RE: [GBlist] vapor barrier at roof

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

Sacie:

The answer of whether to use a vapor barrier on the "warm side" under the
rigid insulation depends.  It depends on your expected relative humidity
inside and on your climate.  Generally a person should avoid a vapor barrier
on two sides of the insulation but this is less critical when using rigid
insulation.  But some roofing manufacturers will not warrant their roofing
system unless a vapor barrier on the inside (in cool climates) is used.  I
suggest you talk to one of your local roofing membrane distributors about
their technical advice.

I just spoke to the folks at Firestone who make roofing membranes.  I am
doing two commercial projects with rigid insulation on the roof and a
membrane on top.  One is an existing indoor swimming pool with precast
concrete roof deck and the other an existing school with renovations and new
additions.  The existing school has a poured in place gypsum roof deck.
Both projects are in the St. Louis, MO area.  I want to make sure we have a
durable roof and want to do what we need to meet wind resistance
requirements and maintain the 20 year warrantee.

The top membrane becomes a vapor barrier much like your metal roof will.
Because of the higher humidity in the pool we will be using a vapor barrier
on top of the concrete roof deck and under the rigid insulation.  Then we
will be using polyisocyanurate rigid board insulation (because of its high
r-value per inch), and then a white TPO membrane on top to provide our
waterproof layer and to reflect the summer heat.  The vapor barrier is a bit
problematic.  6 mil poly would be good (made of virgin vinyl for long term
durability) but the insulation has to be mechanically fastened and would
punch holes in the poly.  By the way all seams and joints in the poly should
be lapped and sealed.  Firestone recommends a vapor retarder created by
mopping on 1 layer of building felt.  The asphaltic mop on will tend to heal
itself around the penetrations from fastening the insulation.

At the school we will not need a bottom vapor barrier because of lower
indoor  relative humidity.

Sacie, you noted <Several responses have indicated that felt is not a vapor
barrier however but kraft paper is, so maybe I'll go that route>.

I have to disagree.  Unless your respondents are referring to something else
that I am not aware of, Kraft paper is a horrible vapor barrier.  Felt,
especially 30 lb. (vs. 15 lb.), is a fair vapor barrier/retarder but the
seams are difficult to seal. It would have to be mopped to be an effective
vapor barrier/retarder.

Sacie Wrote <And because the metal roofing is attached to the roof via
straps of metal,
there won't be the traditional felt on top of the EPS; it would be
redundant to place it both under and over the insulation.>

If you have not already I would consult both SMACNA guidelines and your
metal roofing supplier about whether you should put building felt under the
metal roof.  Most of the time when the metal roof system is not a purlin
system they recommend building felt with rosin paper on top.  I like the
felt as a secondary barrier in case the top roof leaks. (I put felt under
shingle roofs too even on steeper slopes where the manuals say I do not have
to). The metal can get so hot from the sun that the metal will stick to the
felt.  The rosin paper lets the metal roofing slide and expand and contract
as it should without sticking. I do not know if the metal can stick to the
EPS without the rosin paper. Make sure you alow for expansion and
contraction in your metal roof.

I hope this is helpful.

Cheers,
Ralph Bicknese

-----Original Message-----
From: Sacie H Lambertson [mailto:dlambert@grasshoppernet.com]
Sent: Tuesday, July 31, 2001 2:02 PM
To: greenbuilding@crest.org
Subject: Re: [GBlist] vapor barrier


Hmm,
	Still a bit of a conundrum. IF if were possible to really seal the EPS,
then I would leave it at that, and let it be the vapor barrier, but my
experience to date is that this is not entirely practical.  If I were
applying the foam adhesive between each 2' width of the EPS blocks then I
would be assured it would be OK, but my diligent crew, regardless of my
exhortations, will not get it all sealed, therefore I feel I may need a
separate barrier.   (I've just spend hours foaming between sections of the
EPS walls where they supposed to be already sealed).  Several responses
have indicated that felt is not a vapor barrier however but kraft paper is,
so maybe I'll go that route.   As an aside, I'm not all that concerned
about protection of the decking, since our summer weather produces
relatively little rain and we'll be doing the entire roof in sections (and
I have some big tarps).
	And because the metal roofing is attached to the roof via straps of metal,
there won't be the traditional felt on top of the EPS; it would be
redundant to place it both under and over the insulation.  On another
project, when I placed SIPs (which are EPS sandwiched between OSB) on a
roof I placed felt on top (to protect the OSB) and taped the seams of the
OSB underneath with Tyvek-type tape to make sure warm moist air did not
leak from the interior in the winter.  I think a metal roof is particularly
susceptable to too much trapped moisture.
	Thanks to all for your responses, most appreciated.   Sacie Lambertson

______________________________________________________________________
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
______________________________________________________________________


______________________________________________________________________
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
______________________________________________________________________