<div dir="ltr">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.</div>
<div class="gmail_extra"><br clear="all"><div>Gennaro Brooks-Church<br>Director, Eco Brooklyn Inc.<br>Cell: 1 347 244 3016 USA<br><a href="http://www.EcoBrooklyn.com" target="_blank">www.EcoBrooklyn.com</a><br>22 2nd St; Brooklyn, NY 11231<br>
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<br><br><div class="gmail_quote">On Wed, Jan 29, 2014 at 3:58 PM, <span dir="ltr"><<a href="mailto:conservationarchitect@rockbridge.net" target="_blank">conservationarchitect@rockbridge.net</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div><a title="http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing" href="http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing" target="_blank"><font size="4" face="Times New Roman">http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing</font></a></div>
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<div><font size="4" face="Times New Roman">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. </font></div></div>
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<div><font size="4" face="Times New Roman">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.</font></div></div>
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<div><font size="4" face="Times New Roman">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. </font></div></div>
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<div><font size="4" face="Times New Roman">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. </font></div></div>
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<div><font size="4" face="Times New Roman">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. </font></div></div>
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<div><font size="4" face="Times New Roman">Eli </font></div></div>
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<div>I have
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<div><b>From:</b> <a title="conservationarchitect@rockbridge.net" href="mailto:conservationarchitect@rockbridge.net" target="_blank">conservationarchitect@rockbridge.net</a>
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<div><b>Sent:</b> Wednesday, January 22, 2014 11:01 AM</div><div class="im">
<div><b>To:</b> <a title="greenbuilding@lists.bioenergylists.org" href="mailto:greenbuilding@lists.bioenergylists.org" target="_blank">Green Building</a> </div>
<div><b>Subject:</b> [Greenbuilding] Open Cell Failure</div></div></div></div>
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<div><a title="http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing"><font size="4" face="Calibri">http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing</font></a></div>
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<div><font size="4" face="Times New Roman">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? </font></div></div>
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<div><font size="4" face="Times New Roman">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. </font></div></div>
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<div><font size="4" face="Times New Roman">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. </font></div></div>
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<div><font face="Times New Roman"></font><font size="4" face="Calibri"></font> </div></div>
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<div><font size="4" face="Times New Roman">El</font></div></div></div></div></div>
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