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Insulating a crawl space in hot/humid Miami

I have searched the web for what seems like days about crawlspace insulation. I find conflicting information and really no information on my specific climate factors.
1950's house built on above grade crawlspace. Original crawl was vented on all 4 sides. 2 additions added in the 1980's are slab & are on the South, West & North sides of the house. That leaves me with 2 original vents on the East side (about 8 ft apart), 1 on the South & 1 on the West. There is no access from inside the house. The home had original oak floors that had significant termite damage & some tile on top. I removed all bad wood, replaced with plywood, layed tarpaper & went over with walnut. The crawlspace has no insulation. I did put down 8mm plastic sheets on crawlspace floor but have not made it airtight yet (not taped or attached to walls). I did it to tell if my new plumbing was leaking & also to make the space nicer for my workmen. They are less likely to leave debris behind when the space is clean. The ground is not wet but I can tell from digging that there is some moisture. The only pipes in the space are plumbing. All a/c pipes are in the attic.
During the summer months when the a/c is running I have serious cold air leaking into the crawlspace. When you walk past the 2 East facing vents it feels like I have an a/c duct right there. The air coming from those vents is freezing cold & can be felt 3 feet from the vent hole. During the winter I feel nothing when I walk by the vents. So, my assumption is that when I have a/c running I have ALOT of cold air settling into the crawlspace & then blasting out of the side of my house. So my goal is to keep the cold air IN my house.
Possible solution based on research:
Closed cell spray (for potential flooding issues) between floor joists. Only need spray for air leakage and not R value as crawl space is cool.My gut tells me to keep the crawlspace venitlated.
Concerns: Do I keep an airspace (1-2")between sub floor & sprayfoam like they recommend with batt insulation? Should I make the plastic ground cover airtight?
Thank you!!

Re: Insulating a crawl space in hot/humid Miami

You seem to have a leak problem somewhere. Try to locate the source of the cold air leak into the crawlspace.
What type of cooling system do you have? track the ducting to determine where your leak is. You need to fix the leak first and worry about insulation later.

Re: Insulating a crawl space in hot/humid Miami

Thanks for your reply. All of my ductwork is in the attic. We installed a whole new system & were able to really check it when the drywall was removed from the entire ceiling. I agree I have a leak but I'm thinking it's the entire house:eek:

Re: Insulating a crawl space in hot/humid Miami

Insulating a floor is different from insulating a ceiling. The reason for the air space in the floor is completely different than the reason you leave an air space between the batts and the underside of the roof for cathedral ceilings.

But in either case, if you are using closed cell foam, you do not need to have this air space.

For the amount of cold air coming out the east vents to be due to air leakage from the house, it would take either a lot of holes or some pretty big ones. The fact that it is so cold and projects so far from the foundation is a big concern.

Do you have a strong west to east prevailing wind there? Is this projection alway this strong and always from the east vents? You don't feel it from any other vents? Is this cold air projecting out like this when the AC is in an off cycle and the fan is not running?

For this to be only on one side of the house suggests that there is a strong wind blowing in that direction. For the house to be the source, you would expect the same projection from all the vents.

Another thing for the house to be the source would suggest that your AC unit either has no return duct, the return duct is plugged up (from a very dirty filter most likely), and/or, there is a huge gap/hole in the return duct between the filter and the coils. This gap could be inside the furnace where the coils are. If this were the case, then you are using outside air and pressurizing your house. It is also making your AC much less efficient.

I would start with an inspection of your return ducts, filter and the furnace housing to make sure everything is sealed up as it should be. This would reduce the pressure inside your house and blow less air into the crawl space.

You could go straight to the spray foam next, but it would hurt to have someone crawl around under there looking for where cold air might be blowing down into the crawl space. If there is a large hole, it would need to be addressed before spraying the closed cell foam, other wise the foam won't work. It is best to identify the root cause of the problem before applying solutions, better chance of success that way.

It could simply be that the floor is radiating cold into the crawl space, that cold air from the floor drops down and is carried outside by the prevailing wind. My only problem with this theory is the temperature and projection of the cold plume. If it is as bad as you say, that is really strong.

One last theory, and I hope for your sake that this is not true, but you could have an underground fissure that opens up under your house. That could blow a lot of cold air into the space and make a plume like you describe. This would be something you would want to know in advance as it could lead to a sinkhole.

Re: Insulating a crawl space in hot/humid Miami

Although I sure am many, many miles north of FLA. We sure have the same problems. You certainly have AC leaking from your ductwork, possibly more than one. I am presuming that you have duct work below the floor in the crawl space. I have the exact construction here. While I am dealing with heat loss when its 5 below as well as cooling loss in the 90 degree summers with humidity around 85 percent.
First things first, if you are feeling air flow something has to be driving that air mechanically, not just ambient wind, that would be your blower on the HVAC. You must find the leak(s). If what you describe, that you can actually feel cold air blowing, it almost sounds like an elbow, joint, etc. Has actually partially or fully separated. Find the leak, seal it. And while your under there insulate all of your ductwork. Several methods available. I use a pipewrap that you should be able to find at the big blue or orange store. If you can find the commercial grade that is your best bet.

As far as the closed cell isostyrene spray insulation, I highly recommend it. A gap is not only not necessary its almost impossible to achieve. This product creates the best thermal break and is designed to address the moisture issues all in one application. It becomes one with whatever void you are filling it with (monolithic application).

You are on the right track with laying down a vapour barrier on the sand in the crawlspace. You speak of 8mm vapour barrier, knowing a bit about metric conversion 8mm is almost 1/4 of an inch, man that's thick, what you probably have is 8 mil. Which is to mean 8 one thousands of an inch. We can use Supersix here, same moisture barrier for interior wall. I suggest the following installation method, we have been doing it here successfully for years.

1) Run the material 8" up the foundation wall all around the perimeter.

2) Overlap the material 8" as well, I don't tape the joints, weigh down the overlap with stones every 4' or so. I believe there is some science to this method. Sealing the vapor barrier tightly will result in moisture becoming liquid on the underside of the vapour barrier. With out some airflow that moisture will only build up over time exasperating the moisture issues rather than keeping it in check.

3) Those air vents are there for a reason. Rule of thumb is 1 ftsq. net vent space for every 200 ftsq. Of crawlspace area. It is recommended that the vents should be placed at each corner of the foundation wall to promote cross ventilation. If you have any number of builders and engineers sitting down at the same table you will get as many opinions as to when to have the vents open, and when, if at all to close them.
The purpose of circulating air in a crawlspace is to keep the dimensional lumber dry. Presuming the goal is to keep dry the lumber below the floor, sil plates, rim joists, floor joists, flooring, one would think these vents should be closed in the humid times during the year and open during the cooler dryer winter, I personally believe open all year long. It has worked on this property since 1945, no issues.

Hope this has helped, an I haven't digressed too much.

Re: Insulating a crawl space in hot/humid Miami

Good Thread, but I think we haven't hit the solution just yet. I have a house in a similar situation to OP and I have been doing some research. I am not an expert, but from my reading I believe vents in Miami need to be closed up nowadays.

Up north where outside air is less humid for more of the year venting may work. In the past when this house was non airconditioned (for about 20 years) or just cooled slightly by one window unit, that may have been the case as well. But nowadays these houses have central AC that can keep the interior down at 72-75. When outdoor air is so hot and humid so much of the year, it just doesn't seem to make sense to let that air in to dry the crawlspace.

I tried it the past few months this winter, and the crawl space still isn't drying up. So for the summer I will be closing all the vents. I may follow that up with laying plastic on the floor.

I hope to hear other opinions and more info.

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