About This Solo Episode
- Host & Expert: Daniel Stih (Aerospace Engineer, Board-Certified Indoor Environmental Consultant [CIEC/CMC], and Author of What Your Builder Should Know)
- Core Objective: Evaluating whether Energy Recovery Ventilators (ERVs) can effectively dry crawlspaces and identifying the conditions under which they may worsen moisture problems.
- Scientific Framework: Applying psychrometrics, building science, and heat and moisture transfer principles to distinguish between drying a crawlspace and maintaining one that is already dry.
- Primary Topics Explored: Energy Recovery Ventilation, crawlspace moisture control, vapor barriers, conditioned crawlspaces, psychrometrics, and mechanical dehumidification.
The Mold Money Podcast
Why ERVs Don't Belong in Crawlspaces (Until They're Dry)
Energy Recovery Ventilators (ERVs) are excellent at improving indoor air quality and reducing the energy penalty associated with ventilation. Because they transfer both heat and moisture between incoming and outgoing air streams, they're often recommended as an energy-efficient way to ventilate homes.
When it comes to drying a damp crawlspace, however, that same moisture-transfer feature can work against you.
Many homeowners assume that installing an ERV will remove moisture from a crawlspace and prevent mold growth. In reality, an ERV is designed to exchange moisture—not simply remove it. As humid air is exhausted from the crawlspace, some of that moisture is transferred into the incoming outdoor air. The result is a balanced exchange, not an active drying process.
The key question is simple:
Are you trying to dry a crawlspace, or keep an already dry crawlspace dry?
Those are two different problems requiring different solutions.
If a crawlspace is already damp, the first priority is identifying and correcting the moisture source. Ground moisture, plumbing leaks, poor drainage, inadequate vapor barriers, and other building defects must be addressed before ventilation strategies can be effective. In many cases, conditioning the crawlspace or using mechanical dehumidification provides a more effective approach to drying than relying on an ERV alone.
Once the crawlspace has been properly dried and the moisture source has been eliminated, an ERV may help maintain stable conditions as part of an overall ventilation strategy.
In this episode, I explain how ERVs actually work, why they are frequently misunderstood, and how understanding moisture movement can help you choose the right solution instead of an expensive system that may not accomplish what you expect.
Transcript
Today's show is on energy recovery ventilators, ERVs, basically mechanical ventilation systems. Today's just going to be on crawl spaces because some people, some homeowners, a contract recommends, hey, you want to stop mold from growing in your crawl space, keep it ventilated, and oh, by the way, put in ERV.
This sounds practical, unless you really know what an ERV does and break it down.
An ERV exchanges air with the outside, meaning it takes in air, fresh air, and exhausts air at the same time. Incoming air runs through a filter, so there's no allergy issue. It can be set to either run a certain number of minutes per hour or day. I say run them all the time.
Clients often get conflicting opinions on whether this is going to work or not. I had one in Chicago. The contractor suggested using fans to move the air out of the crawlspace because he believes the ERV is not enough - the ERV wasn't working to dry the crawlspace out as he thought it should be.
Why didn't the ERV work? Could the drying been more effective if he had just blown the air out of the crawlspace (without the ERV) or gotten an HRV heat exchanger instead, a heat ventilator instead of an e-ventilator, because energy recovery ventilator, what does it mean, energy?
These are scientists that came up with these terms. They don't seem to be really common sense for the consumers. I think of energy, I think of heat. Energy actually means humidity, entropy. What's the problem with this? An energy recovery ventilator transfers moisture. There's an equal exchange - as air comes in from outside, inside to outside. The exchanger keeps your energy bills in check. With the ERV, that humidity is getting transferred. The issue wit humidity you're trying to get out of your crawl space as you're exhausting that air, it's getting put back in the air that's coming in from outside. It's an equal transfer. There's no intelligence to it.
Now you would think, okay, I want to set this thing balanced, so you know, it maintains a certain level of humidity. No, it doesn't do that. It's equal equal. This works great if it's humid outside and it's dry in your crawl space - as it brings in outdoor air, it's going to transfer that humidity backwards. It's going to go, oh, wait a minute, we need to transfer that humidity back outside.
But if you're using it to dry your crawl space, dry your crawl space out - if you're starting with humidity in your crawl space and it's dry outside - you're working backwards.
This stuff an be a little tricky to understand. You have to think about where is the humidity? Am I trying to keep humidity out? Or am I trying to dry it out?
In this case, an ERV is not the best solution to dry in the crawl space.
Once the crawl space is dry, you can install and run an ERV all the time to keep it dry. If it's damp in the crawl space already, that's not the way to solve the moisture problem.
To dry a crawl space out, identify where the moisture is coming from. Is water collected next to the house when it rains? Is there a plumbing leak? Maybe you need a vapor barrier on the ground, a good one.
The best way to keep your crawl space dry and conditioned is to heat it, ventilate it from the occupied space as if it was a living space. And if you say that's yucky, you didn't install the vapor barrier properly, rather just threw plastic down.
Your better solution, still kind of a bandaid: install power vents to just exhaust air from the crawlspace to the outside. Let the makeup air come from upstairs, meaning don't leave one vent open to the outside in your crawlspace.
You only exhaust air from the crawlspace. You can create a suction and then pull air from upstairs down into it. All the stuff again is a bandaid.
If you have a good crawlspace, you should be able to live down there and be able to heat it and condition it from upstairs. Usually when I get the call, there's a problem because there's a source of dampness that hasn't been identified. You should address that first. An ERV or HRV is not the bandaid not without understanding the whole picture: what you're trying to do, where it's damp, where it's dry, where you live. Is it damp or dry outside, humid or whatnot outside.
Be cautious of a contractor installing something without really understanding how this works and identifying, getting rid of the sources of moisture. Then you can use the ERV to keep things simple.