[Greenbuilding] Natural ventilation

Sun Dec 2 10:37:02 EST 2007

I dont want people to misunderstand what I said. Natural ventilation can work. However, it is quite difficult to reliably deliver ventilation air to normal occupancies.

Houses were built for hundreds of years that had lots of ventilation that occurred accidentally. Most of us grew up in them.
Normal airtightness is measured in Air Changes per Hour at 50 Pa pressure difference. If you divided by 20 you get a rough idea of the ACH under natural conditions. In the past, ACH at 50 numbers were in the range of 10 to 30. This means on would get 0.5 to 1.5 ACH of "natural ventilation" .  This is way more ventilation than needed to remove normal levels of pollutants and provide healthy air. The fact that accidental leaks sometimes provided very bad air by sucking through moldy walls or polluted garages or wet basements was masked by massive ventilation.

The problem is that under non windy conditions or sheltered conditions (ie houses packed together in a sub division or surrounded by a forest) the wind pressure is not available and accidental ventilation drops, perhaps by half. If the temperature outside is not too cold (say it is 40F/5C) or not so hot (say it is 30C/86F) then bouyancy pressures dont drive much ventilation either. So drop accidental ventilation by half again. Under conditions when the temperature is more like 15C/60F, people tend to open windows and doors and the lack of natural driving forces does not matter much.
So, under many hours of the year in many climates, the driving forces are much less than "normal" and so ventilation rates are closer to ACH at 50/40 or ACH at 50/80! 

If you have an air leaky house (which means it is uncomfortable, non-durable and often unhealthy as well as an energy pig) this wont kill you.  20ACH at 50/80= 0.25 ACH which is still likely just enough in 90% of households to provide sufficient fresh air.  Experience and studies suggest that ventilation in the range of 0.2 to 0.35ACH are required for moisture control in cold weather and fresh air all year. This assumes that air is not delivered to each room and relies on incertain distributions between the bedroom (with a closed door) and the living room or the kitchen.

As we have built houses that are more energy efficient and airtight, the ACH at 50 number dropped below 10. New houses in the US are often under 10, and many are under 5. In Canada we have long built airtight houses and slowly the number dropped to below 5.0 and is often an average of 2 to 3. Very well built house have a number of <1.5. My apartment building is 1.0 at 50. Lots of people on this list have built houses that are this tight and much tighter (the best I have heard of is 0.15!).
These numbers resulted in accidental ventilation of well less than 0.2 ACH at 50, and the problems with indoor air quality and moisture became sufficient that mechanical ventilation was added in. Although mechanical ventilation uses energy, it allows houses to be airtight and energy efficient and comfortable and saves a lot of energy in the aggregate.
By providing a guaranteed amount of air to each room or mechanically mixing all air in the residence, the amount of fresh air that needs to be provided can be lowered to what is actually required. This is in the order of 3 to 15 cfm/person, depending on the amount of pollutants you are dealing with. 7.5 cfm /person is a good number if every room gets its own supply, and a minimum of 0.01 cfm/sf is often suggested to remove base level pollutants.

So what role does natural ventilation play? If you mean people will open the window to allow air in and out, then it wont work unless you have an exceptional client (and you may). Most people leave their windows closed on cold winter nights, and hot humid summer days. They still need fresh air. Almost as many people will not reliably open windows under even more normal conditions: they are not used to actively monitoring their air quality and responding to it. As a professional, it is unreasonable for me to assume that an occupant of an apartment, say, will operate the ventilation system in a manner that protects themselves, their guests, or the building enclosure from harm. So some ventilation must be provided without occupant input.

How could we get reliable natural ventilation? Well we could use a chimney that captures wind pressures and stack effect/buoyancy pressures and allow air supply in each room via a baffled air inlet high on the wall that would ensure that cold enetering air has mixed to be warmer before it contacts a person. The challenge is that we need a control mechanism that prevents excess ventilation in cold windy weather (when the hourly average driving forces on a house can be 10-20 Pa) and yet allows ventilation when the pressures are small (2 Pa say) under moderate and still conditions.  This can be done in single storey houses, but is more difficult in multi-storey because the pressure in the building varies over its height.
I don't know of a single commercially available system in any country that does this. I have seen attempts (aerodynamically tuned inlet dampers for example) and think of a number of ways to do this that are untested. Of course, the $150 dollar Panasonic fan is usually a lot easier :) 

So it is hard for me to tell someone how to do natural ventilation in houses and even harder in multi-unit apartments.
In commercial applications it is more feasible because it is more affordable to use controls to open and close inlet and outlet vents based on a computerised assessment of measured pressures, flows, CO2, RH, etc. Even in these cases I find that there are a significant number of hours of the year that would require very large ventilation openings or some boost from a fan. From a capital and operations cost, the fan boost, or hybrid nat vent systems always win.

Natural ventilation cooling can work when the temperature and air moisture content outdoors is less than indoors. In places with the need for a lot of cooling (the US south for example) this is not that common. When it is cool and dry outdoors, you can get free cooling, but a well designed building should not need cooling when it is cool and dry outside, right? Trying to ventilate when it is warm (say 75F/23C) and humid (outdoor RH over 50%) will not help meet comfort standards which are typically in the <76F/24C and RH<60% range. If you live in a dry climate, the temperature swings between day and night will often allow you to cool thermal mass in the building at night by ventilation with cool night air, and then absorb the heat of the day into the mass (and you dont ventilate more than the minimum during the day). All of these strategies are hard to design for because of the highly variable weather conditions, variable air flow resistances, and the tight comfort standards m
ost clients impose (eg ASHRAE 55). All have and can work with careful and sophiticated design and broader comfort conditions. Note: Ceiling fans, which cool the body by encouraging the 90F/28C skin to radiate to air that is cooler and drier really works well but are not "natural ventilation".

Lots of new buildings work today with accidental ventilation which is driven by natural forces. In most cases these buildings work by over ventilating most of the time, and being lucky enough to produce few enough pollutants to avoid problems during the low driving force hours. It is almost impossible in the design stage for a professional to sign off on a substantiated calculation that guarantees sufficient ventilation as required by ASHRAE 62 or equivalent code standards. People do it and get away with it. However, the risks are real, and the energy and comfort penalty can be significant.

Sorry for the essay: the natural ventilation thing has been growing lately and I have been answering a lot of these questions.

Stephen Collette wrote:
> John and all,
> 
> Thanks John.
> 
> It's unfortunate that you are so super smart, as I would love to  
> figure out a way to prove this point the other way. I'm disappointed  
> as I wish it were not so.
> 
> Just to clarify, are you saying that in any temperature/climate,  
> assuming the bulk of us on this list in North America, would not gain  
> any benefit from natural ventilation, or are your thoughts mostly for  
> us in the colder climates such as Canada?  Again, I'm not challenging,  
> as well..., if anyone is going to be right, it's you, but I just want  
> to learn more about natural ventilation. Especially since it was so  
> touted and attempted upon with the straw bale buildings I worked on in  
> the past.
> 
> Are there also any books you or others may recommend that I could  
> learn some more about this topic, even if they are only commercial  
> designs?
> 
> Thanks a heap.
> 
> Stephen
> 
> Stephen Collette BBEC, LEED AP
> Principal
> Your Healthy House - Indoor Environmental Testing & Building Consulting
> www.yourhealthyhouse.ca
> stephen at yourhealthyhouse.ca
> 705.652.5159
>>
>> Once you have an airtight house and quality (read efficient)  
>> ventilation system, natural ventilation does not really save a  
>> worthwhile amount of energy in houses but causes all kinds of  
>> challenges for construction design and operation while remaining  
>> unreliable.
>> In large buildings, natural ventilation may be worth the effort, but  
>> in a typical SFH, hardly.
>>>
>> Dr John Straube, P.Eng.
>> Associate Professor
>> Dept of Civil Engineering & School of Architecture
>> University of Waterloo
>> Waterloo, ON Canada
> 
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-- 
Dr John Straube, P.Eng.
Associate Professor
Dept of Civil Engineering & School of Architecture
University of Waterloo
Waterloo, ON Canada