[Greenbuilding] Open Cell Failure

Wed Jan 29 14:58:02 CST 2014

http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing
I found this article quite compelling.  I hope more of our building science pros on this list will weigh in on this issue.  After considering this article and the blogs that followed, I have made some observations.  
If the open cell is absorbing humidity, but blocking air, it has a dehumidifying affect on the air inside the thermal envelope.  Buoyancy of humidity is the driving force, making the top of thermal envelope (horizontal)more vulnerable than the walls (vertical).  I believe the problem was created in the example of the article by the building felt that created a vapor barrier that blocked the continued vapor movement out of the thermal barrier.  With an open pathway blocked, the concentration of vapor increases, lowering the dew point.  However, if the top of insulation opened into a ventilated air channel, the vapor would have the opportunity to rise out of the thermal envelope and avoid problems.  With a vertical path for air in a drain dry space above insulation, we have an opportunity to actively ventilate the space if our sensors show a problematic build up of moisture.
I have become an advocate for expanded polystyrene (EPS) because of the lower embodied energy compared to Extruded Polystyrene (XPS).  I have a source for EPS that can be formed to custom size, avoiding the trash from partial cuts. Also, per R unit, it is the same price as fiberglass. Like open cell, EPS is vapor permeable but an air barrier.  If vapor with the air is allowed to pass through, the top environment can be manipulated with a fan and possibly even heat if the conditions warrant it to give us a way the enhance evaporation without the use of refrigerant driven dehumidification.  My assembly  is framing, sheathing, EPS, furring strips, purlins, corrugated metal roof. Drain dry channel would hook up to soffit vent and ridge vent.   
The closed cell foam will block the transport of vapor out of thermal envelope.  In winter conditions where humidity is very low that can be a benefit.  However, in summer conditions, an interior means of dehumidifying is required in mixed and humid climates.  That probably is the best choice is the assembly includes a vapor barrier above the insulation.  
I am still probing for a discussion on the viability of buildings that are managed without active dehumidification (air conditioners or dehumidifiers) in mixed and humid climates.  It seems like a vapor permeable foam with ventilation above is one method for reducing humidity without refrigerant driven dehumidification.  
Eli 
I have 
From: conservationarchitect at rockbridge.net 
Sent: Wednesday, January 22, 2014 11:01 AM
To: Green Building 
Subject: [Greenbuilding] Open Cell Failure

http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing
I am always on the lookout for failures with the new methods insulating buildings that have not been time tested.  The link above has an article that describes buildings in South Carolina where they found rotting osb sheathing where open cell foam was installed on the bottom.  I understand that open cell foam is an air barrier, but vapor permeable.  However, generally, I had thought, although now reconsidering, that vapor attached to air was prevented from going through.  This article sites a study that believes the vapor is penetrating and eventually getting to dew point and condensing.  Can vapor condense inside the foam?  
My bet for the cause of these problems is that the roof shingles and flashing could have delivered bulk moisture (liquid water) to the surface of the osb.  In a typical air permeable insulation assembly such as fiberglass, warm air is still passing to dry out such delivered liquid, which protects the structure such that there is no observed problems. This is one of the problems with truly air tight thermal barriers.  This problem has been observed on SIPs that have siding installed without a drain-dry space to allow for drainage or evaporation where wind and rain pushed liquid into the cracks of the siding and no air ventilation or leaking heat dried it out, causing rotting.  
Whether the source of humidity is vapor migrating through open cell foam, air and vapor migrating through flaws in the installation, or liquid delivered by flaws in the flashing and roofing, developing a drain-dry space above the assembly would allow that moisture to dry.  If the source of moisture is determined to vapor passing through the foam, a closed cell foam vapor barrier would be a superior, though more expensive choice.  I am an advocate for putting the lower priced eps foam, also air permeable, on top of the roof deck allowing it to stay warm (above dew point) and putting purlins on top that corrugated metal roofing is installed on.   
El

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