[Greenbuilding] [BULK] Re: Double Stud walls (was re: How green is cellulose infill? / thermal bridging?)

[Paul Newby]

> Fiberglass insulation, IMHO, is enough reason to gut a house.  It makes 
> really good nesting material for mice, but is not much use for anything 
> else.

Maybe there's a business selling it to pet stores... ;->

> You could insulate the studs conventionally, and then add 1.5 inches of 
> foam to the inside, under the sheetrock.  This would cost a lot less 
> than adding another stud layer, and would help with the thermal bridging 
> issues.  Insulation, remember, has diminishing returns. 

Adding another stud layer costs a bit, but you pay a lot for
the high insulation density foam gives you... comparing polyurethane
foam with Roxul, about 4x as much per unit of R value, and
the difference more than offsets the cost of the extra stud
layer (reduces the cost advantage of Roxul to a factor of 3,
in this case, assuming nominal R28).  But the 1.5" of foam
is certainly simpler (BTW, what kind of foam do you have in mind?)

Of course, there's a floor area cost to adding a stud layer, but
it's hard to put a dollar value on that... it could be large
if the loss in space makes room layouts more difficult, but
otherwise the real cost is probably pretty small.

>>Not sure I get that... you mean existing trim is a problem?
> In this case there really isn't any.
> 
> When I’ve  wrapped the inside of a brick building with foam, there was 
> always little details of trim around windows and doors.  Somehow, the 
> foam was never the same thickness in any two spots, either it rides up 
> on some invisible bump underneath, or the walls aren’t straight to begin 
> with.  We ended up fiddling around with window trim an inordinate amount 
> of time. 

I see what you mean.  The time lost to fussing with the trim
is an additional cost that you don't have to pay if you add
an extra stud layer.

> Make your interior sheetrock layer an air barrier.  Get meticulous about 
> caulking around any penetrations, such as electrical boxes.  Use cans of 
> spray foam behind electrical boxes, caulk between the wall and ceiling 
> sheets, caulk the gap between the wall and the floor before base trim 
> goes on.  If you can seal this up, you eliminate the #1 way moisture 
> gets into the wall, by bulk transport. 

Good point.

Thanks,
Paul N.