Conditioned crawl space
shammah98
| Posted in Energy Efficiency and Durability on
We recently had a contractor close up our crawl. they placed 10mil poly on the floor and spray foamed the walls. conditionied supply air is added to the space, and a return is vented outside. I do not seem to be noticing a difference though. I have remote sensors down there, and the humidity is pretty steady at 69-70% and the temperature pretty much correllates with the outdoor temp (maybe a little cooler during the day) I thought the humidity would be lower and the temps more approximate the living space temps, or at least more stable compared to outside, since it’s now “insulated” and in the building envelope?
thanks
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Replies
Shammah,
Two questions:
1. How long has it been since the work was performed? If you formerly had a damp crawl space, it can take several months for the damp crawl space to dry out.
2. Is your home air conditioned? If so, your air conditioning system will help dry out the crawl space. If not, it's no surprise that the conditions down there aren't changing.
Thanks Martin. work was completed in the last month. since then the weather has been fairly warm. we are in a dry climate. home is air conditioned, so a/c air is being supplied to the crawl.
Shammah,
I think you should wait 6 months before you jump to any conclusions.
Ok. didn't realize it takes that long to notice an effect. also our crawl was not damp before. in fact, the humidity has gone up down there since we had the work done. thanks again
Hello
so it's been almost 6 months since this project was completed. some minor improvements, but not much. As a recap
- 2 storey home, 3243sq ft, new construction. temp/comfort issues, high heating/cooling biils. past winter lots of ice dam issues and window condensation. Got energy audit. approx 4.2 ACH/50 blower door. R38 attic insulation (blown +batts) several recommendations made.
so far we've
added more blown in to attic (where accessible. some areas cant be reached, knee walls, etc)
encapsulated crawl . 10 mil poly on floor, spray foam up walls. supply air to crawl with a vent out and up through ceiling. there is also a register in first floor connecting crawl to living space..
some air sealing around exterior doors
air balancing
post work blower door leakage 1793 cfm @50Pa (crawlspace adds about 2000 sq ft of conditioned space)
Our main issue is the house still uncomfortable temp wise. we must still be loosing a lot of heat to the attic, as when I leave for work in the morning, I notice all the other homes in the area have frost on their roofs, but ours is melted (similar orientation, roof color). And finally the crawl still seems to have relatively high RH. (60%)
RH of the 1st and 2nd floor is in the 35-43% range (depending on if we're cooking/bathing, etc). I've checked teh plastic, and I don't see any gaps, etc. The contractor admits the RH is on the high end, but not sure why. they are suggesting adding a fan at the joist area, blowing out, to depressurize the space. it's currently under positive pressure. Im not so sure about this idea. when I run a dehumidifier down there, it does drop to the low 30s, matching the rest of the living space.
Any ideas?
thanks
Shammah,
A few points:
1. What do you mean when you write that your crawl space has a "vent out and up through ceiling"? Can you describe this better?
2. You wrote that workers did "some air sealing around exterior doors." But that's not the most important site for air leakage. When workers "added more blown-in [insulation] to the attic," did those workers perform air sealing work on the attic floor before insulating? Air leaks in attics matter far more than air leaks around doors.
Hi Martin
the hvac guy added a register to one of the main supply trunks in the crawl. They then ran a piece of rigid duct from the crawl, up through the floor/attic and vented outside. The end in the crawl is just open. I guess the idea was as the supply duct pressurizes the crawl, the pressure will be relieved by this duct/vent going out? The end that's outside has a seal that's supposed to prevent air from coming back in to the crawl. This was the system in place for several months, but the humidity levels were not dropping, thats when we added the floor/transfer grille between crawlspace and living area. NOw the crawl RH% closer approximates the living area RH (still higher), but we also get some of that crawl smell now, where the vent is.
Sealing the attic floors was also one of the recommendations after the energy audit. We deferred that for now, just due to cost. Also, there are some areas of the attic that are inaccessible (or very difficult to access). so the air sealing was limited to easily accessible areas for now, even though we understood this was likely not where most of the leakage was occuring. We were also told that encapsulating the crawl, would lessen the stack effect, and consequently air leakage through the ceiling, etc
thanks
Shammah,
The type of vertical vent pipe to the outdoors that you describe is unconventional, and is probably the source of the crawl space moisture. The stack effect creates a powerful draft in this type of vertical pipe, and the stack effect will pull outdoor air into the crawl space through unsealed cracks. In some weather conditions, these leaks will introduce moisture into your crawl space.
You need to permanently seal this vertical vent pipe.
I would seal up the crawlspace and run the dehumidifier in it - you (and many others) know this works.
Thanks Martin. so air can be drawn into the crawl even though the space is under positive pressure? I guess it's not always under positive pressure, since the hvac system isnt running 24/7...the hvac contractor's response was that the vent has a gasket and flap that prevents air from flowing the other way. IN any way, sealing it should not be a problem, I will just need to tape over the open end in the crawl. Im just upset that I spent all the money for nothing :(
On sealing the attic floor, do you recommend spray foam?
thanks
Yes Jon, it does work to run the dehumidifier. I was just under the impression it would not be necessary, and it adds more energy expense. It's good to know I have that option though
thanks
Shammah,
Here is a link to an article that explains what you need to know about crawl spaces. (Note that you don't want a vertical pipe that encourages air exchange with the outdoors):
Building an Unvented Crawl Space
If you have a supply register in the crawl space connected to the forced-air heating and cooling system, that's the source of dry air. The return air path (back to the furnace) is usually via the grille installed in the floor above the crawl space (and from there to a return-air grille somewhere in a common area of your home).
Here is a link to an article about sealing leaks in your attic floor:
Air Sealing an Attic
thanks Martin. reading the article...it sounds like an option to the grille in the floor is using a exhaust fan to the outside? or am i misunderstanding that? I thinking of solutions to avoid the smell from the crawl
Shammah,
If you don't want to install a register attached to a forced-air system duct in your crawl space, you don't have to. Instead, you can install an exhaust fan in your rim joist. (Size the exhaust fan carefully, according to the recommendations in my article.)
The makeup air for this exhaust fan is provided by the grille in the floor above your crawl space. This guarantees that the air entering the crawl space is conditioned -- heated in the winter, and air conditioned during the summer -- because it's coming from the house above. This approach also reduces the chance that crawl space odors will enter your house.
Removing moisture takes energy whether it's done by a dehumidifier or by your AC system. And in most situations, there are long periods (weeks or months) where neither AC or heating are needed. Where is crawlspace dehumidification supposed to come from in these periods?
Jon,
During the swing seasons, when temperatures are mild, the indoor air is usually dry enough to remove moisture from a damp crawl space, as long as the exhaust fan in the rim joist is operating properly.
Later in the summer, when the humidity rises, most homes operate an air conditioner, so that the indoor air entering the crawl space is capable of drying out the crawl space.
You're right, of course, that this is an additional load on the air conditioning system.
In my experience, there is enough open window swing season weather where the dew point is tolerable upstairs but is too low when cooled somewhat (as a crawlspace does). The result without dehumidification is mold smell.
Everyone has different weather, amounts of insulation, tolerance for living space dew point, availability of AC, bulk water issues, sensitivity to mold smell, etc. But other than possible code issues, why not recommend the sealed crawlspace/basement solution that always works?
Jon,
Q. "Why not recommend the sealed crawlspace/basement solution that always works?"
A. That's what I did in my article (Building an Unvented Crawl Space). Do you disagree with my recommendations?
what should i be targeting for RH%?
thanks
Limiting it to no more than 60-65% is very safe from a mold-growth point of view. Above 75% RH the risk grows rapidly.
Martin, I think your article would be clearer if the advantages of permanent dehumidifier use were discussed in the article (vs talk of it as a fallback option buried in the comments).
I expect that further discussion of the energy use issue would also be helpful. For example, all of these add energy use beyond removing moisture:
powering an exhaust fan
conditioning the air removed by an exhaust fan
heating/cooling the crawlspace (even with no exhaust fan)
running the AC when the living space conditions are OK but crawlspace humidity is too high
On the other hand, if one plans on using permanent crawlspace dehumidification, then ideally both the crawlspace walls and the floor would be well air sealed.
Jon,
The air exchange alternatives mentioned in the article -- either an exhaust fan in the rim joist, or a register connected to the forced air system -- provide air mixing and odor reduction (and the first of these options is better at odor reduction than the second). Those measures are code-mandated, so one of the two approaches is mandatory.
In most cases, these methods of air exchange are adequate to handle moisture. In cases where they aren't, I recommend the use of a portable dehumidifier. The operation of a dehumidifier is usually a temporary measure. Once the RH in the crawl space is under control, it's a good idea to remove the dehumidifier, because it's an energy hog.
A dehumidifier also provides effective odor reduction. Code, sometimes yes, sometimes no. "energy hog" in a well sealed area - let's see the figures, including the offsets I mentioned above.
Jon,
If you're happy with your dehumidifier, of course you should continue to use it.
But if a crawl space remains dry without the use of a dehumidifier, we can all agree that "no need to operate a dehumidifier" requires less electricity than "operating a dehumidifier."
As I wrote in one of my blogs:
"Lauren Mattison and Dave Korn, two researchers employed by an environmental consulting company in Massachusetts called Cadmus, recently conducted a study of residential dehumidifiers. The researchers monitored the energy consumption of 21 residential dehumidifiers in the Northeast and mid-Atlantic regions over a three-month period. According to the study, the average dehumidifier uses about 1,000 kWh per year — twice as much as an Energy Star refrigerator. In homes with dehumidifiers, the amount of electricity used by the dehumidifier equals 9% of the electricity consumption in an average home."
I have no idea what you meant by this sentence: "Code, sometimes yes, sometimes no." If you mean that some jurisdictions in the U.S. have no building codes, you're right, of course.
Not accounting for offsetting costs of using a dehumidifier (like turning off the exhaust fan, not using air conditioned by the house AC, etc - see above) and quoting average dehumidifier electrical use across all kinds of uses makes for nonsense statements and figures.
If your completely sealed (no exhaust fan use, closed register) crawlspace stays nicely dry by itself, then don't worry, the dehumidifier will never turn on.
Jon R,
I said it before, and I'll say it again: building codes mandate one of two options: a continuously operating exhaust fan, or a forced air register.
Assuming you want your house to be code-compliant, you can't "offset" the energy use of one of these features, since one of these approaches is mandatory.
If, for whatever reason, you want to disregard the building code -- either because your house is located somewhere without any code enforcement, or simply because you (correctly) assume that code officials don't check on compliance -- you can, of course, experiment with whatever approach you want.
Martin,
In regards to crawlspaces, code has often lagged behind best practice. Dehumidification in conjunction with encapsulation is a proven strategy for keeping crawlspace humidity in the safe zone.
Dehumidifiers do use energy, absolutely, but this can be minimized with good encapsulation and reasonable RH targets. When we talk about optimal approaches for crawlspaces, we should think about not just dehumidifier energy use but the energy penalties associated with supply and exhaust strategies.
Some knowledgable building scientists like Allison Bailes list dehumidification as a code-approved third option for conditioning crawlspaces. It's been a while since I've read these codes but I think there may be some room for interpretation here.
I've encapsulated my crawl space and wondered if I can detach the flex duct going to a small bathroom and use that to feed the crawl space with conditioned air instead of cutting a hole in the main trunk and the vent would then be there to act as the transfer grill. It's located just a few feet from the main return grill for the house HVAC. All the duct work is in the crawl space. The house is 1000sqft and the heat pump is 2.5ton 16 seer