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

[Paul Newby]

One more little kick at this can...

If there are two layers of Roxul - one in the old stud
wall and another in the second interior wall, the insulation
being more or less split between the two layers, then you
can't put a vapour barrier in the middle.  However, it's
tempting to put a moisture-permeable air barrier between
the two walls, which does provide some additional protection
against moisture penetration by air infiltration.
But it's a bit of a hassle to do this - think it's
worth the trouble?

Paul N.

>> 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.
> 
> 
> _______________________________________________
> Greenbuilding email list
> List info: http://listserv.repp.org/mailman/listinfo/greenbuilding_listserv.repp.org
> List email: Greenbuilding at listserv.repp.org
> Managed by BuildingGreen, Inc. http://www.buildinggreen.com
>       publisher of Environmental Building News and GreenSpec(r)
> Hosted and archived by REPP / CREST http://www.crest.org
> 
>