I've had two types of doors in this situation. One wasn't really a door. It was a piece of plywood that screwed in/out to something. Not really convenient. In the current house the builder did exactly as your builder indicated--cutting down a full size interior door. We've never really noticed any problems with air infiltration in the area, and the space is above a garage that gets cold.

I would think cutting down a solid wood door is a pretty good solution actually.

What's wrong with custom?

Two skins, and a frame filled with rigid foam.

The IRC does not require a vertical door into an attic to be any better insulated than your front door, but it would certainly be a benefit to add more foam insulation to the back side.

http://media.iccsafe.org/news/icc-enews/2017v14n15/2015_irc_sigchanges_1102.2.4.pdf

IMO, building a flat panel custom door is not much more work than cutting down a door and gluing the upper rail back in as long as you do not expect furniture quality on the parts you don't normally see. This is not a door that has to withstand kids running in and out of either.,

A (northern US) U-factor of .32 is equal to an R-value of 3.125 under the simple math. That is about ½" of closed cell foam insulation. A typical 1¾" thick foam filled exterior door may be about R7, so any insulation you can add to the back side of a door is a bonus above code.

Seal the frame with weatherstripping and do what Worthy suggested - put a couple of layers of XPS slightly larger than the door itself glued to the back on an inswing door.

I've decide to just go ahead and cut one to size. There is no point in ordering a custom one. I'm going to make the opening 3ft by 5ft.

As you state cut a solid core door to size. Continuous geared (piano type) hinge. Use jambs ALL 4 SIDES with weatherstrip stop. Used to do these all the time for builders.

Thanks guys for all the info! I'll post pics once i have it up.

We are doing this exact thing in our build. Cutting a regular door down to size, gluing additional insulation to the back and weatherstripping the jamb. Our attic is spray foamed so it's not too hot/cold in there but every bit helps. Ours is also hidden inside a closet so it doesn't even have to look pretty.

use a sheet of plywood cut to fit, insulate attic side with reflective foam

sheathing. weatherstrip door.

cutting down a door is a wasteful thing, and you compromise the

structural integrity of the door.

expensive way to do it IMO, and as wood is R-1 per inch...you really

aren't gaining anything.

best of luck