Another Roof Insulation Thread

[AndrewDouglasBird]

I've been reading a lot on this topic and am getting a little bit of conflicting information.

I'm wanting to insulate my garage which has a 2x4 rafter roof and no ceiling. Rafters are 24" on center. The previous owner insulated most of it already with fiberglass batt insulation, no air channel against the roof and no vapor barrier on the warm side. 1/4 of it has no insulation, but I'd like to finish the job.

I do not want to put up a ceiling, not worth the money on this old garage, plus I use the area for storage (has a wood deck right now).

If I keep the insulation like it is, with no vapor barrier and no airspace against the roof, warm, moist air will condense on the roof sheathing, correct? The only way to prevent this would be to insulate and then install a vapor barrier to seal off everything so no moisture can get in, correct?

I know spray in expanding foam would be ideal, but it is too cost prohibitive.

 

[AndrewDouglasBird]

107 views and not a single reply? :crickets:

 

[stm317]

Is the existing insulation faced or unfaced? If it's a "faced" insulation, then the facing should act as a vapor barrier.
Does the current roof have any venting? Soffit vents with baffles installed that lead up to a ridge vent is probably ideal for keeping the underside of the sheathing dry. If there's no venting, then you'll need to add some (regular roof vents, gable vents, soffit/ridge vents, etc). Without proper venting, moisture will collect, even if you have a vapor barrier installed.
If you like the advantages of foam, but don't like the cost of spray foam, you might look at rigid foam panels. They offer higher R-value per inch of thickness (which is good considering you only have 2X4 rafters), and they typically cost less than spray foam would. Obviously, they don't seal everything in a continuous blanket, but the seams can be taped off to get it close to the same performance as spray foam.