Thermal Barrier/Insulation requirement?

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Author Message Robin E. Snyder Senior Member Username: robin Post Number: 524 Registered: 08-2004 Posted on Friday, April 04, 2014 - 02:48 pm:    I am working on a shell space and the intended wall design is: Exterior tile over cement board (over vapor permeable waterproofing) with metal studs with unfaced mineral wool insulation between the studs and then foil-faced rigid extruded polystyrene on the interior side of the studs. We need the foil facing to act as our vapor barrier. Because it is a shell project at this time, the Architect is not supposed to cover the insulation with any type of gypsum board. Here's the question - can this foil faced XPS remain exposed to the interior? Eventually, it will get covered by the retail tenant, but that tenant has not been selected yet. Dave Metzger Senior Member Username: davemetzger Post Number: 508 Registered: 07-2001 Posted on Friday, April 04, 2014 - 04:29 pm:    Code requires that foam plastics exposed to the building interior be covered by a minimum 1/2 inch thick thermal barrier (eg gypsum board). Assuming the building owner is required to provide a code-compliant building to the tenants, whether or not a separate certificate of occupancy for the shell space is needed before the space can be rented to tenants, I'd think that the foil-faced XPS cannot remain exposed but would need to be covered by a thermal barrier. ken hercenberg Senior Member Username: khercenberg Post Number: 748 Registered: 12-2006 Posted on Friday, April 04, 2014 - 04:40 pm:    There are a number of coatings that can be applied. See http://www.4specs.com/s/07/07-2129.html for some possibilities. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 653 Registered: 10-2007 Posted on Friday, April 04, 2014 - 04:48 pm:    If the gypsum board is left off does this change the location of the dew point and thus contribute to corrosion? anon (Unregistered Guest) Unregistered guest Posted on Friday, April 04, 2014 - 05:42 pm:    Robin, If you use foil-faced polyisocyanurate board insulation by Thermax or Atlas, then you do not need to cover this with a thermal barrier - these products have been tested as thermal barriers, and have all the documentation you need to show that building code requirement is satisfied. as for the comment about the dew point - this is not important as the assembly has been described. Because the multiple layers of "vapor retarders" in the assembly (foil facing, waterproofing over cement board) are thermally isolated, there IS NO POTENTIAL for condensation - there are no cold surfaces for vapor to condense on. Air transport is a much greater concern. Vapor diffusion except in the most extreme climates (think Fairbanks, AK) is almost always a non-issue. A vapor retarder is there for DIFFUSION, not for air transport. You still need to identify the air barrier in the system and ensure that it is continuous - or you could have condensation issues. Dave Metzger Senior Member Username: davemetzger Post Number: 509 Registered: 07-2001 Posted on Sunday, April 06, 2014 - 04:33 pm:    Dow's website, http://building.dow.com/na/en/thermaxtws/consider/fire.htm, notes the following: (2006 IBC) "2603.52 Thermal Barrier: Any foam plastic must be separated from the interior of a building by an approved thermal barrier per Section 2603.4. The THERMAX™ Wall System incorporates a 5/8" Type X gypsum layer on the interior of the steel studs to serve this purpose." That says to me that Thermax does not serve as a thermal barrier. Anon, can you please refer us to the specific documentation that demonstrates that Thermax satisfies the requirements of IBC as a thermal barrier? Colin Gilboy Senior Member Username: colin Post Number: 370 Registered: 09-2005 Posted on Sunday, April 06, 2014 - 06:15 pm:    I researched this when I insulated our too cold storage area. Here is a link I found today: http://building.dow.com/na/en/applications/building/walls/eiw.htm Colin Gilboy Publisher, 4specs.com 435.200.5775 - Utah 800.369.8008 anon (Unregistered Guest) Unregistered guest Posted on Monday, April 07, 2014 - 11:33 am:    Yep. Thanks Colin! You can now take that 5/8 inch gypsum board covering the Thermax out of your spec now, David Metzger! Robin E. Snyder Senior Member Username: robin Post Number: 527 Registered: 08-2004 Posted on Monday, April 07, 2014 - 11:53 am:    From what I can tell, the Thermax doesn't have a facing that serves as a vapor barrier though, does it? Ronald L. Geren, FCSI, AIA, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 1208 Registered: 03-2003 Posted on Monday, April 07, 2014 - 11:57 am:    It has aluminum facers and aluminum is a vapor retarder. Ron Geren, FCSI, AIA, CCS, CCCA, SCIP www.specsandcodes.com Robin E. Snyder Senior Member Username: robin Post Number: 528 Registered: 08-2004 Posted on Monday, April 07, 2014 - 12:00 pm:    you're correct - I saw a data sheet that had the permeance listed as over 2, but it must have been the wrong data sheet. Just found the permeance as <0.03, so this should work. Thanks everyone, I will forward to the Architect for their consideration. Brian Payne, AIA Senior Member Username: brian_payne Post Number: 31 Registered: 01-2014 Posted on Tuesday, April 08, 2014 - 10:41 am:    One of the benefits of using Thermax is definitely its ability to be used for shell construction. We did this two years ago above a freestanding emergency department and had no problems justifying it with the AHJs. Dave Metzger Senior Member Username: davemetzger Post Number: 510 Registered: 07-2001 Posted on Tuesday, April 08, 2014 - 11:21 am:    Thank you Colin and Anon. One of the best things about specifying and 4specs is I always can learn something I didn't know before.