RE: [GBlist] Roof vapor barriers.

["Ralph Bicknese" <ralph.bicknese@christnerinc.com>]

Sacie:

Careful.  This is one of the more hotly debated questions in residential
energy efficient building today. It is difficult to be more precise without
knowing the basic details of the building (drawings help) and the climate.

Len King wrote <Nothing should stop moisture vapor from
passing thru the ceiling and roof to the outdoors. A non permeable barrier
on the roof surface side will allow moisture to collect in the roof material
and insulation (if a cathedral) rendering it inefficient and eventually
causing damage.  A barrier on the ceiling side will stop the moisture in the
finish material of the interior ceiling and likewise cause eventual damage.>

I have a full vapor barrier under my roof just behind my gypsum board finish
and I can assure you I do not have any of the moisture damage that you have
suggested.  I designed and built the house (well I built much of it) myself,
including the vapor barrier and gypsum board so I can attest to what is
there.   My house is in St. Louis, MO, still a heating climate (even though
it is 97 degrees F, 87% rh today).  BUT if I was building in a hot humid
climate (say for instance in Florida) I would not provide a vapor barrier
inside.  The rule of thumb is to place the vapor barrier on the warm side.
(Inside in winter dominated climates).

If I understand correctly Sacie's roof does not have an attic or cavity
between joists.  She is applying rigid insulation outside on top of t&g roof
decking.  To get a bit more technical, the point at which condensation will
occur is determined by the relative humidity and the temperatures inside and
outside. A dew point study can be done to determine where condensation will
occur.

Now to generalize, and I am assuming that this is primarily a cold winter
climate situation. Because of the relatively high r value per inch of EPS,
about R-5 per inch, and the fact that Sacie has said she is providing a
pretty thick layer of insulation, it is possible (but not highly likely
unless the building is unusually humid) that condensation could occur on the
underside of the insulation, outside the vapor barrier.  In that case a
vapor barrier on the underside of the insulation might help keep that
condensation from dripping into the building.  It would be better to keep it
outside.  It is more likely the dew point temp will be reached within the
insulation.  If the building is well insulated it is less likely that
condensation will occur inside the building. If the building is very tight
mechanical ventilation (air to air heat exchanger might be warranted to
provide sufficient fresh air and help control humidity.

Because this roof has a metal roof it will not breathe from inside to out.
Metal is a much better vapor barrier than even poly. You might want to
consider cool roof design though where the metal roof "floats" above the
insulation on sleepers.  You can then vent the metal roof (perhaps with
soffit vents and ridge vents) and have a much cooler roof.  I would still
use a vapor barrier underneath given the conditions you have stated.

Cheers,
Ralph Bicknese


-----Original Message-----
From: King & Liang [mailto:lenking@blueridge.net]
Sent: Tuesday, July 31, 2001 3:25 PM
To: Greenbuilding
Subject: [GBlist] Roof vapor barriers.


the following was written:

>I need a vapor barrier above a t&g decking ceiling under a
> thick layer of EPS roof insulation and metal roof.  I would like to use
> taped 30# felt rather than taped 6 mil poly because it will be much safer
> to walk on while the insulation and hold downs are being attached.  Will
> this provide an effective vapor barrier?

<<<It sounds like you will have a waterproof and vapor proof barrier above
the
insulation (e.g. the roofing underpayment as well as the roofing) and now
want to also install one underneath the roof insulation; sort of like
sandwiching the EPS insulation between a top and bottom layer of vapor
barrier> If so, I don't know if that is such a good idea.>>>

Absolutely. The roof must 'Breath'. Nothing should stop moisture vapor from
passing thru the ceiling and roof to the outdoors. A non permeable barrier
on the roof surface side will allow moisture to collect in the roof material
and insulation (if a catherdral) rendering it inefficient and eventually
causing damage.  A barrier on the ceiling side will stop the moisture in the
finish material of the interior ceiling and likewise cause eventual damage.
All of this is due to the fact that the interior warmer air rises to the
ceiling /roof carrying the moisture with it. Stopping it, although good for
thermal loss, is bad for the moisture problem. It must pass through. Attic
spaces have some other aspects but the principles are generally the same. So
to the original question- building paper is the way to go.


Len King, NCARB
Lake Lure, NC.


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This greenbuilding dialogue is sponsored by REPP/CREST, creator of
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______________________________________________________________________
This greenbuilding dialogue is sponsored by REPP/CREST, creator of
Solstice http://www.crest.org, and BuildingGreen, Inc., publisher of
Environmental Building News and GreenSpec http://www.BuildingGreen.com
______________________________________________________________________