Always done slab-on-grade in the thin-shell concrete domes. For the first time I’m designing one with a wood joist floor. (Client wants something more forgiving on old knees.) I’m gonna talk to the inspector, but wanted to get other insights, too. We’re insulating with the continuous polyurethane right to bottom of footer. And we have continuous concrete. While crawlspaces are normally vented from what I read, I’m wondering if that’s recommended in a case like this–if we should cut a hole into an airtight space. I’d anticipate 6 mil poly over the ground. I’ll be using Warmboard for the rfh. No hvac in crawl space, though there’d be plumbing.
Given that, vent or not vent the crawlspace?
Replies
Vent it, but to the INSIDE.
FHB had an article on in an issue that has since been "recalled", $153. [Check it out, in the back issues section it is missing. Don't know of any other reason than they recalled that issue.]
Also JLC had a very detailed article on this with in the last year. You can buy it for $5 from there web site.
Forgot about that article. Thanks. Just reread it and saw the detailing on sealing the liner. Interesting. The issue was recalled? Didn't know that could happen.
We'll have just a foot or so between the joists and the ground, so "crawl" is an exaggeration, I guess. Ever see any code issues with venting it to the inside?
" The issue was recalled? Didn't know that could happen."
I didn't either. But what else can explain the missing issue.
The "late" editor, AndyE could not explain it. (Conspiracy note is this the reason that he is gone). Nor could a couple of people from New Media (the people that do the web site).
The issue was recalled? Didn't know that could happen.
It can't. Don't know what the glitch is, but you can't recall a magazine.
You know, I don't work there anymore. Why do I still answer these questions? Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
"I don't work there anymore. Why do I still answer these questions? "
It's part of the Breaktime addiction. (-:Women and cats will do as they please, and men and dogs should relax and get Used to the idea. [Robert A. Heinlein]
>Why do I still answer these questions?
It's the power rush of being able to intimidate us with that hammer! How's the real world treating you?
Edited 3/16/2004 9:29 pm ET by Cloud Hidden
The real world's treating me good so far. I've taken some time off, but went back to work yesterday. Did 10 hours today. Got free pizza for lunch. How much better does it get?Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
>How much better does it get?
Well, was there beer?
No beer at lunch, but after work, the HO and I quaffed some sort Czech pilsner which wasn't bad. Although recently my favorite has been Smutty Nose Brown Dog Ale. But OP's always works.Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.
Cloud,
You are pressurizing the living quarters so a small out of the way vent will pressuize the crawl space.
Dirt is not an air barrior, it just slows down and filters air. If the crawl space is pressuized all the radon will be pushed down and out of the shell to come up outside.
Eventually all the soil under the shell will be dried out, relatively speaking, from the dry air pushed thru it, that would make the soil a better insulator.
Not so?
SamT
>If the crawl space is pressuized all the radon will be pushed down and out of the shell to come up outside.
So you're suggesting not putting a vapor barrier in the crawlspace?
Oops, sorry to take so long, this one slipped benath my radar.
>>>If the crawl space is pressuized all the radon will be pushed down and out of the >>>shell to come up outside.
>So you're suggesting not putting a vapor barrier in the crawlspace?
I just don't think it will be needed.
SamT
Cloud, please leave your self a minimun of 18" between dirt and joist in your crawlspace, (code in my area) Any beams, to carry floor joist, please dig out a nice large half circle under there location, all along the length of the beam. While your down there, have your electrician install a couple of lights. I have never understood, when people say "Nobody ever going be here" I've spent a good part of my life, either, in the attic or in the crawl. Jim J
Thanks for the suggestions. We should have enough room to create 18" without a problem. Site access stinks, so smoothing dirt out is easier than bringing in the required fill/gravel for a slab.
I think Bill Hartman is right. Sending air down there with the HVAC system seems to make the most sense to me. (With a return air vent, if the area is airtight)
Letting warm outside air into a cool environment seems wrong to me.
Just MHO.............
Soldiers usually win the battles and generals get the credit for them. [Napoleon Bonaparte]
Where is the building going to be and what kind of heat?
Be sure to read the crawl opinions at http://www.buildingscience.com/
Two issues to consider - excessive moisture and radon.
In my area a conditioned crawl works best - line the ground with viz-queen, insulate the exterior crawl walls (which you already have covered) and add a register or two from the distribution ducts into the crawl. This only works with forced air heat, of course.
And concerns may be different in different climates.
My opinion is that you should have vent a little to the outside - keep from getting positive pressure in the crawl which could force air from the crawl into the house itself.
I don't like the idea of putting a return in the crawl, myself. Even the best of crawls can get nasty air at times (and bugs!) and if not balanced properly, at least in theory you can increase radon levels in the crawl by creating a negative pressure zone there.
Radon levels vary of course, and are of differing levels of concern, but the same approach will typically minimize them.
_______________________
Tool Donations Sought
I'm matching tool donors to a church mission to Haiti - we're shipping a bus converted to a medical facility in (now it looks like) April and can fill it with clothes, tools and all sorts of stuff needed in that poorest of all countries. A few hand tools or power tools can provide a livelihood for an otherwise destitute family. Please email me if you have tools to donate.
Thanks to Jeff and David and Jim and Rich and Steven and Mark and Jason and Shep and Jen and Mike and Joe and Bill and Ken for their offers!
Several donations have arrived! Thanks and God bless!
Edited 3/16/2004 12:37 pm ET by Bob Walker
I'll check out Building Science. Heat is rfh since we'll be using Warmboard. No A/C. Will use an ERV/HRV for fresh air. I'd like very much to avoid putting holes in the shell below floor level for two reasons: a) we'd be taking an airtight/waterproof building and cutting holes in it that would make it non-airtight and would now require more insulation for the floor and more protection against air infiltration; and b) the owners wanna berm a bit and in their ideal, wouldn't have any of the foundation exposed enough to have vents. Wonder if there's a way to negatively pressurize the crawlspace (or positively pressurize the house, which is what's done in my house) with a register(s) so that the fresh air comes in the hrv/erv and goes out a vent in the crawlspace (berming around it) so that radon/foul air is pushed out...
Two houses actually...one in NC Great Smokies and one in Durango, CO.
There are some crawl space vents which are humidity activated. And there is a make-up air vent which is essentially no more than a barametric damper (sr. moment on the name --- ahh: skuttle vent - see http://www.skuttle.com/216.html) which might be pressed into service.
And I think you might wish to consult directly with the folks at Building Science.
_______________________
Tool Donations Sought
I'm matching tool donors to a church mission to Haiti - we're shipping a bus converted to a medical facility in (now it looks like) April and can fill it with clothes, tools and all sorts of stuff needed in that poorest of all countries. A few hand tools or power tools can provide a livelihood for an otherwise destitute family. Please email me if you have tools to donate.
Thanks to Jeff and David and Jim and Rich and Steven and Mark and Jason and Shep and Jen and Mike and Joe and Bill and Ken for their offers!
Several donations have arrived! Thanks and God bless!
Bob, I build in Virginia and crawl spaces are prominent here. I have done a number of conditioned crawl spaces and have stopped because of too many problems- often unexplained. One, in particular, was mostly below adjacent grade. I waterproofed to the top of the block in my brick/block foundation. We ran slotted drains inside and outside of the wall. We sealed the walls completely and added two supply vents with a heat exchanger to allow the crawl air to evacuate. I have sent people under the structure three times with 100% chlorox solutions to scrub the joists. I have added two dehumidifiers and I cannot get the RH below 70%. I have paid two engineers to go under the house and they offer no clue as to why we continue to have moisture. This Spring we have cut in vents and the moisture seems to have abated. Keep in mind that mold in a tight house can be toxic(I'm sure you've read the horror stories.
Other concerns, I don't like creating any space in a structure that cannot be inspected. We have termites here.
Here's a thought: use a radiant slab for your base. Place sleepers (probably treated to avoid dry-rot) on the slab and run your flooring on top of that. You'll have your flexibility and the price should be comparable. A wooden floor structure here is running something like $4.00 vs. $3.25 for the slab. You will also save on waterproofing ( necessary if your interior grade is above the exterior), foundation wall insulation, which should offset any fill for your foundation.
Good luck!
I have added two dehumidifiers and I cannot get the RH below 70%. I have paid two engineers to go under the house and they offer no clue as to why we continue to have moisture. This Spring we have cut in vents and the moisture seems to have abated.
Hi Bob, what part of Va? When you had 70% RH, what was the air temp? That is often the main problem. Venting will help in that situation. Dehumidifiers are picky about air temp, although if you can get them to work they will heat up the space.
The one time I added a dehumidifier to a crawl space it worked like a dream. But it wasn't a cold space. And got a whole lot warmer with the dehumidifier running, further decreasing the humidity. PAHS Designer/Builder- Bury it!
I live in Richmond, Virginia. Crawl space was 70 or so... conditioned crawl. I've done a half dozen with no problems but could never solve this one. If I don't understand it (and my engineers can't explain it, I prefer not to do it. Don't hope to do another.
Wonder if there's a way to negatively pressurize the crawlspace (or positively pressurize the house, which is what's done in my house) with a register(s) so that the fresh air comes in the hrv/erv and goes out a vent in the crawlspace (berming around it) so that radon/foul air is pushed out...
Not at all difficult. A tiny exhaust fan from the crawl space will work fine. To positively pressurize, all you need is a small differential in the heat exchanger fans, the way all houses should be built. You always want leaks to go out. In this case, also down into the crawl space.
For Durango shouldn't be any problem. For NC, you sure don't want to add humid air to the crawl space. Is there a whole house dehumidifier planned like you live with? Surely you are including something to control humidity. As long as the house isn't humid, no problem with pushing air from the house into the crawl space.
If there was a basement, rather than a crawl space, your clients presumably wouldn't mind including the space in their air system plans. A crawl space is the same issue. I'm sure you know that if you have frequent air changes, radon's a moot point. Humidity is another matter.PAHS Designer/Builder- Bury it!
Install the vents, close them off if the season, weather or moisture warrants.
"Install the vents, close them off if the season, weather or moisture warrants."
That's asking a lot of the homeowner. To do this, they would have to;
1. Understand the potential moisture problem.
2. Believe that the problem is important enough to warrant action.
3. Change the vents repeatedly, and conditions warrant it - That may be several times a week, as temperature and humidity change.
4. REMEMBER to do it.
I think you're asking the impossible. Positive venting with the HVAC system will "idiot-proof" it, and require no attention from the homeowners.There are two kinds of people, those who do the work and those who take the credit. Try to be in the first group; there is less competition there. [Indira Gandhi]