The higher the number the better it will insulate or "save the most heat" There are limitations however i.e. wall thickness, 2x4 or 2x6. Ceiling insulating is limited by attic space. You want a vented air space above your insulation. The 15" or 15 1/4" is not going to make much difference. The 15 1/4 will give you a better fit if your using friction fit insulation vs. faced insulation. To sumarize use the highest R number that is designed for your particular wall thickness. I would use at least 25 in the ceiling. Vent the attic. Use a 3 mil vapor barrier all around. Hope this helps.

are you going to heat/cool this shed? if not, then i recommend NOT insulating it. i have seen insulated sheds get extremely hot during the summer with no AC. the insulation not only keeps out cold, but it holds heat in and that heat will continue to build every day. once it heats up, it takes forever to get it cool again!

With all due respect, your above recommendation not to insulate a building because it will not cool and continue to build heat is simply not true. It will hold the heat longer but will also be at a lower temp during the heat of the day as long as the attic is ventilated correctly. If your statement were true than summer nonairconditioned homes would be better off uninsulated. If you don't believe me go up into a nonairconditioned buildings attic during the heat of the day and see how warm it is.

with all due respect, it WILL continue to build heat. the first few days won't be bad, but it won't cool off much at night and eventually will get highly unbearable. i once saw an old refrigerator trailor used as a storage shed that routinely got to over 160 inside during the summer. we propped the door open on it and left it open overnight, at 7am the next morning it was still 125 inside, even though the outdoor temps were inthe low 70's. and this trailer was not airtight, it had a couple of window units in it, but they were not turned on for a while and locked up. so even with a little air movement it still got too hot to be of use.

in areas where heat is a concern, LESS insulation is best if you are not running AC. down here all the old homes that never were built for AC routinely had only minimal if any insulation so that the heat of the day could move away fromthe interior of the house. the houses will cool down almost instantly after sunset, where as an air conditioned house with excellent insulation will stay hot long into the night if the AC is off. my AC went out a couple years ago during July. even with the windows open my house would stay 20 degrees above outdoor temps all night long due to having so much insulation. it takes a while to cool all that insulation down once it heats up.

the attic will have more heat than the ground level of a house, since heat rises. plus the roof heats up and some of this leads to heating the attic. think of your electric oven, on certain settings only the upper element is on, yet the oven still heats up all over.

So using your oven example lets say the element represents the heat generated by the sun, the oven is the attic and the bottom drawer is your house. Now remove the insulation between the oven and the drawer and see how warm the drawer gets.
The refrigerator trailer has no air ventilation between the heated metal and the insulated area like an attic serves. Also the R value would be much less than in a home.
I do agree with you about an insulated house is going to cool slower but then wouldn't an insulated house warm slower?
Attics are warm because heat rises but the extreme heat comes from the sun beating down on the roof and I cannot understand why anyone would not want to insulate between your useable space and that extreme heat generated by the sun. I guess we will have to agree to disagree.
Anyone else out there have any thoughts. Also I was wondering if the original poster is going to insulate their shed or not?

an insulated house does fully warm slower. the insulation's purpose is to lower the heat gain/loss, not to eliminate it. once it does finally heat up, it takes forever to cool down.

as a better example, think of a drink cooler. fill it with ice and rinks and it is fine for a while. but leave it sitting outside in the sun and within a few days the inside of the cooler is very hot.

my shop is 18x18. when we moved in the house it was uninsualted. i put a window unit in the wall and it could cool the shop down in about 30 minutes. the next summer i started insulating the shop, put up rigid foam insulation onthe ceiling, R-13 in the walls. now if the AC has not been on for a few days it cannot cool it back down. i have to open both doors and get a squirrel cage running to blow teh hot air out so the AC can do it's job. on the hottest day of this year it was 102 outside. my shop was 157! and i do not have a south facing wall, since my well house is the same lenght as the shop and 1 ft south of it.

i agree that you should insulate, i am just pointing out that if you do you must also cool it.

I to agree that it should be insulated for the Winter, but for the Summer I have 2 windows that I can open and let fresh air in along with opening the 2 doors.

Yes my biggest thing is insulateing for Winter when you need a heater to bring the heat up to stay warm.

as long as you can have airflow inthe summer, it should be ok. jsut don't leave it sealed up or it will be unbearable!