crawlspace vents

[bighammer]

We are currently framing on a crawlspace style basement, however, we are in iowa and the plan calls for foundation vents. If i were to insulate the walls with 2" styrofoam and insulate the floor system, could i get by without putting those damn vents in?? If it was my own, i know i wouldn't want the "iowa" winter wind blowing underneath my floor system. If i do this would i have to be worried about moisture?? Any good comments are welcome. This is the first crawlspace i have ever worked with, are you really saving any money???

[Rich]

Unvented crawlspaces are getting more popular and code approved in many areas. Here is the skinny on what is required for unvented crawlspaces
http://resourcecenter.pnl.gov/cocoon/morf/ResourceCenter/article//118

[bighammer]

Thank you for the reply - what do you think would be better...1. Insulate floor joists or 2. Insulate the concrete floor. If i insulated the floor haven't i created an attic type situation?? Would that require venting?? I am using 11 7/8" i-joists, 16" on center and could take 3/4" styrofoam between the joists and than fill the rest with fiberglass and cap with subfloor. Let me know what you think

[Richard A Hetzel]

If your floor system is properly insulated, why is it any different from Iowa winters blowing against your walls (which are insulated far less), or at your windows and doors (which are virtually uninsulated by comparison), or across your roof and in your attic?

The risks of an unvented crawl space far outweigh the possible gains. Conditioned crawlspaces are a great idea IF they can be properly sealed, and if the vapor retarder is properly installed. Those are two big IFs.

If you do a vented crawl space, there is no law against closing the vents in extreme cold or high cold winds. Just don't forget to open them again, unless you like dripping wet insulation.

Oh, and be careful about placing rigid polystyrene foam across the bottom of the joists, because you could well create at least a partial second vapor retarder, and unless you like mold and decay, that would not be advisable.

[bighammer]

Thank you Rich, you bring up so very good points, I am sure glad I have a an outlet like this to help us nail benders....

[bighammer]

I was thinking more about my current crawlspace. In a typical basement we will apply 1" "bead-board" to the concrete wall and than leave a min. 1" air gap before we frame up perimeter 2"x4" walls with fiberglass insulation.

I am not going to do any of this in a crawlspace, so it will have more "Iowa wind" causing temperature and moisture issues. I have the concrete crews coming back to pour the floor this week. The only vapor barrier I will have is going to be in my rigid foam in my floor system.

I just want to make sure that I am going to do this "right". I think if we put foam on the interior on the concrete walls (without vents) and insulate the floor system. What about piping and ductwork?? I will have insulated duct work under my joists. Have you ever had anyone run duct work thru the joist using some type of floor truss??

I just can't get to exicited about having the winter "iowa wind" blowing underneath by crawlspace. I realize you can clos them up in the winter, however, in the summer months are the vents enough to dehunidify the crawl space. In iowa i wonder if the outdoor air can lead to increased moisture levels.

[Richard A Hetzel]

Foam in the floor system won't be a good vapor retarder. You should use 6-mil polyethylene sheet under your concrete floor, with all joints and penetrations taped. If you install any kind of plastic foam on the inside of the crawl space walls, they must be covered with gypsum board...the foam cannot remain exposed.

The climate where I live is probably similar or worse than yours. My crawl space vents remain open all year, except for the two vents which are right near all the plumbing. If I leave those open, the pipes freeze in the worst cold, so I close only them. My floors are warm, and the insulation remains dry. I don't understand trying any other way...this is simple and it works, and uses no energy.

[Don_P]

Yes you can have problems in the summer if the crawl is below dew point, it is rare but I've seen it happen. More ventilation can usually warm it up above dew point. A fan hooked to a humidistat can provide mechanical ventilation. There is nothing wrong with a non vented crawl anymore than a basement if you think about it that way.

[Richard A Hetzel]

What's wrong with a non-vented crawl space is that it takes energy to condition it. There are a million properly-built ventilated crawl spaces that work just fine and use zero energy. There is no magic to it...just build it right and forget about it, because it will work forever.

[bighammer]

i will understand more once, this one get off ground level. I will get plastic under my concrete (black??) radon migation? and than install vents in the foundation. Thanks for your help. We start framing Thursday, I'll let you know how it goes.

[wag]

In temperature and humidity dynamic climates, you would be better off insulating and sealing the crawlspace from the outside conditions. But you must still let it breath a little, so do not vapor barrier the above floor. Use treated lumber/galvanized fasteners if you plan to be there long term, ...for a comfort level.

[Richard A Hetzel]

The above is faulty advice, so follow it at your own risk, especially if you like musty crawl space odors wafting up through your soaking wet insulation and floor. It violates almost every fundamental about crawl spaces.

[bighammer]

thanks for sharing your help Richard - I have drawn these plans for a basement, and the home owner would like a crawlspace, and never being around them, I have had some troubles understanding them, but thank you for your help....

[Don_P]

Here's more bathroom reading, thanks for getting me reading back over there Rich
http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/view?searchterm=basement insulation

[Rich]

A wealth of information there for sure.

[bighammer]

on this crawlspace, i got the floor system completed with the insulation , joists, and the vents installed

I used 15 foundation vents and was thinking as we installed tham, holy cows, the air is going to be whistling thru this , but whatever....

I actually took the advise from the forum here and my local lumberyard.....plus I am the general, so really it was whatever I wanted to do...=)

I insulated the sidewalls and used a gypsum based product on the sidewalls and than we set our floor system with a bead-board type pruduct attached to the underside of the i-joist than used that "gypsum"type product to cover that (all seams taped)and filled the cavity with fiberglass and than used my subfloor.

i got inspected this morning the genuis told me he would like to of seen rigid foam underneath the floor system to help as a barrier from moisture, if i would of done that, i could of just taped the seams and not worried about the gypsum.

any comments......

[Richard A Hetzel]

The genius is fill of prunes. If you create a "barrier against moisture" [B]under[B] your floor joists, you how have two vapor retarders, and rather than protecting against moisture, the foam on the underside will trap moisture within the joists, which will wet the insulation, which will be reduced to about R=0 in effectiveness.

You want the underside of the joists to be open, so that any moisture which collects in the insulation can escape, which is what the vents are there for. I worry about the bead board under the joists. I'm sure it will act as at least a partial second vapor retarder.

Insulating the sidewalls of the crawl space is a waste of money, because the crawl space will be the roughly same temperature as the outside air, as it should be.

Crawl space construction is very simple. Why make it so complicated?

[Don_P]

Yes, there is a wealth of information over there Rich...

[bighammer]

i agree that the sidewalls of the crawlspace might be overkill, however, if we insulate the crawlspace we have made an attic and can run regular duct and than we can also put mechanial equipment down there.(mechanical subcontractor requested that) i think it is proper to have something underneath the joist...it protects the bottom side of the joist from the damp enviorment. i will let you know in 5 years what it looks like, trust me i'll keep an eye on this one....i do value all your opinions and i think that it is awesome to have an outlet like this....thanks again

[Richard A Hetzel]

it is proper to have something underneath the joist...it protects the bottom side of the joist from the damp enviorment.
If whatever you put there does what you want it to do, you've got two vapor retarders and some possibly big problems. That insulation has to vent, that's why the vents are there.

You have received quite a bit of bad advice, from the building inspector genius and from the mechanical contractors. If they were planning to install equipment down in the crawl space, or even uninsulated ductwork, they should have advised you to build a "conditioned crawl space", without vents and with heat and mechanical ventilation.

[wag]

The above is faulty advice, so follow it at your own risk, especially if you like musty crawl space odors wafting up through your soaking wet insulation and floor. It violates almost every fundamental about crawl spaces.

Fundamentals change with the times, despite minds lagging behind. Why insulate the floor if you insulate the sides and lay down a the vapor barrier? What you want down there is stable temps and humidy. You tell me a better way to do that?

[Richard A Hetzel]

A better way to do that it not to have 15 vents. Another better way to do that is to conform to the building code, which requires that a crawl space either be vented, or "conditioned" meaning at least heated and mechanically ventilated. There is another method, which uses a small mechanical unit which in essence "steals" a tiny amount of conditioned air from the house and delivers it to the crawl space.

Fundamentals never change with the times. Methods of achieving fundamental correctness might.

[wag]

Codes change and vary, so their correlation to anything "fundamental" is dubious. Vents allow outside conditions to come in and out of the crawl space. If you have two vents, less will be let in, but less will also escape during warm, dry days. I don't see how the number of vents means much in the condensation equation. Sure, mechanical systems do it more intelligently, but that type of complexity will cost you initially and over time. My way is simple, just as good I believe, and keeps critters out. But as I said, use treated lumber and a durable subfloor (e.g., advantec or you can also go treated plywood) if you want the additional comfort level. But I would not put a vapor barrier in the flooring system. You're better off allowing it to breath some to prevent build up of moisture on one side or the other.

[Richard A Hetzel]

Your way is excellent if you like mold and structural decay. It will trap moisture in the crawl space and not let it out.

[wag]

The structure below would be only as moldy as the inside of the house. You can keep the inside of your house less moldy than the outside of it.

[Richard A Hetzel]

A sealed unventilated space without a vapor retarder between it and the house will trap moisture which is created in the house and will migrate to that space. When moisture collects, mold occurs, and so does structural decay. You're not thinking the issue out properly. And in any event, your way does not conform to current code.

I lived in a rental house where the owners had sealed off the crawl space vents. Someone went into the crawl space to inspect it, and when he came out, he showed me the shoulder of his jacket, which was soaking wet. He said he had only just touched the insulation, and was instantly soaked, and that the insulation was all dripping wet down there. There were joists and beams which had been previously rotted and were sistered. It's entirely predictable.

That's what your method will produce. The "fundamentals" don't come from code writers, they come from the laws of physics, which are beyond anyone's control. When someone comes along and changes the laws of physics, I may listen, but that someone isn't you, my friend.

[wag]

Then assuming you are correct, if you were to go ahead and seal the crawlspace anyway, would it then be better to vapor barrier the earth inside the crawlspace -- thereby sealing in the moisture generated by the house; or to not vapor barrier that earth, which would then allow the house moisture to seep down into the earth?

I just ordered a humidity gauge. There is a sealed, vapor-barriered crawlspace I know of that I will check. I will let you know what I find.

[Richard A Hetzel]

Moisture doesn't seep down into the earth, because the earth itself is a bigger source of moisture, so if you don't put vapor retarders where they belong, you will have moisture migrating up from the earth plus moisture migrating down from the house, and you will get the soaking wet insulation I promised you.

Don't you be telling us about fundamentals if you expect moisture to seep into the earth.

[wag]

The fundamentals are: moisture will seek equilibrium. If, as you said, the insulation was dripping wep, then the moisture barrier would be creating that situation. It would be impossible for the ground to yield upward that amount of moisture to create dripping wet insulation. Moisture sinks, does not rise, especially cold moisture. I actually question if your story is even true. But if it were, subsequently, ... if the house moisture could sink and be trapped in the sealed crawlspace, then a vapor barrier under the subfloor would be out of the question. Are you advocating open vents and no vapor barrier under the subfloor?

But in any event, the humidity gauge has been shipped, and I will let you know what I find.

[Richard A Hetzel]

I am advocating open vents, and vapor retarders both on the earth floor and also directly under the subfloor. And so is the rest of the educated world.

You want to keep vapor from migrating from either direction...up from the earth, and also down from the house. Vapor retarders retard the migration, they don't stop it completely. Some vapor will get through from either direction, and the vents serve to allow the vapors to pass to the outside air before it can collect in the crawl space and create mold and structural decay.

As far as your questioning whether my story is true, let me just say that the words I have in mind as a response to that are frowned upon on this forum. Too bad ignorance isn't equally frowned upon.

[wag]

How many old houses have you torn down for salvage materials? Let me tell you, you will never salvage a subfloor with a vapor barrier under it. I've pulled up a couple of them in funky, nasty pieces.

And my new humidity gauge told me that my enclosed, sealed crawlspace was 2 percent more dry than the living space above it. This was just after a wet, rainy day. I didn't want to drill that hole to test it, but at least I know I am right.

[Richard A Hetzel]

I haven't salvaged any, but over almost 50 years, we've taken floors down to the subfloor where there were vapor retarders below, and the subfloors were universally perfect unless there was a leak or something like that. Maybe the ones that needed to be salvaged were bad for some other reason. I've never seen a bad subfloor where there was a vapor retarder.

I have, however, lived in a rental house where the crawl space vents were sealed, and the insulation down there was just dripping with water, and there were rotted beams and joists which had been sistered, and mold all over the place. That's what a sealed unventilated unconditioned crawl space will get you, and without a vapor retarder, it would be even worse. Fundamentals. They never fail.

[handyman4life]

Dark and damp crawl spaces are ideal for spiders and snakes, but not so good for a house. This crawl space ventilator has built-in sensors that measure and monitor temperature and humidity under your house and outside, too.