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Passive House Airtightness WITHOUT Continuous Ventilation

Steven Burke

Senior Director, Sustainability

Suffolk Construction

LEEDuser Basic Member

0 thumbs up

November 2, 2020

Hi All, this is primarily directed at our Sustainable MEP folks, but please anyone with insight feel free to weigh in.

I am increasingly seeing designs and specifications asking for Passive House levels of airtightness as verified by blower door testing, but with traditional ventilation system designs.

To me, Passive House US has 2 main components:

1) Super insulated airtight buildings, and

2) Continuous, balanced ventilation with high efficiency energy recovery.

The latter of those two ensures good IAQ, and it makes me very nervous that we are reaching for the airtightness goals without the continuous ventilation piece.

After all, Sick Building Syndrome (SBS) became a thing in the 70’s for precisely that reason: Highly insulated, airtight buildings, that didn’t have good enough ventilation. Presumably our technologies and understanding of building science has collectively evolved to the point that the mistakes we made that led to SBS won’t be repeated, but again I just get nervous about these crazy airtight buildings at PH levels for airtightness but with just minimum code ventilation rates. And, I was nervous about it before we had a global pandemic.

I am a huge advocate for airtightness for a lot of reasons, but not without the caveats of understanding the knock-on impacts related to it for things like controlling humidity and general IAQ in the building.

Do you SMEPer’s think my concerns are overblown? When this came up most recently on a project, PH airtightness was listed as a desired energy conservation measure, but there was no discussion of amendments to the ventilation system design. I expressed my reservations to the Mech. Engineer, and they said their code compliant ventilation designs aren’t relying on infiltration for IAQ and their system would be good whether we had regular code airtightness or PH airtightness. But what do you think I should I be using to make my own determinations on critiquing the mechanical design in these situations (this is probably the 4th project in the last year this issue has come up):

# of Air Changes / Hour in all spaces, CO2 monitor setpoints for Demand Controlled Ventilation, efficiency rating of the filters in the system, recirculated air to outdoor air ratios, all the above, etc.?

We could always use ongoing air quality monitoring to address issues after the project is built, but I’d like to avoid those issues ahead of time if possible.

Interested to hear your thoughts, and a P.S. thanks to Luke, Kim, and Pete for their great Winter Covid HVAC presentation last week.

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Luke Leung

Principal

Skidmore, Owings & Merrill LLP

LEEDuser Basic Member

0 thumbs up

Mon, 11/02/2020 - 04:44

Steve, Great insights! Note

code required ventilation rate is for human bioeffluence control (I do not smell you), not necessarily for healthy ventilation.
"clean" air not necessarily has to come from outside, e.g. California wild fire and outdoor air contaminants
Both indoor or outdoor air can have potential issues.

So, a few things:

Code required outside air as a minimum
Is the HVAC unit control by a thermostat (e.g. furnace) that will turn off when temperature is satisfied and bring no outside into the building? If that is true, some override maybe required, especially during pandemic
What is the indoor filter provision? For many reasons, it may want MERV-13 or better, especially during pandemic, if you are in areas with wild fire, maybe MERV-15 and beyond, you may not use it all the time, but it is there, if you want to use it. Also filter efficiencies can deteriorates with time, so air monitoring is better, if possible.
Consider "smart" control to tell the occupants which air is better? outdoor or indoor? Human is not necessarily a good detector of air quality
What are the activities indoor? e.g. Is it gas cooking? Electric cooking? Do the potential contaminants have local exhaust?
There are some uncertainties, e.g. diversity of indoor microbes, that may suggested it is a good idea to open the window when thermal and air quality is correct to "air" indoor

Dan Piselli

Director of Sustainability, Senior Associate

FXCollaborative

LEEDuser Basic Member

1 thumbs up

Mon, 11/02/2020 - 13:25

I'm unsure about specific ventilation strategies, but SBS is also caused by mold and indoor pollutants.

Infiltration increases the risk of mold in walls, and can convey dust & other pollutants from wall cavities to the inhabited space. Better airtightness reduces those risks.
PH level thermal bridge mitigation, and proper vapor barrier placement reduces the risk of interior condensation and mold.
Specification of healthy materials (low-VOC, non-Red list, etc.) helps with indoor pollutant control.

Pete Jefferson

Principal

BranchPattern

LEEDuser Premium Member

0 thumbs up

Mon, 11/02/2020 - 13:32

Steven - I don't think your concerns are overblown. We have such widespread problems with ventilation, I'm pretty wary of anything that potentially compromises it. If by "not continuous" you mean that fresh air is supposed to cycle for a certain # of minutes per hour, well, I think that falls into the potential for over-complication. It might be code-compliant, but what are the odds that a user 10 years from now realizes that's how it's supposed to work? Also, by not supplying outside air continuously, you start messing with the building pressure, which depending on the type of building and internal uses, could be an issue. But to your main point, I've got reservations about sealing up a new building super tight and just doing the basic level of ASHRAE 62 ventilation - especially one that hasn't done a really robust building flush. Getting those flushes done always seems to be a challenge, especially with DOAS systems because of the duration of time required to flush it. So that's also something to potentially plan for. My own personal opinion is that I'd probably be looking at ASHRAE 62 + 30%, with some air quality monitors, and providing some form of carbon filter (even just a combo particulate/carbon) for the first year to help remove some of the VOCs that can be present even with good materials spec'ing. Then again, that's where I'm at even with non-PH buildings too.

Patrick Murphy

Director of Sustainable Design

Vanderweil Engineers

LEEDuser Premium Member

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Mon, 11/02/2020 - 15:11

I'll add to Pete and Luke's feedback that because Passive House buildings are so tight, less ventilation air is needed for building pressurization. That means more of the ventilation air brought into the building can be used for energy recovery. The effectiveness of typical energy recovery ventilators/wheels/etc. is largely driven by the balance (or lack thereof) of air on the intake and relief sides of the heat exchange medium. The closer these two airflow are to one another, the higher the effectiveness and greater the energy savings. There's probably a balance to increasing ventilation air (as Pete suggests) without an energy penalty because of this increased energy recovery effectiveness. Ask your engineers to consider this when selecting and modeling the ventilation system. Also, to Luke's point, the controls determining when the ventilation system runs are quite important. There's a reason CHPC instructors tell aspiring CPHCs to duct ventilation straight into the occupied space: if you duct the ventilation into a recirculation heating/cooling unit you're dependent on that unit to run and distribute the ventilation throughout the space/building. If that unit cycles on/off, the ventilation won't be distributed effectively.

David Posada

Integrated Design & LEED Specialist

SERA Architects

LEEDuser Expert

1361 thumbs up

Mon, 11/02/2020 - 20:39

Steven - is your question directed at residential (multifamily) or commercial buildings, or both? What climate zone(s)? Many good comments above, especially Dan's point about the sick building syndrome risk factors. PH levels of air tightness in the envelope *can* improve moisture management and reduce condensation & mold risks by limiting air movement from transporting moisture through the envelope. However, some PH envelope assemblies, especially ones with foam insulation, rely on drying to the interior for any moisture from vapor drive or unplanned leaks. These assemblies may require minimum ventilation rates in the majority of spaces to allow that inward drying. I’ve seen PH assemblies where WUFI analysis showed the impact of interior ventilation rates (and a range of interior moisture load assumptions) and in some cases good ventilation rates were an important part of the envelope moisture management. I’d suggest that in a “perfect world,” PH levels of airtightness and code ventilation could be okay, if we believed that code ventilation was sufficient to provide IAQ; but the discussion points out many reasons to doubt that would be the case. As you alluded, PH levels of air tightness address two different concerns: 1. energy consumption for heating cold infiltrated air, and 2. the durability issues of air-transported moisture through the envelope. With less-airtight windows, door thresholds, dampers, etc that is more likely to cause heat loss and thermal comfort issues than interstitial condensation issues, whereas less airtight envelope assemblies may have more risk for durability, along with the heat loss concerns.