Replies

1. 
   DanH | Dec 23, 2014 10:20pm | #1

   Before we can answer -- where is this structure??

2. 
   DanH | Dec 24, 2014 02:02am | #2

   The half-inch gap itself is not a serious problem, and any insulation is better than no insulation (which I assume is the current situation).  However, you might want to consider using 5.5" insulation instead of the 3.5" stuff.  You would not, of course, get the full insulation value of 5.5" when you compress it to 4", but you will do better than with 3.5" insulation.

   If you installed 1/2" foam board in the cavities before the fiberglass you'd run a moderate risk of having a (misnamed) "double vapor barrier problem" and capturing moisture between the fiberglass and the foam.  You could minimize this risk by installing a tight vapor barrier (poly sheeting, stapled across the joists and with all seams taped) just behind your new drywall.  (And this isn't a bad idea even if you don't use the foam board.)

   The full-bore, all-out solution, of course, would be to get foam sprayed into the cavity, but that would be something of a waste if the rest of the house is left uninsulated.

3. 
   Mike_Mahan | Dec 24, 2014 11:05am | #3

   Another option

   You might want to consider dense pack cellulose to fill the entire 4" cavity.

4. 
   MRuiz | Dec 24, 2014 12:37pm | #4

   DanH,

   thank you for the detailed response.  I will then go with the 5.5" insulation and I had been considering installing a poly vapor barrier over it and now I see it is worthwhile.  Thanks again!

5. 
   catmandeux | Dec 24, 2014 02:03pm | #5

   Another option is mineral wool insulation:

   I am a big fan of mineral wool insulation  ( i.e. Roxul, Stone Wool, Thermafibre...)  A little better in the R value than  fibre glass, easier to install, fireproof, sound absorbing, not as susceptible to wind washing.

   Big advantage in your case is it can be sliced easily and accurately , so you could take a regular batt, and slice it  4 inch thick, and stack these pieces in the wall for a perfect fit.  The insulation is not affected by the way it is installed.

   Alternately , install a 5.5 inch batt, and slice flush with the face of the studs. Use a three of the 1.5 thick cut offs to fill the next cavity.  No waste, no significant overcompression.

   There are special knives for mineral wool, but a long serrated kitchen knive works just as well for occasional use.

6. 
   user-2588683 | Jan 05, 2015 11:01pm | #6

   True 4 inch insulation.

   Just wondering - would there be a downside to putting 3"+ polyiso board between the studs? It seemed like a simple, economical solution to me, and that's what I did - good or bad.