colliehauler wrote:
pianotuna wrote:
Think about how a vapor barrier is installed in a house.
That was the point of me asking. They use unfaced and I wondered about using faced and humidity build up in a closed roof, no attic to vent.
This is the problem. RVs have the vapor barrier/retarder on the wrong side of a wall or ceiling compared to buildings. If using batt insulation in a ceiling in a building, you require a gap or space for air movement and it needs to be vented to remove moisture, whether it's a vaulted ceiling or has an attic.
When batt insulation absorbs moisture, the R-value drops. Not good. And in cold weather you can have a high humidity level if you don't take measures to remove moisture-laden air.
The very best insulation job you could do is spray foam. It doesn't require an air gap to the underside of a roof deck. It encapsulates everything and leaves no un-insulated cavities. Costly for a small area like in an RV unless you could perhaps tow it to a job site somewhere where they're doing a spray foam job. If you lucked out and found someone, I'd do the underside of the floor too.
If it were me, I'd look at using closed cell rigid foam. Glue it to the underside of the roof deck and build it up in layers using PL400. Fill any voids with spray foam in a can. You'll end up with a very well insulated ceiling that way. I bought a ton of rigid foam for our garage floor at a building recycling place for a fraction of the new cost. Just don't use open cell rigid foam.
For spray foam or rigid foam, you'd of course need the ceiling down.
RV manufacturers do a lousy job of insulation ceilings. Ours has voids all over, isn't very thick and is compressed all over. The open cell foam they use in walls is also a poor choice as moisture migrates right through it to the exterior fiberglass where it will condense.
