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Foil faced polyiso as exterior continuous insulation in rainscreen assembly with liquid applied weather resistive barrier (WRB) behind it.

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MSmythe0627

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Aug 13, 2022
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Location
Philadelphia
Hello. First time posting and appreciate any help. I am located in the Northeast (zone 4a) and am working on a project with a rainscreen wall assembly. Per ASHRAE 90.1, we have foil faced polyiso as our continuous insulation that creates the drainage plane in our drained and vented cavity of our rain screen behind our exterior metal panel. Our metal panel is held off by thermally broken z-girts. We were calling for the seams of the polyiso to be sealed so that water can’t be absorbed into the cut ends of the polyiso. I know this also forms a class 1 vapor retarder. Behind the polyiso, the contractor has already installed a separate liquid applied WRB to the face of exterior gypsum board sheathing. The gyp sheathing is on metal studs with cavities filled with Batt insulation. I have 2 questions:

1.) ASHRAE calls for continuous insulation, which by nature of the polyiso product forms a class 1 vapor retarder on the exterior of the wall assembly. Does this present an issue in a zone 4A location? We have called for latex paint as the interior finish which is also a class III vapor retarder to allow for the wall to “dry” to the interior.

2.) is there any issue having the sealed edge continuous polyiso insulation with the separate WRB behind it? The perm rating of the WRB is 19. The contractor also had said he was not intending to seal the polyiso seams since the WRB was behind it and would handle any water. I believe this is incorrect and he would still need to seal the seams.

Again, any insights would help. Thanks!
 
Your situation is complex and needs the involvement of the manufacturer of the polyisocyanurate insulation and possibly a building envelope consultant who can perform a hygrothermal analysis of the wall assembly. There are six issues I can identify:
  1. Is the insulation a part of an approved NFPA 285 assembly?
  2. Is the insulation manufactured for use as continuous insulation within a rainscreen system?
  3. Is the outer surface of the insulation designed to be a weather-resistive barrier and complies with the requirements of the building code?
  4. Is the Class I vapor retarder located in the best location?
  5. Will the wall dry out with multiple materials having varying perm ratings?
  6. Does your jurisdiction require an air barrier system?
If you do not have answers to these questions, seek consultants who can provide the expertise to provide the answers.

Personally, I do not like using polyisocyanurate insulation (or any foam insulation) as continuous insulation for exterior walls (think Grenfell Tower). With foam continuous insulation systems, you are extremely limited in available wall assemblies because of NFPA 285.

I would rather use mineral wool board insulation. It is fire- and moisture-resistant and is not subject to NFPA 285. Although mineral wool has a lower R-value per inch than polyisocyanurate, the difference will only add about 1/2 inch to the overall insulation thickness. In Climate Zone 4, you need continuous insulation having an R-value of 7.5 for metal-framed walls. Polyisocyanurate insulation loses R-value over time, so you must use what is called the long-term thermal resistance (LTTR) value. Although polyisocyanurate insulation will have an initial R-value of 6.5 per inch, the LTTR value will be about 8.6 at 1-1/2 inches thick (approximately 5.7 per inch). Mineral wool, which is not affected by LTTR, will have an R-value of about 8.4 at 2 inches thick (4.2 per inch).
 
MSmythe0627 said:
The contractor also had said he was not intending to seal the polyiso seams since the WRB was behind it and would handle any water. I believe this is incorrect and he would still need to seal the seams.
Click to expand...
My opinion is that the contractor is on the right track - use the liquid applied WRB as your air/vapor barrier, and do not tape the seams in the foam. The minor amount of water that gets through the rain screen will not penetrate the building and will be able to dry out of the ventilated space.
 
Polyisocyanurate insulation is a form of polyurethane foam used frequently as a roofing membrane, especially in my area of the country. Thus, it is relatively moisture-resistant as mentioned in a previous post. Its drawback is its combustibility.
 
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