Re: [GBlist] Roof vapor barriers.

["John Salmen" <terrain3@home.com>]

problem with a cool metal roof (venting it slightly) is that you increase
the potential for condensation and have to rely only on the roofing felt to
direct this away. While nightime condensation is not huge it still is a
continuous form of moisture. There will always be some condensation even
with the metal tight but there will also always be some air movement just
due to the expansion and contraction of metal, ec. Running the metal tight
to the insulation (WITH A BUILDING FELT BETWEEN) provides the best practice
I think as it reduces the area and potential for condensation. The rosin
paper I think is good practice but should not be substituted for the felt.

An assembly is really only as weak as its weakest point. In this case it the
metal roofing and the potential for condensation on the underside - because
of this weakness you have to provide a roofing felt in the design as a
2ndary barrier. but you also have to minimize the amount of work this 2ndary
barrier is doing as it is the next weakest element. So it makes sense to
minimize the first weakness by providing a layer of insulation next to the
metal and reduce the potential for condensation and the amount of work the
roofing felt is doing.  The next layer of material is good in that it is
vapour impermeable but this is a factor of depth so also it does have the
ability to take in and release small amounts of moisture - and does not
deteriorate in doing so. The final layer of wood is also good in dealing
with small amounts of moisture without serious effect unlike something like
gypsum. This assembly has the potential to dry in small amounts to both the
interior and the exterior depending on the pressures active - which makes it
a good assembly for a climate with excessive humidity as well as colder
weather. I would not in practice double up on the vb in the interior as it
is a waste of material and interfers with the work that eps is better
capable of doing in the assembly.

I hope this is clear as it is rather important to consider how each material
is working and works with other materials.

John Salmen
TERRAIN EDS



A hot roof assembly
----- Original Message -----
From: "Ralph Bicknese" <ralph.bicknese@christnerinc.com>
To: "King & Liang" <lenking@blueridge.net>; "Greenbuilding"
<greenbuilding@crest.org>
Sent: Tuesday, July 31, 2001 2:16 PM
Subject: RE: [GBlist] Roof vapor barriers.


> 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
>



______________________________________________________________________
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
______________________________________________________________________