[Greenbuilding] Veneer Foam Panels

Wed Jun 25 06:54:51 CDT 2014

On Jun 24, 2014, at 12:38 PM, conservation architect <elitalking at rockbridge.net> wrote:

> Thanks John for replying to my message.  I work in a mixed climate of Western Virginia. 
>  
> Coming from you, I take your comments about the fluctuation of humidity in framing outside the thermal envelope very seriously.  My thought was that I am creating a similar condition to drafty old houses that never rot because they have lots of air flowing through them.  Therefore whatever weather driven moisture gets through the siding will dry out.  My intention was to put flashing at the bottom of the wall to drain any bulk liquid.
I understand the desire, but you are not really recreating the same conditions: air is not flowing into the house as well as out of it, the interior house conditions are modern, e.g. warm and more humid in cold weather, the wood is not the same quality as what was used, etc etc.  Hence, protecting the framing and bringing it indoor is actually the more durable approach, and likely more durable than those 100 yr old durable houses
>  However, I have not considered the impact of fluctuating humidity on the dimension of the framing that my proposed panels were to cover to the interior.  So I may reconsider.  John, in you considerable experience have you seen the interior approach tried similar to what I describe?
We see variations all the time.  Thousands of retrofits with interior insulation are done each year.  they are not obvious failures! They are just challenging to build right and at higher risk than 1. normal walls and 2. exterior insulated walls.
>  
> I have renovated my home that had conventional fiberglass batts and plywood sheathing by installing 3” iso foam and mounting  3”x3/4” sheathing strips to mount plywood siding to.  This is similar to what you recommend.   In the future with this approach I would likely use 2x3’s for furring.  If the attic is kept inside the thermal envelope, I will continue the insulation over the roof sheathing avoiding thermal bridge at eave.  If using eps foam, I would ventilate with purlins under metal roof and design positive draft ventilation to carry away any interior driven vapor that would migrate through the eps foam. 
>  
> This house in South Carolina had trouble with rotting osb above a rafter cavity filled with vapor permeable open cell foam.  http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing
> This article does not give firm conclusions.  The solution this author proposes of creating a vapor armor using closed cell foam, is one approach.  However, I believe the problem was that building paper acted as a vapor barrier, raising the dew point and hence condensation and rotting.  I would propose that this problem would not have occurred if the vapor had space to draft out once it had migrated through the foam.  If installed properly, the interior draft would only be vapor, not air.  The air draft would come completely from exterior air.  Therefore the ventilation above the foam does not increase air draft in the building.  I am curious to  know if vapor that condenses inside a vapor permeable foam can evaporate again and continue drafting through once the temperature has risen. 
>  
> John’s Comment:
> The approach does solve the rather major thermal bridges of the floor penetrations throough the wall.  It does breaks the thermal bridge of the plates and studs though. 
>  
> My Comment:
> A multiple story building would still have thermal bridging of joist penetrating the thermal envelope.  I would have professional insulators install open cell foam.  However at the bottom floor and at the top flat ceiling allows perfect continuity(no thermal bridges).   The problems I have become aware of with tight houses that have siding nailed directly to sheathing that that has solid foam on the other side, is that weather driven moisture gets trapped in them and the common practice draft through the house is not available to dry it out, causing rot. 
I agree with observations.  But if we use a vented gap over foam of proper thickness, or better yet, also use rock wool insulated sheathing, drink potential is massive
>  
> My incentive for the method I was describing was to eliminate one layer of framing by making the structure and the drain dry plain the same.  Also, I was looking for the panels to allow a possible retrofit application.  However, your comments give me cause for concern. 
There are many people trying to develop exterior and interior panels.  Problems with interior are described— hard to get continuity (not impossible just hard) and panels put all the joints at one spot.  Multiple layers reduce through joints, and exterior location makes continuity practically easier.

>  
> Eli
>  
>  
> -----Original Message-----
> From: John Straube
> Sent: Monday, June 23, 2014 9:05 PM
> To: Green Building
> Subject: Re: [Greenbuilding] Veneer Foam Panels
>  
> The approach does solve the rather major thermal bridges of the floor penetrations throough the wall.  It does breaks the thermal bridge of the plates and studs though.
> It places the main structure to the exterior, and hence exposes it wild swings in temp and RH with attendant movement and durability issues. Major problem with all interior insulation solutions.
> I dont know what “drafting air” through the studs means but it does not sound like a good idea :)
>  
> Which leads to the question, why would you try to do this? It is so easy to place the interior finish, veneer grade plywood, drywall, MDF, etc. on the interior of the studs of a normal high performance wood frame enclosure (see building science.com).  Why not do that?  It is easy to add EPS to the exterior of the framing, no gluing needed. This protects the framing from temperature and humidity swings. It allows the air -water barrier to be placed over the framing, and yet protects the air-water barrier from the worse temperature extremes, thereby adding longevity and reducing the risk of cold weather condensation.  All that is needed is to add furring strips screwed through the exterior insulation, otherwise almost everything is the same as normal construction.  And exterior insulation improves the durability and performance all round.
>  
> John
>  
> On Jun 23, 2014, at 7:34 PM, <conservationarchitect at rockbridge.net> <conservationarchitect at rockbridge.net> wrote:
>  
> > I am interested in bonding a veneer grade plywood to a 4 to 6 inch thick panel of EPS foam that could be set to the interior of building framing to achieve a continuous thermal barrier.  I am wanting the beauty of the wood to replace the need for conventional sheetrock so this can be the finished surface.  I think that the thickness of the foam bonded to the plywood makes both more rigid and able to take some loading.  Although, unlike SIPs I want the main structure to be separate from the insulation (thermal barrier).  I am wondering if using low investment methods such as spreading foam appropriate panel adhesive over eps foam laid flat would achieve adequate bonding.  I am also wondering if humidity is a factor in bonding and or dimensional stability.  I would seal the seams. The seams would be covered with a finish grade wood strip.
> > 
> > If I had the structural sheathing (osb or plywood) to the interior side of conventional wood framing, siding could be mounted directly onto the framing to allow for drafting in the cavity which can serve as the drain dry plain that is needed if we are not drafting heated air through the wall.
> > 
> > The intention is to set these panels on the floor framing to be primarily self supporting avoiding the need for the long fasteners that drive up cost and likely reduce thermal performance.  In some cases, I would use the flat ceiling for the top of the thermal barrier.  I was interested in installing some exposed ceiling rafters that would be supported by a ledger mounted to the wall foam-plywood panels.  I would set veneer plywood above and support thick layer of foam.  The loads would be only the rafters, foam and the plywood.  Live load and dead loads from the main roof are carried by conventional wood trusses.  These ceiling rafters wedge the wall paneling in place.  This ceiling assembly can be set in place without bonding.  This also avoids the long fasteners.  I would top off the ceiling foam from the attic side with fire retarded cellulose to serve as the 15 minute fire barrier required by code.
> > _______________________________________________
> > Greenbuilding mailing list
> > to Send a Message to the list, use the email address
> > Greenbuilding at bioenergylists.org
> >
> > to UNSUBSCRIBE or Change your List Settings use the web page
> > http://lists.bioenergylists.org/mailman/listinfo/greenbuilding_lists.bioenergylists.org
>  
> John Straube
> www.JohnStraube.com
>  
>  
> _______________________________________________
> Greenbuilding mailing list
> to Send a Message to the list, use the email address
> Greenbuilding at bioenergylists.org
>  
> to UNSUBSCRIBE or Change your List Settings use the web page
> http://lists.bioenergylists.org/mailman/listinfo/greenbuilding_lists.bioenergylists.org
>  
> _______________________________________________
> Greenbuilding mailing list
> to Send a Message to the list, use the email address
> Greenbuilding at bioenergylists.org
> 
> to UNSUBSCRIBE or Change your List Settings use the web page
> http://lists.bioenergylists.org/mailman/listinfo/greenbuilding_lists.bioenergylists.org

John Straube
www.JohnStraube.com