skip to Main Content

Can an Exhaust Fan Control Humidity in a Spray Foam Attic?

The Exhaust Fan In My Spray Foam Attic

Here in the South, we love our humidity.   It makes us glisten in the summer.  But we also love our air conditioning  and low humidity inside our homes.  To save on air conditioning costs, more and more homes have attics encapsulated with spray foam insulation to bring the HVAC systems and ductwork inside the conditioned space.  But there’s a problem.

High humidity in a spray foam attic

A few years ago I wrote about the topic of high humidity in spray foam attics.  When you encapsulate the attic with spray foam insulation and don’t do anything to condition the air in the attic, the humidity can get very high.  It also stratifies, with the highest relative humidity near the ridge.  The 2016 article I wrote on this topic shows the data for a house here in Atlanta.

With closed-cell spray foam on the roof deck, you have humidity in the attic but it doesn’t get to the OSB decking.  With open-cell spray foam, however, the moisture goes into and out of the roof decking every day.  During the daylight hours, especially when the sun hits the roof, moisture is driven out of the decking, through the foam, and into the attic air.  At night, the moisture makes its way back through the vapor permeable spray foam and soaks into the OSB again.  Do this long enough and you can damage the roof.

Using an exhaust fan in the encapsulated attic

Since that time, we’ve been experimenting with ways to keep the attic humidity low.  One of the methods we’ve tried is using a small fan to exhaust air from the attic.  If the attic is properly sealed to the outdoors (and many spray foam attics aren’t), the replacement air comes from the conditioned space below the attic and is cool and dry.  (Caution:  I’m talking about a fan that moves a small amount of air, maybe 100 cubic feet per minute, not a powered attic ventilator, many of which move more than 1,000 cfm.)

One of the houses we’ve tried this with is my own house in Atlanta.  When I bought it, the attic had poorly installed spray foam insulation.  In the fall of 2019, I had Woodman Insulation* come put a lot more spray foam up there for me using SES open-cell spray foam.*  Before they arrived, though, I did a lot of prep work in the attic.

One part of that prep was installing an exhaust fan.  I used a Panasonic FV-0511VQ1 WhisperCeiling fan.**  It’s efficient, quiet, and operates at either 50, 80, or 110 cubic feet per minute (cfm) of air flow.

The exhaust fan roughed in before getting more spray foam in the attic
The exhaust fan roughed in before I got more spray foam insulation in the attic

I also set up a HOBO data logger near the ridge.  It has magnets on the back so I stuck it to the remainder of my furnace flue.  (After installing mini-split heat pumps, I had the furnace and gas meter removed.)  The white box to the right of the data logger is a remote sensor for the temperature and relative humidity monitor** that I keep an eye on in the den below.  You can see the exhaust fan in the background of the photo.

HOBO data logger near the ridge of a spray foam attic, with the exhaust fan nearby
HOBO data logger near the ridge of a spray foam attic, with the exhaust fan nearby

Humidity data from summer 2020

The graph below shows some of the data I recorded last July and August.  The upper yellow dots are the outdoor high temperature for each day, the blue curve is the attic temperature, and the orange curve is the attic relative humidity.

Temperature and humidity data for spray foam attic in summer 2020
Temperature and humidity data for spray foam attic in summer 2020. The attic exhaust fan ran continuously until 5 August and then was turned off.

The first thing to notice is that the relative humidity didn’t get as high as the house I wrote about in my 2016 article.  The relative humidity at the ridge of that other house got up over 80% on five of the seven days we monitored it in June 2014.  It even hit 90% on one of those days.  In contrast, my attic’s relative humidity got into the low to mid 70 percent range at worst and stayed in the 60s during the hot part of most days.

The graph above shows relative humidity.  Below I’ve posted the graph with dew point temperature.  Interestingly, they both fall pretty much at the same place on the chart even though one is a percentage and the other a temperature.

Temperature and dew point data for spray foam attic in summer 2020
Temperature and dew point data for spray foam attic in summer 2020

The second thing to notice is that the first part of the graph shows the data with the exhaust fan running continuously at 50 cfm.  At 1 pm on 5 August, I turned it off.  The first couple of days after I turned off the fan were the same as the preceding days.  Then the relative humidity went up.  Instead of dropping into the 50s at night, the relative humidity went down to only about 60%.  The maximum humidity went over 70% on four of the days with the fan off.

Does it work?

The short answer is that I need more data.  There does seem to be higher attic humidity with the fan turned off, but the outdoor temperature also got higher during that time.  The good news is that in my house, the humidity is mostly reasonable even with the fan turned off.  I don’t like the spikes over 70%, but the attic doesn’t spend much time there.

My data logger is still up there recording data, so I just need some heat and humidity to see how it performs this year.  As I mentioned above, I had the exhaust fan set for 50 cfm when I ran it last summer.  The building code doesn’t require any conditioning of the air in a spray foam attic.  For rooflines insulated with air permeable insulation (the fluffy stuff – fiberglass, cellulose, rock wool), the International Residential Code requires 50 cfm of supply air per 1,000 square feet of attic floor area.  With 2,300 square feet for my attic, I’d need 115 cfm, which is conveniently close to my fan’s maximum rate of 110 cfm.  I’ll do that experiment this summer and get back to you.

The big drawback with exhaust ventilation (literally!) in a humid climate is that an equal amount of outdoor air is pulled into the house through the building enclosure.  Using it in an encapsulated attic helps the attic by exhausting humid air near the ridge and replacing it with conditioned house air.  Then the house has to do with a little extra humidity.  It’s possible it could also grow mold inside your walls.  A better way to exhaust from the attic would be to do it through an energy recovery ventilator (ERV), which is what I’ll do once I get my new Zehnder ERV installed.

The exhaust fan definitely works for one thing, though.  It’s not bad in my house, but I do notice a smell sometimes when the fan’s not running.  With the fan running, there’s no odor in the house at all, even when it’s on the lowest setting of 50 cfm.


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.



* In full disclosure, Woodman Insulation and SES provided their spray foam and the installation at no cost.

** This is an Amazon Associate link. You pay the same price you would pay normally, but Energy Vanguard makes a small commission if you buy after using the link.


Related Articles

Humidity in a Spray Foam Attic

Make Dew Point Your Friend for Humidity

Spray Foam Insulation at the Edge – A Forensic Analysis


NOTE: Comments are moderated. Your comment will not appear below until approved.

This Post Has 44 Comments

  1. I have a comparable setup in FL — an open-cell SPF attic, a WhisperValue DC, and a temp/humidity logger. Both are near the attic ridge. The fan went up to deal with residual SPF odor. My sloppy HVAC duct connections have been purging the attic’s humidity just fine. Looking over my data logs, I have pretty much the same results that you’ve seen… The vent fan hasn’t had a huge impact.

    One thing I do notice is the post-fan daily humidity spikes don’t peak as high as before. I had been seeing sharp, brief spikes to ~70% like clockwork at 4PM. Post-fan, these spikes peak at 58-62%. The 30-day mean humidity, however, is unchanged at 52%.

    Right after the fan went up I fired up the poor man’s blower door (i.e. engaged every possible negative pressure source such as bath fans, range hood, etc) and went to work with my finely calibrated air-movement detectors (ten sweaty fingers).

    Two cans of Great Stuff later (1.5 cans toward the house and the other 0.5 in my hair, stuck to my arm, and embedded in my clothing) I’d sealed-up many leaks including a long run of leakage at the base of the gable and one softball-sized leak near the master bedroom.

    So it’s possible the fan had nothing to do with the diminished humidity peaks and the sealing effort should get the credit. At some point this month I’ll shut the fan off entirely, see what happens, and report back.

  2. The plots are interesting. I am assuming that the yellow dots (high temperatures for each day) are from a local weather station. Those temperatures do not correlate well with the attic temperature or RH. I am guessing that solar radiation has a lot to do with it too.

    I would sure like to see the time scale expanded so that we can see the time of day that the attic temperature and humidity typically peak (high and low). It would also be interesting to see the temperature and RH data converted to dewpoint to see that daily profile.

    1. Roy, yes, I grabbed the outdoor highs from Weather Underground. I’d say the outdoor temperatures mostly correlate pretty well with just a couple of exceptions.

      You got your wish, or part of it anyway. I added a graph with dew point.

      1. The dewpoint information is interesting. Can you give me a rough idea of the time-of-day when the dewpoint and drybulb temperature peaks and valleys occur?

        1. Sorry, I meant to tell you the peak times before. I looked at the graph and they usually peak around 5 pm and are heading down by about 7 pm, although once or twice the humidity and temperature were still high at close to 9 pm. The dry bulb attic temperature bottoms out at about 7:30 am and the RH at 10 to 11 am.

  3. Nice article and very timely. Maybe you could ‘link’ the article I wrote for you back in 2012 where you make the comment about those foamed attics not being sealed well…and all those new big holes! Haha! We have done thousands of those predrywall tests since then…and not much has changed. From the touch-up we complete at predrywall, with an average of 6 cans of foam, we normally improve the ACH50 by 30-50%! Those holes to outside (infiltration points) are hiding in those ‘sealed’ attics!

    The smell issue is real, but we have only dealt with it in open-cell attics and it only seems bad when the humidity is high. If it weren’t for the smells…you could simply pull air from the home and dump it back into the occupied space in a non-offensive location as to not bring in that wetter outside air and create the ‘negative pressure’ on the home. At 50-100cfm, its likely a non-issue, but better safe than sorry since physics doesn’t care about your health, your home’s health or its durability….its our responsibility to learn and believe the BS (Building Science).

    When the home has a sealed attic and sealed crawlspace, we have set up transfer (exhaust) fans to pull air from the sealed crawlspaces and dump that colder, drier into the sealed attics with an additional fan in the attic sending that air back down into the crawlspace on the opposite side via a duplicate fan and duct. These fans are balanced which seems to work well!

    We have also seen great improvements by dumping supply air into the attics (without an active return in the attic)…thinking “Cool Hand Luke Meets Attics”! This conditions them better, provides some circulation/mixing and brings those dewpoints down! We like to see one installed on each side of the attics to aid in circulation…or some circulation fans to help the air mixing if its a large attic that is ‘chopped up’ architecturally (I just thought about Joe Lstiburek crumbling up a piece of paper to show us how architects design roofs plans these days…haha)!

    1. Craig, some of it may be, yes. But when you encapsulate an attic, you’re encapsulating air that otherwise might be vented to the outside. In an older house, there could be dirt, dead animals, and nasty stuff that’s been stored up there for decades. In a new house, you have lots of materials that are still off-gassing. In short, it’s hard to say what all the odor is from.

      1. Any chance the smell is moisture trapped (or slow to leave) the open cell/sheathing and it’s maybe not moldy but “damp” smelling?

  4. I had a similar issues albeit in a crawl space where my heat/AC ducts runs. My duct like everyone else’s is not leak proof, so during heating operations the crawl space was being heated and when the A/C was running the space was being cooled. This wasted energy in both cases.

    Initially I thought the moisture around the foundation due to poor drainage design during the initial construction was the issue. I revised all the downspout discharging and directional flow of the rain water flow with drainage boxes and discharge piping away from the foundation . This did reduce the humidity a bit but did little to eliminate humidity in the crawl space. My next option was to seal the crawl space so outside air would not enter the crawls space. I caulked all the sill boards, insulated and blocked off all the outside vents. This effort Improved the humidity during some conditions but stood could not maintain my humidity requirements of around 50%.

    Three years ago I finally bit the bullet and installed a crawl space dehumidifier in the center of the crawl space, WOW! Problem solved. The dehumidifier has a remote control panel that displays temperature and humidity. I also installed a temperature and humidity monitoring device near the access door that is also the lowest elevation of the crawl space. Both the remote control panel located in my living space and the monitoring device at the access door read consistently 48-52% humidity and 60-65 degrees year round.

    Understood this is a different application but sealing the attics and installing this type of dehumidifier with an external drain should maintain a 50% humidity level all the time. These types of humidors have a blower that operates all the time and keeps air circulation while removing any unwanted moisture. The desired humidity can be set and monitored on the controller. My hardwood floors are no longer cold during the winter as well. We also see out living space humidity is much more stable without the humidity being transferred from the crawl space by seeping into the duct when no positive pressure is present from the heat/AC blowing.

    We also get a lot of comments on how clean the house smells from friends and family.

    Sorry if I have wasted anyone’s time if you do not feel this so a relative post.

    FYI, this type of humidifier is not cheap. Costa around $1,400, but for me it is well worth it. Crawl space dehumidifiers are built much more robust than any normal dehumidifier.


    1. Frank, thanks for your comment. It’s certainly relevant to this discussion because encapsulated attics and crawl spaces are both buffer spaces in which the air needs to be conditioned in some way to control humidity and odors.

      Two questions for you: Where are you located? You don’t say, but it must be somewhere in the humid parts. Which dehumidifier did you install?

  5. Interesting topic and glad that you mentioned the possible negative pressure in the home that could result. I’m and HVAC contractor and just did training this morning with my techs and as we talked about house pressure I said a slight positive house pressure is better then slight negative to keep the humidity out in the summer here in Birmingham, AL.
    I sprayed foamed (open cell) the attic in my last house and my current house. In my last house I added a 4″ collar with damper to both supply and return in the attic and it keep the humidity down but I was uneasy about the possible VOCs in the attic. I have not addressed the new attic but I do have temp/RH/VOC monitor in the attic to see how it does this summer before I do anyting. My first option was to cut in two transfer grilles to let it balance to the living area.

    Is there a way to “seal” the spray foam so that it does not emit the odor? I had read about intumescent paint used to seal it for hire ratings and wondered if that would help the smell.


    1. Tyler, I don’t know of a way to seal the VOCs and odor in, but that doesn’t mean there isn’t a way. Maybe someone with experience in that area will comment.

      On the supply and return in an SPF attic, I agree with what Curt said below. In addition, putting a return in means you pull in attic air and send it into the house. Putting only a supply in also sends attic air into the house because of the positive pressure. That’s why I like the exhaust fan or, ideally, using the exhaust from an ERV.

    2. I’ve contemplated this as well. I reckon a coating that keeps VOCs inside would also be a vapor barrier. If there’s a synthetic membrane roof underlayment, now we have a second vapor barrier. Roof deck is now the meat in a mildew sandwich? Just spitballing here…

  6. While adding a small supply and / or return (If odor not an issue) might seem a no brainer to control attic humidity there can be a code / safety issue in terms of the attic then being directly conditioned space…in some jurisdictions inspector may demand that the foam get an ignition barrier…possibly even be covered with drywall!

  7. Did you consider a vapor diffusion port at the ridge? Gather you didn’t do that prior to upgrading the spray foam, could it be done from outside? That would be an alternative, right?

    1. James, no, I didn’t consider that. I trust Joe Lstiburek that they work when done properly but chose to control the humidity mechanically. Also, our roof deck is 1×6 solid wood and not as susceptible to moisture damage as OSB.

  8. Great article. One of the first things I did after purchasing my first home about 5 years ago was to vacuum out all of the nasty old insulation and spray the roof deck with open cell foam. This made an enormous difference to the comfort and efficiency of the 1960s home, but in retrospect, I would have gone about things differently had I known better at the time. I noticed very high humidity soon after installation – so much so that the ridge foam was soaking up humidity like a sponge. I opened a couple of ducts into the attic and that seems to have solved the issue, although I have yet to install data loggers to confirm. However, the ducts into the attic did transfer the attic smell into the home which was unpleasant and probably unhealthy! Fortunately, after a few years the smell is virtually gone.

    Funny how the insulation contractors tell you all the wonders of spray foam and totally ignore its pitfalls. I wonder how many rotted roofs we’ll be seeing in the coming years due to insulation contractors lack of knowledge or attention the important issue of attic humidity. I imagine very few homeowners ever go up into their attic after the spray foam is installed.

    If I could do it again, I might still vacuum out the nasty 1960s insulation and seal the attic, but instead of spray foam, do a new roof with insulation boards on top of the roof decking. That would allow a higher r-value and avoid some of the issues that come with spray foam. One issue that I still need to address is sealing holes that the insulation contractor did not. But frustratingly, the globby nature of spray foam makes it very difficult to visually inspect for obvious holes in confined spaces. I think it would actually be easier to start with a bare attic and use canned foam and insulation board to seal up all the obvious holes.

    1. Ken, yeah, encapsulating the attic with spray foam and then walking away, especially in a humid climate, is a recipe for disaster. Putting supply air in the attic is definitely one way to control humidity, but as you noticed, it pushes odors into the house. Here’s a nice article from Joe Lstiburek on this topic:

      Cool Hand Luke Meets Attics

    2. Ken, one way to locate holes or leaks is a thermal camera, they have become rather cheap over the past several years. I purchase a FLIR thermal imaging camera, premium cost supplier, that attaches to a mobile phone for $325 three years ago. It will make locating heat and cold escaping from your all areas or components of your home. Lots of uses for this device. You can get cheaper thermal cameras from other manufacturers.

      Frank Hancock

      1. Thanks, Richard! The hygrometer you linked to is on an Amazon “lightning deal” right now for $12.50. I purchased one and will give it a try! If it works well for me I’ll buy several more for other locations in the attic and crawlspace.

        1. Ken, let me know how you like it. I was leery about their accuracy but they both agree with every other temp/humidity measuring device I have at my disposal.

  9. We automatically include the cost of blower door directed spray foam quality control (2 guys; 3-8 hours) on all new construction and deep remodels for which we provide HVAC equipment or ductwork. We have the equipment and experience in house using it to locate, identify and seal (small gaps) in spray foam. Big gaps get outlined in embarrassingly prominent neon spray paint for the foam sub to remediate.

    We carry three different embarrassingly prominent neon spray paint colors for those jobs needing multiple iterations of test – seal – retest – seal – reretest – seal.

    If anyone balks (owner, builder, foam sub) we simply inform the team that without confirming the foam is substantially airtight, we are unable to guarantee ideal humidity control, comfort performance and moisture damage resilience of the project.

    Our customers “get it”. Our best builder clients are those that have been burned in the past by costly moisture control failures and highly value the additional layer of protection our procedures provide. I regard it is such an important step that we would withdraw from a project if someone was dumb enough not to test and confirm infiltration performance.

    Every now and then for extra credit we sub to mold remediation contractors in either or both of forensic and corrective roles.

    1. Speaking of which… Your guys left a can of orange Krylon in my attic. I’ll make sure they take it home next time I call for service. 😁

  10. I got inspired by this article and put z-wave temp + humidity sensors (logged wirelessly via SmartThings) in various places around our 3 year old custom build (that’s when I started reading this blog – learned so much and I drove our custom builder nuts … )

    I was shocked to find out that in our open web trusses (between first and second floor) the humidity is 75-80 % at 76 degrees. I live in Houston, and dew points outside are around 75 right now and it has been windy the past days. Our house was insulated with BIBS (AC is in conditioned closet on second floor thanks to this website) and there are some tricky transitions at the rim joists where first floor roof lines hit second floor walls, but I made sure they were all closed (no big holes per Lstiburek) and the blower door results were about 2250cfm for 3800 sf at 10 ft height (I believe around 3-4 ACH50 which passed code here in Houston.

    Do I have a problem (mold growth for instance)? Should I be concerned about dew points of 80% rH @ 77F in the truss cavity when the living areas below and above it are at 55 % rH @ 76 F?

    1. @Robert, given the information you provided, I would question the accuracy of the RH logger located in the floor truss cavity. I suggest putting the logger that’s reading 55% beside the floor truss logger for comparison. Accuracy is a concern with low cost RH sensors.

      If your truss cavities do have substantially higher RH readings (80% @ 77F = 70 degree dew point!), you need to hire someone who knows how to locate and seal those leakage paths! If you have any ceiling supplies mounted in the floor truss cavity, mold may be growing on the back the drywall surrounding the boots and on the backside of the boots themselves.

  11. I have enjoyed very much reading your articles. I am a building a new home and want to make it as energy efficient as possible. We have planned to condition the crawl space and ventilate the house with an ERV. I was also looking at the attic being conditioned. No duct work in the attic. My HVAC guy is discouraging this because we live in zone 4A. Any thoughts on what we should do. Thanks.
    Tom Lowry

    1. @Tom, congratulations on banning ducts from your attic! Given that, I generally advise against directly conditioned attics. This not only adds considerable first-cost, but increases heating and cooling costs due to the increased exposed surface area and conditioned volume. An indirectly conditioned attic (insulated roof deck, no direct supply air) would have a lower lifecycle cost. In this case, the attic acts as a thermal buffer. This configuration makes the most sense when there’s no other way to get ducts out of the attic.

      Vented attics generally have the lowest lifecycle cost, assuming the ceiling plane is field-verified tight. A lot depends on roof and ceiling complexity and roof pitch (i.e., surface area). If the ceiling is tight, a properly vented attic with a well insulated ceiling will outperform an encapsulated attic. It doesn’t cost a lot extra to exceed code-mandated minimum R-values when using blown-in ceiling insulation. One exception would be flat roofs, which inherently have poor ventilation characteristics.

      1. Thank you very much. This solves my attic problem and concerns. Our crawl space will be very adequate. We plan to encapsulate and insulate the walls. All duct work will be located there. What would be the best way to ventilate and control the humidity? We were thinking about an ERV. Any better way? Roof is standing seam metal.

      2. David, do you combine a wiring chase below these vented attics? I ask as someone who just added and (hopefully) sealed a bunch of Lessco boxes over slimline recessed lighting (and quite a few “regular” light boxes with all the attendant switch wires etc). As I crawled, swore and bruised, my mind returned to the decision not to create a conditioned attic (with insulation under a new metal roof), when we looked at the budget and said, “Ok, so we’ll establish the air barrier at the ceiling” …. So as I write I have a visceral appreciation of the effort needed to maintain the air barrier at the ceiling 🙂

        So only chance I’d ever pursue a vented attic with blown in insulation again would be if there were both no ducts and no wiring in with the blown in insulation. So are you using a “plenum truss” for ducts, bathvents/vent hoods, with a wiring/lighting chase below the air barrier (ie sort of two ceilings), or are the ducts and wiring in a conditioned crawl space?

        1. @James, as noted in my prior reply, an indirectly conditioned attic may be warranted if there’s no way to get ducts out of the attic. But I always try to find a way to bring ducts into conditioned space. For single floor slab-on-grade homes, I’ve specified drop-chases and built-up chases. Every house is different. Whenever possible, I specify ductless returns (i.e., no direct return ducts). This offers several advantages over fully ducted returns, not the least of which is that it makes it a lot easier to bring ducts inside.

          Thanks for pointing out some of the challenges involved in air sealing a ceiling! I think most folks prefer to hire an experienced air sealing crew for that, but sometimes you just gotta do it yourself! And as I said, the decision to encapsulate depends among other things on ceiling complexity — i.e., how difficult/expensive it will be to get a tight ceiling.

          Regarding recessed lights… I advise clients to use surface-mount fixtures wherever possible (LEDs offer many new options for that), but traditional ceiling cans (even AT rated ones) need to be covered and sealed with an air-tight cap (e.g., CanCap, Tenmat Recessed Light Cover, diy XPS box, etc.). OTOH, I don’t see why you’d want to run lateral wiring chases in the context of this discussion.

          BTW, an important yet often overlooked detail is to seal the gaps between ceiling drywall and top plates (including interior wall plates). As the house settles, these gaps can become a major leakage path from the attic to the wall cavities.

      3. David & Tom, an indirectly conditioned attic in climate zone 4A can create moisture problems. If you encapsulate the attic by putting the insulation and air barrier at the roofline, you have to do something with the air in the attic. That’s the whole reason for this article. As I showed in my previous article, the humidity in some encapsulated attics can be much worse than it is in mine. See Humidity in a Spray Foam Attic. The life cycle cost goes out the window if you have rip off your roof and replace it.

        As David says, if you don’t have ducts in the attic, your best option is to insulate and air seal the attic floor.

    2. @Tom wrote: “We plan to encapsulate and insulate the walls. All duct work will be located there. What would be the best way to ventilate and control the humidity?”

      Not sure if you’re referring to house or crawl… Whole-house ventilation is beyond this discussion, but the ERV is a reasonable strategy. BTW, I strongly advise against relying on ERV as primary exhaust for showers or other high moisture areas.

      Humidity control in a sealed crawl space is dictated by IRC Section R408.3 (unless trumped by state/local code). The IRC provides several options.

      In reality, the need for humidity control in a sealed crawl varies greatly depending on site & foundation drainage, and how well the crawl is sealed (see note below). Keeping moisture out in the first place is always the best strategy. I recommend going with the easiest, least expensive option: adding a crawl space supply vent sized to deliver 1 cfm per 50 ft2 of under-floor area and a return air path to conditioned space (transfer grille or jump duct). Then monitor crawl RH and adjust the supply as needed ($5 balancing damper). In any case, you don’t want to use an ERV for a crawl as it will add moisture when the dew point is higher outside than in the crawl!

      NOTE: Sealed crawlspace subcontractors love to sell dehumidifiers — they either don’t respect the client’s wallet or don’t have much confidence in their product. I advise clients to hire someone who will guarantee “leakage-to-the-outside” (typically verified by a 3rd party; requires 2 blower doors — one to test crawl and one to null leakage between house & crawl).

      1. Your information was very helpful. I was referring to the crawl space which we are going to encapsulate. The house will be tight so we will need some kind of ventilation. Thank you very much.

      2. I agree completely with David! The crawlspace on my 1960s home was professionally encapsulated shortly before I purchased the home. They did a pretty good job of the encapsulation, but it appears that they oversold the product to the widowed, elderly homeowner. They did NOT replace the rickety old (original?) crawlspace door which had a rotten frame and leaked like a sieve. Instead, they installed an expensive dehumidifier and sump pump. Additionally, they totally ignored the source of the problem: water infiltration due to poor landscape drainage.

        After purchasing the home I spent quite a bit of effort improving the landscape drainage. Had this been done first, the urgent need for crawlspace encapsulation may have been mitigated. Additionally, I installed a well-sealed crawlspace door. I then unplugged the vastly oversized dehumidifier and the sump pump has not run since. I’ve monitored the crawlspace humidity levels with a cheap hygrometer and it constantly stays between 50 and 60%. I have not needed to supply conditioned air or install a return in the crawlspace.

  12. The crawl space will be 2800 sq ft. Average height 5 ft. We will have geothermal HVAC will all duct work in the crawl space along with a tankless hot water heater. The house will be tight in the crawl space, out side walls of living space and ceiling of the living area. my majoer concern is ventilation in the living area and in the crawl space. Would a Ultra-Aire ventilating dehumidifier work in the crawl space.

    1. @Tom, you do NOT want to introduce outside air into to a closed crawl space! Ventilation is for people!

      One of the building code options for managing RH in a closed crawl is to install an exhaust fan with return pathway from the house. In that case, the fan “ventilates” the crawl with drier air from the house (see R408.3.2.1 @ IRC link in my previous comment). I recommend that option in retrofit situations where crawl has some residual nastiness from before it was closed.

      In new construction, I recommend the supply air vent option because it costs less and is easy to adjust. The contractor may need to use a dehumidifier temporarily for the initial dry-out during construction, but as I stated in my previous comment, you shouldn’t need a permanent dehumidifier if everything is done properly. If the “supply air vent” option doesn’t keep RH under control, you can close the supply vent and install an exhaust fan. (Depending on how your stem walls are built, you may want to box out for an exhaust duct in case you later decide to switch to that option.) Permanent dehumidifiers should only be considered as a last resort!

      Ideally, you want to keep RH in the crawl below about 60%, noting that short-term excursions as high as 70% are NOT a problem (since crawl is generally cooler than house). Folks who install humidifiers and set them at 55% or lower are just wasting energy!

      BTW, if you have residual odors from construction (e.g., spray foam in rim bands), you can purge the odors with a high capacity window or floor fan. Ditto for the house itself. Run it several hours a day for a few weeks as needed. Pick and choose when to run the fan based on outside conditions.

      1. Tom,

        David covered what to do about crawl space air very well. Here’s an article I wrote in 2013 on this topic, with lots of good info in the comments there, too:

        What Is the Best Way to Deal with Crawl Space Air?

        Regarding dehumidifiers, Ultra-Aire is one possibility, but Therma-Stor, the company that makes Ultra-Aire, makes the Santa Fe line as well, and that’s probably what you’d want for the crawl space. Here are the links to the Sylvane pages for the two different lines:

        Santa Fe dehumidifiers

        Ultra-Aire dehumidifiers

        These are affiliate links. You pay the same price you would pay normally, but Energy Vanguard gets a small commission if you buy after using the link. Here are the regular links:

        1. Thanks Allison. I have already read you article several months ago. With this fresh perceptive I am going to read it again including the comments. My HVAC seems very knowledgeable. I better understand the path I need to go and because of you and David, I now know the right questions to ask.
          Thank you very much.

      2. Thank you David. As you have probably surmised I am a novice at this. However after reading your notes and studying the IRC section R408.3 I am beginning to get a grasp of what I need to do. You have been very helpful and I really appreciate the advice.

Leave a Reply

Your email address will not be published. Required fields are marked *

Back To Top