Convincing a code official about location of Vapor Retarder
So I am working with a client renovating their attic space for a master bedroom. The existing knee walls has kraft face batt insulation with no sheathing of any kind on the attic side of the wall. Which we all know renders the batt insulation useless.I am proposing to install 2″ of foil faced polyiso on the attic side. I am able to find the Legacy report from the manufacturer stating it can be left exposed in these types of conditions. And I was just reading the Q&A about the Correct use of foam insulation. (https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/15208/explain-correct-usage-rigid-insulation-exteri?page=1). It confirms everything I am doing.
However now the code official is concerned that the vapor barrier is not on the warm side of the cavity as the code states. How do you convince the code official that by using a foam insulation, that you do not want to have a vapor retarder installed on the interior side of the wall. Besides BuildingScience.com, are there any reports that could be used as documentation?
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So, this man comes into a bar with a monkey on his shoulder....
I thought you were starting a joke, but I guess not.
Either you get an engineer's stamp on the plans or you stick with the code requirement for vapor barrier location.
Joshua,
More information, your location, what about the rafters, knee walls might be the least of it, define the thermal boundary.
Not sure where you are - is a vapor retarder required by code? If so - your kraft paper should qualify. As for the attic side sheathing, they make a few rigid foam products that are not vapor barriers or forget the added R-Value and just go with thermoply or something as an air barrier on the attic side.
The project is in Central Ohio, (Climate zone 5A). And yes, a vapor retarder is required on the warm side (interior) of the wall. So yes, the current kraft facing will qualify, however the code official is concerned with a barrier on both sides of the wall. I mean yes, we can use an XPS product with a higher perm rating, but then we need a 15 minute thermal barrier on the attic side since it still can be accessed. So that is increasing our costs. Plus I like the foil faced polyiso because it gives us a radiant barrier against our hot attic space.
Joshua, you may want to look at Building Science.com's information sheet: IRC FAQ: Insulating Sheathing Vapor Retarder Requirements. It addresses IRC vapor retarder code compliance- specifically on exterior walls, but I assume that the the information can be applied to knee walls.
In Climate Zone 5, "class III" vapor retarders (latex or enamel paint) are permitted if:
-Vented cladding is used over OSB/plywood/fiberboard/gypsum sheathing
- R-5+ insulated sheathing is used over 2x4 wall
- R-7.5+ insulated sheathing is used over 2x6 wall
Here is the link to the BSC info Sheet:
http://www.buildingscience.com/documents/information-sheets/irc-faqs/IRC-FAQ-insulating-sheathing-vapor-retarder-requirements/