Help, my house is covered by sticky orange frogs

CC BY 2.0 Lloyd Alter

Building science has evolved so much in the last few years; it used to be that a building would be covered in black tar paper and siding would just get nailed over that. Now, in a good building envelope, everything is engineered. Everything is argued about. Everything is more complicated than you think.

Take the vapor barrier. This is a sheet of 6 mil polyethylene on the inside behind the drywall. Good builders install it really carefully, seal it around electric boxes. It is supposed to keep water vapor from moving from inside the home into the insulation, where at some point close to the outside it hits the dew point, condenses and soaks your insulation and rots your framing. Absolute accepted practice, and where I live in Ontario, Canada, a legal requirement of the Building Code.

However, a lot of smart people think they are bad practice. Moisture is driven from the warm side to the cool side, and in an air conditioned house (something not common in Ontario when the building code was written) the moisture is just as likely to want to come inward rather than outward. Joe Lstiburek writes:

Vapor barriers are a cold climate artifact that have diffused into other climates more from ignorance than need. The history of cold climate vapor barriers itself is a story based more on personalities than physics.... It is frightening indeed that construction practices can be so dramatically influenced by so little research and reassuring indeed that the inherent robustness of most building assemblies has been able to tolerate such foolishness.

In Green Building Advisor, Carl Seville says much the same thing.

They are a disaster in warm humid climates, and even in cold climates, if there is any air conditioning being used. There is a big opportunity for vapor drive to the interior where it will condense inside the wall cavity. Vapor barriers in extreme cold climates can serve a purpose if they are installed without any gaps or perforations — if there are, moisture laden air will flow right through any gaps, minimizing its effectiveness.

Fortunately our house is not going to be air conditioned so the poly on the warm side probably won't do any harm. Joe Lstiburek told Patrick the carpenter at one of the Training Camps that we should be complaining to the building inspector and to get the code changed, but it is easier just to go with the vapor flow.

© Vaproshield Wrapshield detail

On the cool side, the exterior sheathing is covered with this very fancy and expensive Wrapshield SA (self adhered) that is stuck over everything. It is a "vapor permeable air barrier and at 50 perms it allows damp sheathing to dry quickly." It will let any water vapor that gets into the wall out, while keeping any moisture that gets behind the exterior cladding from getting in. Our design has sheathing set off on hats an inch away from the wrapshield with open joints, so there is lots of room for any water to drain out.

Most builders just staple a sheet of Tyvek over the exterior, but Greening Homes' wall nerd Janette wouldn't hear of it. She insisted on the complete seal with this stuff that stays put without staples. It certainly looks impressive.

The fundamental principle is that moisture is driven from warm to cool. In Canada the vapor barrier probably won't do any harm, but south of the border you should think twice.

Wall detail:

© Workshop Architecture

Tags: Green Building | Materials


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