Flutterby™! : vapor barriers and drainage planes

The recent shed construction started a heated debate between me and a friend who's a designer in the building trade about vapor barriers. She was adamant that they go underneath the siding, I was equally adamant that they go inside the studs (at least in climates where you primarily heat). Several of her contractor friends agreed with her.

My reasoning is that on any barrier between hot and cool, moisture will condense on the hot side. If you put an air permeable insulation (like fiberglass) outside the vapor barrier, you don't have a temperature gradient over the barrier. If you put the insulation inside the barrier, the warm air will be able to migrate through the insulation and deposit moisture on the inside of the vapor barrier, leading to mold.

I don't, unfortunately, have a copy of the building code, but I found numerous sources that supported my notion, including the fact that the California Building Standards Commission has Part 6 of the 2001 California Energy Code online, and everywhere it requires a vapor barrier, it requires it to be on the inside.

A few days ago, though, I found the answer. There's something called a "drainage plane", and it's an air permeable membrane that goes between the sheathing and finish of a house, various manufacturers are trying to sell assorted plastics and paper coated plastics as "building wrap" for this purpose but it appears that the best material apppears to be just tar paper, as the various softeners used in stuccos and various outgasses from popular woods deteriorate the plastic versions over a shorter time.

The idea is that you put on an exterior sheething, this layer, an air gap, and then your external finish material.

The motivation for drainage planes appears to be more modern building materials, prefab fascias, brick facades and such, even metal and vinyl sidings, although they also look like a really good idea under traditional wood lap or panel siding. In fact, although it's hard to tell without a copy of the code in front of me, they appear to be required behind all materials by the International Code Council building code since 2004. I don't see where they first occurred for wood sidings, but there's also a little controversy over this because that air gap with a potentially flammable sheathing provides a ventilated fire space that, unlike the code requires in between studs, isn't interrupted by fire breaks.

One fascinating document I came upon in this is a document talking about changes in the 2003-04 cycle of the International Residential Code - Building, with some cool insight into the politics of how building codes get created.

So, the up shot? What we should have done with the shed is two layers, a layer of 3/8" ply sheating on the studs, tar paper, an air gap, and then the T-111 grooved panel siding on the outside. I'm not stressed over it, it's an 80 square foot building, but next time... In the interim, the solution seems to be to run flashing down in between the sheathing and the side beams, and loosely lay tar paper on the inside of the siding panels. Then put in a layer of fiberglass, and a 6 mil polyethylene vapor barrier, then wallboard.

In the mean time, my respect for a building industry that can't tell a vapor barrier from a drainage plane has sunk further.