Replies

1. 
   DanH | Dec 16, 2012 06:39pm | #1

   First off, I don't quite understand the purpose of a firewall in a DETACHED garage.  Please explain.

   You can always, if the Roxul is "free", cram two thicknesses into the walls.  This will, not, despite some predictions to the contrary, cause the world to end -- you'll get at least the insulation value of 3.5" of the stuff, maybe a little more.

   You only need a vapor barrier if there is significant humidity generated inside the building, and it tends to get pretty cold outside.  Unless you're cooking or showering in the building or you'll be throwing 3-day-long bacchanalias there, there are unlikely to be sources of "significant humidity" in a garage, plus your location suggests that the temperature differential isn't great enough for a serious problem.

1. 
   adroitboy | Dec 16, 2012 08:02pm | #2

   The firewall is required because the garage was built too close to the lot line...60 years ago. 

   I'm not sure if you've handled Roxul much (I'd never heard of it before this), but I don' tknow that 2 thicknesses of it would work. While it's not rigid like foam, it is very firm. Compressing 4" in each bay would be tough. It also would be expensive, since all that's available cheaply is enough to do the wall once. It sounds like a vapor barrier isn't super important as the inside will be very similar to the outside and nobody is living in there.

   Unfortunately the question as to having a gap causing potential problems is still unanswered. If it's OK, I'm fine to have less insulation on that wall. 

1. 
   DanH | Dec 16, 2012 09:03pm | #3

   If you don't need the insulation to keep warm then whatever satisfies the inspector is fine.  Put a little extra effort into being sure that there are no gaps around the edges.  Don't forget to tape the drywall seams.

2. 
   rdesigns | Dec 17, 2012 09:38am | #4

   Are these rockwool batts the kind encased in paper? If so, you'll get their full potential R-value in a stud bay, even if they don't fill the space.

   If they are paper-faced on one side only, be sure to install the unfaced side tight to the outside wall sheathing.to get the most insulation value.

   The reason is that unfaced batts allow convection currents to flow within the batts, and this can carry away a lot of heat.

1. 
   adroitboy | Dec 19, 2012 10:23am | #5

   No facing on the batts. They look like this. 

   From what I'm hearing, it sounds like the gap is not a concern for reasons other than reduction in insulating effectiveness. I'd love to insulate better, but I'm not really able to spend a lot of money for a building that is not occupied full-time. I'll probably do cellulose or whatever is least expensive for the rest of it in the near future. This will get me past the inspection and the rest can be done later. 

3. 
   myodb | Dec 28, 2012 01:13pm | #6

   It seems you have two things going on.  One is to create a fire resistive wall.  The second to have some insulation value in the wall.

   The reason I say this is that there are many rated wall assemblies that do not require any insulation in them.  UL is the primary tester of rated wall assemblies but there are other companies that use their testing or their own mock-ups to come up with a listed assembly.  Listed is the key word here.  As long as the assembly meets the hourly rating required and is listed by someone (UL, Warnock Hershey, OmegaPoint Labs, etc.) it should be acceptable to use.  Just to be sure, run it past your inspector first.

   The second issue being insulation, you don't seem too concerned about the R value you will achieve and maybe for a non or semi occupied space, that's O.K.  Regardless, I would place the insulation against the sheathing and install a minimum 4 mil. vapor barrier over the warm-in-winter side of the assembly just to be safe.  Plastic is cheap, and you don't want to find out later that the pool shooting friends drinking cocktails have created excess vapor now getting trapped in the wall.