Closed crawlspace with dehumidifier and foundation insulation
I am still in the planning stages of building my new house. I would like a closed crawlspace (I’m in a mixed humid climate) with a dehumidifier and foundation wall insulation. See page 35 (which is actually page 41 on the pdf page counter) http://www.advancedenergy.org/portal/crawl_spaces/pdfs/Closed%20Crawl%20Spaces_An%20Introduction%20for%20the%20Southeast.pdf for a sample design of what I’m going with. I have two main questions about this design choice:
1. In the study (see section 6, page 49 for their research), they used supply air to condition the crawlspace. Overall they realized 18% less hvac energy use with the conditioned crawl space. Is there any reason to believe using a dehumidifier instead of supply air would significantly alter the energy savings?
2. As I just mentioned, in their research they used supply air to condition the crawlspace, but they also recommend using a dehumidifier as an alternative. I’m a little bit concerned about cold floors in the winter time with this method. Does anyone have any experience here? Thanks
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Replies
Michael,
In some cases, a dehumidifier can be useful for lowering humidity levels in the sealed crawl space of a newly built home. However, I see the use of a dehumidifier in this location as a temporary measure.
Once the crawl space has dried out, it should be possible to remove the dehumidifier from the crawl space. That's good, because dehumidifiers are energy hogs.
A sealed crawl space needs some air exchange. Building codes require sealed crawl spaces to be connected with the conditioned space above (usually with a floor-mounted grille). Assuming you have installed such a grille and you intend to comply with the building code, you now have two options for conditioning the crawl space: you can either (a) install a properly sized, continuously operating exhaust fan in your rim joist, or (b) install a forced air supply register in your crawl space.
More details on implementing these two options for conditioning your crawl space can be found in this article: Building an Unvented Crawl Space.
I am assuming that the air exchange requirement if for both a heated and unheated crawl space? I see code is 1 CFM/50 sq ft of floor area (which is about 100 to 150 cubic ft of space - being shallower than 8 feet). And is the proviso that they must be air and moisture sealed on the 5 planes facing outwards?
Martin, what do you recommend:
1. Exhaust only approach where the crawl space air is pulled out a crawl space vent with an in-line fan, and new conditioned air is pulled down by negative pressure from the conditioned space above through a passive register.
2. Passive or mechanically assisted circulating of the crawl space air using two or more in-floor registers with one assisted by a low flow register fan; making a circular path for air movement into the main house.
3. Since I know you consider exfiltration often to be an adequate exhaust pathway, I suppose there is 3rd approach: use an in-line fan blowing conditioned air downwards into the space with the old air exhausted by fugitive pathways/exfiltration. In this option, there would be no crawl space air being sent into the house.
I would think, given stack effect et al, that there would have to be a mechanical assit to the air flow, no matter which venting approach is used, otherwise the air exchange in a passive approach would tend to flow up and replenish itself from outdoors instead of with conditoned (heated or dehumidified) air. And if one wants not to heat the crawl space above the main house comfort levels, exhausting into the house would cause a cool breeze. Maybe the best approach is determined by climate zone and season.
Flitch
Flitch,
First of all, my own preference is not to build a crawl space. As I wrote in my article:
"It’s important to emphasize that even a well-detailed crawl space represents a problematic foundation design. Bill Rose, a renowned building scientist and a research architect at the Building Research Council at the University of Illinois, remains skeptical of crawl spaces. “I’m very cautious about crawl space construction — maybe it ought to be abandoned,” said Rose. “It would cost as much as a basement to get a crawl space right.”
"One problem with sealed crawl spaces: the air quality in such a crawl space may be poor unless the crawl space is equipped with an exhaust fan that runs continuously. Of course, such a fan can be part of a whole-house exhaust ventilation system; but when (not if) the fan eventually conks out, the homeowner is unlikely to notice. That raises concerns over air quality in the crawl space — and also in the home above.
"According to the previously cited Home Energy article, “Additional radon testing showed that radon levels in the closed crawls — with a relatively low dilution rate — were roughly 10 times the levels measured in the vented crawls.”
"If a builder chooses to condition a sealed crawl space using “Option 2” from the two code-approved options — that is, by installing a forced-air register rather than an exhaust fan in the crawl space — it’s easy for any radon or moisture in the crawl space to circulate throughout the house."
You asked, "I am assuming that the air exchange requirement is for both a heated and unheated crawl space?" Heating has nothing to do with it. These are code requirements for unvented crawl spaces.
While you propose three methods for providing air circulation in a sealed crawl space, the code allows only two methods. The first permitted method corresponds roughly with your suggestion #1.
The second option allowed by the code requires that "...the following is provided for the under-floor space: ... 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 insulated in accordance with Section N1102.2 of this code."
There is actually a third option in the code -- using the entire crawl space as an "under-floor plenum" -- but that option shouldn't be used, especially since under-floor plenums are now "prohibited in new structures" according to M1601.5.
The advanced energy designs (http://www.advancedenergy.org/portal/crawl_spaces/pdfs/Closed%20Crawl%20Spaces_An%20Introduction%20for%20the%20Southeast.pdf) have no transfer grills and they claim this method works fine.
Here http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/70974/What-Is-the-Best-Way-to-Deal-with-Crawl-Space-Air they prefer to use a standalone dehumidifier, since supply air may not provide adequate dehumidification in swing seasons. I like that option since it doesn't negatively pressurize the house (although advanced energy says "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."), but I guess there is an energy penalty to doing that. It was suggested in the comments on the energy vanguard blog that if you have a dehumidifier and ducts in your crawlspace already, the cost of adding some supply air is very small. You could use supply air to condition the crawlspace so the dehumidifier wouldn't need to run, but in swing seasons if the humidity level gets too high, it can kick on.
I'm my situation, I wonder if supply air will be enough. It's a two story house with the bedrooms upstairs. At night when we are all upstairs sleeping I see no reason to cool the downstairs, so the temperature on the thermostat will be set higher.
Michael,
If you are still in the planning stages for a new house, you have lots of options.
One option, of course, is to choose a foundation system that makes more sense than a crawl space.
In general, installing a dehumidifier in a crawl space is a desperate, last-ditch effort. There is absolutely no reason to think you will have to do that until you have actually built your house and monitored your crawl space to see if you have any problems. Many crawl spaces are just fine without any active features at all.
You also have the luxury of getting the construction details right, so that you don't have moisture problems in your crawl space. A few important points:
1. Make sure that your crawl space floor is above grade.
2. Make sure that your grade slopes away from your house in all directions.
3. Make sure that you include footing drains that lead to daylight.
As I said, don't start out assuming that you need a crawl space dehumidifier. And concerning the recommendations of Advanced Energy, my only reaction is that it's usually safer to follow the building code than it is to violate the building code. But if you can negotiate with your building inspector successfully, you may be able to follow Advanced Energy's recommendations -- assuming they make sense to you.
When does a crawl space become a basement? and do the same rules apply? I have a 6 ft unconditioned basement and plan to heat the first(only) floor with a mini split. Are there air exchange requirements?
Don't be deterred by Martin's dislike of crawlspace construction: I think it's a regional issue. Here in the Carolina Piedmont we have a similar prejudice against the kind of basement that seems to function well in the North. If your climate and terrain are anything like ours (we're in Advanced Energy's home territory) the unvented insulated crawl space with a small supply air vent from the main HVAC system has many advantages and is generally considered standard best practice in the industry.
I am in the Hampton Roads area of Virginia, so not too far from the Piedmont region.
Does anyone know why Advanced Energy's design doesn't include a transfer grill, but the building code does?
It sounds like I'm leaning closer to supply air with a transfer grill now.
Shawn,
Q. "When does a crawl space become a basement?"
A. I'm not aware of any code provision that clarifies the distinction, but perhaps another GBA reader has more information on the issue.
In the absence of a relevant code provision that applies, I imagine that your local code official will make the determination. You can be sure that if your code official is 6'4" tall, then your 6' basement will be ruled a crawl space.
From my personal glossary if it has a stairwell with a real door to access it from the first floor and is being used for more than place to install some of the mechanical equipment it's a basement, even if it's munchkin-height and the inspector needs to duck.
Code won't call it a basement space if it's anything less than 7' for "habitable space, or 6' 8" for other basement areas, per R305.1 :
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_3_sec005.htm
In my book if access is only via a hatch from the interior or a hatch/ door from the exterior it's a crawlspace, even if it's 7' tall, unless it's access is large enough to usable for something like storage, etc.
"Exhaust-only" seems a more comfortable idea since it keeps out known or unknown "bad air" (i.e. radioactive emitters like radon are colorless, tasteless and odorless; VOC's gassing off of paint cans and the like are no fun either; fugitive CO2 from combustion appliances). Additionally, the space is usually kept significantly colder than the living space.
So a return air pathways seem like a bad idea.
And given the technology and design of crawl space systems/materials these days, they are likely to be quite tight, thereby limiting the rate and volume of exfiltration --- not wasting heated and cooled air --- which seems the one risk to the exhaust-only option.
We have done all of the above and we have finally landed on the exhaust fan approach for the reasons you stated above. With tighter construction and closed crawls, we are hearing of more radon issues in our area and we are not known for radon issues here (Charlotte, NC - also a mixed/humid climate). We build fairly large homes (9-12 months construction schedule) and continue to have problems of controlling the moisture in the crawl during construction. We have also worried that small plumbing leaks going unnoticed can cause problems in the crawl. We assume all crawl air is bad air so best to exhaust it from the crawl rather than push it up into the house by pressurizing the crawl with a supply duct. It is very important that all penetrations are sealed, seal at mud sill and band joist and crawl door is weather-stripped and closes tightly - need to be 100% sure that the incoming air is coming from the house and not from the exterior.
We are also leaning back towards leaving the insulation at the floor joists rather than the exterior foundation walls - we call this a closed crawl rather than a conditioned crawl. Cold floor complaints combined with increased hardwood floor issues is pushing us in this direction. If you look at the Pinehurst study Advanced Energy did - there is actually an energy penalty in the winter on a conditioned crawl. Closed crawls outperform vented crawls in both energy savings as well as moisture control. There is very little difference in energy savings between a closed and a conditioned crawl.
Additional benefits to the exhaust only approach - will lower the neutral pressure plane of the stack effect and and in some cases can help with fresh air requirements such as ASHRAE 62.2. We have always worried that we were not going to get enough passive infiltration through the subfloor from the house above so we install dataloggers to monitor the crawl conditions and have always thought that we may need to create a pathway if the crawl was not staying dry - so far we have never had to do that. I would not worry too much about wasting your conditioned/heated air - at 1 CFM/SF the fan is running at a very low speed - less than any fresh air requirements or an ERV would typically run at. Usually under 50 CFM.
We have used several different fans - the best one is an all PVC fan - do not need to worry about condensation as you would with a fan with metal housing and you can use PVC for your exhaust vent too which makes install much easier than rigid duct like we used to use with the aluminum fans. Most of the fans that are designed to run continuously start at 100CFM so we purchase a dial type switch and dial it down to about 50% or so.
If your HVAC is in your crawl - you obviously need sealed combustion if you are using gas.
Danny,
Thanks for your comments.
I'm still a little puzzled about why builders choose to to build crawl space foundations.
"We are hearing of more radon issues..."
"We continue to have problems of controlling the moisture in the crawl during construction."
"We have also worried that small plumbing leaks going unnoticed can cause problems in the crawl."
"We assume all crawl air is bad air."
"Cold floor complaints..."
"Increased hardwood floor issues..."
"We install data-loggers to monitor the crawl conditions..."
Please tell me again why these foundations make more sense than slabs or basements?
I don't buy the idea that a well built and dressed up crawl space is anywhere near the cost of a well dressed, full basement.
Almost every basement in the Great Lakes region is wet (would be better to call it the Great Swamps region). Clay is dense and where its bedrock it is often only a few feet down. And if you are in a city or a development, narrow setbacks result in drainange rules that force one to pipe your rain gutters into the basement's perimeter drain. That starts off as a disaster and never gets better. Its chronic and its conventional.
Concrete is a sponge.
Flitch,
If you are convinced that it's difficult to build a dry basement where you live, I suggest that you build a slab-on-grade foundation.
Martin,
My state, North Carolina, allows a dehumidifier-only solution.
2012 North Carolina Residential Code, Section R409.5, Closed Crawl Spaces, says, "allowed [space moisture control] methods are listed below in Sections R409.5.1 through R409.5.5. At least one method shall be provided."
None of the methods require a connection with the conditioned space above.
Section R409.5.1 reads, "Dehumidifier. A permanently installed dehumidifier shall be provided in the crawl space. The minimum rated capacity per day is 15 pints (7.1 liters). Condensate discharge shall be drained to daylight or interior condensate pump. Permanently installed dehumidifier shall be provided with an electrical outlet."
Reference: http://ecodes.biz/ecodes_support/free_resources/2012NorthCarolina/Residential/Part_I_Residential/12NC_residential.html
Disclaimer: I'm a homeowner, not a building professional.
Stewart,
I'm grateful for the information you provided. I've learned something.
The moral of the story is that jurisdictions in the U.S. employ a hodge-podge of local building codes, and often enforce versions of model codes that have been modified to meet local practices and preferences. I appreciate the reminder.
Response to Martin's comment, #13:
I’m still a little puzzled why Martin is apparently suggesting that basement and slab construction systems are somehow intrinsically less susceptible than a well-detailed crawl space to the kinds of issues Danny Kelly raises. To take just one example, why is a small unnoticed plumbing leak more of a problem in a crawl space, where it will generally cause zero consequential damage to finish or structure, than one encased in (for example) framing, sheetrock, tile and cellulose? As to why why good, responsible, intelligent builders continue to prefer crawl spaces in appropriate regional circumstances, how about a clean dry mechanical and electrical distribution and equipment space that is inspectable, upgradable, is relatively inexpensive to construct and suitable for almost all topographical conditions of the region. North Carolina code requires daylight drainage for a full basement - this is not achievable on 90% of the lots here. Slab construction is certainly possible on most lots but it comes with its own set of significant disadvantages. In his excellent recent article delineating ten points in favor of the Pretty Good House he writes "Study buildings in your climate zone that are attractive, simple, and energy-efficient. Pay attention to their specifications. If possible, talk to the residents and find out whether the buildings are working well.” I suggest he takes his own good advice.
James,
I lived for six months in South Korea, where I lived with a Korean family. We ate rice, kimchi, seaweed and fish for breakfast, every day. (Delicious.) Many people in North Carolina, hearing this story, will say, "Who would want to eat rice, kimchi, seaweed, and fish for breakfast?"
Crawl spaces are a little like breakfast. Here in New England, we shake our heads, wondering, "I wonder why they like crawl spaces so much in North Carolina?" And in North Carolina, they say, "What's wrong with crawl spaces?" This is the type of discussion that has no winners or losers. It's pointless. I'm sorry I started it.
I'll answer one of your questions.
Q. "Why is a small unnoticed plumbing leak more of a problem in a crawl space than a basement?"
A. Because people who live in houses with basements visit the basement at least once a week, and they notice the puddle. People with crawl spaces often go a full year without visiting their crawl space.
Martin
Thank you for answering a question I didn't ask. There was no reference to a basement in my question. My comparison was between a small unnoticed leak in the exposed and accessible plumbing in a crawl space and a concealed and inaccessible leak in a finished space elsewhere in the house. The concealed leak could equally persist for a year or more before being noticed. Plumbing leaks can happen anywhere. That they can happen where they are unlikely to cause damage and are easy to locate and fix is hardly an argument against the crawl space. In fact as an annual inspection can turn up evidence of a concealed leak elsewhere in the structure it's rather an argument in its support.
James,
I get it now; I'm sorry if my original reading of your question was too hasty. You are extending the discussion somewhat beyond the original question of crawl spaces vs. basements and slabs. Fair enough.
Plumbing leaks in concealed locations are always a problem. No argument there. It's nice to end the discussion on a point of agreement.
Martin,
"You are extending the discussion somewhat beyond the original question of crawl spaces vs. basements and slabs. Fair enough."
Well not really. Using the leaky pipe only as an example I was just suggesting that the comments from Danny Kelly were about a reputable contractor working to make his crawl spaces the best they can be and it's a distortion to quote them as a reason to drop the crawl space altogether. Apparently at least Danny is not ready to give up on them. The concerns he cites are easily matched by equivalent problems with other foundation systems. To put it simply, in the area where I have worked for the last two decades below-grade northern-style basements are generally not even an option and slab foundations are frequently neither my nor my clients preferred options, for reasons which it would be tedious to go into now. So we mostly build with crawl spaces, we know how to do them well (we've encountered few if any of the problems Danny Kelly cites, for whatever reason) and they represent a reliable standard of best practice for our area. Whether this is relevant to Mr. Keoghan's situation is up to him to decide. A distance even of a hundred miles or so can make an enormous difference not only in climate and terrain but also in culture, skills availability and local best practices, so I repeat my suggestion he consult with reliable LOCAL professionals before making that decision. I believe that too would be a significant point of agreement between us.
James,
Local practices are sometimes, but not always, worth emulating. Just because a practice is locally common, doesn't mean that it shouldn't be evaluated on its merits.
That said, crawl space foundations are clearly working for you, and it sounds like you are happy with the results. So that's good.
"Local practices are sometimes, but not always, worth emulating. Just because a practice is locally common, doesn't mean that it shouldn't be evaluated on its merits."
I agree, and I've never suggested otherwise. There are three common approaches to traditional local building practices. You can dismiss and disdain them as the habits of old fuddy-duddies who don't know any better. You can adopt them uncritically because what was good enough for grandaddy is good enough for me. Or you can treat them with critical respect to see what you can learn from them in the light of current building science knowledge, material and skills availability and changing needs. I've never advocated any but the third approach. In searching for the best building practices of our time and place we're part of an ancient and continuing tradition, and if we're enabled to see farther than those who came before us it's it's only because we stand on their shoulders.
Any recommendations for fans in the 20cfm range, my crawl space square footage is just over 900.
Terrell,
The Panasonic FV-04WS2 WhisperSupply wall fan is rated at 20 cfm:
http://www.westsidewholesale.com/panasonic-fv-04ws2.html?keyword=Google_Shopping&mr:trackingCode=38D645C4-940F-E411-9D8F-BC305BF933C0&mr:referralID=NA&mr:device=c&mr:adType=plaonline&mr:ad=56682931285&mr:keyword=&mr:match=&mr:filter=98771795605&gclid=CjwKEAiA_4emBRCxi8_f2cWWjFcSJAB-v1qynUGOjLTOW-OvX1UQH-vQ9q2owDuCGTjw__J32mcZjhoCeajw_wcB