| Author |
Message |
Tracy Van Niel, FCSI, CCS
Senior Member
Username: tracy_van_niel
Post Number: 335
Registered: 04-2002
| | Posted on Thursday, July 31, 2014 - 11:18 am: |    |
I have been asked by the project team to put this question out to y'all for input and wise words.
We are preparing documents for a small band room addition to an existing school building in northwest Ohio. The space will not be humidified and is mostly open except for a couple of practice rooms and storage.
The criteria documents that were prepared by another firm have defined the roof sandwich as metal deck, vapor barrier, nail base insulation, roof underlayment, then finally shingles.
The concern in-house is whether the vapor barrier between the metal deck and the nail base insulation is needed (and what should it be if it is) or whether we would be better off using something like Ice & Water Shield over the entire roof deck (instead of just the eaves with 'roll roofing' over the remainder of the roof area) and delete the lower vapor barrier all together.
Thoughts?
Thanks for your help.
Tracy L. Van Niel, FCSI, CCS |
ken hercenberg
Senior Member
Username: khercenberg
Post Number: 805
Registered: 12-2006
| | Posted on Thursday, July 31, 2014 - 11:40 am: | |
Won't your mechanical fasteners turn your barrier membrane into swiss cheese anyway?
Typically NRCA requires a vapor barrier that far north at flat roofs but I don't recall the requirements at steep slopes. If you use the peel-and-stick, where would you locate it? Over the nail base? Again, won't that result in hundreds of penetrations? I know manufacturers claim that the membrane will pucker and seal around the fasteners, but I don't know how real that is. Anything less than 60 mils typically doesn't self-heal properly and cold weather will slow the flow so it may not work too well in winter when melting snow will find its way along the fasteners. A good spray-applied membrane such as the GE or Dow silicone air and water barriers might work at the deck before installing the nail base insulation but they are vapor permeable. You'll need a continuous flat substrate over the deck to be able to install a sheet membrane under the nail base insulation. If the roof underlayment is not vapor permeable (most sheet membranes are not vapor permeable), the addition of a vapor barrier under the nail base insulation will create a moisture chamber which will create more problems than it will solve. If the membrane under the nail base insulation is meant to serve as a drainable plane, you will need some means to get rid of water that will form there.
Sorry. Raising more questions than answers for you. |
Dave Metzger
Senior Member
Username: davemetzger
Post Number: 523
Registered: 07-2001
| | Posted on Thursday, July 31, 2014 - 12:03 pm: | |
More questions in addition to Ken's.
What is the roof slope? If less than 4:12, you'd want a self-adhesive underlayment under the shingles to act as waterproofing. As long as fasteners through the self-adhesive (peel-n-stick) membrane are not withdrawn, I have not heard of problems with water through fastener penetrations.
As Ken noted, peel-n-stick membranes are almost all vapor-impermeable, and its location on the cold side of the insulation could be an issue. However, this is frequently done, so may not be an issue if the addition is well-ventilated (you note it will not be humidified.) |
anon (Unregistered Guest)
Unregistered guest
| | Posted on Thursday, July 31, 2014 - 04:00 pm: | |
So much confuse.
Metal decking is a vapor retarder.
Most closed cell roofing insulations are vapor retarders as well (depending on type and thickness). If the nailable portion of the nailable base insulation is composite wood, then that is also a vapor retarder. So, you have 3 layers of vapor retardancy in the assembly without adding a layer specifically called a "vapor retarder."
Many people are confused about the purpose of a vapor retarder. A vapor retarder is used to slow the transfer of water, in vapor form, due to diffusion. it is not meant to be used as an air barrier. If a vapor retarder is 99 percent intact, it is 99 percent effective. If, on the other hand, an air barrier is 99 percent intact (1 percent holes) it is severely compromised.
The absolute best source of information on whether or not you need a vapor retarder in a roofing assembly can be found here:
http://www.rci-online.org/interface/2013-CTS-teitsma.pdf
You do not need another vapor retarder in your assembly. |
Tracy Van Niel, FCSI, CCS
Senior Member
Username: tracy_van_niel
Post Number: 336
Registered: 04-2002
| | Posted on Friday, August 01, 2014 - 09:37 am: | |
Thanks for the input, everyone. I'm going to pass this thread along to the project team to see what they'd like to do.
I appreciate your help!
Tracy L. Van Niel, FCSI, CCS |
J. Peter Jordan
Senior Member
Username: jpjordan
Post Number: 748
Registered: 05-2004
| | Posted on Friday, August 01, 2014 - 10:26 am: | |
Most people are probably confused about the peel-n-stick products like Grace's Ice and Water Shield. Their purpose is to provide a layer of waterproofing under the roof when the primary roof material is compromised by ice build up at the eaves (ice dams). Most of us probably see this put under the primary roofing material (shingles, tiles, or metal panels) over the entire roof, not just at the edge so it is easy to forget the original purpose.
Although I see the product referred to as "Ice and Water Shield" on the Drawings, I usually specify a high-temperature (HT) product, and there are comparable products from several different manufacturers (I believe there are two available from Grace). The more correct term for these products is "UNDERLAYMENT" and it should be thought of as a replacement for the No. 15 or No. 30 asphalt felts that are probably still used in some construction. The primary purpose of these products is to shed water (liquid) that may get under the primary roofing material. |
Sheldon Wolfe
Senior Member
Username: sheldon_wolfe
Post Number: 783
Registered: 01-2003
| | Posted on Friday, August 01, 2014 - 10:57 am: | |
Ice & Water Shield is a registered trademark of Grace. As Peter says, use "underlayment"; even Grace calls its product roofing underlayment.
This type of product was not intended to replace roofing felt, so two types of underlayment often are used together, making it necessary to differentiate between them. |
ken hercenberg
Senior Member
Username: khercenberg
Post Number: 807
Registered: 12-2006
| | Posted on Friday, August 01, 2014 - 11:09 am: | |
Anon, based on the article you posted we find that metal deck is, in fact, not a vapor retarder unless the joints between deck sections, penetrations, and the perimeter are sealed. Additionally while XPS board can serve as a vapor retarder when joints, penetrations, and perimeter are taped, that is rarely done. Composite wood is not recognized as a vapor retarder, nor is gypsum sheathing. The fact is that most conventional roofs are not designed to 'dry out'. Adding a vapor retarding membrane prevents the roof from 'drying in' but may prevent vapor from migrating from the interior of the building into the roof system, especially in Regions 5 - 7. PRMA assemblies are still probably the best way to resolve this situation but most roof systems are still conventional from what I see. In this case of a shingle roof, PRMA is obviously not an option.
As to the underlayment membranes, I agree that most of us tend to line the entire roof, not just use the membrane as ice dams. I prefer the butyl rubber products since they are more flexible in cold weather and less likely to 'run' in hot temperatures. I agree with Dave and Peter that their primary purpose is to serve as waterproofing but they do add a vapor retarder into the system. It is for that reason that I usually prefer to see it placed under the insulation instead of over the nail base layer except as an ice dam. In those conditions, I would probably prefer to see a 30# felt under the shingles and over most of the nail base except at ice dam conditions with the membrane under the insulation. |
Ronald L. Geren, FCSI, AIA, CCS, CCCA, SCIP
Senior Member
Username: specman
Post Number: 1250
Registered: 03-2003
| | Posted on Friday, August 01, 2014 - 01:10 pm: | |
Sheldon, although the product was intended for a specific purpose, since it complies with ASTM D 1970, it can be now used in lieu of the required felts per the 2012 IBC.
This was added as an exception to a requirement that was added regarding underlayments in high-wind locations. This requirement was added to each steep-slope roofing material.
In my opinion, these were a poorly inserted requirements. If self-adhered underlayments are suitable for high-wind locations, then they should be suitable in areas that are not classified as high-wind. Therefore, it should have been added as an acceptable underlayment in each of the underlayment sections.
Ron Geren, FCSI, AIA, CCS, CCCA, SCIP
www.specsandcodes.com |
Wayne Yancey
Senior Member
Username: wayne_yancey
Post Number: 672
Registered: 01-2008
| | Posted on Friday, August 01, 2014 - 01:37 pm: | |
Tracy,
I recommend keeping the vapor barrier on the warm-in-winter side of the continuous roof insulation and connect to the wall air/vapor barrier.
Wayne |
anon (Unregistered Guest)
Unregistered guest
| | Posted on Friday, August 01, 2014 - 12:50 pm: | |
Of course metal decking is a vapor retarder! Metal decking has a perm rating of well below 1.0 (which is a Class III vapor retarder in the IBC). As are composite wood products and closed cell insulations. Here's a handy chart that illustrates that MANY building materials meet the code definition of vapor retarder:
http://www.uaf.edu/files/ces/publications-db/catalog/eeh/EEM-00259.pdf
Please check the latest version of the building code. You will see that vapor retarders are now classified based on differing perm ratings, and required based on the wall assembly. Here's a little primer for you on that issue. You will see in this link that rigid insulations are Class II vapor retarders:
http://energy.gov/sites/prod/files/2014/01/f6/4_3b_ba_innov_vaporretarderclassification_011713.pdf
Your argument that seams and fasteners render the material NOT a vapor retarder is weak. If you follow that logic, then no plastic sheet placed between gypsum board and studs can be a vapor retarder either, because of all the screw/nail penetrations that go through it to attach the gypsum board to the studs.
Again, I submit to you that you are confusing "air barrier" with "vapor retarder." They are NOT the same thing, and are NOT interchangeable! I encourage you to read the paper I provided the link to as well as Lstiburek's excellent and many free-to-the-public articles on the subject.
It is dangerous to hand out building science related advice if not fully up to speed on the science. |
Sheldon Wolfe
Senior Member
Username: sheldon_wolfe
Post Number: 784
Registered: 01-2003
| | Posted on Friday, August 01, 2014 - 05:06 pm: | |
Ron, we regularly use peel & stick just about everywhere. As others have noted, we often use the high-temperature varieties. In residential construction, in which I (thankfully!) am not involved, I wouldn't be surprised to see it used only in areas where ice dams would occur, with the rest of the sheathing covered by roofing felt.
I'm sure there is good reason to not use it in areas not classified as high-wind. It might be something as simple as saving jobs in the roofing felt industry. |
Wayne Yancey
Senior Member
Username: wayne_yancey
Post Number: 673
Registered: 01-2008
| | Posted on Friday, August 01, 2014 - 05:13 pm: | |
But what about the penetrations and terminations?
The penetrations and terminations indeed!
All y'all make this to complicated.
Please go to http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall?topic=doctypes/insights
Wayne |
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