Controlling crawl space humidity

[samclem]

I think the first reference was a typo, go to this link, looks like it's a spray foam, not rigid:

http://www.insulstar.com/uploads/InsulstarBrch_11-5-10.pdf

Ahh, thanks. It looks like it may need to have a coating (Aldocoat 757) to meet fire codes, and that the termite question (detailing required, inspection strips, etc) depend on local codes and if the home is in an areea where the "probability of infestation is 'very heavy'"". Otherwise, from what I've read in a few places the product itself is a reputable one. It will all boil down to the details and the installers.

Oh, if applied over 1.5" thick the Insulstar should do a sufficient job of stopping water vapor infiltration through the walls, you won't need anything else to stop the water vapor. At 1.5" it will still be a little short on insulating capability (about R-10) compared to the walls in modern homes. I'd shoot for a bit more if you can afford the price. And ask him if he'll apply the Boracare with Moldcare (or similar) penetrating borate product to the wood two days before he does the spray. He shouldn't charge you much for this--it's a few trips around the edge, thoroughly dousing everything with a garden-type pump sprayer. The materials will cost about $200 for a house like yours.

 

[sheehs1]

I just spent over 4 hours with a representative from JES - Baement-Crawlspace-Foundation Repair Specialists. They are mostly in Virginia but are part of a national dealer network for Basement Systems. There are 450 dealers nationwide so perhaps you guys have heard of them. I called them in to learn more and because I was not confident the other guy was going to do a good job. This company seems to know what they are talking about and have engineers on board. However, they really do not address the mold saying the mold will die when the humidity to brought down and stays down consistently. A bit surprised because I called them in to do a "conditioned space and they recommended a semi-conditioned space with dehumidifiers.....saying that is what they always recommend and in fact, this guy says he has only done one real conditioned space in 3 years.
Their quote for this semi-conditioned space.....was sky high and a whopping...$18,812 for 2,163 feet of crawl space.
This is what they were going to do..
1. Remove all debris including that old air conditioning unit
2. Remove insulation - quoted at $1.00 a square foot
3. Install 20 mil Cleanspace Encapsulation System that is guaranteed for 25 years. If it is torn or messed up for any reason they fix it for free.
4. Install 2 inch Silverglow insulation on walls, caulk, seal all openings
5. Install vent covers with gaskets
6. Install 2 sealed crawl doors
7. 2 Sanidry Dehumidifiers with condensation pans and pipe thru foundation
These units were quoted at $1,800 a piece
The company was founded in 1993. Anyone know anything about Basement Systems and their dealers?

 

[travelover]

........ Anyone know anything about Basement Systems and their dealers?

I think you should hire Samclem - he's probably do it for $15,000

 

[samclem]

The company was founded in 1993. Anyone know anything about Basement Systems and their dealers?

No, but that doesn't mean anything, they could be great. If you're at all tempted to go with these guys, check them out with BBB and (especially) Angie's List (it's well worth the membership if you aren't a member now).

That's a lot of moola.

There doesn't appear to be anything techncally wrong with their approach--reducing the crawlspace humidity will stop the mold problem. They just want to install dedicated dehumidifiers to remove the moisture rather than relying on the dehumidifier you already own (your home's air conditioner). Their approach will certainly work. The question is whether you want to pay the high buck installation and the ongoing costs for those dehumidifiers (electricity, maintenance, etc).

Why are they recommending removal of the insulation below the floor? Is it to prevent the freeze-up of their dehumidifiers in the winter? Those dehumidifiers are going to add heat to your crawlspace in the summer, so if you remove the insulation your home's cooling cost will increase.

I think you should hire Samclem - he's probably do it for $15,000

Hmm. Let's see. My Pat Pend formula for estimating the time to do a home repair or improvement:
Estimate the time it will take ("E")
Double "E" and square the result
The answer (Actual Time to Complete Job, "A") is given in the next higher unit of time.

SO, to do the crawlspace the way I described earlier, I think it would take about a week. "E" = 1 week
(2E)^2 = 4. and weeks-->months. So, 4 months will be needed to actually do the work. That's includes all the normal trips to the ER, all the trips to the hardware store, all the unanticipated additional fix-it jobs that need to be done "while I'm right here" etc. Maybe $15K isn't enough . . .

 

[Dimsumkid]

If you're looking at $19k in costs, maybe you'd be better off considering a poured in concrete floor. I don't know the cost, but it seems that will correct many of the issues you're having now.

 

[sheehs1]

Travelover, SamClem and Dimusimkid...thanks for your replies. It is a Lot of moola. So much so that I could go with the other guys' quote of around 4,700 and do it almost 5 times! Huge margin for error I'd say. That said, I will check them out with the BBB.

This guy was good at what he does and he almost had me a couple of times during the 4 /12 hour inspection and sales pitch. But I don't make hasty decisions unless I'm sure.

The thing that confused me is that I specifically called them in for a "conditioned crawl space" and all he pitched was a semi conditioned one with dehumidifiers. He told me a conditioned space is exorbitantly expensive and he has had only 1 client insist on it. This was before he quoted me the almost $19K. Can't imagine what his version of the conditioned space would have been.

So...I need to think a bit. But am still thinking some good plastic, seal the walls, seal the vents, route some air from my own HVAC system is a good start.

SamClem...I still have questions about the vents necessary for a conditioned space.
Some say it is not as if my crawl has to be exactly like my house and that all I need is enough air down there to create a bit of positive pressure.
This guy today was telling me I'd probably have to replace my HVAC systems to heat and cool an additional 2,000 square feet of crawl.
My brother in law says the former...which is just enough to create a positive pressure.
City code has guidelines for conditioned air space which is 1 cubic foot per minute for each 50 square feet. What is the best way to make sure this is what I get and that it is done correctly?

 

[ERD50]

If you're looking at $19k in costs, maybe you'd be better off considering a poured in concrete floor. I don't know the cost, but it seems that will correct many of the issues you're having now.

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

 

[samclem]

SamClem...I still have questions about the vents necessary for a conditioned space.
Some say it is not as if my crawl has to be exactly like my house and that all I need is enough air down there to create a bit of positive pressure.
This guy today was telling me I'd probably have to replace my HVAC systems to heat and cool an additional 2,000 square feet of crawl.
My brother in law says the former...which is just enough to create a positive pressure.
City code has guidelines for conditioned air space which is 1 cubic foot per minute for each 50 square feet. What is the best way to make sure this is what I get and that it is done correctly?

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%.

 

[Dimsumkid]

This guy today was telling me I'd probably have to replace my HVAC systems to heat and cool an additional 2,000 square feet of crawl.

I think this guy is right, this is like adding a big 2000 sq. ft. addition onto your house. You'd have to consider all the additional costs with heating/cooling or new equipment this will add on. I guess I don't see the point of putting heat and AC down there, since this was meant for keeping people comfortable. The AC does 2 things - cools and dehumidifies, you only need the latter. I can see doing all the vapor barriers and insulation, but I'd prefer just dehumidification. I wonder why you can't just get two 50 - 60 pint units (or bigger) from the hardware stores that sell for around $200-300 each. Set them up in the space and route the water thru plastic tubing to a drain or outside? The ongoing costs are for dehumidifying only.

I run a 40 pt dehumidifier on auto control in my basement all year round to help reduce the moisture in the air.

 

[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.

 

[harley]

I think this guy is right, this is like adding a big 2000 sq. ft. addition onto your house. You'd have to consider all the additional costs with heating/cooling or new equipment this will add on. I guess I don't see the point of putting heat and AC down there, since this was meant for keeping people comfortable. The AC does 2 things - cools and dehumidifies, you only need the latter. I can see doing all the vapor barriers and insulation, but I'd prefer just dehumidification. I wonder why you can't just get two 50 - 60 pint units (or bigger) from the hardware stores that sell for around $200-300 each. Set them up in the space and route the water thru plastic tubing to a drain or outside? The ongoing costs are for dehumidifying only.

I run a 40 pt dehumidifier on auto control in my basement all year round to help reduce the moisture in the air.

It may be 2000 sq ft, but it's certainly not the same as adding a 2000 sq ft addition. I believe the OP said the crawlspace is 42" high. Most of my rooms are a little bit taller. And that's actually a really tall crawlspace. I wonder if he was measuring to the bottom of the stus or the top. If there's insulation between the studs the cubic footage could be even smaller.

 

[samclem]

It may be 2000 sq ft, but it's certainly not the same as adding a 2000 sq ft addition. I believe the OP said the crawlspace is 42" high. Most of my rooms are a little bit taller. And that's actually a really tall crawlspace. I wonder if he was measuring to the bottom of the stus or the top. If there's insulation between the studs the cubic footage could be even smaller.

Right, but even then, strictly speaking, the cubic feet of the house has nothing to do with the size of the AC unit needed. The size of the AC unit depends only on the amount of heat the house gains on the hottest days (and the amount of water that must be removed, but that's seldom actually calculated). For example, let's say his floor is 2000 sq feet and insulated to R13. That's where the heat is being transferred now. If his crawlspace walls are 1000 sq feet in area (for example) and that becomes the new building envelop, and if they are insulated to R13, then he'll be gaining far less heat than he was before, and the AC unit can be smaller. Plus, with the better vapor sealing his AC unit will be dealing with less moisture than it does today. (All this ignores the heat lost to the floor of the crawlspace, which tends to be very minor)

 

[travelover]

I'm starting to agree with Telly. The complaint is excessive condensation in the crawlspace ducts. For far less than $19K, you could have those ducts sealed tightly and insulated well, then throw a sheet of plastic over the dirt floor to minimize ground moisture entering the conditioned space.

 

[ERD50]

I'm starting to agree with Telly. The complaint is excessive condensation in the crawlspace ducts.

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

 

[Dimsumkid]

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

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.

 

[ERD50]

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]

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]

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]

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]

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]

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]

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]

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.

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.

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.

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.