Since neither of have any chemical sensitivities, our goal was to produce a house that was quite healthy, but didn't necessarily do the extra things required for an occupant whose immune system couldn't deal with a small quantity of toxics.  In our experience people bombard themselves with toxic materials regularly, and although there is likely a health effect, it is almost always many years away and some people manage to escape unscathed into old age.  Our theory was to reduce our exposure to most contaminants, but not to worry if we ran into situations where alternatives are either expensive or inconvenient.  In many cases the exposure to contaminants is more due to the operation of the house, than its building materials, and we already pay much attention to those issues, although toxic cleaners and weed killers do end up in the house on occasion.  Like most people, we aren't perfect.

In terms of the building itself, our biggest concern is designing and building as to reduce the possibility of mold growth anywhere in the house.  This means keeping groundwater and rainwater out, preventing water vapor from entering the walls (and allowing them to dry out when it does), and working to prevent mold growth around sinks and showers.  Mold will grow whenever the relative humidity is over 60%, and although this does occur, it doesn't happen frequently: only when the relative humidity outside is 60% or above and it is relatively warm out.  Cold air holds much less water than warm air, so on cold days, when outside air leaks in and is heated the relative humidity drops. In periods of continued temperatures near or below freezing, the relative humidity of a house can drop so low that a humidifier is necessary to keep skin from drying out.

Building codes already have a significant number of rules concerning water, and for the most part they help keep a house mold free.  The foundation will have a damproofing installed on the exterior and drain installed around it and be backfilled with gravel, while the sill plate will have a gasket to prevent water from moving upward by capillary action into the framing.  Both roofing and siding will have #30 tar paper or equivilant installed behind them, and all windows and doors will have proper water barriers and drip caps.

What usually causes mold in a building isn't rainwater or groundwater entering, but moisture from the home itself: often in kitchens and bathrooms, but sometimes due to the moisture that inevitable ends up in walls not being able to dry out.  Because Seattle is primary a heating climate, water vapor is mostly driven to the outside, so we want to make the exterior wall breathable. This is not easy, because due to sheer wall requirements (ie the structural materials that resist wind and earthquakes), the outside generally must be plywood or OSB sheathing, both of which are considered impermeable materials when dry.  Depending on the quantity of water that is moving through the wall and the outside temperature, condensation can occur on the inside of the sheathing, which in time may evaporate back into the insulation or penetrate the wood.  If it does penetrate the wood, it should be able to dry to the outside by passing thru the permeable tar paper.  One issue is how it gets thru the siding, since anything with paint on it tends to be highly impermeable- one possibility is using the rain screen approach, which involves nailing furring strips (eg low quality 1x4s) onto the sheathing and then attaching the siding to them, leaving an air gap which acts as both a drainage plane for any water that gets through the siding and a place for vapor to come out of the wall.

Sinks and showers are a much bigger problem, and the most common source of mold- we'd guess that at least half of all houses have some amount of mold in one of those locations.  The most important thing is to avoid porous surfaces that will be we regularly-in particular if tile is used near a sink or in the shower, the grout must be well sealed.  No grout sealant lasts for ever (or even a particularly long time), so it is generally recommended that excess water be removed from wet areas after each use.   While this is a simple and very effective mold prevention strategy, in practice most of us are often in much to big a hurry to do this reliably.  To complement these strategies, an exhaust fan should be run for approximately a half and hour after a shower to remove the excess moisture from the air.  At this time, we are looking into quite timers and alternative grout sealants besides silicone, which lasts at most a year.

Ventilation&Filtration

Our current design has the ventilation system integrated in with the heating system, and run all day long.  We don't believe this level of ventilation is necessary, and are planning on using a programmable timer to run it only part of the day, with the exact amount being dependent on what level of natural air leakage the blower door test indicates we have.  Based on this our plan, we may set the ventilation system to supply what we will need on a typical day, or possibly adjust is seasonally.

The particular ventilation unit is a Venmar HEPA 3000 Heat Recovery Ventilation and Filtration unit.  This unit operates in two modes at once: it circulates air from within the house while filtering it and it exchanges some inside air with outside air.  It has two speeds of operation: high (270CFM circulation and 105CFM of fresh air) and low (180CFM of circulation and 70CFM of fresh air).  Since the standard is that occupants need about 15CFM each, the ventilation unit supplies much more fresh air than is needed, even at low speed, even assuming there is no natural ventilation.  Even if the house comes out very tight (say 1/10 ACH), it will still have significant natural ventilation for much of the year (28 CFM at 1/10 ACH).  

As of now we are undecided on how we will deal with this. 

A HEPA filter is a very effective filtration system (see filtration in the tutorial), and should remove most allegrens as well as the majority of the dust.  In addition to this filter, we will install at least a 4" pleated media filter in the main duct, so that when the heater is running, we will also get some filtration (much more than the standard heater filter, but not as effective as the HEPA filter.