You are in a great location for learning about cob. I am sure you are aware of the activities on Mayne Island. Take a short hop and see cob in action. http://www.cobworks.com/
That will tell you more than anything what cob will do in your weather/climate.
As far as my understanding of strawbale (admittedly my data is old. I gave up on the idea of strawbale about 8 years ago.) is that the very high compression of the straw keeps moisture in the form of vapor from migrating too deeply into the bale.
Realizing that my information was old I did some reading. It seems maximum breathability is the recommendation.
There are no historical precedents of bales being used with moisture barriers, and consequently there is no data on how the two perform together. Most historical data for unwrapped bale walls demonstrates the importance of walls of maximum breathability: a mansion in Huntsville, Alabama, has successfully endured Southern humidity since 1938; a 1978 building near Rockport, Washington, receives up to 75 inches of rain a yea
r; and an unplastered building near Tonasket, Washington, with no foundation and unplastered walls shows no apparent deterioration of the bales since 1984. Recent bale structures in northern New York (humid winters) and Nova Scotia (cold humid winters) have been monitored and demonstrate good performance in these difficult climates.
To better clarify my point on moisture with cob:
Cob is a form of earthen building developed in the British Isles, a very wet area famous for beautiful mists and vicious storms. Yet the native cob buildings in these places have withstood centuries of harsh weather and still remain in use today. When I built a cob studio Pensacola, Florida in 2002 many people were skeptical that it would survive our 98% humidity and frequent hurricanes. However, the building has had no problems despite soaking rains throughout the building process and a hurricane that destroyed many of the surrounding wooden buildings. Even in humid climates cob does not rot, grow mold, get eaten by termites, or melt on the rain.
Cob is very porous and can absorb a tremendous amount of water without softening. In fact, unless it is completely submerged, cob will never just “melt”. It will however erode over time if it is exposed to direct rain. The solution to protecting cob from moisture is “a good hat and boots”. In other words, a cob house needs a good roof with wide eves and the bottom of a cob wall must sit on a non-absorbent stem wall so that it cannot wick moisture up from the ground or be splashed by water coming off of the roof. Cob walls are very thick, often two feet or more, so even the heaviest rain will never soak through more than the outer layers of plaster. Even if your walls get soaked with rain occasionally (like in a really big storm) they will be fine as long as they can dry back out. For this reason it is very import to never plaster a cob wall with a non-breathable coating like cement stucco.
As to your question on gradient, I am going to say that when the humidity is the highest on the Island, Winter, the structure will be heated from the inside creating a high or steep gradient (migrating outward.) The time of year when the gradient would be shallow would be the warmer months when humidity is at its low point. I am confused by your statement that "in the winter ... interior moisture content is high." From a relative humidity perspective, the air is less humid inside than outside presuming there is some heat differential to the outdoors. The warmer it is the lower the relative humidity. So the air inside the walls should be less humid by volume than the air outside. Am I wrong here? If so I apologize.
If in fact the inside is less humid than outdoors the air density gradient will be from low humidity to high humidity, even though it seems counter intuitive. http://www.usatoday.com/weather/wdensity.htm
Humidity and air density
Most people who haven't studied physics or chemistry find it hard to believe that humid air is lighter, or less dense, than dry air. How can the air become lighter if we add water vapor to it?
Scientists have known this for a long time. The first was Isaac Newton, who stated that humid air is less dense than dry air in 1717 in his book, Optics. But, other scientists didn't generally understand this until later in that century.
To see why humid air is less dense than dry air, we need to turn to one of the laws of nature the Italian physicist Amadeo Avogadro discovered in the early 1800s. In simple terms, he found that a fixed volume of gas, say one cubic meter, at the same temperature and pressure, would always have the same number of molecules no matter what gas is in the container. Most beginning chemistry books explain how this works.
Imagine a cubic foot of perfectly dry air. It contains about 78% nitrogen molecules, which each have a molecular weight of 28 (2 atoms with atomic weight 14) . Another 21% of the air is oxygen, with each molecule having a molecular weight of 32 (2 stoms with atomic weight 16). The final one percent is a mixture of other gases, which we won't worry about.
Molecules are free to move in and out of our cubic foot of air. What Avogadro discovered leads us to conclude that if we added water vapor molecules to our cubic foot of air, some of the nitrogen and oxygen molecules would leave — remember, the total number of molecules in our cubic foot of air stays the same.
The water molecules, which replace nitrogen or oxygen, have a molecular weight of 18. (One oxygen atom with atomic weight of 16, and two hydrogen atoms each with atomic weight of 1). This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic foot; that is, it's density decreases.
Wait a minute, you might say, "I know water's heavier than air." True, liquid water is heavier, or more dense, than air. But, the water that makes the air humid isn't liquid. It's water vapor, which is a gas that is lighter than nitrogen or oxygen. (Related: Understanding water in the atmosphere).
Since heat will convect from hot to cold and air density will flow from high (less humid) to low (humid) the gradient should be the highest when the temperature differential is highest in the wet winter months, even when their is high humidity outside as is the case in the Pacific Northwest.