Question About Fiberglass Insulation

[Old Moparz]

I am hoping to find out if it's okay to add insulation to a cathedral ceiling that is already insulated. The rafters are 2x8's, & the R19 fiberglass insulation almost entirely fills the space between the sheet rock & the plywood roof deck. I can't add to it because I will lose what little air space I have that I need to ventilate with. There is a vapor barrier on the living space side of the insulation, so it forces me to question whether or not I can add more insulation to the face of the existing sheet rock & create a newly insulated ceiling, under the existing insulated ceiling.

Does anyone know if this idea will cause moisture problems or not add to the R value at all?

The work I am doing to my house is in this thread.....

http://www.dodgecharger.com/forum/index.php/topic,75815.0.html

I am going to install a new ceiling made of lightweight T&G cedar no matter what, but if I can get a much better R value in the process I'd love to do it.

Thanks.   

[dodgecharger-fan]

Whatever you do, you want to keep that vapour barrier on the warm, moist side of the structure.
If you bury it with more insulation, you'll just end up rotting that new insulation.

You could look into some foam board insulation just be sure to buy the tape to seal the joints.

There's some good application notes here: http://building.dow.com/na/en/applications/building/walls/rigid.htm
Check out the tongue and groove styrofoam in the first section - there's a handful of applications described when you click that link, including some "over existing <wood/wall/studs/etc.>" applications.

[Old Moparz]

Whatever you do, you want to keep that vapour barrier on the warm, moist side of the structure.
If you bury it with more insulation, you'll just end up rotting that new insulation.

You could look into some foam board insulation just be sure to buy the tape to seal the joints.

There's some good application notes here: http://building.dow.com/na/en/applications/building/walls/rigid.htm
Check out the tongue and groove styrofoam in the first section - there's a handful of applications described when you click that link, including some "over existing <wood/wall/studs/etc.>" applications.

I always knew the vapor barrier had to be on the living space side, but if the barrier is in between layers, it'll be a problem with condensation? I'm not actually doubting you, but I was thinking that since there is already an insulated space by the original insulation keeping the cold air away from the heated interior, wouldn't that prevent the moisture issue?

Maybe I'm over thinking it.   

Or not thinking.