[Greenbuilding] Open Cell Failure

[John Salmen]

I like the term predictive failure that was used to describe roofing
technology. Mostly in reference to commercial flat roof construction but it
makes sense. What it basically means is that we can assume that each
material in an assembly will be compromised to some degree. The degree of
failure depends on the materials ability to cope.  Moisture studies in
plywood and osb have shown basically that plywood can withstand repeated
wettings and dry significantly enough to cope. OSB can be more moisture
resistant but once saturated has a limited ability to transport the moisture
out.  This basically means that OSB has to be fully protected from moisture
- meaning a fully adhered waterproofing membrane over (one of the
conclusions or  recommendations from a Building Science study) and a thick
enough open cell foam barrier inside to act as a vb. (if relying on open
cell foam - minimum 4" usually).  Still a shaky assembly if the water
barrier or vapour barrier is compromised you have a material that will fail
in the middle. I've started liking engineered wood product but don't think
osb should be used on a roof.

 

Plywood is different in that it transports moisture to exterior faces. My
thought is that in such an assembly it would be a benefit to have an open
cell foam - but that is just a thought as I've not seen any moisture studies
on this.

 

 

From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org]
On Behalf Of conservationarchitect at rockbridge.net
Sent: January-22-14 8:01 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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