The Impact of Air Tightness on Ventilation in Passive House Design

When many first hear of the Passive House blower door test requirement of 0.6 ACH at 50 Pascals of test pressure, the first thing that comes to mind is the difficulty of designing and executing air barrier details that deliver this. They may not stop to consider how this level of air tightness impacts the strategies they typically employ for ventilation.

Very air-tight buildings use less space conditioning energy because the amount of outside air that leaks into and out of the building is much lower, requiring much less energy to heat, cool and dehumidify.

Among existing homes in the Northeast, perhaps 20% to 30% or more of the space heating demand only can be due to air infiltration.

But that is not the entire reason for the 0.6 ACH at 50 Pa requirement, which is also intended to reduce the transportation of moisture laden air into building assemblies at a rate which might cause damage to the high R Value assemblies used in colder climates to meet the Passive House Space Heat Demand requirement. This ‘extreme’ level of air tightness has real world implications for ventilation approaches used in existing buildings.

Ventilation can be viewed as having two goals:

1. Elimination of point source moisture and odor loads from the kitchen cooktop and bathrooms.
2. Hygienic air exchange for non-point sources such as CO2, off-gassing from the house itself as well as the things we bring into our homes, people, etc.

If moisture and odor loads are not effectively managed at the source, it is possible they will linger in the building, become more diffuse, and so more difficult to remove. You may have heard the statement ‘the solution to pollution is not dilution,’ in this context. Effectively capturing and eliminating these moisture and odor loads at the source is more important in an air tight building, because the lower air exchange rate may not dilute these loads so quickly.

We’ve been using exhaust extraction from bathrooms via a distributed HRV/ERV based ventilation system to manage moisture and odor loads with very good results, but only if the ventilation system is properly designed, installed and commissioned. Failing to follow through on all three items can result in a situation that can be prohibitively expensive to remediate – such as having to remove drywall to correct issues with poorly implemented flex duct based ventilation systems. But done correctly, in our experience it has worked very well.

The more challenging situation is in effectively managing odors and moisture from the kitchen cooktop.

What’s the difference? Bathroom moisture and odor loads can be effectively managed with perhaps 20-30 CFM of continuous exhaust, and there are a number of HRV/ERV manufacturers who have no problem allowing you to use their machines to do this.

Why are cooktops different?

1. They are not located in a room with a door that can be closed or left ajar to minimize the migration of moisture and odors to the rest of the house. Keeping a bathroom door mostly closed while the bath fan is left on creates an air pressure drop across the door that is very effective in limiting the migration of moisture and odors to the rest of the house, so they may be quickly removed.
2. An exhaust flow of perhaps 20-30 CFM will not be effective in capturing the moisture and odors from a cooktop, unless you are designing a cooktop exhaust hood with a ventilated enclosure (like one you might find in a laboratory.) For more normal looking cooktop exhaust hoods, you might need more, perhaps much more, than 10 times that flow. If you decide to try to use a gas cooktop (which is not always a great idea for health reasons: see Resources at the end of this article) local codes could require even higher cooktop exhaust rates.
3. ‘Make up air,’ a dedicated source of outside air, may be required by building code. For example, International Residential Code (IRC) requires “… make up air at a rate approximately equal to the exhaust air rate …” for exhaust hoods capable of exhausting in excess of 400 CFM. But that would mean another penetration in our Passive House quality air barrier, another thermal bridge, a mechanical damper to potentially fail, etc.

So, if you can get away with an exhaust hood rated at 400 CFM or less, you may not need make up air according to code.

How effective an exhaust hood actually is at removing moisture and odors depends on a variety of variables, which include:

1. How well the airflow ducting from the cooktop hood to outside, including the exterior exhaust hood, is chosen and implemented. It is not difficult to restrict a cooktop hood rated at 400 CFM, to an actual 200 CFM airflow with poor ducting and a restrictive exterior hood.
2. How high above the cooktop the exhaust hood is located.
3. Walls or appliances (e.g. a fridge) or cabinets at the rear and/or sides of the cooktop would help confine the cooktop emissions. The more the air entering the cooktop area is constrained, the higher the air flow speed across the cooktop, and so the more effectively the cooktop emissions will be captured (capture efficiency) and removed.
4. The design of the cooktop: how well can it capture moisture and odors?

So, while it seems clear that more cooktop exhaust air will tend to do a better job at removing cooktop pollutants, what is the impact in the context of a very tight building envelope?

You can consider the cooktop exhaust fan to operate like a blower door, extracting air while working against the pressure drop of the building envelope to outside. This is because the same airflow the cooktop exhaust fan pushes out of the house must somehow enter the house. Add to that a poorly executed cooktop exhaust duct and a restrictive flow hood to outside and you could realize half, or less of the cooktop exhaust hood rated airflow.

Of course you could chose a fan with a flow rating greater than 400 CFM, but then you would be forced by code to install make up air, which would be another penetration to outside. Here, air could leak and create an additional expense, and an electro-mechanical damper could fail at some point.

There are many ways to approach the cooktop problem, but the first step is to be aware of the issue.

Fans do not always deliver the rated airflow working against any pressure – this is a common misconception.

A fan rated at 400 CFM will actually deliver 400 CFM against a specified working pressure. If that pressure is increased, the airflow in most cases be reduced (there are ‘special’ fans that will adapt to different pressures to deliver the same airflow, but these are not commonly found in cooktop exhaust hoods.) The airflow in CFM that a fan will deliver at different working pressures is defined by a graph of pressure vs. airflow, often referred to as a ‘fan curve’.

Building Pressure Balance with Fan Flow Curve Spreadsheet

In the accompanying spreadsheet, a fan flow graph is illustrated, on the vertical “Y” Axis is the pressure in inches of water (in. w.g. ‘inches of water gauge’) and on the horizontal “X” axis is the airflow in CFM.

Note that at high pressures, the airflow is greatly reduced, and at very low pressures the fan airflow is at a maximum – in this case at about 400 CFM. At low airflows, a building leaks a small number of CFM, and at higher airflows, it will leak a greater number of CFM.

So, if you…
1. Do a blower door test at 50 Pa and you obtain a CFM @ 50 Pa number, and

2. You have a given fan flow curve

Is there a way to find out how many CFM that fan can move against that building pressure ? Yes, there is. You can create two equations:

1. One representing the relationship between building pressure and airflow
2. One representing the fan flow curve of pressure and airflow

You can then adjust the airflow until the pressures calculated by the two equations are equal. This means that both equations are ‘in balance’ (the airflow resistance of the building is exactly balancing the ability of the fan to deliver that airflow in CFM at that pressure), so that would be the actual airflow.

Use the Building Pressure Balance with Fan Flow Curve Spreadsheet free tool (below) to get a better grasp on these equations.

The most important ‘takeaway’ is that the rated flow of a fan may be only true at one pressure, and that the fan may deliver less airflow (perhaps much less) at a higher pressure.

Download the FREE Building Pressure Balance with Fan Flow Curve Spreadsheet here:

Resources

Performance assessment of US Residential Cooking Exhaust Hoods with real data, capture efficiency, etc. http://eetd.lbl.gov/sites/all/files/publications/lbnl-5545e.pdf

Hidden Dangers in the air we breathe – LBL – concise summary, includes PM 2.5 as an indoor pollutant, etc. http://newscenter.lbl.gov/2013/04/10/hidden-dangers-in-the-air-we-breathe/

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