Passive House Principles: Balanced Ventilation

Sealing a house up so tight that under a blower door test it out performs a code built home by a factor of 10 is a good thing right?  It can be.  Especially in the sense of heating/cooling.    However it comes at a price.  The price is that unventilated spaces have a lot of issues.  Volatile organic compounds from building products, air contaminants from cooking, body odour, moisture from breathing, showering, washing dishes...all build up inside the building envelope.  The air becomes stale and unhealthy.  With the right environment, condensation can build up bulk water in areas which lead to mold growth which releases spores which you breath and eventually everybody inside the structure has a cough, chronic asthma, headaches, and the list goes on...Did this problem exist in older homes? Nope.  A lot of older homes had nothing in the walls.  Water vapour passed right through from the inside-out or the outside-in.  People didn't have an HR in their HRV (The HR is for "Heat Recovery, and the "V" in HRV is for ventilator).    They just had a V....and that V was called wind!!!  The walls could dry from both directions.  That 18 kW wood cook stove oven in the kitchen was a major contributor to drying also.  It was pumping heat straight through the walls preventing any dew point issues that would lead to condensation.  The homes were inefficient and home owners probably used 10 to 20 cords of wood per year for heating!  The draft created when burning wood acts as a vacuum, sucking air from the room which in turn brought fresh air through the leaky walls, right into the house.

So back to the vapour issue.  The cure to the problem is an HRV, (or an ERV), the differences of which I will discuss  in a later post.  The HRV provides a supply of fresh air to the interior of the structure, while at the same time, expelling the stale polluted air to the outside.  However the simple exchange of air would be like having a window open.  When its hot outside, all that hot air comes inside and has to be cooled.  When it's heating season, all that cold air outside would draft through the open window and make it extremely cold and hard to heat.

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n an HRV, the incoming and outgoing air streams pass through a cross directional recovery core which transfers heat from outgoing to income air.  Efficiencies of up to 80% can be realized using this passive device.  Which is pretty good given that it's really just two fans.  

HRVs aren't fool proof.  The static pressure (air resistance) of the duct system for incoming and outgoing air are rarely the same.  In which case, you can be supplying a volume air to the building at a rate higher than you are expelling the stale air.  In this case the pressure inside the home is higher than the outside.  A pressurized building pushes air out through the cracks: under baseboards, through plugins, through holes in the vapour barrier, and through leaky doors and windows.    That heated air is escaping through the envelope which means the home is losing energy and more heat will need to be applied to bring the home back to  temperature.  There are bigger problems.  Moisture laden air carries a huge potential to condense when it comes into contact with surfaces that have a temperature less then the dew point. 

This can lead to dangerous mold issues inside of stud walls once the temperature of the stud space is at a temperature high enough to encourage mold growth.  So next time you puncture a hole in your vapour barrier by hanging a picture you should think twice about what you are doing to your walls.

Very little water passes through drywall via diffusion.

A hole in drywall moves bulk air carrying moisture. 

If the pressure inside the home is less than the outside pressure you have another problem.  First, cold air leaks in through any crack it can find.  Secondly, now it needs to be heated or cooled.    Moist air from the outside can be sucked into the wall space from the outside, potentially leading to condensation and mold.  

Prescribed ventilation rates should be followed if the HRV is to perform as expected.  ASHRAE 62.2 requires (floor area)/100 + (number of bedrooms +1)*7.5 cfm for the total ventilation.  So for a 2000 sf, 3 bedroom home the total ventilation rate should be about 50-60 cfm.  Based on installations that I have seen, homes are over ventilated.  An over ventilated house is literally spewing energy right out through the HRV and leads to extra energy costs.  Over ventilation may be necessary at times to rid the house of excess moisture or smelly air.  Most HRVs have a turbo function to facilitate increased ventilation rates for a fixed time interval.  The turbo function should be used temporarily instead of over ventilating.

If you have an HRV it needs to be balanced.  The air flow needs to be adjusted to give ventilation rates that are necessary to ensure the health of it's inhabitants.   An HRV, set up properly, provides a single avenue for air to pass:  it's a controlled system with predictable results.  Like a standard code built home, a passive house needs balanced ventilation.  Choosing one with high efficiency is the hard part as very few exist with efficiencies above 80%. I have found a couple of these higher efficiency HRVs, but they are few and far between.  Ill be looking to get the highest efficiency possible.....looks like and ERV will be my choice...although we seem to be stuck in the 80's here in Newfoundland and HVAC guys think that HRVs are the answer.