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

[Lawrence Lile]

>In this case I decided against going outside for various reasons
(e.g., roof overhang is very short on one side and would be hard to
extend).  Also the inside was a major mess, much of the existing
nominal R10 fiberglass insulation 

 

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. 

 


>I considered just foam between the existing studs, but in
this case it seems that thermal bridging from the old framing
(esp. uninsulated headers, three-stud corners etc.) places a tight
upper bound on the achievable level of insulation.  But if
you add another 2x4 layer and then spend a lot of cash on extra
insulation in the straight runs of stud walls, you're still limited by
the layer
of old framing - i.e., if you apply R24 foam on top of uninsulated
headers and three-stud corners to get localized levels of
insulation of R28 or so, adding R40+ insulation to the 7" depth
in the runs of straight walls might not be the most cost-efficient
thing to do.

 

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.  



> Also your trim never fits right...

>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.  

 

*      Don't worry so much about the vapor barrier issue. Use a drainage
plane
> under whatever siding you choose, and try to make your wall airtight.

>Since I'm working inside, I was planning to leave the cladding
and siding alone.  I'm concerned about running the Roxul right against
the inside surface of the cladding, however - although there's
no sign of moisture problems in the existing walls, it seems
counterintuitive not to have some way of dealing with moisture
that may find its way to the boundary of the insulation and
cladding.  Any thoughts on that?



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.  

 

--Lawrence