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JHM161 02-13-2009 09:41 AM

Use of radient barrier insulation
I am looking to increase the efficeny of a new room added to my house. The room is over a 2 car unheated garage. Floor joist of new room are 2 x 10's . From the garage side I installed new r-13 fiberglass insulation with the craft paper facing up to the floor. I used existing r-19 fiberglass insulation, removed the craft paper and filled the remaing space of the floor joist with rods to hold it up. I will add two layers of 5/8 sheetrock as per code. Inside the room I installed encapsulated r-13 with vapor barrier facing in, the 2 x 4 walls. In the cieling which are 2 X 10's, I installed r-30 craft paper facing the room. To possibly increase the efficeny of the heat and central air I was thinking of installing a radient barrier insulation ( Prodex ) around the room stapling it to face of the 2x4's. Installing 1/2 sheetrock directly over it. I was also thinking of installing it on the 2 x 10's and then adding a 3/4 wood strip over it. Then install the sheetrock, so there would be a air gap on the ceiling. The manufacture said both applications are exceptable. Has anyone had any expierence with this product? What about both vapor barriers facing each other? My thought is the prodex will help with the radient heat comming into the room in the summer, and prevet the heat from leaving the room in the winter. Any thoughts would be appreciated

Maintenance 6 02-13-2009 11:39 AM

I can't say how efficiently the radiant barrier would work being in direct contact with other surfaces. My impression has always been that it requires an airspace to be effective. One thing is certain. Radiant barrier has a foil layer which is a highly efficient vapor retarder. If you install it over top of your insulation, then slash the kraft facing every 3 inches to reduce it's vapor retardant capability. Is the double layer of 5/8 drywall a local requirement. I thought a single layer of 5/8" type X was sufficient for code where an occupied space was located over a garage.

JHM161 02-13-2009 12:46 PM

From what the inspector told me 2 layers of 5/8 sheetrock on the ceiling and even on the bottom of the LVL beam. Single layer is OK only if it's attic space above the garage. Not sure if it's the town's code or State of NJ code. On the cieling of the room after installing the prodex I was going to install 3/4 strips for a gap between the prodex and the sheetrock. As for the walls I did not want to pack them out. I kinda thinking 2 vapor barriers together might not be a good idea. Thanks

Dragon 02-13-2009 01:38 PM

The thin radiant foil insulation must have an air gap to work properly. Generally 2 or 3 inches at a minimum. If you sandwich them between something they will no longer work properly. They of course will still work but that R rating drops to the low single digits. This is because they are now working in a conductive situation and they are not designed for that.

It would be much better to use the big sheets of foam if you are looking to bump up that R factor.

Another place people think this foil insulation can be used is under a concrete slab. Absolutly not, unless you are going to somehow elevate your slab to provide the necesary air gap. If you poor the concrete right on this stuff the same things happens as above, it stops working properly.

Good luck!

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