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Designing a Good Ventilation System

Ventilating is easy — it’s ventilating right that’s hard

Posted on Jun 15 2009 by Martin Holladay

UPDATED on June 25, 2018

Most green builders include some type of mechanical ventilation system in every home they build. That’s good. Since green buildings usually have very low levels of air leakage, mechanical ventilation is usually essential.

Unfortunately, several research studies have shown that a high number of mechanical ventilation systems are poorly designed or installed. Among the common problems:

  • Ventilation fans with low airflow because of ducts that are undersized, crimped, convoluted, or excessively long.
  • Ventilation systems that ventilate at too high a rate, or for too many hours per day, resulting in a severe energy penalty.
  • Ventilation systems that waste energy because they depend on inappropriate fans (for example, 800-watt furnace blowers).

It’s disheartening to learn that many green homes waste energy because of poorly designed ventilation systems that were improperly commissioned.

If you’re unfamiliar with residential ventilation systems, it’s a good idea to review the ventilation information in the GreenBuildingAdvisor encyclopedia.

The ASHRAE standard

ASHRAE’s residential ventilation standard (Standard 62.2) sets the minimum ventilation rate at 7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area.

The residential ventilation requirements in the 2018 International Residential Code (IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.) differ from the requirements of the ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. standard, however. According to the 2018 IRC, the minimum ventilation rate is 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. (For more information on code requirements for ventilation, see "An Update on the Residential Ventilation Debate.")

Systems complying with ASHRAE 62.2 have ventilation rates that are relatively low; for example, a 2,000-square-foot house with three occupants requires 83 cfm of mechanical ventilation. That’s about as much airflow as is provided by a typical bath exhaust fan. (Of course, systems complying with the minimum requirements of the 2018 IRC have even lower ventilation rates.)

Since ventilation airflows are typically quite low, ventilation ductwork needs to be impeccably sealed. If ventilation ductwork is leaky, fresh air won’t reach its intended destination.

Prominent building scientists are now debating the merits of the ASHRAE 62.2 ventilation rate. Max Sherman, former chairman of the ASHRAE 62.2 committee, defends the existing ASHRAE formula. On the other hand, Joseph Lstiburek, the well-known building scientist and gadfly, argues that the existing ASHRAE ventilation rate is too high, resulting in unnecessarily high energy costs — especially in hot humid climates, where the introduction of high volumes of outdoor air increases the need for cooling and dehumidification.

Lstiburek and Armin Rudd, a fellow engineer at the Building Science Corporation, advise designers of Building America houses to ventilate at a lower rate. “These [Building America] homes have roughly 50 to 60 percent of the ventilation rate required by ASHRAE standard 62.2,” Rudd has written. “The lack of complaints by occupants indicates that the systems are working to provide indoor air quality acceptable to the occupants.”

The “great rate debate” is far from settled; stay tuned. (For more information on this topic, see two articles: Ventilation Rates and Human Health and How Much Fresh Air Does Your Home Need? On August 7, 2013, Joseph Lstiburek released a new proposed ventilation standard, "Ventilation for New Low-­Rise Residential Buildings.")

Do we really need mechanical ventilation?

As more and more local building codes include ventilation requirements, fewer builders are able to get away with building new homes without mechanical ventilation. However, a few die-hard holdouts defend homes without mechanical ventilation.

One reason why homes without mechanical ventilation systems work better than expected is that many common household appliances act just like exhaust-only ventilation systems. Such appliances include:

  • Power-vented water heaters (50 cfm),
  • Clothes dryers (100 to 225 cfm),
  • Central vacuum cleaners (100 to 200 cfm), and
  • Wood stoves (30 to 50 cfm).

When these appliances are operating, fresh outdoor air enters a house through random cracks to replace the air that is exhausted.

However, homes without ventilation systems are homes of the past. The building science community has reached a consensus: build tight and ventilate right.

What are my choices?

After two decades of experimentation, builders have narrowed ventilation options down to four main options:

  • The simplest system is an exhaust-only ventilation system based on one or more bath exhaust fans.
  • For better fresh air distribution, choose a central-fan-integrated supply ventilation system.
  • For the lowest operating cost, choose a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV) connected to a dedicated duct system.
  • If you don't relish the thought of installing complicated ventilation ductwork, consider installing one or more pairs of innovative Lunos fans from Europe.

Can I install a supply-only ventilation system in a cold climate?

Some builders worry that a supply-only ventilation system (for example, central-fan-integrated supply ventilation) won’t work in a cold climate, because the ventilation fan will drive interior air into building cavities where moisture can condense.

This worry is needless. As energy expert Bruce Harley explains, “The upper portions (walls and ceilings) of every home — typically most of the second floor in two-story homes — already operate under positive air pressure in cold weather, due to the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.. The relatively small airflow of most supply-only ventilation systems (75 cfm to 150 cfm) will have little effect on this situation other than to shift the neutral pressure plane down slightly, in all but the very tightest of homes. … In cold climates, I believe that distributed, supply-only ventilation such as that supplied by a ducted distribution system controlled by an AirCycler, or other ducted low-flow supply ventilation, is vastly preferable to single or multi-port exhaust-only systems, except in extremely tight homes (in which case balanced supply and exhaust ventilation is the best choice).”

What’s wrong with exhaust-only systems?

As Harley’s comments make clear, many energy experts (including Lstiburek) disparage exhaust-only ventilation systems. The main argument against exhaust-only ventilation systems — for example, a Panasonic bath exhaust fan controlled by a timer — is that they don’t provide adequate distribution of fresh air. As a result, some rooms have plenty of fresh air while other rooms remain stuffy. (For more information on this issue, see Ensuring Fresh Air in Bedrooms.)

According to some ventilation experts, ASHRAE 62.2 — which currently lacks any provision requiring fresh-air distribution — should be revised to include a distribution requirement. Armin Rudd has written, “I think distribution of ventilation air is an important issue. Bringing in ventilation air and hoping that it will provide adequate indoor air quality throughout the whole house is just a hope and a prayer.”

Research shows, however, that in some homes — especially small homes with an open floor plan — exhaust-only ventilation systems work well. If the exhaust fan is well chosen — my own favorite is the Panasonic Whisper Green fan, which uses only 11.3 watts to move 80 cfm — exhaust-only ventilation systems have very low installation and operating costs.

If you choose this type of ventilation system, it’s important to remember to undercut the bathroom door.

Do I need passive air inlets?

Most homes with exhaust-only ventilation systems don’t require any passive fresh air inlets in the walls. Unless the house is unusually airtight, fresh air will find its way into the home through random cracks. (For more information on this issue, see Passive Air Inlets Usually Don’t Work.)

A 2000 Vermont study (“A Field Study of Exhaust-Only Ventilation System Performance in Residential New Construction In Vermont”) by Andy Shapiro, David Cawley, and Jeremy King, investigated whether passive fresh air inlets make any sense. The researchers studied 43 new homes (22 of which had passive fresh air vents) with exhaust-only ventilation systems. They wrote, “When the EOV [exhaust-only ventilation] fan was operating, 35% of the vents were exhausting inside air, 48% were supplying outside air, and 17% of the vents were not moving air.” The explanation? “The pressures induced by fans in these [studied homes] … were low relative to pressures induced on a house by natural forces, including wind and temperature-driven stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season..”

Note that there is an exception to this guideline: if your house approaches PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. levels of airtightness (aiming for 1 air change per hour at 50 Pascals or less), an exhaust-only ventilation system may be starved for makeup air. Most Passivhaus homes have a balanced ventilation system (an HRV or an ERV). Builders of very tight homes who prefer to install an exhaust-only ventilation system should consider the installation of one or two passive air inlets.

There's an easy way to check whether your exhaust fan is starved for makeup air: simply measure the exhaust air flow. If you are aiming for 50 cfm of exhaust air flow, and that's what you're getting, then everything is fine. If 50 cfm of air is leaving your house, that means that 50 cfm of outdoor air is simultaneously entering your house.

Central-fan-integrated supply ventilation

For years, the engineers at the Building Science Corporation have been singing the praises of central-fan-integrated supply ventilation systems. These systems can only be used in homes with forced-air heating or cooling systems. The systems include three important components:

  • A duct that introduces outdoor air to the furnace’s return-air plenum;
  • A motorized damper in the fresh air duct;
  • An AirCycler control to monitor the run-time of the furnace blower and to control the motorized damper.

The AirCycler control (also known as a FanCycler) prevents both underventilation and overventilation. When the AirCycler notices that the furnace fan hasn’t operated for a long time, the control turns on the fan to prevent underventilation. When the control notices that the fan has been operating continuously for a long time, the control closes the motorized damper to prevent overventilation.

During the swing seasons — spring and fall — the furnace blower will need to operate occasionally for ventilation purposes, even when there is no call for heat or cooling. In most climates, about 15% of the annual blower run time for such systems will be devoted to ventilation only. If the system is properly commissioned, the furnace will supply a 7% outside air fraction during ventilation mode.

The big downside to central-fan-integrated supply ventilation is that the installer needs to understand how to design and commission the system. HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors capable of this task are rare. Unless the designer of a central-fan-integrated ventilation system takes great care when specifying the furnace and programming blower operation, such a system can have unreasonably high operating costs.

A well-designed central-fan-integrated supply ventilation system needs a furnace with an energy-efficient ECM blower. Such furnaces cost between $1,000 and $1,500 more than conventional furnaces. If you end up using a furnace with a conventional blower motor — that is, one that draws 700 to 800 watts — the ventilation system will incur a big energy penalty. (For purposes of comparison, a Panasonic exhaust fan draws 11.3 watts, and most HRVs draw 100 watts or less).

Duct systems and fans designed for heating and cooling are not optimized for ventilation. While ventilation airflow is typically in the range of 50 to 100 cfm, furnace fans move as much as 1,200 to 1,400 cfm. One study (Robb Aldrich, Chicago, 2005) found that a poorly designed central-fan-integrated supply ventilation system in a house with an 800-watt furnace fan used 347 kWh of electricity for ventilation during a swing-season month. During the same month, an identical home with an exhaust-only ventilation system used only 6% as much electricity for ventilation. Although the researchers were somewhat worried that the exhaust-only ventilation system might be ineffective, the data were reassuring: all of the rooms had very acceptable CO2 readings.

Will cold outdoor air damage my furnace?

Some builders worry that central-fan-integrated supply ventilation systems won’t work in a cold climate, where cold outdoor air might damage the furnace. According to Armin Rudd, such concerns are baseless — as long as the ventilation system is well designed.

Assuming a high outdoor air fraction (15%) and a low outdoor temperature (-30°F), a furnace equipped with a supply-only ventilation system will experience mixed return-air temperatures no colder than 55°F, as long as the thermostat is set to 70°F. Even in Chicago, such systems work well.

Do I really need the AirCycler and motorized damper?

To reduce costs, some builders install the lazy man’s version of a central-fan-integrated supply ventilation system — one that includes a passive fresh air duct to the return-air plenum, but without a motorized damper or AirCycler control.

What’s wrong with this approach?

  • During the swing seasons, when the furnace fan isn’t operating, the house won’t get enough fresh outdoor air, and homeowners may complain of stuffiness.
  • During the rest of the year, when the furnace fan is operating regularly, the house will be overventilated, resulting an a severe energy penalty. During the winter, all that unnecessary cold air will need to be heated; during the summer, all that unnecessary hot air will need to be cooled and dehumidified.

An HRV with dedicated ventilation ductwork

The best ventilation performance and lowest operating cost comes from an HRV or ERV with dedicated ventilation ductwork. Such a “gold standard” system should be designed to pull stale air from bathrooms and laundry rooms, while introducing fresh air to the living room and bedrooms. [Author's postscript: After this article was written, a new type of energy-efficient balanced ventilation system, the CERV, became available in North America. For more information on the CERV, see A Balanced Ventilation System With a Built-In Heat Pump.]

Although HRVs and ERVs save energy compared to exhaust-only or supply-only ventilation systems, they are expensive to install. The high cost of these systems raises questions about their cost-effectiveness, especially in mild climates. To learn more about this issue, see Are HRVs Cost-Effective?

For ventilation purposes, either an HRV or an ERV can work well in any climate. The presumed advantage of ERVs over HRVs in hot, humid climates is not based on research or field data. As Max Sherman has written, “Almost all hot, humid climates have hours when it is dryer outside than inside, and then ERVs actually make the [indoor] moisture problem worse. The net effect this that ERVs are about a wash [compared to HRVs] for humidity control in those climates.” (For more information on this topic, see "HRV or ERV?")

Lunos fans

The Lunos fan is a new type of ventilation fan from Germany. Installed in pairs, the wall-mounted ventilation fans automatically alternate between exhaust mode and supply mode. Because each fan includes a ceramic core, they are able to recover heat from the exhaust air stream.

These fans are particularly useful for retrofit applications, or for any situation where the installation of ductwork would be awkward. For more information, see European Products for Building Tight Homes.

To commission a ventilation system, you need to measure airflow

Anyone who commissions a ventilation system needs to learn how to measure airflow. Manufacturers offer an array of accurate (and expensive) instruments to measure airflow, including $2,000 flow hoods. Builders who need to troubleshoot problems may be interested in several lower-cost methods of measuring airflow, including the use of a home-made flow hood, a method requiring a cardboard box and an old credit card, the garbage-bag method, and a method using a laundry basket or wastebasket.

For more information on these test methods, see these two articles:

Last week’s blog: “Farewell to the Chimney?”

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  1. Renewaire

Jun 16, 2009 8:37 AM ET

Pathway for Replacement Air
by David Bearg, PE, CIH


I was talking with Victor, your old college roommate at a contra dance last night, and he sends his regards.

Any experience with using a duct through a basement wall that terminates in a back as a device for allowing replacement air to be pulled into the house by the operation of the exhaust fans?

Also, I invite you to visit my site and download and read my article on "Achieving and Maintaining Healthy Green Buildings"


Jun 16, 2009 8:38 AM ET

Pathway for Replacement Air
by David Bearg, PE, CIH

Oops, typo:

The outdoor air duct terminates in a BUCKET. The bucket fills with cold air and airs will only enter to replace air that has left the house elsewhere.

David (again)

Jun 16, 2009 8:47 AM ET

Makeup air
by Martin Holladay

Concerning a passive duct that introduces exterior air into a basement: such a duct is a crude source of makeup air, whether or not it terminates in a bucket. As you point out, the duct will be a source of makeup air whenever the house is depressurized. Depressurization can occur for a variety of reasons, including operation of combustion equipment, operation of exhaust fans, or the stack effect.

The trouble with such a passive duct is that it is basically an uncontrolled hole in the house -- never a good idea. All winter long, the stack effect causes the upper half of the typical home to be pressurized. Air at the top of the house is continually escaping through small cracks in the ceiling and upper wall area. If there's a big hole in the basement -- for example, a passive makeup air duct -- the stack effect is greatly increased, and the house will lose air continuously. Of course, a tight ceiling will help. But even a well-sealed house has leaks in the ceiling.

Say hi to Victor when you see him again, David.

Jun 17, 2009 2:06 PM ET

Make up air inlets
by Riversong

You disparage exhaust-only ventilation systems as not providing good distribution of fresh air and then discourage the passive make-up air inlets that would allow positive distribution in the spaces where it's most needed - bedrooms and living rooms.

While it's true that putting deliberate "holes" in the thermal envelope can cause havoc in a poorly-sealed house that experiences stack losses and wind pressurization; in a house that's built tightly to today's air barrier specifications, passive make-up air inlets DO work because a small bath exhaust fan will easily overcome the minimal stack effect pressure.

And an exhaust-only ventilation system is the only option which will maintain a constant negative pressure in the conditioned space, preventing air exfiltration into the thermal envelope. "Balanced" power ventilation systems can pressurize a house because the cold winter air expands when warmed which results in more cfm input than output.

Jun 17, 2009 2:25 PM ET

neutral pressure plane
by Riversong

You say: As energy expert Bruce Harley explains, "...The relatively small airflow of most supply-only ventilation systems...will have little effect on this situation other than to shift the neutral pressure plane down slightly..."

Running an exhaust fan (i.e. increasing negative pressure or decreasing positive pressure) will RAISE the neutral pressure plane. If the entire conditioned space is under negative pressure, the neutral pressure plane will be at the upstairs ceiling (an ideal situation in a cold climate).

Jun 17, 2009 2:28 PM ET

Passive air inlets
by Martin Holladay

I hope my summary of ventilation options is not seen as disparaging exhaust-only ventilation systems. For small houses with an open floor plan, they make a lot of sense. They are affordable and energy efficient.

However, I disagree with two of your points:

1. It is simply untrue that "a small bath exhaust fan will easily overcome the minimal stack effect pressure." The research paper I cited -- “A Field Study of Exhaust-Only Ventilation System Performance in Residential New Construction In Vermont” -- set out to investigate precisely this question. According to measurements made by the researchers, “The pressures induced by fans in these [studied homes] … were low relative to pressures induced on a house by natural forces, including wind and temperature-driven stack effect.” That's why they discovered that, even when the exhaust fan was operating, 35% of the passive air vents were exhausting inside air rather than admitting fresh air, and 17% of the passive air vents were not moving air.

2. It is also (sadly) untrue that "passive make-up air inlets ... allow positive distribution in the spaces where it's most needed - bedrooms and living rooms." I wish it were true, but it isn't. Good fresh air distribution requires the use of either a central-fan-integrated supply ventilation system or an HRV with dedicated ductwork. Even if you pepper your walls with holes (passive air inlets), there's no guarantee than any fresh air will enter the holes. For example, if your bedrooms are on the second floor, there's an excellent chance that the stack effect will undermine the operation of the passive air inlets, allowing conditioned air to escape or preventing any air movement through them at all.

However, these distribution problems don't always rise to a level of concern. Homes with well-designed exhaust-only ventilation systems often have happy occupants without any complaints. The systems do provide fresh air, even if it isn't perfectly distributed, and in many cases these systems are perfectly adequate.

Jun 17, 2009 2:32 PM ET

Exhaust systems, supply systems
by Martin Holladay

You object to my quote from Bruce Harley concerning supply-only ventilation systems. Your counter-example, however, concerns use of an exhaust fan. But Harley is talking about systems with a supply air fan, not an exhaust fan.

Jun 17, 2009 9:38 PM ET

Flawed Study
by Riversong

My mistake on misreading Harley's context.

But your rebuttals of my statements as "simply untrue" is based on a single, highly flawed study. Even a cursory reading of the study (and I just read every word) reveals that the only legitimate conclusion would be that EOV systems don't function well in not very tight, poorly designed homes with shamefully inadequate construction detailing.

45% of the builders failed to program the fan timer, one had a blocked duct and the average measured fan flow was only 67% of factory rating. 70% had a gap between fan housing and ceiling drywall, with one having 5 square inches! Only half the homes even used passive air inlets, with an average of only 3.7 per home - even though the homes averaged almost 2200 SF and 3.3 bedrooms. 19 systems used flex duct rather than rigid, smooth-wall. Only 43% had a discernible pressure difference at the inlet with the fan running, and the average depressurization was only 1 Pascal.

In the last superinsulated house I built (the only one for which I have blower door data), the ACH50 was 2.13 with the air inlets taped and 3.06 with them open. The baseline house pressure (24° outside temp, no wind) was -2.2 Pa. With one bath fan running it was -4 Pa and with both fans -8.6 Pa.

The Panasonic bath fans had measured air flows of 86% and 92% of factory rating. The seven Airlet 100s were passing an average of 7.13 cfm with both bath fans running (though this was with the weatherstripped mud/laundry room door open which allowed almost 11 cfm to enter through the dryer make-up air duct, and there was 18 cfm leaking in through the woodstove combustion air inlet because the woodstove hadn't been installed).

While it's true that, under these conditions the air inlets weren't providing all the make-up airflow for the fan exhaust, and some was leaking through the minimal effective leakage area of the air-tight shell, both the fans and inlets were working as designed - contrary to the study you reference.

Conclusion: a programmed exhaust-only ventilation system with passive make-up inlets in a very tight house will function as intended and can meet the ASHRAE IAQ standards at minimal cost with minimal ducting.

Jun 18, 2009 4:25 AM ET

Where we agree
by Martin Holladay

Thanks for posting your views. Although you disparage the study conducted by Andy Shapiro, David Cawley, and Jeremy King as "highly flawed," and imply that my reliance on their data is misguided, I think the researchers are due a little more respect than you grant them.

Because this was a field study, the researchers measured the actual performance of exhaust ventilation systems in existing new homes. They were not measuring the performance of installations in a lab. As you point out, many of the systems they looked at were poorly designed or poorly installed. Their data are useful, though. One of the reasons I wrote this blog — entirely consistent with their findings — was expressed toward the top of my essay: "It’s disheartening to learn that many green homes waste energy because of poorly designed ventilation systems that were improperly commissioned."

Although you point out that "only half the homes even had passive air inlets," the researchers were fully aware of that fact. Their conclusions concerning the performance of passive air inlets were based only on data from the 22 homes equipped with the devices.

The researchers concluded that random air leaks are a perfectly adequate source of makeup air for exhaust-only ventilation systems. It would appear that the builders who omitted passive air inlets made the right decision, so I'm not sure why you emphasize that "only" half the homes had passive air inlets.

The three researchers took pains to measure any despressurization caused by exhaust fans in these houses. They wrote, "It should be noted that measuring these low pressures was difficult, because the wind could and often did overpower the small pressure induced by the fan. Values would fluctuate widely when the wind was blowing. In only 43% of the cases was it clear that running the fan induced a discernible pressure difference at the passive air vent."

I'd like to sum up by emphasizing our points of agreement:

1. Many builders are currently doing a sloppy job when installing exhaust fans. It's important to get the details right. That means that ducts should not be convoluted, should have as few elbows as possible, and should have sealed joints. Moreover, builders need to seal the gap between the fan housing and the drywall.

2. All of us need to redouble our air sealing efforts. Tight homes perform better than leaky homes. Those who are achieving good results -- including you, Robert -- deserve accolades.

3. Exhaust-only ventilation systems can work well. Most occupants of homes with exhaust-only ventilation systems have no complaints.

Our main disagreement, it appears, concerns the usefulness of passive air inlets. I still feel that the study I cited provides ample evidence that random air leaks are a perfectly adequate source of makeup air for exhaust-only ventilation systems. Robert, it appears that you prefer to install passive air inlets. This disagreement is minor in light of the points on which we can agree.

Jun 18, 2009 12:43 PM ET

I agree with Riversong
by John Brooks

I think that we should be very careful about drawing conclusions from data taken in average new homes and even in Energy Star level homes.
All new homes should be built airtight.PERIOD
Relying on make-up air from an unknown source is a bad idea.
There should be pressure balancing between all spaces between intake and exhaust.
HRV is best .. but I think that Robert's Plan is sound and would/will work in a properly built home.

Jun 18, 2009 1:00 PM ET

Still looking for agreement
by Martin Holladay

I agree that an HRV is best.

Unfortunately, no one has yet been able to build an airtight house. There are a few exceptions, perhaps, including submarines and the Space Shuttle; but in general we measure airtightness with a blower door because all homes leak.

The homes in the study under discussion had air leakage rates that covered a wide range, as is typical for a group of new homes. The best home fell well short of the Passivhaus standard, of course; it measured 2.24 ac/h @ 50 Pa -- about the same as the superinsulated house that Riversong built.

Depending on "random leaks" for makeup air is not as dangerous as some building experts maintain, although scare stories about makeup air from garages and crawl spaces are commonly bandied about at building science conferences. Obviously, it's important to do a very good job air sealing a common wall between an attached garage and living space.

There is even some evidence that air that enters a house through wall leaks can be cleaner than outdoor air, because fiberglass insulation can act as an air filter.

Here's the bottom line: installing a bunch of "fresh air vents" in your walls is all fine and good, but there's no guarantee that outdoor air will follow your instructions and use your designated holes. Those arrows in the ads from air inlet manufacturers aren't "smart arrows." They're more like the "serving suggestion" shown on a box of corn flakes. In most cases, the strawberries don't come with the box.

Jun 18, 2009 1:13 PM ET

We can do it
by John Brooks

We can build better than 2.24 ach50
How well did the air inlets work in the The best house?
I think that...
If you are going to use exhaust ventilation then providing the air inlets would be the wiser choice.
And that goes without saying that you must measure the flow..
If the flow is not there... the problem is not the the inclusion of air inlets. The air inlets would increase your odds of achieving a good outcome.

Jun 18, 2009 2:46 PM ET

Great stuff
by Carl Seville

Thanks for the solid info on this subject. Ventilation is definitely a big issue with tight homes and I have found a wide range of professional opinions on how much and how to accomplish it. Considering that my experience and most of my work is in mixed humid climates, what do you think about using a dehumidifier with a fan cycler function running through the central HVAC ducts for fresh air ventilation? Where it isn't frigid the winter energy penalty would be minor, and the dehumidifier should handle the excess moisture brought in. This should also solve the fan energy issue as long as the blower used is efficient enough.

Jun 18, 2009 3:12 PM ET

Fan Cycler and dehumidifier
by Martin Holladay

Ventilating with a passive air duct connected to your central HVAC ducts is a fine way to ventilate a house, as long as you remember:

1. Only use a furnace or air handler with an ECM blower, since conventional blower motors are energy hogs.

2. The fresh air duct must include a motorized damper controlled by the FanCycler unit.

However, I'm not a proponent of whole-house dehumidification using central HVAC ducts. I base my belief on a great study performed in Houston by Armin Rudd. (Rudd presented the study on October 10, 2002 at the EEBA conference in Phoenix, Arizona. A report of his findings appeared in the January 2003 issue of Energy Design Update.)

In a hot, humid climate, Rudd compared six dehumidification strategies in energy-efficient new homes. These included the use of an Ultra-Aire dehumidifier ($1,250), a Venmar ERV ($1,450), a Carrier 2-stage air conditioner ($1,550 incremental cost over a regular air conditioner). There were other tested strategies as well.

Surprisingly, the most effective dehumidification strategy was the cheapest and simplest: installing a stand-alone Whirlpool dehumidifier in an interior closet. Total installed cost: $500. The fancy equipment cost more to install, used more energy, and was less effective than the simple portable dehumidifier.

Jun 18, 2009 3:51 PM ET

Agree on the stand-alone dehumidifier
by John Brooks

In a Hot/Mixed Humid climate...
I have the forced air type system with air controller and motorized damper and ECM motor as you described.
Works very well during the summer and part of the shoulder season.
I also have a stand-alone dehumidifier ($250 Lowes) for those special occasions.... a few times in winter when Air Cycler was not keeping the RH down low enough...and a few times in the shoulder seasons when the RH was high outside and there was no call for the AC.
Next time I will build a little bit tighter add more wall insulation and go for a dedicated HRV.. (also Cool Recovery Ventilator)... and still use a stand-alone dehumidifier.

Jun 18, 2009 4:17 PM ET

Faster air changes... Flushing
by John Brooks

I think that our fresh air systems should also be "capable" of more airchanges than mandated by 62.2
When the Weather outside is better than inside.. It would be nice to be able to rapidly flush our homes with "better air"

I know what your going to say... just open the windows.
I think that we should consider minimizing our operable windows and use a higher percentage of picture windows...
According to Thorsten Chlupp it is much easier to insulate over the frame of a picture window and really boost performance.
Of course we still need fire escape windows or "little doors" and a way to ventilate if the power goes down.
Our current home is very comfortable..we almost never open the windows.. If we really want to feel like we are outside...WE GO OUTSIDE

Jun 19, 2009 4:45 AM ET

Whole-house fan
by Martin Holladay

The product you are describing exists; it's called a whole-house fan. It's used to flush the air out of the house whenever the outdoor air is more pleasant than the indoor air. It is an exhaust fan designed to be installed in the upstairs ceiling; when used — usually at night, during the summer, to cool the house — downstairs windows are left open.

For decades, my grandfather, a former president of ASHRAE, used a whole-house fan originally installed in the 1930s or 1940. He lived in southern California, and never installed air conditioning in his house. "There's no need for air conditioning. The whole-house fan works better," he used to say.

The best whole-house fan on the market is made by Tamarack Technologies:,Category.asp

A whole-house fan is fundamentally different from a fan used for residential mechanical ventilation because it moves a lot more air. While a mechanical ventilation fan needs to move 40 to 80 cfm, a whole-house fan generally moves 1,000 cfm to 2,000 cfm or more.

Jun 22, 2009 10:03 PM ET

Need More Info
by Carl Seville


thanks for the info on the dehumidification, but it leaves me with some more questions.
1 - Why the motorized damper instead of a barometric one if the house is so tight that it needs fresh air more than the air handler will run? The last few homes I have rated were so tight that they needed the blower running about 2/3 of the time to meet 62.2

2 - If a stand alone dehumidifier is installed in a closet, would it be appropriate to put supply and return ducts in the same closet to circulate the dry air around the house?

Jun 23, 2009 7:12 AM ET

Further details
by Martin Holladay

1. In almost all cases, a barometric damper in the fresh air duct will result in overventilation on the coldest and hottest days of the year — precisely the time when it is most expensive to condition outdoor ventilation air. If properly sized, the furnace will run flat out on the coldest day of the year, and the air conditioner will run flat out on the hottest day of the year. How much ventilation air is being introduced when the HVAC blower runs for 20 or 24 hours a day? That depends -- you'll have to measure it. Armin Rudd likes to set up his systems for a 7% outside air fraction. You need to know the rating of your blower, too. If you have a 1,500 cfm blower and a 7% outside air fraction, you are ventilating at 105 cfm. Depending on the size of your house, that might be just right -- or it might be twice as much outside air as you need. Here's the rub, though: a lot of HVAC installers don't measure the air flow through the fresh air duct when they commission the system. (Because they don't really commission the system.) They just hook up a 6-inch duct and they're done. In that case, you might end up with a 15% or 20% outside air fraction, and you are really overventilating. The outside air fraction needs to be measured, and a damper (not the motorized damper, just a regular damper) needs to be adjusted to get the outside air fraction right. Then, you still need a motorized damper so that the FanCycler control can shut the fresh air duct when the blower has been running for hours at a stretch. Remember, Rudd advocates ventilating at LESS than the ASHRAE 62.2 rate. I'm not saying you should do that, but a lot of people do, especially in hot, humid climates.

2. Here's how Armin Rudd installed the the stand-alone dehumidifiers that performed so well in Houston: "A stand-alone dehumidifier was
installed in a hall closet equipped with a louvered door. This is an inexpensive, tried-and-true system, using a widely available $225 off-the-shelf dehumidifier. Although the closet was not far from the house’s main return-air grille, the dehumidifier was not located directly in the return airstream. The dehumidifier’s drain was tied to the house’s drain system."

Jun 23, 2009 8:26 PM ET

62.2 - Can the experts make up their minds?
by Carl Seville

Thanks for the info. I have read Armin's opinion that 62.2 is too much, and have had discussions with Terry Brennan who says it is the absolute minimum. Who the hell are we supposed to listen to? I respect both of them very much, but it is getting very confusing out there.

Jun 23, 2009 9:04 PM ET

HRV makes more and more sense
by John Brooks

Once we have crossed the hurdle of building airtight (0.6 ACH50) and attack the weak points in the thermal envelope...The HRV really makes sense.

We could ventilate at 62.2 and greater and not have to worry about the "air change penalty"

Jun 24, 2009 6:25 AM ET

Who do we listen to?
by Martin Holladay

In response to your question, "Who do we listen to?," here's some advice:

1. In every house you build, install a mechanical ventilation system capable of providing ventilation that meets ASHRAE 62.2 requirements.

2. In your own home, experiment with lower ventilation rates by adjusting the programming of your ventilation equipment. If you have no complaints — no condensation on your windows during the winter, no mold, no stuffiness, and no smelly-sock odors — you're okay.

3. In your clients' homes, remember this: regardless of how you program the ventilation equipment, once the clients move in, they will operate it any way they want. This is a fact — a sometimes worrisome fact, but a fact nevertheless. They may end up ventilating at a lower rate than 62.2 — perhaps by listening to your advice, or perhaps by experimentation. Or they may run the ventilation equipment for 24/7. You can advise, but you can't compel.

One worrisome element to this issue is that some homes can be damaged by the owners' decision not to ventilate. That's one reason to choose wall and ceiling assemblies that are durable and not prone to condensation. You don't want to depend on your clients' decision to ventilate to maintain the structural integrity of your buildings.

Jun 24, 2009 6:33 AM ET

HRV energy penalties
by Martin Holladay

Every type of mechanical ventilation system, including an HRV, comes with an energy penalty. HRVs require electricity to run; that's an energy penalty. HRV efficiency is always less than 100%; it may be 58%, or it may be 89%, but it's not 100%. Therefore HRVs have a second energy penalty in addition to the electricity they consume: they require the furnace and air conditioner to run more than they would otherwise, to condition the ventilation air.

Two research studies looked at the energy penalty arising from the use of HRVs. With energy costs calculated in 1998, a Lawrence Berkeley study calculated that an HRV in a typical home costs $168 to $193 per year to operate, depending on climate. In 2001, NAHB researchers calculated the cost to operate an HRV at $219 to $266 per year.

The systems studied by these researchers were typical systems, but were not optimized for minimum energy use. If designed and installed by a smart cookie, an HRV system might have a lower energy penalty than the ones studied by these researchers.

Jun 24, 2009 7:12 AM ET

Research Study?
by John Brooks

Of course the HRV must be integrated into the design..
I suspect that the systems studied were tacked onto the houses much like the way we typically install our ac systems.
Look at the HRV in Alex's Super Insulated remodel
(Scroll down about halfway)

I caution again about drawing conclusions from studies conducted in typical homes.
It works in Germany because it is not an afterthought and the houses are designed as systems.

And we (USA) need better HRV's like the Germans..with ECM motors and higher effeciency ratings.

Jul 23, 2009 12:09 PM ET

Passive Dehumidification
by Stephen Bennett

I have a very poor sealed 19th century church I am fixing up and living in.
It is very humid downstairs.
I need a passive system to move warm dry air downward and cool wet air upward.
I am thinking of a simple PVC snorkel with fan but probably need something a bit more developed or efficient. Is there any kit out there or plans to make solve this problem.?

Jul 23, 2009 12:25 PM ET

What's the moisture source?
by Martin Holladay

To answer your question properly, we need more information.

1. The main unanswered question is, What is your moisture source? For example, is it ground moisture entering through a stone floor without a vapor barrier between the floor and the soil below? Or is there another moisture source?

2. What is your climate? Where is this church?

3. During what seasons of the year do you notice this humidity problem?

4. Is the interior of the church conditioned? In other words, is there a heating or air-conditioning system?

5. What makes you think that the air near the ceiling of your church has a lower humidity level than the air near the floor?

Jul 30, 2009 11:04 AM ET

Humidity in church
by Stephen Bennett

The church has two levels
the lower lever is half underground the upper floor is above ground
The warm air from outside comes into the cool underground level and water condensates on the wall and floor which are concrete
I have heat pumps as air conditioners but they dont seem to be able to handle all the moisture
I do not have the money to bu an expensive Compressor type humidifier or the money to pay for the electricty to run it
Each level in the church is 3000 feet
I am searching for an inexpensive way to de humidify
Thanks for the response
any advice would be appreciated

Jul 30, 2009 11:32 AM ET

Your suggested solution won't work
by Martin Holladay

Your problem is that warm, humid summer air contacts the cold concrete walls and concrete floor of the church basement. Predictably, the moisture from the air condenses on these cold surfaces.

Introducing more warm, humid air into your basement with a fan will not solve the problem; it may in fact increase it.

Sometimes there is no avoiding the fact that solving a problem will cost money. You have two choices:

1. Raise the temperature of the condensing surfaces by covering the concrete floor and concrete walls with a layer of extruded polystyrene foam. The foam will have to be protected, of course; I suggest plywood over the floor foam and moisture-resistant wallboard over the wall foam.

2. Remove moisture from the indoor air by running a dehumidifier.

Aug 2, 2009 5:46 PM ET

Does 62.2 require too much
by Riversong

Does 62.2 require too much ventilation or not enough?

That largely depends on the toxicity of both the indoor and outdoor environments, the toxicity of the lifestyle of the occupants, and the occupancy patterns of the residents.

Oct 26, 2009 9:42 AM ET

Low cost ERV???
by BobR


Are you aware of any lower cost ERVs that have a decent payback period?


Oct 26, 2009 9:52 AM ET

Payback period?
by Martin Holladay

Don't expect HRVs, or any other type of mechanical ventilation system, to provide any "payback."
Every type of mechanical ventilation system has an energy penalty. In other words, there is no payback.

That doesn't mean that you shouldn't include mechanical ventilation. You should.

Dec 2, 2009 12:48 AM ET

Make-up Air - Damper Placement
by Huck

We recently moved into our home (in Louisiana) which is tightly sealed. The central room has a 22' ceiling with a large (48") Isokern Magnum fireplace which tends to backdraft unless a window is left partially open. I was thinking about installing a simple barometric passive damper at the neutral pressure plane of the central room (about 11' off the floor). In theory, when the negative pressure rises above the normal neutral plane (because hot air is exhausting through the chimney), the damper would open and prevent backdrafting. This solution is uncomplicated and low cost, but is it too simplistic?

Dec 2, 2009 6:32 AM ET

Barometric damper
by Martin Holladay

The main reason why your barometric damper idea will not work exactly as you envision is wind. (If I remember correctly, Louisiana has wind.) Even a light breeze on the side of the house with the damper will cause infiltration through the barometric damper.

Dec 2, 2009 6:58 PM ET

Ventilation in Hot Humid Climate
by Stephen Colley

Regardless of any jurisdictional requirement to ASHRAE 62.2, I have not heard anything with involving permits being withheld for lack of installing ventilation strategies to address 62.2. That being said, I am inferring from the above insightful discussion that it only becomes a potentially serious issue with houses built extremely tight (around 2 ACH@ 50?). With issues being stale air and damage due to excessive moisture somewhere leading to mold/decay, it seems like the prudent response in tight, well-insulated construction in hot, humid climates (and maybe elsewhere) would be to have easy access to a good quality relative humidity gauge and a portable dehumidifier that can be manually activated. To solve any stale air issues, it may simply be a matter of opening doors and windows briefly. With a tight and well-insulated house, occasional introduction of outdoor air should be relatively quickly conditioned once the stale air problem has been eliminated. For troublesome areas, such as bathrooms, laundry rooms, I would think a fan-timer-light switch could minimize air quality issues.

Dec 3, 2009 9:49 AM ET

Barometric Damper
by Huck


Thanks for the quick reply. The location of the damper would allow me to shield it from direct wind effects because the damper's inlet will be in an attic space above the outside porch (that attic area is well-ventilated). My concern about that location is this: during the summer heat will naturally build up in that space and might push the damper open from behind. This would allow hot, humid air into the living space. Should this be a concern? If not, do you believe that a barometric damper in the location I've described is a viable option?

Dec 3, 2009 10:28 AM ET

Exterior air pressure
by Martin Holladay

If the attic is well ventilated, the attic space will have about the same air pressure as the exterior. When the wind blows, the entire side of the house will be pressurized with respect to the interior. The attic you describe will also be pressurized by wind.

Go ahead and experiment if you want. But such a damper will basically be a hole in your wall.

Jan 11, 2010 10:00 PM ET

passive outdoor air
by Edward Pahler

It would be a clean air improvement if the outdoor air entering the air return system of the home was filtered. It is my impression that a system which needs to be explored is one where an intake with a damper and a filter breathes fresh air into a conditioned system and the exhaust air is released through a passive vent using the stack pressure at the top floor of the house. Any comments?

Jan 12, 2010 6:41 AM ET

Supply-only system
by Martin Holladay

You are describing a supply-only ventilation system, which is more typically used in warm climates than cold climates. The passive vent is unnecessary, however, since the low ventilation rates required by ASHRAE 62.2 do not require huge volumes of air. Normal cracks in the building envelope easily handle the pressure-equalization of a typical supply-only ventilation system.

Those concerned about the need to filter ventilation air can also use an HRV or ERV — that is, a balanced ventilation system.

Jan 19, 2010 10:48 AM ET

Balanced Ventilation Systems
by Interested Onlooker

" "Balanced" power ventilation systems can pressurize a house because the cold winter air expands when warmed which results in more cfm input than output."

Would the use of an HRV heat exchanger help here by cooling (and shrinking) the outgoing air so that the cfms were more nearly matched?

Jan 19, 2010 11:05 AM ET

On balancing balanced systems
by Martin Holladay

Interested Onlooker,
Robert Riversong may wish to respond to your question, which was apparently prompted by his earlier post.

In the meantime, you may be interested to note:
1.The installer of a balanced ventilation system (such as a system with an HRV or ERV) should always commission the system after installation. Commissioning includes balancing. If it's necessary to meet some special requirement of the house, it's possible to "balance" the system in a way that slightly pressurizes or slightly depressurizes the house.

2. Of course, the temperature of the outdoor air changes all the time. But this fact doesn't cause an important enough variation in cfm to matter.

Mar 11, 2010 11:55 AM ET

HRV payback again
by Richard Kaye

Let me ask Bob's question a little differently, and maybe not receive a dismissive answer this time.
Given that a tightly built home will need a mechanical ventilation system of some sort, can I save enough energy with an HRV to make up the additional cost over a simple bath-fan exhaust-only system? Intuitively, it just seems crazy to heat the air and then throw away all the added heat eight times or so a day.

Mar 11, 2010 12:30 PM ET

It depends
by Martin Holladay

The answer to your question depends on the cost of the HRV system, the cost of the exhaust-only system, and the local electricity rates.

I don't have up-to-date estimates for ventilation system operation costs at my fingertips, but I wrote an article on the topic for the May 2002 issue of Energy Design Update. The average of two estimates for HRV operating costs (one in an NAHB Research Center study, the other in an LBNL study) was $0.576 per day ($210 per year), while the average of two estimates for exhaust-only ventilation systems was $0.76 per day ($277 per year).

If an HRV system costs $2,000 to install, and an exhaust-only system costs $500 to install, then the simple payback period for the HRV (based on operating cost savings compared to an exhaust-only system) would be 22 years.

Of course, all of these numbers include assumptions, and all of the assumptions are open to debate and attack. If you are planning your own house, you can do your own math.

Jul 19, 2010 3:34 PM ET

Combination system
by Bob Ellenberg

I have read this series several times and ALWAYS end up with lots of questions in my mind so I decided to post since I see this discussion has been active as recently as March.

Here is my planned system: South Louisiana (hot/humid), 1,100 SF extremely tight heavily insulated house with modest quality windows (will allow some leakage) and about 13% wall area in glass. Sealed envelope, conditioned crawl space. Plan to install central AC with heat strips as heat demand will be low, ducts in the conditioned craw space. Bath fan and kitchen fan for point of pollution exhaust and no supply in the bath room for the central system. I have had excellent past results with whole house type fans in the inbetween seasons. I have used the Tamarack fan referenced above but I also used a simple $110 Broan 470 CFM through the wall unit with an insulated positive closure door near the ceiling in a 1,000 SF unit and it worked great. I plan to also install a stand alone dehumidifier as discussed above but I anticipate the heat it generates will be something of a trade off as I'll have to turn the AC on sooner in the late spring.

Here are my questions: If needed for clean air filtration, I'll install an Ultimate Air unit. But if we have the bath and kitchen fans am I likely to need it? Will those exhaust fans likely cause depressuration problems without a passive inlet (I know the comments above but except for air around the windows, this should be an exceptionally tight house). If I need (or later add) the Ultimate Air unit, during the times of the year that the central system runs for heating and cooling, it should offer sufficient circulation for the filtered air with the supply side of the EVR tied into the central return and the intake near the kitchen area. During the in between season, if the house were closed up and comfortable, I considered that I could flip the main switch off for the air handler and insert a "blank plate" into the filter slot so that the EVR could circulate using the duct system without running the AH or short circuiting the clean air. Is this likely to work?

Lastly, if there is long enough periods where the 470 CFM fan works to keep the house comfortable, would it be worth putting a screen on the sealed insulated crawl space door so that the air was being drawn through there before coming up to the living area? It will be sealed and conditioned but the cooling effect of the ground would certainly result in cooler air down there.

All comments and critiques are appreciated.

Jul 19, 2010 4:19 PM ET

Response to Bob
by Martin Holladay

Q. If we have the bath and kitchen fans, am I likely to need an Ultimate Air unit?

A. I assume your are talking about an ERV. The answer is no -- no one NEEDS an ERV. You install one because you want one, not because you need one.

Q. Will those exhaust fans likely cause depressurization problems without a passive inlet?

A: No.

Q. If I later add the Ultimate Air unit, during the times of the year that the central system runs for heating and cooling, it should offer sufficient circulation for the filtered air with the supply side of the EVR tied into the central return and the intake near the kitchen area. During the in between season, if the house were closed up and comfortable, I considered that I could flip the main switch off for the air handler and insert a "blank plate" into the filter slot so that the EVR could circulate using the duct system without running the AH or short circuiting the clean air. Is this likely to work?

A. I'm not sure what you are aiming to accomplish. I advise anyone who wants to install an ERV to install dedicated ventilation ductwork rather than using HVAC ductwork to deliver ventilation air.

Q. If there is long enough periods where the 470 CFM [whole-house exhaust] fan works to keep the house comfortable, would it be worth putting a screen on the sealed insulated crawl space door so that the air was being drawn through there before coming up to the living area?

A. My guess is that any cooling effect due to drawing in crawl-space air would be short-lived. It wouldn't take long for the exterior air entering the crawl space to raise the temperature of the crawl space. Moreover, by introducing exterior air into the crawl space, you no longer have a sealed crawl space. Instead, you have a mold factory.

Jul 19, 2010 10:29 PM ET

Combination system
by Bob Ellenberg

Martin, thanks for your prompt response.

The idea of blocking the return filter in the HVAC system ductwork was so the ERV could utilitze the duct system during the time of year that the HVAC system was idle. I believe holding down cost should be a major goal of small energy efficient houses so more people will be able to afford them. My idea was to get good circulation without a dedicated duct system.

Maybe I am simply ignorant, but I don't understand why the crawl space would have a greater tendency to have "mold". If it is a sealed insulated part of the buidling envelope that has air circulation normally. Bringing outside air through it as opposed to an open window above would seem to me to promote air circulation there during the season of the year when the central HVAC system is not being used.

Again, thanks for your feedback and all the information you regularly share here.

Jul 20, 2010 5:43 AM ET

Condensation and mold in crawl spaces
by Martin Holladay

If you build a well-detailed closed crawl space with insulated walls and insulation over the dirt or gravel floor, and if you keep the house conditioned, and if the crawl space has a supply register connected to your forced air system, and if you heat your house in winter and cool your house with air conditioning in summer, then the surfaces in your crawl space will be at about the same temperature as the surfaces in your home upstairs.

In that case, as you point out, there won't be much chance of condensation or mold in your crawl space.

If I understand correctly, however, you are suggesting a crawl space without space conditioning (heat or cooling). It sounds like you want to flush the house with a whole-house fan at night. Let's say we're talking about the month of June. The only reason to use such a strategy is if the air conditioner is off. So now your crawl space isn't conditioned.

Your crawl space will, indeed, be the coldest room of your house. Most crawls spaces lack insulation on their floors, so I don't know if you were planning to install insulation on the floor. If you introduce summer air into your crawl space, you'll get condensation on the cooler surfaces in your crawl space -- probably the poly covering the floor.

Jul 20, 2010 8:29 AM ET

Condensation in conditioned crawl space
by Bob Ellenberg

Martin--sorry I was not clear.
I am a builder/designer and the last personal home I built had the type of crawl space you describe except the floor under the membrane was not insulated--just the perimeter and it was in a dryer climate than where this house will be. My past experience with what I call the in between seasons was that I kept the house closed until late afternoon (possible only with excellent insulation and minimum glass exposure to the sun). As the walls and roof that had been heating all day in the sun began to heat the inside more, we slightly opened the windows and turned on the fan. There was usually a couple of hours where it was too warm (but not extreme) and then started to cool down. We would leave it that way until the next morning. The night air would have cooled it well by then and we shut the windows and fan off again. While living in that home I often thought that I should have installed an opening in the crawl space that would operate like opening a window since the crawl space was somewhat like being the lowest floor in the house and was always cooler.

For this house I am planning exactly what you described, central HVAC, supply and return duct in the crawl space, etc. My question (and idea) about pulling air through the crawl space is a concept idea only for what I call the in between seasons where a tight well insulated house can be comfortable with neither heat or cooling operating. At such times I would sometimes ventilate using a small whole house fan as described above and sometimes have it closed up. My thoughts were that pulling air in through the crawl space would not only help the cooling factor but that it would actually help prevent any problems in the crawl space since there is no ventilation there at all when the central HVAC system doesn't run. My concerns revolve around the fact that this will be a hot humid climate. My personal experience has made me a strong proponent of a well executed conditoned crawl space. We will be using a stand alone dehumidifier in this house.
Perhaps my basic question should have been are there potential problems with a conditioned crawl space in a more humid climate and?

Jul 20, 2010 9:41 AM ET

Response to Bob
by Martin Holladay

Q. Are there potential problems with a conditioned crawl space in a more humid climate?

A. No, as long as you keep the crawl space sealed. Once you open a vent and let in the humid exterior air, however, all bets are off.

"My thoughts were that pulling air in through the crawl space would ... actually help prevent any problems in the crawl space since there is no ventilation there at all when the central HVAC system doesn't run." But you're wrong. If you live in a hot humid climate, introducing exterior air into a crawl space causes the problems you are trying to avoid. That's why the crawl space needs to be sealed.

Aug 24, 2010 1:08 PM ET

HRV and ERV's
by Reuven

What happens when you air seal an existing home to 0.2 ACH and then need to install mechanical ventilation? According to BPI, you must install mechanical ventilation to increase ACH to 0.35.

Based on reduced ACH through air sealing the projected savings for the home is $265/yr. Does the installation of an HRV or ERV negate this savings benefit?

Aug 24, 2010 1:17 PM ET

Response to Reuven
by Martin Holladay

You're making an apples-to-oranges comparison. Natural ACH is an estimate. Actual ACH due to air leakage through a leaky envelope varies widely. Leakage will be very high on windy days and on very cold days (due to the stack effect). On days with little wind or when temperatures are very mild, leakage will be much less, and the indoor air will get stuffy.

If you install a mechanical ventilation system, your home will, for the first time, get a dependable source of fresh air, delivered every day of the year -- even when there is no wind or the stack effect is low.

So let's say you decided to stick with natural air leakage. You end up with a house that often has poor indoor air quality, and on other days is over-ventilated. That's not what you want.

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