For a "cold room," typically under a porch, use at least 2 inches of XPS tightly sealed. In Ontario, you don't have to provide further coverage. (At least, that's how the inspectors in TO have interpreted the OBC.) This should prevent condensation. You should also use a well sealed exterior door and provide venting. If this is actually a year-round refrigerated room, it's a different story.

1) If I use cap nails, won't that making the insulation less effective because of the holes? the nails have a plastic cap that will give you a decent seal and will cover the hole left by the nail. as you are nailing into a solid surface (wood) and foam coveres gaps in siding its a non issue as long as you caulk the sides to the studs. 2) energy rater, can you confirm what nerdyshopper said about leaving space between the foam and the drywall? his was a conditioned space application. in homes foamboard can't be left exposed, it has to be behind a fire rated surface like drywall. in his application they used a foil sided foamboard and I guess the radiant barrier (foil) is facing the inside of the wall, putting anything in contact with a radaint barrier is a no no as it doesn't allow the rb to reflect the heat. we put it on exterior walls withthe foil facing the brick, but not to the interior of the wall. yours is a shed.if you plan on finishing the wall out..use a dow blue board and insulate the walls with batts and install sheetrock over them..if its a fancier shed..are you planning on heating and cooling the shed? spending a lot of time in there? what is your location? there is a vapor barrier location criteria. oh and radiant barriers are for hot climates. 3) Any technique for an efficient way us cutting the foam to size so it's gonna be as airtight as possible on irregular shaped dimensions? --> The dimensions are never rectangular. Sides are like: 27" 3/8, 44" 1/4, 28" 1/8 and 43" 3/4 with odd angles. Yep, time and straight edge, and extra razors for the razor knife. cut each to fit tightly and if its too small cut a slice of foam board make it tight fit and nail in place with button cap nails seal with caulk. So you mean caulk every single place where the foam meets the studs? Is it really gonna improve the insulation? exactly. what you are trying to do is make the space comfortable enough to work in. foam sheathing will stop the air flow. if you install it with gaps you will have lots of air infiltration. this is the tradeoff...you do the install of foam sheathing material cost and time or hire a foam company for higher cost and less time. a shop is a diy imo. caulk is cheap. foamboard is easy to work with. insulation doesn't perform because of the air that moves thru it. stop the air movement and it makes a huge difference. how do you plan to finish the ceiling?

We started with existing drywall. We ordered these soundproofing membranes that came in 4x8 sheets that we installed on the existing drywall. My husband said they reminded him of conveyor belt material -- very flexible, thin, and heavy. We secured them with roofing nails (I know -- it sounds odd). We then used acoustic chaulk around the perimeter of the wall, the seams where the sheets abutted, and the nail heads. From there we installed another layer of drywall over the soundproofing membranes. Finally, we just finished the drywall. Like I mentioned, it worked great for us. With a ceiling, I'm not sure what they would recommend, but they are very knowledgeable & helpful.

Then what are you asking? Any new door is going to better than a single pane oldie. A clear view storm and screen set, affords yet more light, and in summer the opportunity for fresh air. That's up to you. A MAJOR cut back to messy/ overgrown landscaping would help as well.