backer-boardcement-boarddrywallshowertile
I am hanging backer board for my shower and have a 5'x 3'2" tall space I need to cover. The backer board comes in 5' x 3' sheets.
My question is should I Use one full sheet and then cut another down to 5'x2" or should I cut two sheets into somewhat even increments so as not to be working with such a small piece of backer board towards the top of the shower near my ceiling.
For example I could cut them both down to 5'x1'7".
The best way to combat the condensation problem is to insulate the riser duct, aka metal duct, that is causing the problem. There are several ways to do the job. The best way is to use a vapor-barrier & spray-on foam insulation, but that can be costly and messy, unless done by a professional. Another is to use Rock-wool to insulate the duct, but often there is not enough room in the wall to use it. The insulation that I prefer to use is a Duct-wrap. It looks like bubble-wrap with small "bubbles" sandwiched between 2 layers of foil-lined plastic. It all depends on how much money you want to spend and how much space there is between the duct and the drywall.
The short-term solution, is to install a duct-wrap. It is available at home improvement stores in the building products department in the insulation section. It generally comes in various lengths at a 4 foot width & can be cut to size with scissors (or a razor-knife, if you are careful). To install it, simply measure the distance between the studs and add at least 12 inches to tuck the insulation behind the duct OR add 4 inches & staple it to the studs like traditional fiber-glass insulation. Cut the duct-wrap to size. You will also need to get metal (duct) tape to seal the seams between the upper piece and the lower piece of duct-wrap. It has a paper-backer to keep the glued side from sticking to itself. You will need to peel the paper-backer off when you install it.
The Long-term solution would be up to you. You could hire someone to take apart your existing ducts and insulate them or you could do the job yourself, either way that is the only solution to your problem. And by the way, cutting a hole on the wall will only make matters worse by giving the humidity a direct path to the now un-insulated ducts.
My experience says to use the rotary tool, but I have always used metal boxes with plaster rings. I have no experience cutting drywall over plastic boxes. I do have three pointers for cutting with a rotary tool:
If you still don't want to use the rotary tool, there is another option. There are tools that attach to plastic electric boxes. You put them on and then position the drywall where it will be installed. Press the drywall against the tool, and it will place small perforations at the outside corners of the box. Connect the dots and cut. The image below is of one of mine, but I haven't had the chance to use it yet.
Best Answer
The reality is that it doesn't matter all that much, and optimizing for whatever allows them to reuse the most material or minimizes cuts should be your first priority (I see you have another uncovered wall that you might want to factor in).
That said, if you are sure you only want to cover the area stated in the question, then it would seem you are in for 2 pieces of backer board no matter how you slice it. Thus, you might consider cutting boards twice so that you will have vertical seams for drainage reasons.