6 mil poly vapor barrieer required by building inspector
I’m about to start building a home in Michigan’s Upper Peninsula in zone 7. I planned for R-40 walls with cedar shingles over cedar breather over 15# felt over OSB over 1 1/2 exterior polyiso board over 8 inch studs filled with blown in fiberglas and air sealed drywall. My inspector just came back from a training session with the impression that Michigan code requires a 6 mil plastic interior vapor barrier and that I shouldn’t use exterior foam with it. Are there alternate interior vapor barriers that I could convince him would satisfy Michigan code (based on IRC)? My wife thinks we shouldn’t fight him and just go with double stud walls and do the plastic vapor barrier.
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It's a mistake to use a double vapor barrier in a cold-climate wall system. The polyiso board and the poly sheeting have perm of approximately 0.03, a Class I vapor barrier (impermeable). My preference is to allow some drying in both directions, but drying in at least one direction is essential to the long-term durability of the thermal envelope.
The 2006 IRC Section N1102.5 Moisture Control under Exception 3, allows for the vapor retarder to be removed “where other means to control condensation are provided.” IRC 2007 supplement has refined the definition of vapor retarder to include three classes:
Class I: 0.1 perm or less (Sheet polyethylene, non-perforated aluminium foil)
Class II: 0.1 perm <= 1.0 perm (Kraft faced fiberglass batts)
Class III: 1.0 perm <= 10 perm (Latex or enamel paint)
A Class I or Class II vapor is needed to be installed on the warm in winter side of the insulation in Climate Zones 5, 6, 7, 8 and Marine 4; however, Class III vapor retarders can now be used instead of Class I or II vapor retarders if the conditions of Table N1102.5.1 Class III vapor retarders are met.
For climate zone 7, these include:
insulating sheathing R ≥10 over 2x4 wall
insulating sheathing R ≥15 over 2x6 wall
Your proposed wall section, with R-9 sheathing over a 2x8 wall would not fall under this exception, but a Class II vapor retarder would be sufficient, which could be latex vapor barrier primer.
And, by the way, your proposed wall system will result in an as-built assembly of approximately R-31, assuming typical 16" oc framing, or R-32 with 24" oc framing. If you want a true R-40 wall, I'd agree with your wife to go with a double-wall or Larsen Truss system with dense-pack cellulose, air-tight-drywall and interior vapor barrier primer.
Thanks for the clarification on the code details. I'll trac k those down and print them out for the inspector. I did want to build a wall that can dry to the inside so I'd prefer not use the poly. My concern with the inspector is reinforced by your listing of the 3 classes of vapor retarders since the example the inspector used from his class was "the instructor specifically stated that kraft faced fiberglass is not a vapor barrier and builders have to use 6 mil poly to meet code." It sounds like he questions the accuracy of this but also sounds like he'll feel he has to enforce it.
http://www.buildingscience.com/documents/information-sheets/irc-faqs/IRC-FAQ-insulating-sheathing-vapor-retarder-requirements/
Thanks Robert, that's helpful information for the inspector. But if I go with your and my wife's advice and do the double stud walls instead of the insulated sheathing, won't I still be required to use poly? I don't see an exception allowing a class III vapor retarder on the linked page you sent and from most of what I've learned, it seems like 6 mil poly isn't a good idea.
The requirement is for a class II vapor retarder, which means no more than 1 perm. That standard can be met by latex vapor retarder primer. I have not used poly sheeting in 20 years.
However, in such a cold climate it is imperative to control winter indoor relative humidity to safe levels (no more than 40%).
The design goal need not be to prevent all condensation (perfect is the enemy of the good), but to minimize it and create a thermal envelope which has maximum drying potential and some safe moisture storage capacity.
Here in the coastal SC area. 6 mil poly was required behind the drytwall for good sense homes in the 90's. I am a repair contractor in this area. The poly is great for my business, bad for Homeowners.
Moisture intrusion from improper flashing, framed opening wraps or lack of, can go on for months or years even without detection until severe structural damage is detected or mold build up, or worst, termite infestation is detected.
Moisture intrusion is typically noticed sooner than later without the poly, repair cost minimal.
A wall can look normal with the poly applied until I remove the drywall, then behind the poly a Homeowner sees their worst nightmare one can have with their home.
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Here in the coastal SC area. 6 mil poly was required behind the drytwall for good sense homes in the 90's. I am a repair contractor in this area. The poly is great for my business, bad for Homeowners.
Moisture intrusion from improper flashing, framed opening wraps or lack of, can go on for months or years even without detection until severe structural damage is detected or mold build up, or worst, termite infestation is detected.
Moisture intrusion is typically noticed sooner than later without the poly, repair cost minimal.
A wall can look normal with the poly applied until I remove the drywall, then behind the poly a Homeowner sees their worst nightmare one can have with their home.
.