Ventilation

Don't let anyone tell you that you can build a house too tight!  The trick is, as the saying goes, "Build it Tight!. Ventilate it right!".  

You must construct the home such that you know where air is entering and exiting the home, and be able to control that air.  When a home has a leaky building envelope, you have no idea where air enters and leaves the home and, worse, you have no way of controlling it.  We've all been in houses where the curtains wave when the wind blows, your feet are cold because air is coming in under baseboards of the exterior walls or thru the floor in pier and beam homes.  It is difficult to keep a house comfortable when this occurs and is expensive to heat and cool it!

As I mentioned in the "Building Envelope" section, we take every opportunity to seal the shell (floor, walls, and attic) of the home as if we were building a boat or hot air balloon.  It is relatively easy using house wrap, tape, rigid insulation, and foam caulk.  Air sealing is probably the biggest bang for your buck when it comes to building a high performance home...  Once the home is "sealed", then you need to address the issue of ventilation so you can keep the house fresh and healthy.

There are several ways to achieve ventilation.  Some methods are considerably more expensive and others are quite affordable.  

ERV's and HRV's

At the top of the scale are units that exhaust air to the outside, draw in fresh air, and while doing so, mix the air streams in a special heat exchanger so the incoming air is closer to the home's interior temperature and humidity.  These are quite useful when outside air gets very cold or is extremely humid.  These units can cost several thousand dollars, but can be justified in certain localities.  These units are referred to as HRV's (Heat Recovery Ventilator and ERV's (Energy Recovery Ventilator).  ERV's are more appropriate in the warmer, more humid, climates.

Exhaust Ventilation

This technique is the simplest and least expensive of the ventilation techniques.  Typically it is accomplished using bathroom exhaust fans that are integrated with timers and/or humidity sensors.  Some of the newer vendor offerings have fans that run full speed for a "settable" period of time, then slow down to slow speed and exhaust a relatively small volume of air on a continuous basis. The continuous ventilation speed is adjustable and it typically set at the volume recommended by the ASHRAE 62.2 ventilation standard (about 45 CFM for a typical Habitat home).  

Although this is the easiest configuration, it results in the home having a slightly negative pressure.  This means that makeup air is being drawn into the home from any path it can find.  Most likely, around door and window seals.  This source of makeup air is unfiltered, and thus, brings in pollen, dust, and other contaminates that may be undesirable.

Supply Ventilation

This ventilation technique utilizes a duct connected to the outside (typically connected to a screened wall cap), and an inline fan in the duct to draw the outside air and force it into the house.  Sometimes the air is filtered before it is discharged into the house.  I've seen it both ways, but I think it should always be filtered.  But this is another filter that may or may not get serviced regularly.

The air stream from the ventilation fan will slightly pressurize the house, which prevents any incoming air leakage from around doors, windows, etc.  So, it the ventilation air is filtered, the homes air should be relatively free of airborne pollen and dust.

In our locale, it gets hot, but is not particularly humid on the average, so we have chosen to use the bathroom exhaust fan with continuous ventilation.  Unit we've chosen allows you to select the CFM for ventilation (approx 30 in our home with 2 people although that is slightly under ASHRAE recommendations).  It also incorporates a motion detector to kick up the fan speed when the bathroom is occupied, then slows down after a preset time (about 10 minutes).  It is the simplest of all the solutions and is quite inexpensive.

It is difficult to find ventilation solutions that scale down to smaller homes.  Typically HRV and ERV solutions work nicely in home of 2K sq ft and larger, but are over sized for smaller homes.  The also require periodic maintenance which can be problematic.  Finding low volume fans that are quiet, efficient, and volume adjustable are also difficult.  That would be a good choice for us, but a viable commercial solution has been evasive thus far.