Hello. I live in Waltham, Massachusetts. We want to add more insulation to our attic floor. We have an attic with loose (unattached) plywood boards on top of the joists. Currently, there is only about 1 inch of insulation between those joists. Using R-rating charts, I have calculated that we should add R-34 unfaced insulation batting to the existing. My question is: After we put in the insulation, it will extend 2 inches or more ABOVE the joists. Can we put back the plywood sheets (thus compressing the new insul. Quite a bit), so we can continue to use our storage space, or will that compromise the R factor too much?

I'm not an expert. However, my understanding is compressing insulation is not a good idea as it reduces the insulation R-value. I'd think the options to consider are:
1. Raise the floor to accommodate more insulation.
2. Use an insulation that has a higher value per inch.
3. Accept the lower value and use a thickness that does not need to be compressed.

I think the questions to ask are:
1. Does anyone know what type of installation has the highest r-value per inch?
2. What is the R-Value of a plywood floor per inch?
3. How deep are your floor joists?

You never stated how deep or high your joist now is without the insul That is a vital piece of information needed to get a complete answer it would really help. When you ark R values thickness is always the first key. And yes compression insul does not down the R value.
If you really want to do this Job 99% right. And you have only about 1" of loose insulation Since its only one inch now I would do this remove the loose insul" in sections and bag or throw it to the side.
Now lets get you a vapor barrier. Now that you are starting fresh some R30 or higher R batts with paper" vapor barrier" that will fit in the joist bay in thickness comfortably.
Install that with the vapor barrier against the ceiling you see from above. You can side staple that paper vapor barrier to the inside of the joist bay. Keep doing this in sections.when you get to a point and it gets crowded now we can use the old loose. Again clean out more bays and use some good mill ploy against the ceiling again. Now throw in the loose insul and you just did a home made insulation with a barrier. This will help you use up the loose insul. You are very concerned about R values. As it stands now compression isn't your only worry Without a vapor barrier the insulation will get moisture and this will just as much if not more greatly reduce the R values now as it stands. Obviously the only option is to reinsulate on top of old, and yes you don't use new faced insul that will trap moisture in between old and new insul. Because: it will gather moisture in the insulation and kill the R value not to mention mold in insul. IE need vapor barrier if you can within reason remove old loose and start right. So think about the extra work BUT save by using old. The best for R value is spray foam ( no air movement in insulation) Blue 3 or 4" thick Dow blue panels expansive. Standard is best of the batts or rolls and cheaper. Is your ply over the whole floor? Its just in spots to walk no that much of a bigger. You can tear off some thickness to reduce the compaction and lay ply down. IF this is a real big concern does your roof have a ridge vent in to get the moisture out of the attic this greatly improves the R value in all of the ceiling insul below. Do you have a soffit for attic to breath and the old fashion to tiny gable vents? I could go on on

Hope this helped answer questions for you. Need more info to get the exacts
Happy Holidays 21 boat!

You never stated how deep or high your joist now is without the insul That is a vital piece of information needed to get a complete answer it would really help. When you ark R values thickness is always the first key. And yes compression insul does not down the R value.
If you really want to do this Job 99% right. And you have only about 1" of loose insulation Since its only one inch now I would do this remove the loose insul" in sections and bag or throw it to the side.
Now lets get you a vapor barrier. Now that you are starting fresh some R30 or higher R batts with paper" vapor barrier" that will fit in the joist bay in thickness comfortably.
Install that with the vapor barrier against the ceiling you see from above. You can side staple that paper vapor barrier to the inside of the joist bay. Keep doing this in sections.when you get to a point and it gets crowded now we can use the old loose. Again clean out more bays and use some good mill ploy against the ceiling again. Now throw in the loose insul and you just did a home made insulation with a barrier. This will help you use up the loose insul. You are very concerned about R values. As it stands now compression isn't your only worry Without a vapor barrier the insulation will get moisture and this will just as much if not more greatly reduce the R values now as it stands. Obviously the only option is to reinsulate on top of old, and yes you don't use new faced insul that will trap moisture in between old and new insul. Because: it will gather moisture in the insulation and kill the R value not to mention mold in insul. IE need vapor barrier if you can within reason remove old loose and start right. So think about the extra work BUT save by using old. The best for R value is spray foam ( no air movement in insulation) Blue 3 or 4" thick Dow blue panels expansive. Standard is best of the batts or rolls and cheaper. Is your ply over the whole floor? Its just in spots to walk no that much of a bigger. You can tear off some thickness to reduce the compaction and lay ply down. IF this is a real big concern does your roof have a ridge vent in to get the moisture out of the attic this greatly improves the R value in all of the ceiling insul below. Do you have a soffit for attic to breath and the old fashion to tiny gable vents? I could go on on

Hope this helped answer questions for you. Need more info to get the exacts
Happy Holidays 21 boat!

21boat, I always hated having to crawl in tight attics lol, I too have an attic project and came here to ask, but after reading I'm pretty sure what I need now.. I do like working with the batts R30.. rafters are 2x8s and original paper R-19 isn't sufficient and the central ac guys hacked the original.. so I will leave the existing.. fill in where needed and float some pieces over the highhats too before running the big batts across the old.. also importantly.. there is soufit and I will allow that to breath for cross ventilation for the upper ridge vent.. thanks again john