I think that the condensation is more from leaky duct takeoffs & plenum to equipment connections that are poorly sealed rather than a gash in ductwrap. although you don't say how large or where the gash is located on the supply plenum ductwrap. and...no it isn't as easy as it looks...if you do it properly. first remove all duct insulation. seal each duct take off with mastic or Hardcast brand mastic #1402 tape. (surfaces must be clean & dry) paint on mastic will take time to dry, mastic tape must be firmly pressed in place to make seal. paint on mastic should be thickly applied.. like thick as a nickel. you'll have to cut the ductwrap to fit around the duct takeoffs. I usually take ducts off, so that I can put the ductwrap with precut holes for takeoffs. take time to seal plenum to equipment. I find that the mastic tape works best for this. you don't say if plenum is upright or trunk line...so most common would be upright. wrap the ductwrap around the plenum, leaving top for seperate piece. tape covers duct staples. so you'll need a duct stapler, duct staples & fsk tape (foil scrim kraft tape...looks like ductwrap without the insulation) double row of staples, fsk covering staples..and I then staple down fsk. ductwrap is sold in 4' wide rolls. buy R-8 only. one side of the ductwrap is blunt cut...meaning ductwrap and insulation are same...the other side the insulation is cut back less than the insulation...this is the side that is stapled to the ductwrap. staples can't penetrate ductwrap & insulation to attach to the ductwrap under the lap..so any areas to be stapled need to be cut back in the same manner. once you get ductwrap on sides of plenum installed, then cut piece for top cap of plenum. make sure insulation is in full contact with the top cap & make the lips (ductwrap without insulation) about 3" wide. use these lips to staple to wrap on sides of plenum. fsk tape, staple. then re-attach the ducts. insulation of ductwrap has to be in full contact with metal of plenum...or it will continue to condensate. make sure you pull it tight enough to be in full contact but don't compress the insulation. duct & plenum to equipment connections has to be completely sealed or it will continue to condensate. ducts attached to takeoffs on plenum...insulation in full contact with both start collar of takeoff & plenum. inner liner (flex) mastic taped to start collar. outer liner of flex covering insulation at plenum & mastic sealed to plenum. some people use lock ties (pundit straps) be aware that compressing insulation with lock ties will cause condensation, and that lock ties alone on start collars will work loose...even with racheting tool...over time.. I like to mastic seal the outer liner (the vapor barrier of the flex) to the vapor barrier of the ductwrap. for ease of install & to save time...mastic tape. it is a hot, uncomfortable job that requires patience & much attention to detail...or it will condensate. look to spend $100+ for ductwrap $50 for duct stapler plus staple costs $30 for hardcast mastic 1402 tape $20 for a bucket of cci 181 mastic & brushes can't remember price of lockties or tool to tighten as I hardly ever buy lockties or use the tool I've had for years. buy two rolls of Hardcast 1402. this tape seals to many materials as long as they are clean & dry... use the other rolls to seal supply boxes, if you take off a supply grill inside the house, you'll see the oversized cut where the supply box penetrates. keep tape within area covered by supply grill. also take down bath fan covers & use mastic tape to seal those oversized cuts. other tapes don't seal like this particular brand of tape, over the years I've tried them all. hardcast 1042 is what works & lasts. if you hire this out...make sure that the work is done as described above..if you diy & have questions.. post or email me. and if existing ductwrap is cold on the bottom..it is probably wet inside. best of luck.

HVAC duct work should not be wet. Duct work for a 4 ton unit when having a 3 ton unit would most likely cause performance problems of the system being able to handle the loads of the structure properly. Because the duct system would not be able to pressurize enough to 'throw' the air thru the vents of the structure. This can also cause longer duct runs to get little to no air. Duct sizing in and of itself does not cause excessive moisture. Because of where this unit is located leads me to believe that condensate drainage is an after thought for your location. Usually duct work is undersized, not oversized... because it costs more $$ to over size duct work and most builders skimp. Some locations require less cooling than heat due to climate. So in fact it could be that the furnace is a 4 ton furnace and the AC is a 3 ton, because for your climate that was all that was needed. Moisture problems can be complicated, just as much as smell. But your duct work should not be wet. In some instances under high humidity conditions, equipment can sweat. But this should only be temporary condition when dew point temp is occasionally hit causing this condensation to form.... but not to the extent of getting duct work soaked.

Given that you're renovating I would say it's a good time to do it. It can raise the costs dramatically depending on a myriad of factors. If what you have is flex duct / going back with flex duct the costs shouldn't be too extreme, but this can lead to boot replacement and vent replacement so that can increase costs further than just duct work alone. You should replace with at least R8 flex duct if you are going back with flex. This can pose challenges because circumference wise these ducts will be larger than what you have now most likely. By using a minimum R8 insulated flex duct, this will increase the efficiency of the system, marginally in most cases depending on climate. It also wouldn't be a bad idea to possibly add an additional new filtered return at a known hot spot / cool spot within the structure. (hot or cool --dead air space that might be getting trapped, by adding a filtered return to this spot it will aid in the comfort factor of the structure.)

The ceiling above the crawlspace is the bottom of the first floor. The warm moist air in the crawl space will rise up into to house if there is no vapor barrier to stop it. I would expect a three year old house to have tight windows and doors. The crawl space might be a source of the excess moisture in the house. What size is the AC? I would think a 2.5 ton single stage, or 3 ton 2-stage AC would be sufficient for a newly constructed 1749 sq. foot single story house. Did the builder provide a load calculation? A 6-10 minute run time might be considered short cycling. How often does this happen in an hour and what is the outside temperature?