My bedroom has been an icebox in the winter with three exterior walls, cathedral ceiling and a garage below. I took out R-30 batts from between those joists, and an insulation guy wanted to blow cellulose in to compact it between the joists.
So are you saying that all I need is a double sided reflective radiant shield – maybe less than a half inch thick – staple it to joists so that it would basically be my garage ceiling, and that’s all I need? And nothing in the 9-inch joist space between the subfloor and the shield?
I do get the theory, but will this actually work?
And as a practical matter, I need to install a regular ceiling for the garage so I would have to install the radiant shield between the joists. What distance should it be from subfloor/pex/heat transfer plates? Do I even need heat transfer plates???
This set of details goes way beyond your original question and represents a much more complicated series of issues. First, cathedral ceilings are perhaps the most difficult residential insulation problem that often bars a good solution. The problem is that the space for introducing insulation is way too limiting for good insulation. No doubt the R-30 that you removed was crammed into the joist framing to the extent they no longer represented an R-30. Additionally, without an air space the insulation would be a better conductor of heat transfer than a resistor. And, you always have to deal with the possibility of moisture / condensation.
Because of the nature of the application and all the mitigating factors that are or could be issues, I will refrain from a specific system. Here are some guidelines that I hope will help you in deciding your final design.
Possible air leaks should be eliminated, perhaps with a spray foam.
A reflective insulation or radiant barrier should be part of the system, making sure that the reflective surfaces face an air space.
If there is a way to ventilated the top side of the radiant barrier, (between it and the underside of the roof), that would be best.
Fiberglass should then be installed at an R-value that would fill the space without crushing.
A good vapor barrier should be installed before the drywall goes on.
Your last question isn’t clear to me, but I’ll try to answer — the radiant barrier should be installed, either on the face of the joists or in between the joists at the mid-point, (the distance isn’t critical, just so there is an air space facing both reflective surfaces). I don’t know what your use of the term “heat transfer plates” unless you are talking about a reflective insulation / radiant barrier. If that is the case, I’ve answered your question. I hope this will help you get a satisfactory comfort level.