[Greenbuilding] Open Cell Failure

Wed Jan 29 15:06:36 CST 2014

A little bit of a side point but I like the way thatched roof buildings in
the tropics don't let water in but are a massive sieve for upward moving
hot air. They increase this movement with a hole in the center. Great
design but I have not figured how to apply that design to a building in
say, NYC, where it needs that design in the summer but NOT in the winter.

Gennaro Brooks-Church
Director, Eco Brooklyn Inc.
Cell: 1 347 244 3016 USA
www.EcoBrooklyn.com
22 2nd St; Brooklyn, NY 11231

On Wed, Jan 29, 2014 at 3:58 PM, <conservationarchitect at rockbridge.net>wrote:

>
>
> 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 <greenbuilding at lists.bioenergylists.org>
> *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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