Impact of Air-Sealing on Ventilation and Moisture Control
Hello,
I have a question about the relationship between ventilation and air-sealing improvements in my home: How do air-sealing improvements interact with ventilation for moisture control?
I live in a cold-humid climate (northern New England, Zone 6A), and we have three bathroom fans (Panasonic Whisper Green Selected) that run continously in the background (~25 CFM each), which can also be turned to a higher speed (120 CFM) with switches. The fans help to control moisture in the bathrooms and also ensure regular exchanges of air in the home, because of radon concerns.
I recently had a blower-door test done to identify areas of air-leakage in the building envelope. I’m still awaiting the results, but overall, the home is in fairly good shape (the attic has already been air-sealed and insulated to R-60). There are some areas for improvement, which could reduce the air-leakage by around 15%.
My question is this: if improvements are made to prevent obvious areas of air infiltration (fireplace chimney flue, a basement bulkhead door, the hatch to the attic), will this actually make a difference in light of the ventilation fans exhausting air from the house. Will the fans not just pull air in from other locations?
(I have some familiarity with heat recovery ventilators and energy recovery ventilators, but they will likely not be a viable or economical option for retrofitting the house, which does not have a ducted heating system. Currently the home is heated with an oil boiler with a hydronic basement distribution system.)
Thank you,
Michael
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You can get sensor-controlled fans that only run when the sensors detect high levels of undesirable substances.
To my ear it sounds like you have an older home that my modern standards is very leaky. My wild guess is over 5 ACH50.
There is zero reason to not give you blower door test results on the day of the test. They know the raw leakage number you know how many square feet are in the home from when you bought it. They measured the ceiling height. So, you have all the numbers and the math take the better part of 30 second to calculate the ACH50 number. Often it seems they go out if their way to not give the home owner the ACH50 number as it is so shockingly abysmal by modern standards.
I think mechanical ventilation in homes with an ACH50 number over 3 is total ridiculous.
Personally, if the ACH50 is over 1 and there is less than 24 person hours of occupation per per day per 1000 SQF ventilation is unnecessary.
I can see how running the bath fan while in the shower and for the next 30 minutes makes sense but 24-7 is crazy unless there is mold growing on the bathroom walls.
My wild guess is you have a humidifier on the furnace because it is so dry in the winter without it. If true that would indicate the house is very leaky.
Walta
Thanks for the responses. Additional context: yes, this is an older home (built in 1964), so not very air-tight by today's standards. However, as noted, I did air-seal the attic after purchasing the house, and the windows are fairly recent double-paned vinyl windows.
The initial inspection of the house revealed mold in the attic, which prompted the installation of the bathroom fans. We also had some mold in the basement this past summer. The local climate tends to be very humid in the summer and very dry in the winter. We have a boiler with a hydronic distribution system, but we are not using a humidifier in the winter.
The recommendations so far have been to forgo ventilation in the house, except when a shower is being used. So you think that air-sealing improvements will only be effective if there is no mechanical ventilation happening?
I'm wondering about finding the right balance between air quality (given radon concerns and moisture problems) and energy efficiency (i.e. reducing heat loss).
If you have a radon concern, get a test. They are cheap. If you have high radon, your bath fans are not a good solution.
Air sealing is still effective with continuous ventilation. It has diminishing returns if you are exchanging 60cfm continuously with no heat recovery. You can get spot ERVs that don't require ducting.
Thanks for your thoughts. We did have a radon test done; it came back just below the acceptable threshold, but it was borderline. Why are continuous ventilation with bath fans are not a good solution? What would be a preferable solution? I will look into spot ERVs.
The short answer is that the energy penalty for continuous ventilation is high relative to the benefit.
Michael, the continuous bathroom fans present some problems. First, mitigating radon in the ceiling of your upper levels (I presume that's where the bathrooms are) means that the radon is being actively pulled into your living space. It's better to at the very least locate the radon exhaust fan inlet in the basement or crawlspace, even better to put it under an impermeable membrane or under the basement floor. Second, those three fans represent an exhaust-only approach to ventilation. Where is the makeup air coming from? As you continue with your excellent airsealing strategy, the need for a balanced approach to ventilation gets more urgent. Panasonic makes spot ERV's that can take the place of bathroom fans, but I'd lean towards a more comprehensive solution. This, along with a dedicated radon system--if you don't pay for it now, you will later when it comes time to sell the house.
I'm not by any means an expert on radon, but my understanding is that if the house has perimeter drains you can mitigate radon by teeing into the drains and venting the tee. This has no energy penalty other than the power to run the fan.
Agree that venting from the upper levels would tend to pull radon through the house.
Thanks, this is very helpful. In terms of implementing balanced ventilation with heat recovery, would you recommend spot ERVs on each level of the house? A more comprehensive solution would be ideal, but the layout of the house offers few opportunities for adding ductwork.
Currently there is one bathroom fan on the 1st floor, and two on the 2nd floor. There is no ventilation in the basement itself. My thinking was that continuous ventilation would prevent radon from accumulating in the air, given that it is hazardous at higher levels of concentration. But the point about the energy penalty for continuous ventilation is important, especially as we prepare to transition from the oil boiler to heat pumps.
Would a spot ERV in the basement be an effective method of radon mitigation? Recommendations for further reading on the subject of radon mitigation are very welcome.
Thank you to all who have commented.
Q: "Would a spot ERV in the basement be an effective method of radon mitigation?"
Well, it might be somewhat effective, but particularly if you use your basement as living space, it's not what you want. The idea with radon mitigation is to create a space of negative pressure that sucks radon-laden air, and then evacuate that directly outdoors. This can be done actively with a fan, or passively with the stack effect (but that's more difficult to design, and best practice has you install a receptacle next to the outlet in case it turns out a fan is needed.) The space you want to evacuate is typically underneath your basement slab, or a crawlspace, or your footer tile or sump pump pit. To pull radon into living space in order to exhaust it is not a smart approach. It incurs an energy penalty to exhaust conditioned air, but worse, you get a chance to breathe the stuff.
Resources: anything on https://www.epa.gov/radon is a good starting point. If you're a member, this great article is behind GBA paywall:
https://www.greenbuildingadvisor.com/article/radon-mitigation
There's lots of data-driven information on this site about ventilation for air quality, start with knowing your ventilation goals (i.e. cfm's per occupant) and your houses's evolving ACH50 numbers. Type "ventilation" in the search bar.