Several years ago I began the process of remodeling my cottage. I had a new roof system installed so my trusses were open prior to winter so I opted to buy some 6" insulation and placed it in the truss cavities as a temporary insulation barrier while I worked on the place over the winter.
I talked to a few insulation contractors and they recommend I go with a blown fiberglass rather than batts in the ceilings so I now have 1000 square feet of 6" (R-19) insulation I can't use. I tried selling it but there's not much of a market out there for used insulation so my question is rather than eat the cost of the insulation couldn't I use the R-19 batts to insulate my 2"x 4" walls.? I'm well aware of the fact I will loose a significant amount of the R-value by compressing the batt but that's not a huge factor as long as I would get close to the R-13 a normal batt would net me. I'm furring the walls out an additional 3/4" with furring strips to attach vertical paneling so the net cavity depth is really 4 1/4" deep. So would you recommend that I use these 6 inch batts in a 4 " wall and what are the consequences of doing so?

I re-sided my cabin a few years back and added a ½” foil faced Polyisocyanurate foam sheathing insulation (perm rating of 1) over the OSB before I sided. So the outside layer (from the inside out) is as follows: 2x4 Stud, ½” OSB, ½” Foil faced insulation board, Tyvec, vinyl siding. 

I’m now in the process of gutting all of the inside walls so I can reinsulated and redo the drywall.

The concern I have is I never considered the perm rating of the ½” foil faced Polyisocyanurate foam sheathing before installed it. They say most brands out there have a rating of 1 or lower which would prevent water vapor from penetrating to the wall cavity. Now if I add a vapor barrier to the inside wall I could possibly trap any moisture that finds its way inside the wall cavity (a certain recipe for mold).

A friend of mine said I should be fine using the logic that water can’t get into the cavity from either side because the perm ratings of the vapor retarders on both sides won’t permit it. If water does find an outlet in it should be able to escape the same way it came in. 

So, just wondering if that logic makes sense to you. The only other option I have would be to eliminate the inside vapor barrier and let the wall breath from the inside but that doesn’t seem right in this climate (northern Wisconsin) where indoor air humidity can fluctuate dramatically. 


Answer from the Insulation Doctor

I absolutely would not recommend installing R-19 blanket insulation in the 4-1/4" space you have. You are correct in assuming you would lose your R-value, however, you would also create undo pressure against your drywall or paneling causing nail pops and bowing.

You could peal the insulation pulling off 2" from the back side. This can be accomplished by pulling off 2" of insulation from one of the 2 ends. The insulation will only peal one way so you need to determine which end to start at. When installing the insulation be sure to place the paper on the inside (heated side). This is not a perfect resolution to your problem but it will suffice.

Note: As a contractor, I would have blown insulation on top of the blanket you had already installed in your ceiling originally.


Regarding your question concerning the double vapor barrier in the outside walls of your cottage, your friend's opinion is correct for the most part. The area you are dealing with is considered to be a dead space when properly insulated. Therefore, you will have no penetration of moisture on either side. I have personally experienced a situation in a similar climate in my own home and have not had any problems. Your only concern would be if you were to have a roof leak, a water pipe break, or extensive water damage in the wall for any reason, in that case you would have to open the wall.