Adding "floor" to unfinished attic with blown-in insulation

Question

I'm in the process of converting our attic into a useable storage space. We have no basement and a small garage so it'll have to be our main storage area, so I'm hoping to make it user-friendly and as easy as possible to move around in.

There is blown-in insulation, and there is conflicting information out there about how to add flooring of some kind without compressing the insulation. The joists are about 30' long and span the entire house, so I'm not 100% sure they are "load bearing", but I don't intend on storing too many heavy things, and if I do to keep them toward the sides of attic where there is more (I think) support.

So my questions are:

1. Can I lay plywood directly on the joists without losing some of the "R-value" of the blown in insulation?
2. Is it safe to use this space as a main storage area since I'm not sure the joists are load bearing?
3. Can I add new "support beams" (the existing beams that span the attic are very flimsy and low and make it difficult to move around)?

The house was built in 1952 and it doesn't seem like anyone else has attempted to store anything up there or use the space.

Answer 1

You don’t give us all the info we need, but I’ll make some assumptions and you let me know if I’m wrong.

The roof and ceiling structure is what we call “stick framing” it is not framed with trusses. The roof joists appear to be 2x8’s at 24” on center. Therefore, I’m assuming the ceiling joists are the same.

2x8’s cannot span 30’ without additional supports. I’ll assume the ceiling joists have walls supporting them at about 15’. If so, the ceiling joists can support about 30 lbs. per square foot (psf) if they’re 24” on center, depending on the species and grade of the joists.

The existing load on the ceiling joists is about 10 psf, so you could add 20 psf, if the roof joists do not rest on the ceiling joists.

However, you have two bigger problems: 1) compressing the insulation will reduce your insulation value, and 2) whatever is stored up there will be on the unheated side of the insulation. This could cause moisture, mold, etc. on whatever is placed up there. Make sure you have adequate ventilation. (Btw, don’t store books up there, they are one of the heaviest things homeowners have. Christmas decorations, luggage, etc. is better.)

Answer 2

I did that, added plywood over blown insulation . It may cause some compression depending on joist size. I have some 2 X 12 joists - no problem . Also some 2 X 6 area , I added 2 X 4 on top to give 9" depth. Also the plywood itself adds some insulation and restricts any convective air flow . So, even with a little compression I think there is a net improvement in insulation. My only problem was electric wire here is not in conduit so I had to push wire around and leave an occasional gap in plywood and make a few cut-outs to avoid pinching wire.