V is for Victory (and Ventilation)

I recently attended another of PG&E’s great series of classes at their Energy Center, this one about ventilation in houses.  The Title 24 building codes regulating energy use in buildings changed this year (see previous blog “2010 title 24 energy updates”).  This class was about the ventilation component of these upgrades, which now require whole house ventilation as a matter of course.

The interesting thing is, although we all think of the forced air heating and cooling systems in our houses as ‘ventilation’ systems, most don’t actually do any ventilation. The instructor, Judy Roberson,  characterized this as “Where’s the V in HVAC”, HVAC being the industry acronym for Heating, Ventilation and Air Conditioning.  She even has a collection of photos of  HVAC contractor advertising, all of which mentions heating and cooling, none of which mentions ventilation.

Ventilation is, specifically, the replacement of stale, odorous, humid air from inside a house with fresh air from outside, and most heating and cooling systems in houses specifically don’t do this – they instead recycle the air, heating and cooling it, but never replacing it.  Fresh air is generally left to the occupants opening windows, or whatever infiltration happens when the wind blows or when someone turns on an exhaust fan.

While these ventilation methods have worked historically, as we have tried to make our houses more energy efficient by sealing up gaps in the building envelope, houses are now having more problems with indoor air quality.  There is relatively little of the small drafts that used to keep houses fresh, so air is left in place, and lack of oxygen, humidity, chemical odors from construction materials, and even mold are becoming a problem. This is the situation which the new code changes are meant to address.

The code allows for several compliance methods: stale air can be either exhausted from the house (fresh air is made up via infiltration through the gaps in the building envelope), or supplied into the house, or some combination of the two.  The ventilation can be continuous at a low level, or, intermittent with higher amounts of ventilation.  So, as of last January, all new and most remodeled houses will need to have these ventilation systems, on a twenty four/seven/365 basis. The older, more conventional exhaust fans are also now mandatory, not optional, in kitchens, laundries and bathrooms.

The main points of the PG&E class included Judy’s bedrock opinion that continuous – not intermittent – ventilation was far better for achieving the desired effect.  It is both more effective (intermittent ventilation can only be worse, never better than continuous), more cost efficient (smaller fans, ducts, simpler controls) and less likely to be annoying to the residents (less noise, no on/off disturbance, less obtrusive air movement).

Incidentally, we are talking about small amounts of air.  The typical bathroom exhaust fan, exhausting 50 to 100 cubic feet per minute (CFM) provides enough ventilation capacity to meet the code for the average house. And in fact, that is what some builders are doing – simply providing a better quality, quieter fan in one bathroom that gets left on all the time.  While this can comply with the minimum standards of the code, and is cheap, there are better options.  Judy’s favorite system was a supply fan which served multiple rooms, particularly bedrooms, where we spend most of our time. 

The supply-type system gives you filtered air from a selected source (not just infiltrated air from anywhere), and positively pressurizes the house so that there is less chance of infiltration from 'dirtier' spaces like attics, crawlspaces and garages.   The electrical usage would generally cost twenty to thirty dollars per year – not too bad considering the positive indoor air quality advantages.  This is the other main point: the energy savings of having a ventilated but ‘tight’ home, which doesn’t leak the much larger quantities of energy that a leaky house does, are much more than the energy used in ventilation.  Thus the title of the class: Ventilate Right, Build Tight.