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My attic is well-insulated with fiberglass insulation strips laid between the joists, silver thermal wrap stretched across the joists, and 18-20 inches of blown insulation on top.

But as I conclusively demonstrated yesterday, there is no flooring to prevent me from plunging through the ceiling and into the room below.

What is the best way add flooring to the attic without compromising the insulation factor? There is too much blown insulation for me to simply compress it all under plywood. Should I add plywood to the existing joists and leave the blown insulation on top?

Two slight complicating factors: my only attic access is through a standard pull-down ladder, so I can't fit whole sheets of plywood. Also, I have an A/C unit (no heat) in the attic that sprawls extensively across the floor. I can work around that, but it makes access more difficult.

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    There's no flooring because there isn't a floor. Common trusses aren't designed to carry the load of a floor and your piles of junk. Add to that your existing HVAC equipment and you need to seriously consider the specifications of the roof system.
    – isherwood
    Jul 2, 2019 at 12:39

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The alternative to Eric Simpson's simple solution is a product like the following. (This particular one is from Amazon, but searching for "loft flooring legs" will find many like it.) enter image description here

Plywood/chipboard is often sold in 2400x600mm (8'x2') sheets precisely for fitting through a hatch to floor a loft.

Also, beware! Roof trusses are usually designed down to a price, which means using the thinnest pieces of wood which can take the load of the roof. They are usually not designed to carry substantial loads like a standard floor and a massive pile of junk on top of it. If you want to put in a pathway and store empty suitcases and spare duvets/sleeping bags that should be fine; a complete floor + piles of old books - not so much. (Thanks to isherwood for the comment.)

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If you compress the insulation you lose its insulating ability so any floor or walkway you install needs to be supported above the insulation. For your radiant barrier (silver foil), you need at least an inch air gap on the hot side so don't put your floor directly on top of it either.

If you have trusses or bracing between your rafters and ceiling you can put horizontal 2x4 or 2x6's between them, then fasten plywood on top for a walkway. I have 1/2" plywood, 24" wide, screwed on top of two 2x4's on edge, supported about every 8 feet for a walkway. I'd make it more substantial if I used the space for storage, but it supports my 200+ lbs fine for an occasional inspection or fan maintenance.

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    Those truss members are not designed to carry a vertical load. They're stressed members of a roof system, not a floor. Ok for occasional maintenance access, not for hundreds of lbs. of storage. The attachment points aren't necessarily up to the task. and they're often just 2x3, which means they'll warp and/or crack under lateral stress.
    – isherwood
    Jul 2, 2019 at 12:41

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