crawl space: to vent or not to vent?

[Lewis Ehrhardt]

I'm upgrading the moisture barrier in my crawl space area with 6 mil poly. I'm gettting mixed instructions.

Old school said we are to maintain good ventilation using vents.

New school says make sure the vapor barrier is sealed aroung the perimeter of the wall, all seams taped and NO vents.

1) Any of you done any research in this area? If so, what advice can you give? Relating to moisture problems, etc.
2) What sort of adhesive can glue poly seams together and last?
3) What type adhesive can I used around the wall perimeter?

Thanks for your input. L

[Dan Friedrichs]

I did a fair bit of research on this upon becoming the owner of a crawlspace for the first time. The vapor barrier in my house is not airtight-sealed, so I vent. However, it appears that the consensus is that if you can make the vapor barrier ~99% airtight, you don't need to vent. Sorry, but I didn't get far enough into the project to figure out how to glue everything tightly...

[Lee Schierer]

I'm definitely not an expert, but if your area is prone to radon, then you would want to vent the crawl space. When tehy did Radon mitigation at my son's house, they sealed the crawl space with rubber sheeting and hooked up a suction line to vent any gas to the outside, but then his crawl space is open to his basement.

[David G Baker]

Lewis,
You didn't let us know where you live. Venting may depend on where you live. When I lived in California the crawl spaces were vented. I now live in Michigan and have a basement under the original structure so no vents, but under the additions there are no vents. I am not positive but I think this may be the norm. In either case, insulation under the floor is a good idea and adding the 6 mil plastic over the ground is a very good idea as a moisture barrier and possible Radon barrier.
If I had closed cell foam under my additions I would consider venting the crawl spaces.

[John Keeton]

We have a crawl under the bedroom wing of the house, and a basement under the main portion. When we built 5 years ago, we used spray closed cell foam for insulation on the basement walls, and also sprayed the crawl concrete walls bringing the foam up on the rim and sill to totally seal and insulate the exterior wall. The floor of the addition is not insulated. However, we leave open the access from the utility area in the basement, and so the space is "conditioned." It has worked great.

I would add that I have not done any radon testing, and Lee makes a good point with that.

[Chris Kennedy]

We recently had our house insulated, and they recommended we make an insulated crawl. We didn't do it for unrelated reasons, but in the meantime, we developed a wood destroying fungus due to a lack of airflow despite being vented. There are heating/cooling ducts under the house that are blocking airflow from the original house to the addition.

Just something to be aware of.

Cheers,

Chris

[Brian Effinger]

I'm an architect, Lewis, and I can say without hesitation, you should insulate your crawlspace and vent it into the basement, turning it into conditioned space. Even if the crawlspace is under the entire house, you should insulate and not vent to the outside air.

[phil harold]

I'm an architect, Lewis, and I can say without hesitation, you should insulate your crawlspace and vent it into the basement, turning it into conditioned space. Even if the crawlspace is under the entire house, you should insulate and not vent to the outside air.

+1

I would fold the poly part way up the foundation wall
use expanding foam caulk around the sill plate
then install 2" blue foam over the foundation walls
no vent to the outside

more than 6 mil I would go with at least 10 or 20 mil

[Lewis Ehrhardt]

Thanks for the advice fellows. I live in South Carolina, where humidity is king! Should I insulate just the perimeter walls (brick) or under floor?

[Ted Calver]

Lewis,
Be sure to allow for termites when you insulate those walls. The guy who looked at insulating my crawl space said they would stop the insulation (foam) short of the sill so any termite tunnels would be visible. Sounded reasonable to me.

[Jim Becker]

This came up during our addition project last year. No ventilation was required if we spray-foamed the perimeter to minimize temperature change and resulting condensation. This was good because it would have been pretty much impossible to vent the foundation given 75+% of the first floor is below grade.

[Neal Clayton]

Thanks for the advice fellows. I live in South Carolina, where humidity is king! Should I insulate just the perimeter walls (brick) or under floor?

i think what they're getting at, and my understanding is, you don't have to vent to the outside but you have to vent somewhere. the moisture comes from the ground, not just the air. they are recommending ground cover, tied to the walls, that is then open to the basement which gets air exchanged by your forced air heat/cooling.

mine is vented to the outside (but it's from 1908, well before forced air).

[David Wong]

I also live in California, and in my area, the local building department required vented crawl spaces.

Brian in an earlier post gave a very key piece of advice that is worth repeating. If you seal and insulate the crawl space, you must turn the crawl space into a conditioned air space. This means venting to a conditioned space, either basement or living space.

In Joseph Lstiburek's excellent book "Builder's Guide to Mixed Climates", he discuss many construction details related to building science. Regarding crawl spaces, he states:

"Crawl spaces should not be vented. Crawl spaces should be constructed as mini basements. They should be part of the conditioned space of the house. That means that they should have a supply HVAC system duct."

There is a bit more in the book on this.

[Peter Stahl]

If you're not going to vent it then you need to keep it really dry. I wouldn't insulate if you get a lot of moisture in there. Needs to be kept dry and unless it's a closed cell sprayed on foam once the insulation gets damp it will trap moisture then droop down. I live in NJ and the county I live wants vents for crawl spaces, couldn't say if it's maditory as I'm not a builder.

[Peter Stahl]

We're thinking about using concrete countertops in our kitchen. If you know where I can get some instructions, the name of a web-site, or how-to book, I would appreciate it very much. We have a ranch style home and the black concrete counter tops would look good in our kitchen. If we can save enough over granite or quartz. Thanks Lewis

Lewis,

You should start a whole new thread for this, it's going to get lost here.