Insulating Rim Joists?

I recently spoke with my town's inspector and he told me that I
shouldn't use rigid foam to insulate Rim Joists. He went on to say
that Batt insulation fills the cavity better and that if I installed Rigid I would need a lot in order to reach the minimum of R-19. I'm
very confused because everything I've read says to use the Rigid. Am i missing something?

 

I used rigid 2" XPS that I held in place with expanding foam to air seal and then stuffed in 6" of the fiberglass junk that I had left over to fill the void. - Far beyond the code.

I am not a great believer in the semi-scientific definition of a vapor barrier since the definition of a vapor barrier is very arbitrary and is just relative. Any extra coat of latex can change a wall to one having a double vapor barrier.

 

There is a big difference between EPS foam which has poor r-value per inch (white beadboard) when compared with ISO or XPS foam which has a much better r-value. Not knowing the whole story, it's hard telling what sort of foam he was referring to. Maybe he misunderstood what type of foam you were planning on using.

 

I just asked him if I had to cover rigid foam with sheetrock in the Rim Joist when insulating this area. He couldnt understand why I wanted to use Rigid foam because he said that I would have to cut every piece exactly to size in order to meet insulation requirements. He said just use Batt insulation, "It fills the whole cavity" "And if you used Rigid you would need a lot for me to pass it." R19. I was surprised that he was so confused by this when everything I've read on the internet says to not use Batt insulation.

 

Batt insulation with out a rigid air barrier and moisture barrier will allow moisture to pass to the rim joist and condense. When rigid insulation is used on an outside wall or inside on that rim joist, there is a ratio between the r-value of the rigid and the r-value of any additional insulation added. To exaggerate, it you installed 1" of rigid (R-5) and then stuffed in 12" of fiberglass (r-38) then the inside surface of that 1" of rigid would still drop below the dew point and moisture/frost would form. By using more rigid or less fiberglass that ratio can approach 50%- 50% which should be more than adequate for your location, NJ I think.

Now, for the fire protection which he didn't answer, Dow Thermax has been tested and approved in some locations to meet the fire codes, but your code official has control. If he doesn't want to see it, then it probably shouldn't be used. The only other detail that he didn't mention is a provision for the termite inspection. Where that is a problem, the top 3" or so of the foundation has to be visible or something removable to make is visible. I'm going by what I have read as we don't see those requirements north of you.

Bud

 

I just spoke with a GC this evening and he said that the insulation needs to be something like Roxul because any insulation under the first floor next to the foundation needs to block a fire from spreading. If this is true how can anyone put Rigid up there?

 

The rigid insulation needs to be covered with a fire barrier like drywall, or metal foil on Thermax, or a very expensive paint. Roxul would be better than fiberglass, but it still permits vapor diffusion, although the amount might not be a problem.

Bud

 

My solution seemed to be correct or at least usable for patch job.

2"of XPS (not EPS) that was applied to the rim joist when the foam air sealing was done and this provided adhesion. Fortunately, I use unfaced fiberglass (because scraps were available and no better insulation, like Roxul, was handy) Everything was covered in the small (4 high) area with 5/8 drywall. No problems after 5 years. - Not exactly by the book that some people read and memorize, but works and makes sense.

Dick