Low humidity is also a problem because it causes dry skin as well as dryness in the nose and throat.  Luckily, if we maintain an indoor relative humidity at 30-50%, it will be enough to keep us from drying out, but not enough to promote the growth of molds and mites (which need about 70% humidity or greater).

Controlling moisture is a science into itself, and even with a solid understanding of the science, the vagaries of weather conditions, building conditions and operation make protecting against all troublesome conditions difficult at best.   The operative rule is to provide as much safety margin as possibly without incurring much extra cost.

Anywhere water can go, it will.  The key to keeping water out is to design the building so that water naturally wants to stay out, allow for the building to dry out if it does key in, and when that is not practical or possible, use mechanical means to remove the water.  There are three main mechanisms for water moving into and out of a house:

• Leaking in (either from ground water or from rain)

• Capillary action (wicking through foundation or siding)

• Vapor transport (carried by air or movement of vapor from warm to cold)

For each mechanism that water can move, we use a different strategy to limit the amount that moves into places we don't want it.

Leaking In
Water can leak into the house either from groundwater coming in through the foundation or rainwater entering through the roof, siding or flashing. The general strategy for groundwater starts with encouraging surface water to drain away from the house by sloping the land away from the house, then for the water that does percolate into the ground, install a drainage system to force it away from the house (gravel and a perforated pipe drain system), and finally by installing a water resistant coating (typically asphalt) on the exterior of the foundation.  Downspouts from gutters should direct the water far from the foundation, not into the foundations perimeter drain system (so as not to overload it).  In the past downspouts were sometimes connected directly to the cities sewer system, but many cities no longer allow this as heavy rains cause the sewer system capacity to be exceeded, and result in raw sewage overflowing somewhere.  Alternative solutions including Green Roofs (see the site page) and Cisterns (see the utilities page).  It is obviously also important to avoid building sites where ground water normally pools near the surface, such as in wetlands and near subsurface streams.

(foundation diagram)

Rainwater can enter the house either via gravity or wind driven, and the typical points of entry are around openings in the envelope: windows and doors, and around areas of roof flashing.  Because roofing and siding are imperfect barriers, a weatherproof barrier (typically tar paper) is always installed as a second line of defense.  A roof with a pitch greater than 3:12 will shed water by gravity and so is inherently a better rain barrier.  Likewise any siding that relies on caulking between panels to prevent rain entry is much more likely to fail-siding element that overlap each other are much better. When wind driven rain is an issue, the siding can be installed as a rain screen, which means that an air gap is created between the siding and its underlying waterproof barrier.   This air gap is made intentionally leaky so that the air pressure in the gap is similar to the air pressure on the outside of the siding, reducing the pressure difference across the siding, and hence the movement of water through the siding.  Rain screen are rarely installed, except with vinyl siding which is always installed in a way that acts as a rain screen.

Capillary Action
Water moves through porous materials (or very narrow gaps in non-porous materials) in the same way a sponge soaks up water, which is all due to water's gravity defying ability to cling to itself.  The primary concern is water moving up through the basement slab and foundation, and most building codes require  capillary breaks to prevent this. Coarse gravel is installed underneath the slab because the relatively large air gaps in it inhibit the movement of water from the underlying soil by capillary action.  (??) Alternatively, a polyethylene sheet can be placed below the slab (?? or also??)

Porous siding material can also exhibit capillary action, which will increase as the material absorbs more water.  Painting the outside of the material helps prevent this, but paints will ultimately fail, especially on wood siding.  Using the rain screen approach allows the inside of the siding to dry out. (??but how much is this really a problem??).  Alternatively, the back side of the siding can be primed so as to reduce the amount of moisture that can get in the siding material, and to inhibit its transfer to the underlying waterproof barrier.

Vapor Transport
There are two main mechanisms for water vapor to move: either carried by air movement of by vapor diffusion through permeable materials from areas of high vapor pressure to areas of lower vapor pressure.  Of the two mechanisms, air movement is by far the larger carrier of water, and hence a much larger concern. In either case, the important issue is not preventing any water from entering the walls, but preventing it enough so that the relative humidity in the wall stays low enough and that condensation does not occur on the colder outside surfaces of the wall (which is typically the inside of the exterior sheathing).

Understanding Relative Humidity and Condensation
In order to understand some of the problems with water movement, it is necessary to understand what relative humidity is and how condensation occurs.  The term relative humidity refers to the percentage of water vapor in the air as compared to the maximum amount of water vapor the air hold.  Its is called relative because the amount of water air can hold changes dramatically with temperature.  Cold air can hold very little water and warm air can hold quite a lot. Condensation occurs when warm air is cooled to the point where the relative humidity would exceed 100%.  Windows are often the first place condensation occurs because they usually have the coldest surface temperature in the house, and so any warm moist air hitting them will cool and deposit liquid water.

The most important issue is whether the warm air on one side of a wall contains enough water so that when its cooled to the temperature on the other side, condensation will occur.  When condensation can't occur, we have little worry about water vapor carried by air movement or vapor transport, although it is still possible for these mechanisms to produce areas in the wall where the relative humidity is above 70%.

In the winter, the outdoor cold air holds little water, even when its raining and the relative humidity outside is near 100%.  When this cold air is brought inside and heated the relative humidity goes down dramatically, sometimes so much that a humidifier is necessary to prevent your skin and nasal passages from drying out.  In these conditions, a house that leaks a lot of air will be dryer than one that is relatively tight, because it will quickly remove and water vapor added by human activity (breathing, showering, cooking etc).  In moderate weather, a ventilation system that brings in fresh air will help keep the humidity down, but in very cold weather, the air is usually so dry that typical human activity still does not raise the interior humidity level high enough.

Vapor Diffusion
Whenever there is a difference in vapor pressure (total water content) between inside and out, the vapor will want to move from the area of higher pressure to the one of lower pressure.  In any house where there is a lot of air leakage, the vapor pressure will be the same inside and out because there is the same amount of water in the air, although at a different relative humidity due to temperature difference (verify this).  However, typically, there is more moisture inside than out, because human activity (showers, cooking, breathing) adds vapor to the air.  Humidifiers and De-humidifiers further change the difference in vapor pressure between inside and out.

As a rule of thumb, when the house is being heated, water typically moves from inside to out, and when the air conditioning is on, water typically moves from outside to in.  In the latter case, the issue mostly occurs in climates with high relative humidity, such that the warm outdoor air has more vapor in it than the cooler indoor air can hold.

Air & Vapor Movement
Because there is often a air pressure difference between inside and outside, the resulting pressure difference will force air through any weakness in the buildings envelope, of which there are often many.  If the relative humidity of the air on the warm side is low enough, or the temperature difference is small enough, no condensation will occur as the air moves to the cold side.  The best defense against air movement carrying excess water into the walls is to seal the walls as tight as possible to minimize the amount of air entry into the walls.  The amount of water moved by vapor diffusion is generally much lower than the amount carried by air movement: an imperfect vapor barrier is still highly effective, but even small holes allowing air movement will carry a lot of water.

The first layer of defense again water vapor movement in heating climates is to keep the indoor relative humidity from getting too high (which also prevents mold growth).  In cooling climates, the outdoor air is warmer, and its humidity can't be controlled.  The next level is to use one of four general strategies for keeping water vapor out of walls (and letting it back out, since it inevitably gets in):

1. Use an interior air & vapor barrier, but make the exterior breathable.  This is the typical approach for heating climates and seeks to contain moisture in the house and then allow any moisture that does enter the wall to dry to the outside.

2. Use an exterior air & vapor barrier, but make the interior breathable.  This is the same strategy as above, but for cooling climates.

3. Make both sides of the wall impermeable.  While this may sound appealing, it ignores the fact that all barriers ultimately fail, and any water that enters this wall system will have a hard time getting out.  Since plywood is considered impermeable, the typical wall assembly falls in this category.  A variation on this strategy is to use exterior foam sheathing so that the inside surface of the sheathing is warmer, reducing the possibility of condensation (since there is now effectively a vapor barrier in the middle of the wall).

4. Make both sides permeable.  This is the approach generally taken in straw bale building, but is generally prohibited by code in many places.  This approach accepts that barrier are imperfect and allows water vapor to move in the wall (the must still be airtight though) and can only work if the water that enters the wall dries out before the relative humidity in the wall gets high enough to cause damage.

There is a significant amount of controversy around which method is best no matter what climate you're in, generally due to the issue of whether water which gets in the wall can get back out.  Keep in mind that even materials that are considered impermeable actually are just low permeability, and that water has a tendency to go everywhere is can, even when its unlikely to do so.

Vented versus Un-vented roofs

The general idea behind vented roofs is have an air gap that allows any moisture that moves thru the insulation to "dry out", rather than be trapped there and eventually cause mold.  Since sheathing (plywood & OSB) are both effectively vapor barriers, the air gap is generally between the insulation and the sheathing.  This rule does not apply to SIPs since, the entire SIP structure is effectively a vapor barrier.  The key to building an un-vented roof is to make sure there is never a "condensing surface": no impermeable surface can be in contact with permeable insulation UNLESS the contact surface is sufficiently insulated from the outside that the temperature at the contact surface will always be above any possible dew point.

House-wraps
A house wrap consists of a layer of tar paper, Tyvek or equivalent product which acts as both a drainage plane (the backup water barrier under the siding), and an air barrier (to prevent air moving into the wall).   Ideally these products must keep out liquid water, while still allowing some vapor to escape, and the wall to dry out in the inevitable case that water gets behind the house-wrap.  In general, beware of products with perforations as these tend to allow too liquid water to get in.  The biggest advantage of tar paper is that its permeability goes up when it gets wet, so if water does temporarily get behind it will dry when the weather improves.  For an in-depth look at house-wraps, there is an article on the University of Massachusetts building materials department website.

General Recommendations

At the moment, the current trend in heating climates is to make both sides impermeable.  First, both inside and out are made fairly air tight, to eliminate air movement, which prevents 98% of the potential problems.  Since sheathing is fairly impermeably, many building codes require the use of a vapor barrier paint on the interior to make the entire wall impermeable.  This requirement is now under revision or being removed because experiments have shown that walls are able to hold all the moisture that does get in during the heating season with no ill effect, and so by eliminating the vapor barrier paint the wall can then dry to the inside during the summer.  In fact, the use of vapor barrier paint may actually cause more problems than it prevents by trapping the moisture that inevitably gets in the wall anyhow.  

Reources

DOE  guide to moisture control:
http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11750 

Building Science Corp guide to moisture control:
http://www.buildingscience.com/buildingphysics/moisturecontrol 

Read about un-vented roofs on the building science corp site: http://www.buildingscienceconsulting.com/resources/roofs/roofs_unvented.htm

Read about roof venting from Home Energy magazine:
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/99/991111.html 

Read Building Science Corp general building info:
http://www.buildingscienceconsulting.com/designsthatwork/default.htm