[Greenbuilding] (not) Re: Outgassing of Sheathing & Insulation fasteners

[John Salmen]

I did one double stud wall which felt like an incredible waste of wood and I worried about moisture in the assembly. There is really nothing easier and faster than laminating 6" sheets of foam to exterior sheathing with a polyurethane foam adhesive. Exceeds nailing for astm standards. Assuming walls and foam boards are relatively flat requires only a few spot nails or bracing to adhere till the initial set - the foam adhesive expands to create continuous contact. 

For outboard strapping ripped exterior ply is flatter and easier to glue to the foam than 1x products with better holding strength. Typically spot fastened with pneumatic staples or a roof nailer. 

Or you can screw the whole assembly on... I can see that it is easier to create a working standard for screwing foam on - but it is like holding h'orderves together with toothpicks - a real chef would use an adhesive food layer. The place for fasteners is sheathing to framing done structurally and siding to furring done cosmetically.

Like John mentioned I do like having a structure sheathed. I will (sigh) use osb if it is well back from any potential weather exposure. Seams can be taped and it does bond better to foam as it typically is a flatter product than plywood sheathing and same chemicals (just never dries if it does get wet).  Taping seams I think eliminates the need for the housewrap which allows for an adhesive rather than mechanical bond. Two staggered layers of 3" foam work better to create an exterior seal but extra work to create a good bond.



-----Original Message-----
From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of John Straube
Sent: November-28-12 10:13 AM
To: archilogic at chaffyahoo.ca; Green Building
Subject: Re: [Greenbuilding] (not) Re: Outgassing of Sheathing & Insulation fasteners

Double stud walls have been used since the late 70s to reach super insulation.  Problems with condensation accumulation at the outer layers have been reported since then. We completed our last forensic investigation on a double stud last winter.  I am sure we will have more.
Using any thick layer of insulation in a wall or roof will cause the exterior sheathing to get very cold.  Very cold sheathing is much more likely to be exposed to condensation for more hours and greater severity than warmer sheathing.  Cold sheathing will also dry much more slowly than warmer sheathing.  Thus, colder sheathing is more susceptible to more damage.  Not a problem if the building is built perfectly, but then no building is.
Exterior insulation, foam plastic or rockwool, is much safer as the framing and sheathing are maintained at warmer temperatures, basically eliminating the chance of condensation in Eli's scenario, and enhancing drying relative to normal construction.  Risk goes down relative to normal construction.
Double stud walls also continue to have some rather significant thermal bridges at floors relative to a complete blanket of insulation.
All of this is laid out in several reports on buildingscience.com under the title of "high-r walls" supported by lots of science, computer models, and case studies if such things are interesting for you.
In short then, 6"-8" of EPS will be much less moisture risk, and slightly better insulation than a stud wall.
Experience has taught those who have built a few of these externally-insulated buildings that they can be easier to build too.  This means one has to learn how to handle the remaining challenges 1. how to find low cost long screws that screw in quickly and easily 2. the furring needed to allow for easy cladding fastening 3. the details at window and door openings required to mee Point 3 is still evolving and improving.

To answer Eli's question though, 7/16" OSB (much more common than 1/2") does have pullout resistance to nails and screws. no predrilling or super coarse threads required. See attached data sheet. A standard 10d ring-shank nail, for example, has an average ultimate load of over 150 pounds. Ring shank are always way stronger in pullout than other choices and approach the pullout of normal wood screws.  Applying a safety factor of 4 gives a safe load of 37 pounds. This is sufficient to resist suction wind loads in most jurisdictions away from coasts if you have furring at 16" and apply nails every 12" vertically.

Screws into 7/16" OSB will have different loads: you will have to test yourself or find a company that reports test data for 7/16" OSB.  There are many companies that will report values for 3/4" plywood (common commerical substrate) but none report data for OSB.

I am not convinced that hitting the studs is so difficult it is worth trying to make use of the OSB.
I like the OSB as a rigid air barrier, taped at joints, protected from extremes in temperature by the insulation. For my own house, I would add a draining housewrap over the OSB as a drainage plane, but one could seal the joints in the EPS with something like BASF's Enershield to control rain.



On 12-11-28 12:00 PM, RT wrote:
> On Wed, 28 Nov 2012 09:48:23 -0500, Eli Talking 
> <elitalking at rockbridge.net> wrote:
>
>> In the house I am  designing, we are planning on putting all the 
>> insulation on the outside, 6” or 8” EPS, still to be determined.
>
>> 2x furring
>
>> I am wanting to know if the  withdrawal strength of .5” plywood or 
>> osb would be sufficient.
>
> I'd have to check some old timber engineering texts for the w/d 
> strengths of the various sheathings(which you could do yourself and 
> probably on the WWWeb these days) but I wouldn't rely upon either for 
> any significant withdrawal resistance unless using a very,very 
> coarse-threaded screw -- so coarse that it'd require pre-drilling 
> humongous holes to accommodate the thread (leaving a large-ish void 
> around the unthreaded shank of the screw where it passes through the 
> 2x furring.
>
> If one is going to be installing 6 or 8 inches of EPS insulation on 
> the outside presumably of a stick-framed 2x4 or 2x6 wall and then 
> install 2x furring on top of that, I do have to wonder why one 
> wouldn't simply do a double-stud wall  (ie 2x4 @16" o/c bearing wall
> + 2x3 @24" o/c curtain wall, spaced apart by a minumum of 3.5" (and
> no practical maximum) of continuous insulation unbroken by framing ... 
> curtain wall and bearing wall joined by 1/2" ply flanges at top plate 
> & sill plate + 3/8" ply or 11mm OSB @ door & window openings ... all 
> assembled on the floor deck and tilted-up together as with single-stud 
> assemblies) and fill the cavity with either batts or blown-in fibrous 
> insulation thereby avoiding the higher cost of the foamed 
> plast-ecchhh! insulation and all of the associated headaches (ie 
> window detailing for one) in having to futz with that foam ?
>
> If using batt insulation, it can be installed in the curtain wall 
> framing and in the continuous insulation layer between the walls while 
> the wall is still laying on the deck, with the air barrier located at 
> the outside plane of the bearing wall.
>
> Once tilted up, such walls are stiff enough and fat enough that they 
> almost don't need any temporary bracing to remain straight and plumb 
> while interior partitions and upper-storey framing are being built.
>
> Also, since the wall air barrier is complete and out of harm's way 
> once the walls are tilted up, services can be installed in the bearing 
> wall without having to worry about what the sub-trades do or don't do.
>
> Of course, if one has the hots for foamed plast-eccchh! insulation, 
> there's no rule that says it can't be inserted in the continuous 
> insulation space between the two walls instead of fibrous insulation.
>

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

www.buildingscience.com