Joints in Cavity Wall XPS insulation

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Author Message Stephen Wilson Senior Member Username: swilson Post Number: 21 Registered: 02-2019 Posted on Monday, February 14, 2022 - 10:20 am:    Hi all, A question came up from one of my coworkers that I wanted to bounce off the experts here. Assume a commercial cavity wall with a structural backup (studs w/sheathing or CMU), air barrier/vapor retarder membrane, XPS insulation, air cavity, and exterior masonry veneer. The question is whether or not you would specify sealing the joints between insulation boards with spray foam. From what I can tell, the XPS manufacturers only require it when you are installing their product as an insulating sheathing. The rationale seems that if you are providing a separate air barrier membrane, then you don't need to seal the board joints since the main benefit is sealing air gaps. Of course, in theory you are helping to improve thermal performance, but in reality it may not be much benefit if the boards are butted tightly. What do you all do in your rigid board insulation specs for situations like these? Thanks! J. Peter Jordan Senior Member Username: jpjordan Post Number: 1171 Registered: 05-2004 Posted on Monday, February 14, 2022 - 10:27 am:    The type of insulation depends on the type of cladding and the NFPA 285 testing. Typically, a metal wall panel would require polyiso or mineral wool board. Masonry assemblies would permit XPS or EPS. I agree that sealing the joints would depend on whether or not you are using the insulation as an air barrier. J. Peter Jordan, FCSI, AIA, CCS, LEED AP, SCIP ken hercenberg Senior Member Username: khercenberg Post Number: 1435 Registered: 12-2006 Posted on Monday, February 14, 2022 - 10:59 am:    Agreed. The other consideration is that when you seal the XPS joints, you create a vapor retarder condition so you should probably run a WUFI to see if that suits your conditions. Typically if you already have an air or vapor retarder, sealing the joints is not required from what I've seen. Greta Eckhardt (Unregistered Guest) Unregistered guest Posted on Tuesday, February 15, 2022 - 12:57 pm:    At one point I did frequently specify sealing the joints of XPS even with a full air barrier membrane on the backup wall. This was because we thought that if the insulation was not tight to the air barrier, there might be airflow through unsealed joints to the airspace behind the insulation, allowing for convection that would circumvent the thermal performance of the insulation. However, I believe that this phenomenon has been shown more recently to have a negligible impact on thermal performance of the wall, so as stated in the original post, as long as the XPS boards have tight butt joints, it may not be worth the labor involved. Therefore more recently I have omitted this requirement. Stephen Wilson Senior Member Username: swilson Post Number: 22 Registered: 02-2019 Posted on Friday, February 18, 2022 - 09:52 am:    Thanks all! Your input is much appreciated. Phil Kabza Senior Member Username: phil_kabza Post Number: 699 Registered: 12-2002 Posted on Friday, February 18, 2022 - 01:20 pm:    It would not hurt to verify that the product used to seal the joints plays nicely with the XPS, chemically speaking. Phil Kabza FCSI CCS AIA SpecGuy Specifications Consultants www.SpecGuy.com phil@specguy.com ken hercenberg Senior Member Username: khercenberg Post Number: 1439 Registered: 12-2006 Posted on Monday, February 21, 2022 - 06:25 pm:    Phil, you made me think of my days working in testing labs when the old guys would try to trick the newbies into bring a styrene cup filled with solvent.