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To Add Vapor Barrier Or Not?

I live in Independence, Oregon (west of Salem) in a house built in 1998. The attic has blown-in insulation. The attached garage has no insulation in the ceiling and I am planning to add some to lower the heat absorption in the summer time. I notice that the blown-in insulation over the main house has no vapor barrier underneath. Why?

I intent to use batts for the garage portion, how should I install it? With or without a VB?

Much appreciated!

Timothy Miller
Re: To Add Vapor Barrier Or Not?

Howdy what type of blown in insulation? Cellulose or fiberglass?
Is the garage roof vented? You might look at ATTICFOIL.COM an instead of insulation add attic foil to the rafters. this product has great applications and is very little cost. The web info videos are very helpful too.

Vapor retarders - are not barriers but have a low moisture transfer rates some very low. The homes interior paint on the walls and ceilings retard quite well some paints even designed to transfer quite less ask local paint stores for specifics. I have applice caulk to the sill and top plates of walls when sheet rock to diminish the convention, chimney effect, an air infiltration along with caulking around ceiling penetrations. The stopping air infiltration greatly reduces moisture transfer too.

Cellulose allows moisture vapor to move threw to the outside an inside of the dwelling depending on the season. This allows it to dry the cavities. If you install a vapor retarder, 6 mill plastic, this stops the movement and can result in rot to the framing members.

Re: To Add Vapor Barrier Or Not?

Thanks for the response. I believe it is fiberglass. The garage and main structure are connected and completely vented. Really, I am curious why the main house doesn't have a VB or VR, is this standard?

Based on what I have read (and I'll look into the attic foil) I am leaning toward not adding any VB or VR because I am not going to seriously heat or cool this space - My intent is to slow the heat transfer from the hot attic to my garage in the summer and during the winter from time to time, I will add some heat from the house (by opening the man door which connects the garage to the utility room and main living area) to take some chill off to work out there etc.


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