INSULATING FLOOR OF ROOM OVER GARAGE

QUESTION FROM BARBARA OF BOXBOROUGH, MA:

I am installing fiberglass batting insulation with kraft paper vapor barrier in the ceiling of an unheated garage. There is a heated, insulated, year round room above the garage.

 Since I live in a cold climate, I should install the kraft paper pointing up toward the warm-in-winter side, correct?

 I will be covering the insulation on the garage ceiling with fire code sheet rock according to building code for a garage under a living space. Should this sheet rock be attached directly to the bottom of the wooden joists that the insulation is between, or should there be strapping installed across the joists first and the sheet rock then attached over the strapping? This would create an air pocket that would allow air to circulate around the bottom of the joists. Is this desireable? Wouldn't the insulation batts then fall down to touch the sheet rock anyway, leaving a gap above the kraft paper between the underside of the floor above and the kraft paper?
 




 
CONTINUING QUESTION:


Thank you for your prompt response to my question. I understand everything you say except for the part about placing a piece of insulation at the ends of the joists to stop the air from coming through the bond plate (end plate) of the structure. 

My office sits in the corner of the room above the garage ceiling that is to be insulated. That part of the ceiling is already insulated, but there is air coming up between the plywood sub floor in spite of being insulated with fiberglass batts, which are covered with plywood (done in 1985). The joists that the batts are in are sitting on top of a header, and I'm wondering if the ends of the joist bays are therefore open and letting cold air circulate freely in there. Does that air flow through the insulation from end to end? And does  that render the insulation completely useless?

The part of that same garage ceiling that is left to insulate on the other side of the garage, has joists that butt up against a header at both ends, so the joist bays are closed. In that circumstance, would I still need to put a piece of insulation at both ends of the joists first, or will just running the batting from end to end work? 

I am not clear as to what a bond plate is ~ would that be the header that the joists butt up against at each end to form an enclosed box? If not, what then? Also not sure of the orientation of the kraft paper on the pieces of insulation to be placed at the ends of the joists to stop the air from coming through the bond plate. Should the kraft paper be vertical at the ends, so the kraft paper acts as a wind break? If so, should the kraft paper be facing toward the inside or outside of the joist bay it is at the end of? 

Thank you so much for helping me understand.

ANSWER FROM THE INSULATION DOCTOR:

You are correct in installing the kraft-backed insulation with the paper side up toward the heated room.  The thing that you want to be sure to do is place a piece of insulation at the ends of the joists to stop the air from coming thru the bond plate (end plate) of the structure.  In other words, you would insulate the ends of the runs most commonly being a 10" x 16" area before installing the insulation in the joists themselves.
 
Regarding the insulation dropping down on the sheet rock, there are insulation supports (a wire rod) that can be frictioned between the joists to hold the insulation from dropping down.  The insulation supports should be available at building supply stores and they are called "insulation supports". They are available in two sizes - 16" or 24" depending on the width of your joists.  They should be placed about every 3'.  You want the insulation as close to the underside of the upper floor as possible without compressing the insulation.  
 
Hope this helps.
 
Bruce Jones
The Insulation Doctor

CONTINUING ANSWER:

First, you are correct in assuming that the header is the bond plate in your case.  To be more specific in architectual language, the board above a window or a doorway, most commonly a sandwich set of 2" x 10" boards, is referred to as a "header".  The board that the joists are nailed into are commonly referred to as a "bond plate".  
 
Second, you will need to remove the paper from the insulation you will be installing to cover the bond plate (header).  I would recommend that you place the insulation in the bond in both ends.  
 
The air coming in by way of the bond plate (header) where you do not have access does not render the insulation useless.  However, it does diminish the effectiveness of the insulation.  You might want to consider trying to drill a small hole in the plywood 3" to 4" back from the header and spraying expandable foam into the cavity.  This type of spray foam is available in most box stores such as Home Depot and Lowes.  The directions on how to use the foam are on the can.
 
Let me know if you have any additional questions or need further information.

Bruce Jones
The Insulation Doctor