I tried searching past threads but came up short. I have old cellulose in attic, and want to increase insulation. Ceiling is sheet rock over plaster. I can get a good price for soem rigid foam, but if I apply that perpendicular to the ceiling joists, will I trap moisture underneath? I could leave a small gap between boards, and not get that much air infiltration ( convection) Thanks.
You'd potentially be inviting a moisture issue to create a layer of cellulose in between the rigid foam and an attic floor that is presumably an incomplete air barrier. Potential air leakage into this cavity from from the living space below could be problematic.
Assuming you have good access to the attic, a better bet would be to thoroughly seal the attic floor with spray foam and caulk, close any chases that are open to the attic, and simply adding more cellulose on top.
I think you're basically asking the wrong question.
You could install enough thickness of foam boards, properly detailed, where moisture accumulation would not be an issue.
But, the best, most cost-effective improvements to your attic are to first find and fix air leaks and second add more insulation.
Foam boards don't usually make economic sense in an open attic. Per R value, foam boards are expensive, and labor-intensive to install, compared to blown insulation.
For what it costs at the home center for 2" blueboard (a little over $1/sf for R-12 or so), you could blow in about R-30 of cellulose.
Incidentally, in my area, paying an insulation contractor to blow insulation in costs about the same as "doing it yourself" so do get some prices.
For info on air sealing, check the ENERGY STAR Attic Air Sealing page: http://www.energystar.gov/index.cfm?...rovement_attic
Rigid on top of cellulose would be risky,for sure. Under the cells and sealed to the ceiling or joists would be good,I think.
I've used it quite a bit to cap exterior wall lines where there isn't room to get much fiber, sometimes placing it from the soffit.
Foundation or crawlspace is the best place for rigid foam though>.:)