at a store (or is it only available to contractors)? I don't have an special equipment if it's needed. We don't have a vapor barrier on the dirt floor (inside the crawlspace). I will look at our local hardware store to see if I may buy it there. During different seasons, I have gone into the crawlspace to look around and try to figure out what's going on. The dirt floor is always dry to the touch and pretty hard (dirt is tightly packed). Also, there seems to be good air circulation from the vents. Given this additional information, do you still believe that the crawlspace is contributing to our humidity problem and vapor barrier is needed? (I'll still plan to put insulation above the crawlspace.) I'll read-up on windows weatherstripping and see how that is installed. Just walking around the house, I can see that some windows have some sort of rubber gaskets around them and others have thin metal frame around them. The rubber gaskets look like that they may have shrank because some of the corners have half inch to one inch gaps while other similar windows don't. Yup, I've heard of dehumidifiers (although I don't know very much about installing them). But, I'm sure I can read-up on how to install them and get it done if I need to. I'd like to understand the cause of our humidity problem and try to address it before I go and address the symptoms by installing a dehumidifier. So far, the new AC/furnace have been doing a good job. Our humidity level has been around 43% to 55%. (It is set to 50% on the thermostat.) The separate thermometer that I have also has a humidity gauge and it confirms what the thermostat is showing. I wonder how it will be this winter when we're not running the AC anymore. weedmeister, thank you for the clarification; I appreciate your input!

I am in AZ - home of the dry heat. Keep the bedroom at 69 during the summer and we love to hunker down under a comforter but don't like the weight of anything too heavy. After buying and discarding countless down comforters, we purchased a silk filled one at Costco. It was one of those traveling show things. We paid $270 plus tax but that was 2 years ago. King 106 x 96. Washes easily, doesn't bunch up line so many of them do. Website is smartsilk.com and it looks like Amazon may carry them. Amazon typically has a generous return policy too. I too could not buy a comforter sight unseen unless I knew I could easily return it.