[Greenbuilding] Double stud vs foam outsulation/single stud ( was re: (not )Outgassing of Sheathing & Insulation fasteners

[John Straube]

Rob, you are correct about one thing: the problems with DSWare always due to some error of design or construction.
However, your "venture" is incorrect.  Lots of plastic insulation, even foil-faced, on the outside increases the drying rates of OSB and plywood sheathing, just the drying direction changes (it dries to the interior).
Most walls of all types work.  Every failed wall has many more examples that succeed.
The real metric is: what percentage of walls of a certain design fail?
We accept that standard 2x6 with OSB and housewrap walls fail at less than about 1% of houses.  DSW's have a higher rate of failure based on limited stats, solid science, and some carefully controlled measurements in the field.  Using lots of insulation on the exterior like Eli suggested is safer than normal houses, based on solid science and numerous detailed field monitoring studies.

There are numerous solutions to the floor thermal bridge in DSWs, but they are rarely built this way for various cost and construction difficulty reasons.

We can make almost any wall work, my comments are based on how to make it simple, repeatable, lower risk at the lowest increase in cost.

As your (and others) whole argument seems to be based on abhorence of foam, simply substitute 6" of Roxul for 6" of EPS.  We have tested both structurally, and both work (actually we tested 4" and 8" of EPS, and 4" and 6" or Roxul). And you get all the benefits of superior moisture performance and thermal performance.

John


On 12-11-28 1:54 PM, RT wrote:
> On Wed, 28 Nov 2012 13:13:24 -0500, John Straube

>
> I have cut open properly detailed/executed DSW during
> renovations/additions and found them to be in perfect condition.
>
>
>> Double stud walls also continue to have some rather significant
>> thermal bridges at floors relative to a complete blanket of
>> insulation.
>
> Again, an issue with improper detailing.
>
> There's no reason the joists can't be stopped at a ribbon joist
> located over the bearing wall which logically would be the innermost
> wall. An unbroken-by-framing layer of insulation (typically a minimum
> > R-20 for a minimum-thickness DSW) would be provided between the
> structural joist header and the curtain wall ribbon joist.
>
> In Eli's case where he's using a frame bearing wall AND 2x furring
> anyway (ie the same volume or more of lumber that would be used to
> make a DSW) why bother with the added cost/futzing needed to deal
> with a 6-8" layer of foamed plastic ?
>
>
>

-- 
Prof. John Straube, Ph.D., P.Eng.
Faculty of Engineering
Dept of Civil Engineering / School of Architecture

www.buildingscience.com