| Author |
Message |
mcc351 (Unregistered Guest)
Unregistered guest
| | Posted on Wednesday, June 08, 2016 - 10:20 am: |    |
For a standard single ply roof over metal deck- I typically go with substrate board, vapor retarder, rigid insulation, cover board, and lastly the single ply membrane.
I recently read where the vapor retarder can actually be bad because it prevents the assembly from drying out. What is everyone's opinion on vapor retarders?
Also, what is the recommended attachment method for the above assembly? I would like to mechanically fasten the first layer of rigid through the substrate board and (unfortunately) through the vapor retarder. Then everything above is fully adhered. This reduces thermally bridging through the fasteners but not sure if this causes issues in trying to meet the various spec requirements (such as FM-I90, SPRI ES-1). Typically, I mechanically fasten everything except the membrane but would prefer the above method if possible. Your thoughts? |
Ronald J. Ray, RA, CCS, CCCA, CSI, AIA
Senior Member
Username: rjray
Post Number: 142
Registered: 04-2004
| | Posted on Wednesday, June 08, 2016 - 10:58 am: | |
Using a vapor retarder in a roof assemble, when not required by the building environment, may have more consequences than not allowing a roof assembly to “dry out.” I suggest you read the discussion on vapor retarders in the recent updates to MasterSpec Evaluations for whatever single ply roofing membrane you are using, as well as read the discussion on vapor retarders in the NRCA Roofing Manual - Membrane Roof Systems.
Traditionally, the attachment method you mentioned (mechanically attach a base layer of insulation or substrate board, and adhered an upper layer of insulation, tapered insulation, cover board, and roof membrane) is considered the preferred method for reducing thermal bridging. If your project requires, or wants, a FM listed assembly, consult FM Approvals RoofNav, available free online, to ensure that whatever system you are proposing meets whatever wind uplift classification you desire (FM 1-60, FM 1-90, or higher.)
If you do included a vapor retarder, you may be able to mechanically attach a substrate board to the steel deck, then adhere the vapor retarder to the substrate board, and adhere the base layer of insulation to the vapor retarder. For FM listed assemblies, ensure that the vapor retarder and attachment method is included in the listed assembly in RoofNav.
ES-1 (ANSI/SPRI/FM 4435/ES-1) is for parapet wall copings and metal edge trim, not for roof assemblies. |
Guest (Unregistered Guest)
Unregistered guest
| | Posted on Wednesday, June 08, 2016 - 11:28 am: | |
You're assembly is probably ok without a vapor retarder. Usually most adhered roof systems are ok, unless you have a high interior RH and a cold climate.
First, you've already got a vapor retarder in your assembly (the metal deck). However, when you point this out you find that designers or consultants really want a vapor retarder to be an air barrier*.
But you've already got one of those too (the roof membrane). So then you find out that what they really want is an air barrier to prevent air from moving into the assembly and around inside the assembly. This would cause warm moist air from inside to potentially condense on the colder roof membrane.
But you're planning on adhering the membrane which will prevent billowing (one of the more common means by which air is moved into and around a roof assembly. Make sure you detail continuity between air barriers and vapor retarders at roof-wall/parapet conditions, etc.
Fasten it however you want or need to in order to meet your performance criteria.
*Sometimes they also claim they want it for a temporary roof, so the building can be dried in faster and any staging/storage taking place on the roof won't damage the actual roof membrane. I say that is means and methods and not my problem as the architect. |
Justatim
Senior Member
Username: justatim
Post Number: 85
Registered: 04-2010
| | Posted on Thursday, June 09, 2016 - 07:24 am: | |
You might consider using a vapor-permeable air barrier in lieu of the vapor barrier. |
Justatim
Senior Member
Username: justatim
Post Number: 86
Registered: 04-2010
| | Posted on Thursday, June 09, 2016 - 07:27 am: | |
Oops, I meant to write "vapor retarder." |
Guest (Unregistered Guest)
Unregistered guest
| | Posted on Thursday, June 09, 2016 - 11:24 am: | |
Are there products for roofing that are vapor-permeable air barriers?
Note that there is already an air barrier in the roof without adding anything extra. C402.4.1.2.1 of the 2012 IECC notes that fully adhered single-ply roof membranes are recognized as air barriers.
Also note that even if you do use a vapor-permeable air barrier, you're probably going to be putting on over the substrate board, which is on metal deck. The metal deck is a vapor retarder, so what would be the point in finding a product that is vapor permeable? Allowing the substrate board to dry out? |
Dave Metzger
Senior Member
Username: davemetzger
Post Number: 643
Registered: 07-2001
| | Posted on Thursday, June 09, 2016 - 02:51 pm: | |
Metal deck itself is a vapor retarder, but the lapped joints between deck sections are the weak points for vapor transmission. |
anon (Unregistered Guest)
Unregistered guest
| | Posted on Thursday, June 09, 2016 - 04:12 pm: | |
Laps in metal decking don't have anything to do with vapor transmission. Laps in metal decking would be an AIR movement issue.
Misunderstanding and interchangeable use of the terms vapor retarder/barrier and air barrier is a major problem. They are not the same thing and are not meant to function the same way. Vapor retarders address vapor migration via diffusion. Air barriers address bulk air transport. A vapor diffusion retarder with 1% holes is still 99% effective. An air barrier with 1% holes is a sieve, and ineffective.
You do not need a vapor retarder in a roofing assembly with metal decking. It is air movement that may cause condensation/moisture problems in such an assembly, not vapor diffusion. |
Wayne Yancey
Senior Member
Username: wayne_yancey
Post Number: 782
Registered: 01-2008
| | Posted on Thursday, June 09, 2016 - 05:44 pm: | |
Thank you, thank you, thank you anon. |
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