My house was built in 1946. When I bought it a few years ago it had very little insulation in the attic. The attic is very tall and the roof slope is very steep, so with time I am planning on expanding the living space into it. Due to shortage of materials following the war, the rafters in the attic were all different sizes and the sparse insulation was mostly missing. Since I bought the house I have installed 2x10 rafters to support future floor. I also cleaned out all of the old insulation and filled all of the rafter bays with cellulose insulation. It worked like a charm. I was able to fill in all of the irregular cavities with cellulose and the whole house is much warmer.
About a year later I wanted to store some stuff in the attic, so I put plywood across the rafters and stored all of my junk there. After some time I noticed that the plywood is getting moist on the underside, so much so that some mold started to grow on it. I immediately removed it and replaced the moist cellulose under the sheets of plywood.
I am not sure where all that moisture is coming from and what I could do to prevent it from accumulating on the plywood sheets. I researched the cellulose insulation, and no moisture barrier was recommended, so I did not install it. The roof has eaves all around, so the attic space is cold. I filled the bays with cellulose all the way to the rim. Should I have left an inch of space between the top of cellulose and the plywood?
What should I do to make sure that my house is properly insulated and that I can continue with the future expansion into the attic?