Approved Vented Rainscreens
What are “other approved vented rain screens” per 2018 IBC 1404.3 ? More specifically, are vented rain screens created with nominal 1″x4″ furring at 16″o.c. with 5/16″ fiber cement plank siding an “approved vented cladding” that would satisfy the criteria for use of a Class III interior vapor retarder in Climate Zone 5B?
Further reading is not necessary. Most is reflection of possible answers from a 2015 Technical Note and a secondary forked tongue Code-Talk question.
A 2015 Technical Note 6 Vapor Retarders provides some encouragement under “Code Considerations” with the sentence:
“Vented cladding generally does not include stucco, adhered masonry
veneer, insulated vinyl siding, or lap siding (such as fiber cement or solid wood) without furring.”
Is that the same as “Vented cladding generally does include fiber cement or solid wood lap siding with furring”, and if so, is the 3/4-in. typical furring depth sufficient?”
Table 3 and its companion Figure 4 suggest a Class II vapor retarder for vented cladding and no exterior c.i. as well as for unvented cladding and no exterior c.i.
That leaves me scratching a hole in my head wondering whether Class III (achieved with vapor retarding latex primer) is enough or not. In time we’ll be in Climate Zone 4B, with more cooling season. The limited guidance in Technical Note 6 does not seem to address the multiple assemblies possible. In any case, I see a Class 1 interior vapor retarder becoming even more of a liability. I just wish Code did not appear to allow it.
In the instant case, permeable mineral wool exterior insulation R6.3 is now planned, along with the rain screen which ought to be vented given the siding material, but also a 6mil polyethylene interior vapor retarder that is more than a burr in the saddle. I am pushing to increase the insulation to R8.4 and meet the 27% in Table 3 of “Martin’s 6 Rules” (the 2×6 stud cavity fluff is mineral wool batt R23.1) and definitively vent the rain screen which I hope mitigates solar induced vapor drive I have been reading about as I seek the holy grail.
I have another related code question, pertaining to the following from 2018 IBC:
on the exterior side of the frame wall.”
What is or is not allowed if the foam plastic insulating sheathing is R 6 rather than R7.5 in Climate Zone 5B? I am afraid “code” allows idiocy, and I think Technical Note 6 was issued to mitigate idiocy, but not prevent it.
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Replies
Eddy,
I'm not familiar with the IBC, but I took a quick look out of curiosity and I'm a mystified as you. What appears to be a defined term "Approved Cladding" should surely have some note as to what they are, or where you can for out.
Thanks for lending credibility to my inability, but I'd rather you gently point out the reference to the table to the sub note that illuminates all.
If that section is any indication of the general opacity of the IBC, I'm sure glad we don't use it here. All I can suggest is passing the problem to to your local Building inspector. Good luck!