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If you want to add more insulation to this system, I would look to the outside as opposed to inside. Your upstairs rooms have ceilings that are supported directly by the rafters leaving you will almost no room to improve the dimensional area you have to insulate. So, the cost of removing and replacing all that sheetrock and providing spray foam, furring, the moving, the cleaning, etc. will likely average out to what it would cost you to replace your roof with a nailbase panel system. If venting is something you want, they make those too. A vented panel I recently used had a thickness of 6" with a 1" airspace and an R-value of 28. R-28 + your existing R-19 gives you an R-47... Here's an image of a roof with an insulation board layer and then a nailbase panel top layer: http://www.flickr.com/photos/hunterpanels/5654956569/
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Last edited by AGWhitehouse; 12-21-2011 at 11:46 PM.