Bathroom exhaust fans are what we typically do. But a few point source exhaust fans is typically not enough to provide adequate whole-house ventilation. The goal of whole-house ventilation is to provide balanced ventilation that is well distributed and mixed with the air throughout the house.
Note: This is Part 1 in a series. To see Part 2: Choosing a Whole-House Ventilation Strategy, click here.
"There is nothing like the cure of fresh air for cases of bladder infection, paranoia, and Cartesian thinking."—Rawi Hage
You can blame cave men for the confusion around how to ventilate the homes you build or remodel. They started the whole mess when, right after they discovered fire, they found out that if they brought those flames into their cave, they'd be warmer. Then they had to figure out how to deal with the smoke, and the ventilation industry was born.
Of course, if you have an industry, you've got to have standards, which tell you how to do it right. And you've got to have building codes that adopt, and possibly modify, those standards. Then there are the voluntary programs, with their guidelines based on codes and standards.
At each level, parties representing various interests push for their ideas to be adopted. They also sometimes fight zealously against requirements they oppose. Sometimes they do so voluntarily. Sometimes they're employed by a trade association or company whose work is affected by the standard, code, or program in question.
With all the confusion in the air about how much we should ventilate our homes and which methods are best, it's a good idea to stop and take a look at the fundamental issues.
Why Do We Need Standards?
In the 17th century a fellow named John Mayow put small animals in jars with candles and studied how long it took them to die of asphyxiation. By doing so, he proved the existence of a special component of air that keeps us alive. What he termed "nitro-aerial spirit" we now call "oxygen." Combustion and breathing both use it up in an enclosed container, so we need some way of replenishing this "spirit." Knowing what's necessary to keep us alive is a start, but there's so much more to ventilation and indoor air quality than that.
In the 20th century, ventilation research had advanced to studying humans in confined boxes. Rather than asphyxiating the subjects, however, this time the researchers ran controlled amounts of ventilation air through the boxes. The objective was to see what minimum ventilation rate was sufficient for trained smelling judges to find the odor of the exiting air acceptable. It was fascinating work really, and they looked at a number of variables: social class, bathing frequency, age, and air space per occupant, among others.
But there's more to good indoor air quality than preventing asphyxiation in an unsmelly house. The next step is learning about the other hazards in the air we breathe—things like carbon monoxide, volatile organic compounds, formaldehyde, and particulate matter. That's where we are now. There's a lot of research going on into how much of this stuff is in the air in our homes and what we can do about it. Not all of them are best solved with ventilation, however. Source control is often the best option.
As we make homes more and more airtight, ventilation becomes more and more important. We need to keep replenishing the oxygen that keeps us alive and removing the carbon dioxide that makes us drowsy. We also need to flush out the odors and other disagreeables in the air to keep us healthy and happy.
Standards provide some guidance on how to do that. They also help insulate you from lawsuits.
Standards, Codes, and Programs
Let's see if we can sort out this morass of information and relationships so they make some sense. It all starts with knowledge, and Donald Rumsfeld gave us a great way to think of it when he talked about the known knowns, the known unknowns, and the unknown unknowns.
We certainly know a lot more than we did about indoor air quality and ventilation back in Mayow's day. Then, it was mostly unknown unknowns. They didn't know much, and they didn't even know enough to know how much they didn't know. Today, the known knowns are much greater, as are the known unknowns. I'd like to say the unknown unknowns are fewer, but that might be a bit delusional.
As our knowledge base grows, we use it more and more to provide guidance on what we do. That's where standards come in. Standards are focused bits of guidance for specific, scope-limited areas, like how to design a duct system, test an automatic ice maker, or ventilate a low-rise residential building for acceptable indoor air quality. The main standard in the U.S. that covers that last topic is ASHRAE Standard 62.2. (Standard 62.1 covers ventilation in everything that's not low-rise residential, although there's a scope-change happening now. High-rise residential buildings are moving to 62.2.)
A standard is just a set of recommendations, though, and has no power on its own. It becomes useful only to the extent that it gets adopted for use in codes and programs. ASHRAE has a lot of standards, some widely used, others not so much.
Codes come in two flavors. Model codes are like standards. They're a compendium of guidelines without any real power until they get adopted. The International Code Council (ICC) publishes model codes, like the International Residential Code (IRC) and the International Energy Conservation Code (IECC). State and local codes are based on model codes. For example, the state I'm from, Georgia, has an energy code based on the 2009 IECC, but we also have a set of supplements and amendments that goes along with it.
Programs are generally voluntary and also refer to standards as well as codes. The ENERGY STAR new homes program has a requirement that insulation levels in qualifying homes meet or exceed the 2009 IECC requirements. (If your state is already on a stricter code, this doesn't allow you to do less.) The program also requires that the ventilation systems in the home meet the requirements of ASHRAE 62.2-2010. If a home you're building is getting certified in that program, you're subject to both of those requirements whether or not they're part of your local code.
What's the Right Number?
The science behind how much we should ventilate goes back at least to 1836, when Thomas Tredgold calculated each person in a building needs 4 cubic feet per minute (cfm) just to stay alive. Since then, the recommended ventilation rates have gone as high as 60 cfm per person, but have come back down.
The 2013 version of ASHRAE 62.2 has a lot of provisions, but let's take a simplified look at the ventilation rates in the standard. It requires 7.5 cfm per person plus 3 cfm per 100 square feet of conditioned floor area. That first part is not based on the actual number of people living in the home. You may not know that for a new home anyway, so the number of people is defined as the number of bedrooms plus one. There's also a provision for reducing that number a bit if the house has a blower door test. At a measured infiltration rate of about 5 ACH50 or higher, the second part of the formula drops to about 1 cfm per 100 square feet, which is what the 2010 version of Standard 62.2 called for. (This is a rough estimate as the calculation for the infiltration credit isn't easy. There are spreadsheets that help you do that if you want the credit.)
Let's say you build a really tight house, though, and aren't going to be able to get any credit for infiltration. How much would you need to ventilate if the house has three bedrooms and 2500 square feet? Three bedrooms would mean 4 people, so the first part of the calculation would be 4 x 7.5 = 30 cfm. Using the 2013 standard, the second part would be 3 x 25 = 75 cfm, yielding a total required ventilation rate of 105 cfm.
If you need to meet the 2010 standard, the first part would be the same (30 cfm), but the second part would be based on 1 cfm per 100 square feet: 1 x 25 = 25 cfm. The total required ventilation under this scenario would be 55 cfm.
Therein lies the rub. One of the main issues that's brought so much attention to ASHRAE 62.2 over the past year is that difference between the 2010 and 2013 versions of the standard. The reason the number went up so dramatically is that versions through 2010 said you needed 3 cfm per 100 square feet, but automatically gave everyone an infiltration credit of 2 cfm per 100 square feet. There's some interesting history behind that, but all you really need to know is the committee took away the default infiltration credit, and required ventilation rates have about doubled unless you qualify for a reduction with your blower door results.
Enter Dr. Joseph Lstiburek, who was on the 62.2 committee until a few years ago. His company, Building Science Corporation, has designed and studied the ventilation systems for tens of thousands of houses over the past couple of decades, and he has a beef with the higher rates. His main complaints are:
- Comfort – In humid climates and in cold weather, the higher rates can lead to air that's too humid or too dry.
- Energy – More ventilation means more outdoor air that needs to be conditioned. Also, to solve the comfort problems in humid and cold climates, you have to use even more energy.
- Science – The higher rates aren't based on solid science showing that more ventilation is necessary. (I've covered this in some detail in the Energy Vanguard Blog.)
- System type – Exhaust-only, supply-only, and balanced ventilation all have their pros and cons, but they don't ventilate a house equally.
To address these concerns, Lstiburek introduced his own standard, which he calls BSC-01. It's not a standard in the sense that ASHRAE's standards are, because it hasn't gone through any kind of consensus process and isn't ANSI certified. It's more like a set of guidelines or recommendations, but based on BSC's vast experience with those tens of thousands of new homes they consulted on.
The BSC-01 base ventilation rates are the same as the 2010 version of ASHRAE Standard 62.2, so that a 2500 square foot, 3 bedroom house would have a base rate of 55 cfm. But under BSC-01, that base rate is modified by how much the ventilation air is distributed throughout the house, balanced between exhaust and supply, and mixed with the other air in the house. With a balanced ventilation system that distributes the air throughout the house, the base rate is sufficient. If you also mix that air, you can cut the ventilation rate by 25%; with a system that balances, distributes, and mixes, you’d need only 41 cfm. If the system is not balanced, not distributed (e.g. a single-point exhaust fan), and not mixed, this house would need 83 cfm (1.5 x 55).
Credit: Building Science Corporation
The BSC-01 base ventilation rates are the same as the 2010 version of standard 62.2, so that 2500 square foot, 3 bedroom house would have a base rate of 55 cfm. But—and this is the crucial part—that base rate is modified by how much the ventilation air is distributed throughout the house, balanced between exhaust and supply, and mixed with the other air in the house. The table above shows how it works.
Let's see how this affects that 2500 square foot, 3 bedroom house. The base rate is 55 cfm, as we calculated above. With a balanced ventilation system that distributes the air throughout the house, that's how much Lstiburek says you need. If you also mix that air, you can cut that by 25% so with a system that balances, distributes, and mixes, you'd need only 41 cfm. If the system had none of those qualities, this house would need 83 cfm (1.5 x 55).
How Will it All Shake Out?
Lstiburek pushed hard on the issue over the past year. He didn't succeed in getting any programs or codes to adopt BSC-01, but he did put pressure on the ASHRAE 62.2 committee. At the ACI conference in Detroit, he and several committee members held a panel discussion of the issue, and most in the room believed Lstiburek had the more persuasive arguments.
Near the end of that discussion, Lstiburek announced that he'd like to be back on the 62.2 committee and has since been reinstated as a member. This is good news for home builders, weatherization crews, and energy auditors because they share many of the complaints on Lstiburek's list. In the next article in this series, we'll dive into the different system types and explore their pros and cons.
Allison Bailes ownsEnergy Vanguard, a home performance and training firm in Decatur, Ga.