| Author |
Message |
Dave Metzger
Senior Member
Username: davemetzger
Post Number: 444
Registered: 07-2001
| | Posted on Wednesday, October 31, 2012 - 06:18 am: |    |
I have a roof detail for which I'd appreciate some advice from my 4specs colleagues.
The condition is a composite steel deck/concrete roof deck, with a hot fluid-applied roofing membrane and an extensive planted roof assembly above. Code requires an R-value of R-38 for the roof; the architect wants to use 2"-3" of extruded polystyrene above the membrane, with spray-applied closed cell plastic foam below the steel deck to provide the balance of the necessary insulation.
This raises the following questions:
1. The roofing membrane above and the steel deck/closed cell foam below effectively provide a vapor retarder on both top and bottom of the slab. This can trap moisture in the slab, leading to failure of the roofing membrane. Even if an open-cell foam or glass fiber/mineral fiber insulation was used below the deck, the steel deck would still be a vapor retarder. How real of a problem is this? Can the slab be vented from the top (through the roof membrane), and would this be effective through a planted roof?
2. The plastic foam below the steel deck requires the use of a thermal barrier, of 1/2" gypsum board or another approved material. Would FRT plywood or FRT particle board be suitable? (Obviously the use of glass or mineral fiber insulation means a thermal barrier is not needed, but also requires substantially more insulation).
3. It seems that the most effective location for the thermal barrier would be in direct contact with the plastic foam; however, I do not see that the code requires such direct contact. So, how much of a gap between the foam and the thermal barrier can there be before the thermal barrier is no longer effective--2"? 6"? 12"?
Thanks for the help. |
Jeffrey Wilson CSI CCS
Senior Member
Username: wilsonconsulting
Post Number: 84
Registered: 03-2006
| | Posted on Wednesday, October 31, 2012 - 08:10 am: | |
I understand the concern about trapping moisture, but the condition w/ composite deck & roofing membrane is so common it must not be a problem. I don't have an explanation, but it seems to work.
The solution to a thermal barrier over the spray foam insulation in the plenum could be a sprayed cementitious fire protective coating such as Grace's Monokote Z-3306. There are also similar products marketed as thermal barriers for direct application to foam insulation, and I have spec'd this condition many times.
I created a master section for the sprayed thermal barrier (078117 "Cementitious Fire Protective Coating") which I'll be happy to share (Jeff at WilsonConsultingInc dot com). |
John Bunzick, CCS, CCCA, LEED AP
Senior Member
Username: bunzick
Post Number: 1461
Registered: 03-2002
| | Posted on Wednesday, October 31, 2012 - 10:54 am: | |
Dave,
This essential question is true even when using membranes other than hot-fluid that have a low vapor transmission rate. My prior firm had done a lot of them, and I think they had even had HAM analysis done on some of them and found no issue.
As to the thermal barrier over the foam. I'm assuming that the condition requires a thermal barrier, and not an ignition barrier. I think the latter is more generally found in residential construction. Assuming a thermal barrier is needed, it seems likely to me that FR treated plywood would not work. When I last researched this, alternatives to the gypsum needed to meet NFPA 286. Other products that meet this requirement are often offered by (or suggested by) the foam insulation manufacturers. The ones that we had listed included:
Thermal Product Research, Fireshell F10E
Isolotek International, TB Cafco TB-415
Demilec USA, LLC.; BlazeLok IB.
Hope this helps. |
Lynn Javoroski FCSI CCS LEED® AP SCIP Affiliate
Senior Member
Username: lynn_javoroski
Post Number: 1575
Registered: 07-2002
| | Posted on Wednesday, October 31, 2012 - 11:05 am: | |
I've spec'd this one over closed-cell foam:
W.R. Grace & Co. – Conn.; Monokote Z 3306 |
ken hercenberg
Senior Member
Username: khercenberg
Post Number: 356
Registered: 12-2006
| | Posted on Wednesday, October 31, 2012 - 11:09 am: | |
Interesting design. Still, having only R10-15 above the membrane leaves a lot of insulating value under deck making condensation a possible concern. I always heard the rule of thumb was 2/3 outboard, 1/3 inboard; this is reversed.
All that insulation beneath will prevent conditioning of the deck from below, minimizing the benefits inherent with the PRMA concept regardless of perceived cost benefits. I'd question if the costs-vs.-benefits balance. What type of use is the building? Do you have high humidity inside?
FRT wood products would have to be tested to meet IBC requirements for use of foam plastics. I don't know of any wood board products that have been tested. My personal preference is Ure-K though International Cellulose has some 'prettier' options if you're leaving it exposed. As you know, you can find thermal barriers here - http://www.4specs.com/s/07/07-2129.html.
Spacing isn't dictated that I know of. As long as the 1/2 inch gyp board is continuous and inboard, it shouldn't be an issue. To my understanding, you can't have anything in that space, so a gyp board ceiling with MEP devices in between isn't normally permitted though I think the Code will permit you to use a fire-rated assembly as your thermal barrier. Sounds like a question for your Code official if that's the intent. How are you suspending it? Keep in mind that any 1/2 inch gyp board is acceptable, even non-Type X, so you can use the new lightweight products and still comply with Code despite the fact that some will not block a 15 minute burn.
My kneejerk reaction is to resort to cellulose fiber below deck, especially if you're leaving it exposed. I'm not a big fan of either foam plastics or fibers used at interiors. |
Ronald L. Geren, AIA, CSI, CCS, CCCA, SCIP
Senior Member
Username: specman
Post Number: 1057
Registered: 03-2003
| | Posted on Wednesday, October 31, 2012 - 11:32 am: | |
I think the problem that Dave is focused on is the concrete deck--a prime source of moisture. The NRCA Roofing and Waterproofing Manual does not address this roof deck condition. The Roofing Manual installations that come close to the type that Dave describes is the lightweight insulating concrete decks and structural concrete decks.
The Manual states that lightweight insulating concrete be vented from below. However, composite metal decking with concrete would not allow this and the open-cell polyurethane would seal off any vented openings, even if it could be done.
The Manual also states that structural concrete decks can dry from below, as well; but in Dave's case the composite metal deck will not allow sufficient drying from below.
As Dave indicates, this is a double vapor retarder condition with a source of moisture in between. Maybe installation of stack vents may help--NRCA states that stack vents at 1 per 10 squares for lightweight insulating concrete is not sufficient, but the moisture content in standard concrete may be more manageable with stack vents.
That's one approach. Another is to consider the composite metal deck and concrete as being similar to a concrete slab-on-grade with a vapor retarder directly below and the application of a nonvapor-permeable floor finish applied on top--a condition frequently encountered in construction. If a flooring system can withstand a sufficiently cured and dryed out slab-on-grade, then fluid-applied roofing could possibly do the same.
Ron Geren, AIA, CSI, CCS, CCCA, SCIP
www.specsandcodes.com |
ken hercenberg
Senior Member
Username: khercenberg
Post Number: 358
Registered: 12-2006
| | Posted on Wednesday, October 31, 2012 - 01:44 pm: | |
Has anyone had good experiences with stack vents? I have not. Too often they seem like they let water in.
I'm not too concerned about hot-fluid-applied membranes failing due to moisture vapor migration when applied directly to concrete substrate. First, that system often goes over concrete that is cured but not dry and it doesn't tend to fail under those conditions even though there is vapor drive.If you had to wait for the concrete to dry, you'd never be able to apply the membrane in many parts of the country. That's one benefit of using that system. Second, blisters that do form often result from heat hitting the membrane causing vapor to migrate upwards, causing separation between the membrane and substrate. Having insulation,soil, and plants above the membrane should help preclude that from being an issue regardless of what is under the deck. In fact, having continuous insulation under the deck may reduce vapor drive from below, but I can't say that for certain.
I am never in favor of creating a moisture chamber. Still, is the concern that you'll rust out the deck or the reinforcing steel or that you'll cause the membrane to fail? If the latter, I'd suggest that you ask the membrane manufacturer to sign off on the design to make sure the warranty isn't voided as a result of your concerns. I'd also appreciate knowing what feedback you get. |
Dave Metzger
Senior Member
Username: davemetzger
Post Number: 445
Registered: 07-2001
| | Posted on Wednesday, October 31, 2012 - 02:45 pm: | |
Thanks for the information, folks, it was useful. I'll try to convince the architect to increase the outboard insulation.
Dave |
|
4specs.com Home Page