Insulation Question

[z33kyle]

I am currently planning on adding some storage space above my two car, 19x21 ft garage in the attic. At the moment there is no insulation above the garage just bare rafters. I would like to lay down plywood so I can move the junk cluttering my spare bedroom up there. Since I am laying down flooring I would like to go ahead and put in some insulation while I am up there, and also pre-wire for more garage lighting, but that is for another thread. There is no sense putting down flooring then pulling it up once I decide to insulate.

I would like some experienced opinions to make sure I am not making a mistake. This is what I have in mind. I believe my rafters are 2x8s on 16" centers. What I was thinking about doing is blowing insulation down in between the rafters and then putting the flooring down. I know you have to have a certain depth of insulation to achieve a certain R-value but I figured some insulation is better than nothing, right? Would this be the best way to go or should I make some changes in my plans?

 

[bczygan]

First some questions. Is the space above the garage substantial? In other words, can you stand up in the space? What kind of storage will there be? Heavy stuff or just light things? Do you want these things to be in the heated space? I assume the 2x8's you mention are your ceiling JOISTS (they run horizontal from wall to wall) and that you have no trusses filling up the space with 2x members, but rather, RAFTERS sloping down from the ridge to the eave. I assume you are you going to heat the garage at some point. If you want the storage space unheated, then what you are doing will work. Use visqueen on the bottom of the joists for a vapor barrier if no drywall has been installed yet. If drywall exists, use a vapor barrier paint on it. If you want the storage space to be heated, different placement of the insulation is needed. The span and capacity of the 2x8's should be checked before loading with stored materials if they are very heavy.

 

[z33kyle]

The space above the garage will not be climate controlled, just the garage interior. The garage is finished with drywall, and there is substantial room in the attic. Only light items will be stored up there. The heaviest thing being put in the attic will probably be some boxes of clothes and the random car or motorcycle part.