vapor barrier requirements

[Andy M]

this has probably been thoroughly discussed in other threads, I cannot find anywhere in the IBC where vapor barriers or vapor retarders are required in commercial buildings. In my case I have a wood framed building, typical wall assembly is wood studs, gypsum interior, insulated sheathing exterior with fiber cement siding. I cannot seem to identify a code requirement for vapor barriers. Historically in my part of the country - the northeast - polyethelyne sheets are installed on the interior face of studs beneath the gypsum, although I have done a couple projects without it and not encountered any problems. Am I missing something or is the vapor barrier optional?

 

[Yikes]

Per IBC 1805, it can be omitted in commercial spaces if it is "not detrimental" - see bold emphasis below. But this is subjective: who determines where/when it is "not detrimental"?

Most building science experts will tell you that in this era of highly insulated and sealed energy-efficient structures, and air conditioned space will have a low humidity 'microclimate' that tries to pull up moisture up from the soil through the slab - - even in dry desert climates. If you have any flooring with water-based adhesives, it will be "detrimental"!

That said, I typically omit it in unconditioned, ventilated parking garages with a low water table.

IBC 1805.1 General​

Walls or portions thereof that retain earth and enclose interior spaces and floors below grade shall be waterproofed and dampproofed in accordance with this section, with the exception of those spaces containing groups other than residential and institutional where such omission is not detrimental to the building or occupancy.​

Ventilation for crawl spaces shall comply with Section 1202.4.​

​

IBC 1805.2.1 Floors​

Dampproofing materials for floors shall be installed between the floor and the base course required by Section 1805.4.1, except where a separate floor is provided above a concrete slab.​

Where installed beneath the slab, dampproofing shall consist of not less than 6-mil (0.006 inch; 0.152 mm) polyethylene with joints lapped not less than 6 inches (152 mm), or other approved methods or materials. Where permitted to be installed on top of the slab, dampproofing shall consist of mopped-on bitumen, not less than 4-mil (0.004 inch; 0.102 mm) polyethylene, or other approved methods or materials. Joints in the membrane shall be lapped and sealed in accordance with the manufacturer's installation instructions.
​

 

[SCBO1]

Check 2021 IECC code, C402.5.1 Air barriers

 

[Tim Mailloux]

Refer to 2021 IBC section 1404.4 for vapor retarder requirements.
If all your exterior wall insulation is all continuous insulation outside of the exterior wall studs you can use a combination air and vapor barrier (AVB) on the exterior face of sheathing. If you have insulation in the stud cavity I call for a vapor permeable air & water barrier (AWB) on the exterior face of sheathing and then a smart vapor barrier in the interior face of studs under the drywall. These smart vapor barriers are able to change their permeability based on the seasons to allow both outward and inward drying of the wall cavity.

 

[classicT]

Others have provided a couple IBC sections to look at, but I'd also say to look at your energy code as well.

 

[Andy M]

Refer to 2021 IBC section 1404.4 for vapor retarder requirements.
If all your exterior wall insulation is all continuous insulation outside of the exterior wall studs you can use a combination air and vapor barrier (AVB) on the exterior face of sheathing. If you have insulation in the stud cavity I call for a vapor permeable air & water barrier (AWB) on the exterior face of sheathing and then a smart vapor barrier in the interior face of studs under the drywall. These smart vapor barriers are able to change their permeability based on the seasons to allow both outward and inward drying of the wall cavity.

Thank you, somehow I missed that section. It's exactly what I was searching for