Controlling crawl space humidity

[ERD50]

Quote:

Originally Posted by Dimsumkid Here's an issue that AC won't address. Right now, the temperature is in the 50-60's and in the mornings, the humidity reaches 80+%, you're definitely not running the house AC and probably not even heat. If you do run the heat for a small amount of time, but may not on long enough to reduce the humidity (and do you have a humidifier built on the heater, most do). I know the issue brought up was for the hot summer months, but you get high humidity readings in spring and fall. I wouldn't be surprised in winter too.

Correct, as I said, if there is high humidity in non-A/C season, this won't completely address the problem.

However, I suspect that the A/C season is the biggest humidity problem - maybe some smaller ( <$1800!) dehumidifiers running when the A/C is off and humidity is high would do the job? Dehumidifiers are really an inefficient approach during the A/C season - yes, they de-humidify, but they dump heat into the space, while you are cooling the adjoining space.

Since those de-humidifiers would not be running so many hours annually, I'm sure some less expensive units could fit the bill.

-ERD50

[Dimsumkid]

Quote:

Originally Posted by ERD50 Correct, as I said, if there is high humidity in non-A/C season, this won't completely address the problem. However, I suspect that the A/C season is the biggest humidity problem - maybe some smaller ( <$1800!) dehumidifiers running when the A/C is off and humidity is high would do the job? Dehumidifiers are really an inefficient approach during the A/C season - yes, they de-humidify, but they dump heat into the space, while you are cooling the adjoining space. Since those de-humidifiers would not be running so many hours annually, I'm sure some less expensive units could fit the bill. -ERD50

I think the original issue brought up was the sweating from the AC ducts in the crawl space. Why not try a 50-60pt. dehumidifier in the space to make the humidity closer to a moderate level, this should stop the AC sweating issue down there. The amount of heat given off isn't going to be noticeable in this much space. As mentioned before, I would put a humidity meter in there to check the levels. Once you start cutting into supply and return ducts, these are open windows of mixing open air to the rest of the house when the system isn't running at all.

I actually tried a dehumidifier in our master bedroom this summer, it did raise temp room temp by a few degrees, but this was a small space compared to this crawl space. The humidity went down from 88% to 60% overnight, but the unit I used was too noisy to keep using it. The temp was 80 and it was comfortable enough to not need AC.

[Telly]

Quote:

Originally Posted by sheehs1 Am interested in others' solution to this problem...if you have it. We live in the very humid south. The AC ducts under my house drip with condensation July August and September.........

And there is the basics of the problem.

All across the South, in every state, there are post and beam foundation houses with crawl spaces. Have been for years. They aren't all molding away. And they don't have conditioned crawl spaces or dehumidifiers running under them. I don't see why VA should be any different than the deep-South states. The dew point in many of these states can get into the 70's F with no problem.

Having poorly insulated poorly sealed A/C ducts running through the crawl space can introduce a new problem. It can create a chiller system, where the ductwork temperature is lower than the dewpoint temp. Which will cause condensation, then dripping of water to the crawlspace floor. This is a big change in the crawl space weather system. There is an endless supply of humid air from outside that migrates in, so the humidity will keep going up until it is near 100 % in ALL of the crawlspace air, if there is enough dripping water, as this will raise the dewpoint temperature. It would be like creating a giant dehumidifier (but the condensing half only!), and never emptying the condensate tank while it runs and runs.

Without the dripping water, a properly-vented crawlspace would be roughly near the dewpoint outside, and would vary with the weather conditions, probably with some time delay factor due to the restrictions on airflow.

I still think the real problem is those cold ducts, or else almost every pier and beam house in the South would have major problems. Shoddy workmanship abounds in many trades areas, and shoddy ductwork is over represented!

[sheehs1]

Quote:

Originally Posted by samclem Luckily, the amount of venting needed for the crawlspace can be easily accomodated--those supply ducts that are sweating now can just be tapped and registers put in (do it on the main trunk, not on outlying branches so you don't end up with insufficient cooling to some rooms.). The return air from the crawlspace can either go directly into a return air duct (if it's easily available) or you could add an opening that leads back to the main building envelope--and thence to the return air grill in your house. I don't know why the guy would suggest you might need a bigger AC unit. AC units aren't (or shouldn't be) sized according to the cubic feet of the house, they should be sized according to the house's overall expected heat gain (a so-called Manual J computation). Right now heat leaks through the floor of the house in the summer. If you insulate the crawlspace walls to an appropriate R-value, the total heat loss will be less than you're experiencing now through your floor. In addition, moisture is now entering your home through the floor (you can bet that the paper vapor barrier on your present fiberglass insulation probably isn't in great shape). All that moisture coming in puts a load on your AC system that won't be there once this crawlspace is sealed up. I really think they want to sell you those dehumidifiers and a Cadillac insulation job. How much AC air will need to be exchanged with the crawlspace to keep it dry? Well, if it's sealed up correctly against water vapor, it should be about as tight as the rest of your home. And if it gets the same air circulation as the rest of your home (air changes per hour) and the rest of your house isn't moldy, neither will the crawlspace be moldy. From a practical perspective, you could start there. For the first year I'd probably buy a hygrometer and measure the RH down there. In the summer, strat with the registers wide open (the crawlspace will be coolest relative to the house, and driest), then progressively close them until the RH down there only rarely gets above about 65-70%.

Most I've talked with are talking about only 2 to 4 vents in the crawl space. Enough to create a positive pressure. Don't see how this will make the crawl space feel exactly like the house. Most individual rooms have 3 and 4 vents.

Maybe I should start with the 3 or 4. I can always add more vents as I monitor the situation...right?

HVAC company says they normally put the vents in the trunk lines to spread out the air flow. They are talking about using something like dryer vents so they open when system runs and closes when it doesn't. Are these type vents appropriate?

They are not planning on an air return....again because they are planning on just creating a positive pressure...allowing for air to escape up thru the floors or other unseen cracks and crevices.

Not sure I would want that air returned into the house.....until I have more of a handle on the mold situation. Am I wrong about this?

Doing a rough calculation on cubic square feet.
2,000 sq feet X 3.5 feet (height) = 7,000 cubic square feet.

Not about to pay some national dealer franchise $19K for this. It will probably end up costing me about $5K for this solution which is the conditioned space without the dehumidifiers. And if I have to add on some stuff after monitoring, well I should still be well below that $19K.
Thanks to all....I think I've just about got my arms around this project.

[sheehs1]

Quote:

Originally Posted by ERD50 It would seem so. For reference, here in N IL, we get heat/humidity as bad as the OP, but just not for as long. My basement has no special insulation, and is fairly 'leaky'. Yet, two standard in and out vents in the ducts are enough to reduce the humidity in there on the most humid days ( we get some 90/90's here). In fact, during a warm spell, one of my "should I turn on the A/C?" decision points is if the humidity is rising in the basement. Maybe I should close off the intake vent, and attempt to create a slight positive pressure? That would seem to make sense - any air being drawn in is hot, humid, and could condense upon hitting the cooler space. I'll try that next year. It doesn't seem to take that much air flow from the A/C to keep it dry enough. Unless your A/C is currently right on the edge, I'd think it could handle the slight extra load. Even if you had to upgrade, sounds cheaper than two(!) $1800(!) humidifiers. Or does this space need to be dehumidified during non-A/C season? Here in N IL, the non A/C season is not so humid, outside of rainy days. -ERD50

ERD...space does not need to be dehumidified...except during this hot humid summer months...typically July thru September. Particularly so after many hurricanes or rain events and we get a lot of those.

[sheehs1]

Quote:

Originally Posted by packrat44 Last week I had a HVAC system installed in the crawl space of my vacation house (20 years with only a wood stove for heat). The installer placed a condensation tray with pump to remove the condensation to the outside that will result due to running an AC. You might want to check and see if your air handler has a condsention tray and pump. If yes, is it working. My installer said they often can give problems. Also, ensure the R value is adequate around the duct work and all joints on the insulation are sealed. Next week I am going to apply 2" rigid insulation (foil back) to the main trunk on both the supply and return air side and seal all joints with aluminum tape. I have already insulated the round laterals with fiberglass insulation.

Have a condensation pan but I think it works on gravity or fall. Pipe from that thru the foundation wall and water is dripping out constantly so...seems to be working.

[sheehs1]

Quote:

Originally Posted by Telly If I were doing it, I would take a completely different approach. Assuming that the ductwork in the crawlspace is sheet metal, I would clean up the connections with a solvent, then seal all the connections 100% with the HVAC mastic or UL-324a tape. Then I would carefully insulate every inch of duct with the fiberglass duct insulation pads that have the heavy plastic vapor barrier on them. It comes on a big roll and is about 4 feet wide or so. One edge has a flap that gets taped over the edge of the next piece to get a complete seal. It has to be taped together with the proper tape, or else it will peel off over time. Can not tape over kinks or mismatches in the jacket edges, needs to be flat on both sides of where the tape will go to get a good seal. The ductwork has to be 100% sealed BEFORE the insulation is put on, or else air leaking from the ductwork will migrate through the insulation to the outer jacket, and condense there. And the outer jacket needs to be sealed very well at all the joints to prevent outside air getting into the insulation, and condensing. To get proper insulation coverage, you have to be able to get your hands around all of the horizontal ducts, so if they are up against floor joists, then some duct rework is needed to lower them to get access above them, and to make sure that the insulation will not get compressed against a joist, which would lower the R-value. I then would put 6 mil plastic down on the ground, tape all seams with the 2 or 3" wide polyethylene tape, and lap the plastic at least 6" up the foundation walls. And I would keep all the foundation vents open in the summer.

Telly....I am no longer certain that all the condensation is just from the AC duct work. Water table is high here in south eastern Virginia. After studying this problem for several years I am convinced part of the problem is the humid air going under the house from the vents as well as water vapor seeping up from the ground. Doesn't matter how many times I put plastic down which has been twice now, people screw it up when they go under my house. It is currently balled up in large spots. I already have insulated duct work but I understand your approach to insulate further.

[sheehs1]

Quote:

Originally Posted by ERD50 The condensation is a symptom of the excess moisture. You need to get rid of the moisutre to avoid mold and/or musty nasty stuff. Plastic sheet alone is probably not enough in his area. I think you need to get some dry air down there. I like the idea of slight pressurization of the dehumidified air from the A/C, assuming that is running often enough to do the job. -ERD50

It runs often enough. I keep my house very cool during the summer months. Can't stand the heat or humidity :-)

[sheehs1]

Thanks everyone...at least I know I do not have termites...! I am sure with all the people down there if I had them...I would know!

[samclem]

The info at this link answers most questions on the details of constructing conditioned crawlspaces. This would be a good guide for your discussions with whoever is going to do the work.

Quote:

Originally Posted by sheehs1 Most I've talked with are talking about only 2 to 4 vents in the crawl space. Enough to create a positive pressure. Don't see how this will make the crawl space feel exactly like the house. Most individual rooms have 3 and 4 vents. Maybe I should start with the 3 or 4. I can always add more vents as I monitor the situation...right?

See the options in the document at the link above (page 4). Note that the "transfer grills" mentioned are simply an opening or duct which allows air to flow from your crawlspace to the house. Any of the methods shown on page 4 will work, I'd probably favor Method D. Regardless of the method chosen, you need 20 CFM of supply air per 1000 sq ft of crawlspace. The amount of air that comes out of each register depends on the pressure supplied by your AC fan. If your system is designed properly now, you could figure out the CFM per inch of register area by finding the CFM for your AC fan (at the speed it normally runs in your house, some have multiple settings) and then divide this by the sum of all effective area of all the registers in the house (ones that are partially closed= estimate effective area). If this sounds like too much work, you can just use a rule of thumb: in a properly designed system, a 4" x 10" register supplies about 30-80 CFM depending on fan speed. So, if your basement is 2000 sq ft, you'd need 40 cfm of supply air = about two of these registers. Buy ones you can close off in case later you determine your system is working really well and you want to reduce the flow to the crawlspace.

Quote:

Originally Posted by sheehs1 HVAC company says they normally put the vents in the trunk lines to spread out the air flow. They are talking about using something like dryer vents so they open when system runs and closes when it doesn't. Are these type vents appropriate?

You could install such a thing (a "backflow damper"), but it's really not needed and it's another thing to fail and get stuck in the closed position someday. Regarding the location of the supply air registers-- you'd like to establish a flow pattern in the basement from the supply air registers to the return air point(s) (see below). The air should leave the registers and cover almost all the crawlspace before reaching a return air grill/opening. If it's convenient, then spreading out the supply air registers on the branch lines is okay. What you want to avoid, however, is starving a room register by installing one of these crawlspace registers. If you've got a room that gets only marginal cooling already (maybe a room that has a lot of windows and gains a lot of heat, supplied by a single 4"x8" register fed by 40' long 8" dia duct), you'll make matters worse by installing another register on that branch.

Quote:

Originally Posted by sheehs1 They are not planning on an air return....again because they are planning on just creating a positive pressure...allowing for air to escape up thru the floors or other unseen cracks and crevices. Not sure I would want that air returned into the house.....until I have more of a handle on the mold situation. Am I wrong about this?

Yep, I think you want that air coming back to the house, otherwise you'll be pumping nice cool air out into the neighborhood all summer. If you don't deliberately build in a way to get the air back to the HVAC system, then much of it fill wind it's way outdoors, and the remainder will come up through your floor into the house. Every cu ft that goes to the outside of your house depressurizes your home's envelope, which will cause air to leak in somewhere else (driving up your AC bills, bringing in pollen and outdoor pollutants, etc). Taken to extremes (unlikely in this case), this depressurization can even cause problems from backdrafting natural gas flues with accompanying CO "issues." See figure 5 on pg 5 of the linked doc for a diagram of a transfer grill--this (these) would be best placed in a hallway or other spot where there will never be a door closed between it and the home's return air plenum. Or, if the return air plenum itself is in the crawlspace, you could just put the grill there. Again, you don't want the return air grill close to the supply air registers in the crawlspace--this would create a "short circuit" loop and the really damp air in the crawlspace won't be picked up in the return air duct and sent to the AC evaporator coil to have the moisture removed.

Note that if you develop positive pressure in the crawlspace (relative to the rest of your house, perhaps because you didn't include transfer grills, etc), it means the supply air registers in the crawlspace will be pushing against this greater pressure. The "backpressure" will be less than this at the other registers (in your rooms) and that's where the air will flow. So, not providing a way for the air to get out of the crawlspace and back to your AC unit will reduce the amount of air that gets dehumidified through your AC unit (the main reason you're doing all this work). I think you want almost neutral pressure down there--virtually the same as the rest of your house.

Regarding the mold: I'm not an expert. I do know that mold spores are everywhere all the time. But, if this were my project, I'd probably wet down any visible mold with one of the commercial mold eradication products (available at Lowes/HD), then I'd count on the low humidity to prevent any future problems. The Scotch Filtrete furnace filters claim to capture mold spores, maybe I'd use those over the return air grill to capture any mold spores for a year or so. Again, none of this is based on any thorough research of reputable studies (to the degree that's available at all in the mold biz!). Know that if you don't provide transfer grills and instead just let the air find it's way into the house through cracks and crevices, it will still be the same air (including any mold or dust from the crawlspace) as if you'd provided a dedicated duct, except you won't have any way to filter it (if such is your wish). It might not be a bad idea to put a fan down there to stir the air up for a few weeks after you get this work done, that should help dry the place out a bit faster (the circulation will help the air pick up more water from the floor, etc between cycles of the AC).

Also, as the wood dries out the floor may behave in "new" ways. If it's plywood or OSB you probably won't have any problems, but if you've got planks anywhere . . .

Radon: See figure 13 in the attached document. If there's any chance you might have a radon issue in your house, it's smart (and cheap) to put in a manifold for removing this soil gas. Just a few trenches in the dirt about 6" deep lay in some 4" perforated pipe all connected to a single pipe sticking up at a convenient spot for a vent flue to be installed later (if needed). Cover the pipe with coarse gravel, then lay your plastic all over the dirt floor and continue with the work. Then, after the the crawlspace is done, I'd close all vents to it and measure the radon level for a few days (kit: about $20). If there's a problem you can buy the fan and run the flue line out through a wall to a point above the roof line.

Again, be sure to do your own research so you understand exactly what you are trying to achieve, and recognize that my opinions are just as "authoritative" as any other random gurgling you'll find on the interwebs. It sounds like you are well on your way.

[ERD50]

That's some great research there samclem. I have not read thru the links yet, but I like the idea of the two 4x10 registers. Good point on wanting a return, so the conditioned air does not all go to waste - but I wonder if sizing the return a little smaller than the sources might be good, just to provide a very slight positive pressure, just enough to keep outside, hot humid air flowing in?

Hmm, I'm thinking I might hook up a manometer so I can see that I create a very slight positive pressure for my basement. Would be good to check in winter, to assure no back-draft potential for the furnace (which has a sensor, but best to not have the problem in the first place).


For sheehs1 - consider something like this:

Amazon.com: La Crosse Technology WS-9037U-IT Wireless Forecast Station with Pressure History: Home & Garden

I have a similar unit from LaCrosse that I'm very happy with ( I have three remotes now, outside, porch, and I move the third around to wherever I want to monitor), but my remotes do not report humidity. Put one of those in the crawl, and you can monitor and know that things are as expected w/o having to expose yourself to cobwebs. Plus, since it just takes a glance, you are more likely to actually check it.

-ERD50

[samclem]

Quote:

Originally Posted by ERD50 Good point on wanting a return, so the conditioned air does not all go to waste - but I wonder if sizing the return a little smaller than the sources might be good, just to provide a very slight positive pressure, just enough to keep outside, hot humid air flowing in?

I think the problem is that, barring deliberate forced introduction of outside makeup air, any positive pressure in the basement can only be achieved by producing negative pressure in the rest of the building envelope. If the positive pressure in the basement is keeping moist outside air out, then the negative pressure upstairs is encouraging more of it to come in. And that warm, moist air outside air is going to be sucked into the walls and other places where it will meet the cool back side of gypsum board, etc. And then we get mold.

If we were providing positive pressure to the whole house via a heat recovery ventilator, etc, that would be a different situation.

I think . . .

[sheehs1]

Quote:

Originally Posted by samclem I think the problem is that, barring deliberate forced introduction of outside makeup air, any positive pressure in the basement can only be achieved by producing negative pressure in the rest of the building envelope. If the positive pressure in the basement is keeping moist outside air out, then the negative pressure upstairs is encouraging more of it to come in. And that warm, moist air outside air is going to be sucked into the walls and other places where it will meet the cool back side of gypsum board, etc. And then we get mold. If we were providing positive pressure to the whole house via a heat recovery ventilator, etc, that would be a different situation. I think . . .

I get this. Makes sense SamClem. I've printed off your link and your long response regarding the other points......and will go thru both.

You are right...if a return is not provided then the crawl space air will just come into the house anyway. They say they have never used a return but I'm going to copy this paper and as you said, reference it when I talk with the HVAC people

They've agreed to spray the entire crawl with Microban 2 days prior to the start of the project. and since we are going into cooler months it should be dryer when the actual work is performed.

THe old HVAC company has agree to come and take out the HVAC system they left under my house at no charge....but can not tell me when. Don't want to start the project until that is out.

I've asked each company (three of them) and they tell me we don't have a radon problem here. ummm.... Think I will call the building inspectors office and asked them that question.

I'll post again once I get thru the paper . Much appreciated SamClem!

[sheehs1]

Read everything SamClem. Method D page 4, the supply/return option seems to make the most sense.
Wonder why...not one contractor company I've spoken with seems to do it this way and two of them wanted to encapsulate and put in dehumidifiers.
Have forwarded to the main contractor and asked if he could or would do it as Method D shows.

Found this link for the EPA on national Radon levels for anyone reading this thread that may be interested.

EPA Map of Radon Zones | Radon | US EPA

My question regarding this is while radon is low in my area, does encapsulating the crawl space create a situation where it will build up.? Building inspector hasn't returned my call yet.

My twin sister and husband put a radon monitor in their crawl space but live slightly west of Richmond Va. where radon levels are high.

[samclem]

Quote:

Originally Posted by sheehs1 My question regarding this is while radon is low in my area, does encapsulating the crawl space create a situation where it will build up.? Building inspector hasn't returned my call yet.

It is risky to make a determination about your own home's radon levels based on what's happening in the neighborhood. While there are patterns, every house is different. If I were in your shoes, I might buy a radon test kit, put it in my crawlspace now (with no plastic, etc) and keep all the vents closed. Wait the required time (3 days?) then send it off for a reading. It's quite likely the radon levels will be elevated, and if they are, I'd put in the pipes before putting the plastic down. If the radon levels weren't elevated, I'd feel relatively sure that there wouldn't be a problem after installation of the plastic. After the job was done I'd test again: if radon levels were high I'd be happy that I'd laid the pipes in the ground and I'd get a radon remediation guy out or put in the flue and fan myself (if allowed by local code, yada yada).

Truthfully, putting in the pipes and gravel will be relatively easy and good insurance in case you ever do have radon problems. OTOH, if you want to take your chances, installing them later would just mean cutting the plastic, laying the pipes and gravel, and laying/taping new plastic down on top of the pipes/gravel. It wouldn't be hard (compared to jackhammering a slab, etc) and you don't have to do it now if you don't want to.

[ERD50]

Quote:

Originally Posted by samclem I think the problem is that, barring deliberate forced introduction of outside makeup air, any positive pressure in the basement can only be achieved by producing negative pressure in the rest of the building envelope. If the positive pressure in the basement is keeping moist outside air out, then the negative pressure upstairs is encouraging more of it to come in. And that warm, moist air outside air is going to be sucked into the walls and other places where it will meet the cool back side of gypsum board, etc. And then we get mold. If we were providing positive pressure to the whole house via a heat recovery ventilator, etc, that would be a different situation. I think . . .

And I think... that you are correct.

My problem was that I was thinking of the crawl space in isolation. It is part of the whole house system, as your analysis points out. That air has to be made up somewhere, and if we pressurize the crawl it will be made up through leaks on the upper floors. That said, pulling in a small amount of air into the house ought to be 'less bad' than pulling it into the crawl. Since we probably can't control this precisely, I think I'd prefer to err on the side of a very slight pressurization to the crawl - some vents between the crawl and the house would help equalize this so that we are not blowing large amounts of air out the crawl (which might be part of the links you provided, have not read them yet).

I have no idea if a manometer is sensitive enough to detect these small pressure changes? I see that you can set it at an angle to increase sensitivity. I could experiment easily, and will do it this heating season.

-ERD50

[samclem]

Quote:

Originally Posted by ERD50 I think I'd prefer to err on the side of a very slight pressurization to the crawl - some vents between the crawl and the house would help equalize this so that we are not blowing large amounts of air out the crawl (which might be part of the links you provided, have not read them yet).

Yep, the return air duct description is in the doc at the link. If one wanted the crawlspace to be under slight positive pressure, I think the best way to achieve that would be though direct measurement of pressures like you mentioned and closing off the return ducts from the crawlspace to the rest of the house. Of course, it will only be under positive pressure while the fan is running, which would normally be a small portion of the total time (unless one just keeps the fan running to circulate air, etc).

[Tadpole]

This is an old thread but I have a similar crawl space question.

We recently bought a 1958 rancher in Bellingham, WA. I had better insulation put in the attic and had insulation put in the previously uninsulated crawl space. One of my major goals was insulating the heating ducts because by the time heat got to the bedrooms the air was cool.
I noticed after insulating the crawl space that the water was running colder than before. The water pipes were covered with the new insulation but they are not being heated by the lost heat from the heating ducts.

It occurred to me that when the house was built gas was so cheap that people compensated for lost heat in the crawl space by setting the thermostat higher. So it might have been a key element of the design that the heating ducts kept the water pipes warm in the winter.

Have I goofed by having the insulation added to the crawl space? Do I need to run the water during below freezing weather. Can stuffing the vents with insulation during the few below freezing days we have help? Temperatures here usually don't go extremely low.

[Dimsumkid]

Tadpole,
Your heating ducts wouldn't be used to heat your water pipes, at least not to any level you'd notice. You may be having other issues at work here, have you checked your water temp using an instant read thermometer? Is the heat set to 120 degrees? Is your water heater really old? If you're concerned about freezing pipes, all you need to do is open the basement sink to a slow trickle to prevent the water pipes from freezing.

[ERD50]

Quote:

Originally Posted by Tadpole Have I goofed by having the insulation added to the crawl space? Do I need to run the water during below freezing weather. Can stuffing the vents with insulation during the few below freezing days we have help? Temperatures here usually don't go extremely low.

Put a remote, wireless thermometer down near the pipes. You can monitor it and see. It would take some significant time for the water to freeze if you are occasionally just dipping a bit below 32F . It has to drop the water from supply temps (50-60?), and then it takes 144x the energy to pull that last degree out to go from above freezing to forming ice. Running the water at night and first thing AM would probably be OK.

But if it is below 32F for extended times, you'll want to start getting serious and do something to warm it up, and/or trickle some water.

It makes little difference what the outside temp is - there are so many variables, you need to know the temp where the pipes are. Get the thermometer.

What would stuffing the vents do? IF anything, that would cut the airflow, the ducts would be cooler and there would be less heating of the crawl (unless there are vents to the crawl in those lines, so the air would be driven down there)?

-ERD50