A major issue in Minnesota construction has been the material used in exterior wall construction. The use OSB as the vertical wall sheathing with #15 felts as the weather and wind barrier over the OSB has been recently observed at a project here in Minnesota. I disagreed with this solution for several reasons which are very relevant to the long term sustainability of the exterior walls of this and all projects.
In northern climates, like Minnesota, with a high number of heating degree days, the air movement pressure is from the inside out, during the majority of the year. This fact can be used to the advantage of the exterior wall if this wall meets two requirements. First, there is a vapor/air barrier on the warm side of the wall, which is very tightly sealed and second that the exterior of the wall (to the outside of the insulation and therefore the dew point) has a high perm rating. These two requirements combined, allow the exterior wall to dry, to the outside, during the heating months.
If the first requirement is not meet, the wall will accumulate a large amount of water vapor inside the wall. A 1” X 1” wall area with a vapor barrier will transmit .1 grains per hour of water vapor through air diffusion. That same 1” X 1” area, open to the inside wall, will transmit 1,000 grains of water vapor per hour through air infiltration (7,000 grains of water vapor equals a pint of water). The sealing of this vapor/air barrier is therefore critical. Some water vapor will get into the wall, even with a tightly sealed vapor/air barrier due to water vapor diffusion but since the vapor pressure is to the outside, in the heating season, and the amount of transmission due to diffusion is so low, this is not critical if the sheathing and house wrap can let the vapor pass. Further, this drying process reverses the vapor direction of the summer months and any accumulation of vapor that the summer has brought into the wall is taken back out. If the sheathing does not allow for rapid water vapor transmission, the vapor condenses on the inside surface of the sheathing during the winter months and when it warms this ice will turn to water which soaks the sheathing. It’s a fact that in Minnesota, where walls have had water problems and have had OSB sheathing, the entire wall, sheathing, studs and all have been compromised. In the case of walls with other sheathing materials, the wall damage has been more localized and less structural in nature.
The perm rating of a material indicates the ability of the material to allow vapor to pass, a low perm, not much will pass. A poly film has a perm rating of around .1, good for the inside vapor barrier. Gypsum board has a perm rating of around 35. Tyvek house wrap has a perm rating of 50, excellent for the outside air barrier. OSB sheathing has a perm rating of around 2 and #15 felts a perm rating of around 5. This means that the water vapor will have a hard time escaping to the outside during the heating months from the exterior wall. Further, these two products are not very good air barriers, meaning that during the warm months, warm humid air will be forced into the wall through the overlaps (the felts have been taped some of the time, but not consistently and the tape fails to adhere well to felts) and cracks between the OSB and during the cold months this creates a thermal air break in the wall with cold air being forced into the wall by wind pressure and cooling the wall components. Bulk water intrusion into this wall will be a serious problem, as the wall will not dry out.
The most ideal exterior wall in our climate would be siding, Tyvek, strudri-brace sheathing with sprayed closed-cell foam insulation directly on the inside of the sheathing, then the gypsum wallboard. The sprayed insulation acts as the insulation, a vapor barrier and an air barrier all in one, furthermore this is on the outside of any wall penetrations, so it is a continuous barrier. The next best wall would be siding, Tyvek, sturdy brace sheathing (perm rating of 28), standard batt insulation, 6 mil vapor barrier, and gypsum board. With this wall, it would be a good idea to spray foam the rim joists, where taping and making the poly continuous is very hard.
For these reasons always consider making a breathable wall a requirement for your next project, as the exterior wall construction is so important to the long term sustainability of projects.
Miles & Associates
“On a journey of continuous improvement”