We get asked all the time if a crawl space fan ventilation system will keep a crawl space dry versus a crawl space dehumidifier? Well, the answer might surprise you so take a look at this crawl.
If you have a home with a crawl space—or are building or buying one—you have several options on what to do with that particular foundation type. Most crawl spaces are vented to the outdoors, but over the past decade, encapsulating the crawl space (as shown below) has gained favor among builders of green and energy efficient homes. It's often seen as the best way to eliminate the moisture problems that often result from vented crawl spaces. But what do you do about the air down there?Before we address that question, however, let me point out that encapsulated crawl spaces are great for humid climates, like the Southeastern US. In a dry climate, it may or not be worth the cost to encapsulate. So, as always, do what's most suitable for your.Basically, your options are to do one of the following:.
The tried and false method of venting the crawl space to the outdoors. Putting a little bit of supply air from the HVAC system into the crawl space. Using an exhaust fan to move air from the crawl space to outside. Installing a dehumidifierLet's take a look at each of these methods.
Vent the crawl space to the outsideDo this in a humid climate, and you're asking for trouble. The outside air that you bring into the crawl actually has more moisture in it than the crawl space air you're venting to the outside much of the time.
Although the, a look into just about any vented crawl space in the Southeastern US proves that venting to the outside is often a disaster.The photo above is from a house that was only a year or two old. Although the home builder had installed closed cell spray foam insulation in all the above-grade walls and the attic, he left the crawl space vented to the outside. The result was mold growing on the HVAC system.Let's just say this is your worst option unless you're in a dry climate and quickly move on to the better choices.Conclusion: This is your worst option in humid climates. It's probably also worst for most climates if the HVAC systems and ductwork are located in the crawl space. Supply air from the HVAC systemIf you encapsulate the crawl space, adding a little bit of air from the HVAC system is probably the most common method used.
If there's already a duct system down there, it's easy and inexpensive to do. The air from the system helps to dry out the crawl space air.But it also puts the crawl space under a slight positive pressure and the house under a slight negative pressure. Is that a problem? Not according to 's Closed Crawl Spaces guide:In Advanced Energy field tests, the small crawl space airflow causes a negligible pressure effect that is far exceeded by the effects of duct leakage, stack pressure or wind-induced pressures in the building.How much supply air do you need to add to the crawl space? Advanced Energy recommends 1 cubic foot per minute (cfm) for each 30 square feet of crawl space floor area. The International Residential Code (IRC) recommends 1 cfm per 50 square feet:Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulatedIf you go this route, it may work well in the summer if the air conditioner is sized properly.
If the system runs long enough, it can provide enough dry air to the crawl space to keep the relative humidity below 70%, where you want it.In the swing seasons, however, and in homes with oversized air conditioners, this method may not work well. Another thing about this method that I'm not crazy about is that you're conditioning the crawl space based on the conditions in the finished space above. The crawl space has its own conditions, though, and may or may not respond well to controls in another place.Conclusion: Although it's the most common method of treating the air in encapsulated crawl spaces, it may or may not work well. Exhaust air to the outsideFirst, note that I'm not talking about the crazy idea to put large fans in the crawl space to exchange large quantities of air between the outdoors and the crawl. (Yes, companies really do promote this, and it's a quick way to rot the wood in your crawl space.) I'm talking about a small exhaust fan, as described in the IRC.
Here's the language from the code:Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7m2) of crawlspace floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulatedSo a 2000 square foot crawl space would need a 40 cfm exhaust fan. It's pulling just a little bit of air from the crawl space. The problem is that you don't really know where the makeup air for the crawl space is coming from. Yes, the code specifies a pathway for air to move into the crawl space from the house, but in the diagrams. If the band joist isn't sealed well, for example, it may pull a lot of the air from outdoors, which isn't really what you want.Conclusion: It could work but is probably less reliable than supply air from the HVAC system. Install a dehumidifierIn this method, you use a standalone dehumidifier for the crawl space. (Again, we're talking about encapsulated crawl spaces only; although some people try,.) It's controlled by the conditions in the crawl space, not the house above, so it this method will do the best job of keeping the crawl space dry.Brad Brinke of in Virginia Beach responded to my question about this topic on: 'We have a very good track record with the dehumidifier here in Virginia.
We install a humidistat and fan system to help the dehumidifier operate efficiently. We tried using supply air from the home and it did not work.' (See the.)If you want dry air in your crawl space, a dehumidifier is the way to go. Yes, they do have drawbacks. You have to buy additional equipment.
They require maintenance. If the drain fails, you may have a bulk water problem in the crawl space.If it's a small crawl space, a standard dehumidifier that you can buy for $200 to $300 may be sufficient. For medium to large crawl spaces, however, it's best to go with a more robust model, like the ones made. Their is made for basements and crawl spaces. The photo above shows an older Santa Fe model made just for encapsulated crawl spaces.Conclusion: Although more expensive, installing a dehumidifier is the best method for treating the air in an encapsulated crawl space.
Should you connect the crawl space to the house with a transfer grille?Both of the quotes from the building code that I mentioned above include the following language.including a return air pathway to the common area (such as a duct or transfer grille)What they're saying is that if you pressurize the crawl space with supply air or depressurize it with an exhaust fan, they want the crawl space to be able to communicate with the house above to relieve the pressure. The easiest way to accomplish this is with a transfer grille right in the floor, as you see below.Advanced Energy isn't a fan of this method, instead recommending that the entire floor be air sealed to isolate the crawl space from the house.
There are some good reasons for this. If, for example, the intent of the opening is for house air to move downward (as in the case of the exhaust fan), the stack effect may win out if the crawl space isn't perfectly sealed to the outside.Another potential problem is pollutants in the crawl space air migrating into the house. These could be soil gases such as radon or fumes from paints, pesticides, or fuel stored in the crawl space. Also, if the crawl space access door is left open, it may not be discovered for some time. Meanwhile, the house is directly connected to the outside through that transfer grille.Is it ever OK to install a transfer grille?
In the house where I took the photo above, there's no access to the crawl space from outside, and the owner doesn't store hazardous chemicals down there (unless you consider wine hazardous). The bottom lineThe best way to deal with the air in a crawl space is to encapsulate it and install a dehumidifier. Adding supply air from the HVAC system is tricky and may let the crawl space humidity go too high in spring and fall.Now, go out there and turn those netherworlds into betterworlds!Related ArticlesExternal Resources.
Good article Allison.Reed, the only problem that we have with your approach down here in the 'lowcounty' of South Carolina is that termite companies want to inspect all of the wood. They are dropping termite bonds left and right these days for improperly done crawlspaces where insulators are covering the stem walls all the way up to the subfloor, or even spray foaming the subfloor and covering down the band board. This promotes a great air seal, but hides a real problem. 'Capping' the joists from below would work well, but it would hide a lot of the inspection ability and can create other issues with termite bonding of the home.Thanks,Jamie KayeElm Energy Group. #1 After 30 years of moisture/mold problem solving and 12 Summer Camps I understand dewpoint and crawlspace environments. But why did I (embarrassingly today) encapsulate my small crawlspace in 1967? To get rid of the damn chipmunks.
When I encapsulated there was a musty smell and a light white dusting on the floor joists. Six months after encapsulation the space was bone dry with no musty smell.
The crawlspace dewpoint was about 2°F higher than the room above. My crawlspace had no supply, return or exhaust air & no dehumidifier. Near-dewpoint equilibrium was apparently due to moisture diffusion though the hardwood floor and perhaps a little supply air duct leakage.#2 The eastern part of Canada within 300 miles of the great lakes (and the U.S.
Below) has summertime dewpoint temperatures of 68°F +/- 3°F (not much different from Atlanta). I recall a Canadian code or design guide which appropriately said to close crawlspace vents in the summer unless you wanted to humidify the crawlspace.#3 Providing cold supply air to the crawlspace will result in a crawlspace RH higher than that in the home. In some homes this could be near the mold proliferation RH!#4 Perhaps it is time to research and test basic encapsulated crawlspaces (a) without supply/return/exhaust and (b) with only a transfer grille.#5 If a dehumidifier is used it should be distant from bedrooms above because light sleepers often complain about the change in sound level as the dehumidifier cycles.
Bailes, you need to escape your southern parochialism and visit places where hockey is played by real hockey players. Transfer grilles work just fine in such locations.The point needs to be made that moisture has to be removed from the crawlspace.
That can only be done with dilution with drier air or with a dehumidifier. With the former approach some type of air change is needed with the conditioned part of the house where dehumidification is occurring either via the operation of an air conditioned or via dilution ventilation with outside air when the outside air is drier than the inside air.If your crawlspace is 'clean' connecting it to the rest of the house is ok. If it is not clean then you should not do so. In trashed crawlspaces sucking on them with a continuous exhaust fan works well. In clean crawlspaces up north transfer grilles work just fine. In between and in the south a very small amount of return air works fine with a transfer grille as does a small amount of supply air with a separate fan that does not blow cold air from the HVAC system (that Mr.
Gatley guy knows a few things about physics) but blows house air that is warmer and drier. Allison,Hope you (or Joe L.
If he is “listening”) can help answer a crawlspace question for me out in the cold dry region of Santa Fe, NM. Slabs are most common out here, but I’m helping to spec a home to be built at Santa Fe Community College that will be transferred to a home site.
Because the floor system will be open-webbed trusses, getting insulation against the warm floor (and keeping it there) is problematic. The stem walls will be ICFs with an R-22 value.I have suggested we have an unvented crawlspace with a sealed membrane on the dirt that is also sealed to the ICF stem walls to keep radon out of the envelope. The main reason for the unvented space is not to keep moisture out but to negate the need for insulation in the floor assembly. We are at 6000 HDD so cold dry air in the winter is more of an issue than moisture.There will be plumbing runs and ductwork in the crawl space but no mechanical equipment. Access to the space will be a trap door from the home’s interior. The band joist will be sprayed foam.Am I on the right track here?
Another great article, Allison. But I see a dehumidifier as a method-of-last-resort. After spending an extraordinary amount of effort trying to reduce consumption, I just can't see the logic of that if there's another method that can be effective.This is one of those building science topics where smart people may disagree.
Airing it out in forums like this is a great way to move toward best practice.For the pros following this thread, here's a link to a thread Allison started last year in the LinkedIn RESNET BPI group on HERS interpretation of diagnostic testing (ducts & envelope) in homes with closed crawls: (a must-read for HERS raters)And here's a more recent thread on ACH50 calculations and crawl space drying. Lstiburek put me in my place for suggesting that Advanced Energy developed the concept of closing a crawlspace (but HE was wrong about NC BBQ). Barry wrote: I have seen the energy savings claims of 18% by 'conditioning' the crawl space. I remain skeptical. I suspect that there are some missing data in the calculations.You should be skeptical. Energy savings claims are very much dependent on climate and circumstances, in particular, whether ducts are in the crawl.
Also, there's a trade-off between insulated walls vs. Floor in colder climates, which is a nice segue to Kim's situation. But if energy savings and first-cost is paramount, best to eliminate the crawl and either build a finished basement or slab-on-grade.@Kim, you'll also need to mechanical fasten and seal the membrane to the piers. In a cold climate and no mechanicals in the crawl, the floors may get a bit cool (due to influence of cooler ground). I have a project in eastern Long Island with radiant floors over a closed crawl, with AC ducts in the crawl. If the thermal envelope aligns with foundation wall rather than floor, the radiant system will lose too much heat to the crawl, increasing heating costs. They decided to insulate both walls and floor, but I advised some air exchange between crawl and house.
Since the floor isn't a pressure boundary, I suggested batts rather than foam (imperfect, but adequately reduces heat loss to the crawl). I'm also interested in what others have to say about this. Allison,Your customarily thoughtful and informative observations are always welcome.I try to precede all my interweb comments with something positive. It has a lulling effect:-).All seriousness aside, I encapsulated my CS here in semi-arid Carson City several years ago, and because the builder of our '02 home had conveniently (and mysteriously, according to the HVAC contractor) specified a 4' duct from the attic distribution system, a bit of ventilation air was easy to provide. I chose to add a return grill in the closet CS access hatch, after confirming that, with the hatch in place, no air (undetectable with wet finger:-) came down the 4' supply.Exterior vents were foam sealed and additional insulation was added to a few wall areas in need.
The vapor barrier is probably not as continuous as it needs to be, but regular visits to the netherworld have revealed no condensation issues. Annual radon tests (free through the Agricultural Extension Office) have the been just below the threshold but at age 69, I will doubtless die from something beside lung cancer anyway.Benefits include bare-foot comfortable hardwood floors, no more mice wintering over through the hole in the vent left by the cable guy (three hours clean-up with a shop vac), no more dirt in the CS blown in by our Washoe Zephyers and no annual fussing with vent dampers. Energy use changes have been undetectable on a purely anecdotal basis.Totally DIY but without the supply duct, would have been a big deal. With the supply duct in place, I'd estimate a 4 hour job by a pro.No actual engineering was involved in this project and any unintended negative consequences have been below the threshold of detection.Best wishes. 'Let he that hath AC supply in his crawlspace cast the first supply duct.' Reed D.: Yes, I've heard of people doing that.
I met a guy from New Jersey (or was it Delaware?) who's been doing it and is working on marketing a whole system to do it that way. One issue you have to be careful with there is what happens when there's a plumbing leak above that pressure boundary? If you use foamboard to at the bottom of the joists, you've got to have a drain in it somewhere, or maybe many somewheres.Jamie K.: Good point. I know someone in Hilton Head who promotes this idea, too, but I don't know if he's still doing it. Maybe he'll comment here and let us know.M.
Johnson: Yes, that hybrid approach would get you the best of both worlds. Then the DH would be down there when you need it during cool, rainy periods in summer, too (like we had most of this summer in Atlanta).Barry W.: The 18% savings number you cite came from Advanced Energy's Princeville study, which you can read about in.Matthew R.: I don't quite understand your system. How will a sub-poly ventilation system affect the air above the poly? Don G.: 'I understand dewpoint and crawlspace environments.' That's a bit of an understatement for someone who wrote what may be.
Your 5 points contain a lot of great points. About your home, though, I think maybe you got lucky. Not all homes get such a happy result without doing something with the air.Dennis D.: Thanks!Joseph L.: Indeed I do.escape my Southern parochialism, that is. In fact, that photo of the transfer grille was taken in New England. Perhaps you've seen it? Perhaps I should have just let you write the article because you said more clearly in two paragraphs what I tried to say in 30.Kim S.: That Lstiburek fellow knows what he's talking most of the time, although it did take him 58 years to learn.
Here's a list ofon the Building Science Corp. Website.David B.: Thanks! Here are the clickable links to those other discussions. I sure would like to hear an explanation of what conditions are expected in say, Tom's scenario. Humidity problems become awfully clear.after. someone explains them using the right principles of physics.I have a general idea of dewpoints to expect from indoors air, and also from outdoors air in my region (hot-humid South).
The dewpoint of ERV exhaust air is of interest to me, as well as the conditions we must avoid in crawl spaces. Are ERVs any better than 40% effective in transferring humidity from one air stream to another? I would love it if someone would explain using some details.My first guess which is probably wrong: indoor air 60F dew point, outdoor air 70F, ERV exhaust maybe 64?
Is that such a difference that blowing ERV exhaust into the crawl space would lead to harm? Yo DAndre,'.and the complete air barrier sounds like the best solution over any kind of grille to the conditioned space.'
With a 'complete air barrier in place', how would you get any circulation in the CS? What am I missing?As noted in my earlier post, I'm pushing (pulling?:-) air down a 4' supply and back up through a grill in the access hatch. No inappropriate storage in the CS and regular Radon checks are nominal.
I call it my 'crawlment'.Again, non-existant Northern Nevada humidity.YMMVBest wishes. @Mark, unfortunately, ERV manufacturers only publish total efficiency at summer conditions (sensible + latent), so there's no way to know how much moisture is rejected in the opposing airstream.Indoor dewpoint generally should not exceed mid-50's, at least not for any length of time.
But outdoor dew points are often in the mid-70's in much of the country. So mid-60's might be a fair assumption for exhaust stream, with AC picking up the excess moisture introduced in the fresh air stream.The crawl is likely to be significantly cooler than the outside, so dumping air with 64F dewpoint into a crawl that's 75F would create a relative humidity of 70%, which is high enough to grow mold. Also, if you use ERV to exhaust a bathroom during a shower, 60% of the interior moisture (using your example) would be pushed into the crawl. Shower steam may have a dew point in the 80F range!Check out.
@Tom: I have done some research using Weather Underground website. For example using airport 'KIGX' in Chapel Hill, you can see July dew point averaged 70F.@David: Something has been nagging at me about indoor dew point. You said it 'should be mid-50's' which corresponds to 45% RH at 78F temperature. I believe lots of homes in Houston run at 78F but have higher RH. When the subject is preventing crawl space hazards, I submit one should not assume anything better than mediocre conditions. It will be the more pathological 20% or so of homes which show problems.
So the question I see is what would be the indoor dew point in the worser part of the market, not the ones behaving well. I wish I knew all the stuff you know, and thanks for listening. @Tom, you can look at historical dew point graphs at wunderground.com. Monthly view is best for that, although you can also set a custom range.Last July, the peak dew point at nearest station was 76F:In your previous post, I thought you were trying to further recover the lost sensible heat from the ERV. There's obviously no way that's not going to exacerbate the humidity issue.If it were my home and the crawl was clean, I would do a 'pretty good job' sealing the floor and install a very small transfer fan between house and crawl.
I would control the fan with a humidistat set to about 50% in winter and about 65% in mid-summer (depending on peak crawl temperature and peak house RH). A dehumidifier may be necessary to dry out the crawl in the beginning, after construction wetting. @Mark, I didn't see your post when I posted my last comment. We were on the same page (literally).Regarding dew point, I agree that many homes have higher humidity than desirable, but for purposes of my point (bad idea to dump ERV exhaust into closed crawl), that's even worse.In general, if stat is set to 78F or higher 24/7, there's no problem with dew points into the low 60's.
You just want to keep the RH below 60%), but that's going to make it more difficult to manage crawl moisture.Anyone who recommends solutions for these types of problems needs to be proficient in psychrometrics. In fact, that goes for anyone involved in building science and especially mechanical design.