I have an existing 2-car garage which I'm in the process of
insulating. There is no living space above and it will not be a
living space when complete. This will become my workshop.

The gable roof framing is 2x4 struts, not joists and rafters as
normal. Because of the maze of strut members, it will be very
difficult to retrofit a vapour barrier inside the attic. One
possibility would be to take down all the plasterboard in the ceiling,
install the vapour barrier and resheath but I don't want to do this.

If I insulate the garage attic to R-26 with Rockwool batting (two R-13
batt layers layed perpendicular to each other), can I avoid installing
any vapour barrier and still retain the R-value of the insulation?

On Fri, 17 Jun 2005 00:10:25 -0400, someone wrote:


If I insulate the garage attic to R-26 with Rockwool batting (two R-13
batt layers layed perpendicular to each other), can I avoid installing
any vapour barrier and still retain the R-value of the insulation?

As you may already understand, the reason for a vapor barier is that,
without it, moisture from the air on the heated side will pass into
the insulation where at some point the temp is below the dew point so
the vapor will condense out as liquid, potentially soaking your
insulation. Frost or ice buildup is a usual result in cold climates;
by Spring you can have enough to actually have liquid coming through
the ceiling (in extreme cases).

SOME kind of vapor resistance is needed. You may be able to
adequately treat the garage ceiling to accomplish this, especially if
the unheated volume above is well ventilated.


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