venting to a crawlspace

[TNprogrammer]

I'm looking into sealing up my 3 foot crawlspace in order to improve the climate of my house. A couple of companies (such as Cleanspace) mention that sealing up the foundation vents and opening up an HVAC air and return line under the house is the best way, thus "conditioning" the space. My crawlspace is roughly 1200 sq. ft. How many vents would I need to open to the space and will this put undue stress on my downstairs unit? We did replace our downstairs unit 2 years ago and oversized it by 1/2 ton.

[pushkins]

How do I say this delicately .....? DON'T DO IT !!!!!!!!!!

Unless.....

Your crawl space has a complete from foundation wall to foundation wall concrete slab (that I doubt it has).

The argument of closing off crawl spaces has been going on for the last 10 years or so, and there are some SERIOUS issues to address before doing so.
IF your crawl space has a concrete slab from foundation to foundation AND the sill plate on your foundation is sealed AND you can guarantee a 100% sealing of the foundation vents AND you can guarantee 100% protection from ANY moisture in this crawl space ( just like the rest of your house) then go right ahead, as this crawl space would just be another room @ 1200 square feet only 3' high.
IF you cannot....then your in for a world of hurt in both the short and then long term.
1. I doubt your crawl space is covered with a concrete slab.
2. I doubt anyone can guarantee 100% no moisture in a crawl space.
3. Almost impossible to seal up existing foundation vents.
4. Complete sealed house building practices are only now slowly becoming standard practice. (Foam under sill plates etc...etc...)
5. Adding conditioned air to your crawl space (without the above measures) will cost you a fortune, as it will forever be trying to dry out ANY moisture in this area.
6. Adding vents to an otherwise not sealed system will encourage vermin like cockroaches, spiders... etc... and give them a free entry into your home, via ducts open to a crawl space.
7. the extra 1/2 ton won't even come close to doing 1200 sq. feet.

I'm having trouble understanding your comment..."improve the climate of my house" a properly insulated crawl space has NO bearing on the "climate" of a home.
It would be far cheaper to ensure your crawl space has at least R19 insulation.


Airflow as well as a 6mil plastic sheet over the open ground in a conventional crawl space is IMPERATIVE to the health and well being of the home, free flowing air helps prevent mold growth, free flowing air helps keep moisture levels below 19% anything higher encourages termites....the list goes on and on.

I just a month ago finished building a huge addition for a client that wanted a "sealed crawl space", to pass code alone added an extra $12,000 to the cost of the home, inspectors wanted a full concrete slab and footing (in one piece) with the foundation wall built on top of the slab. ALL sills had to be sealed with foam sill plate sealer. Basically I built a basement just 3' tall.
The 2500 sq. foot addition required a 3 ton A/C unit to condition the addition as well as the crawl.
A good estimate for the A/C unit requirements for the crawl space is to simply multiply the additional sq. foot by 33% (rougthly a third of the standard living area. In your case 1/2 ton won't do the job.

[TNprogrammer]

Pushkins,
Thanks for the reply. You've just confirmed what my gut was telling me. The real problem I have is that the plastic sheeting that is under the house is in bad shape. It has been moved all over the place and quite a bit of dirt is exposed. The builder went with a loose spray on layer of insualtion along the cinder block. My original plan was to just peel off the crappy insulation, put 4" foam board all along the cinder block, then put down new 6 mil plastic to cover the dirt floor under the house. I was going to attach the plastic to the cinder block starting about 4" down from the sill plate, drape it over the foam board and then cover the ground. The house never gets standing water underneath. The lot is actually very sloped, so 3 feet in height is an average. It is really about 5.5 feet at the front of the house and about 2.5 at the back.
The whole idea of sealing the crawlspace didn't really enter my head until I started reading about what some of the "space containment " companies were doing. You see any problems with my original plan? Thanks again.

[EDIT] One thing I meant to mention:
There is no insulation between my first floor joists. They are completely open to the subfloor.

[TNprogrammer]

OK...here are some pics. I'm really wondering if that insulation the builder put on the walls is good to have under the house. It just seems like it would get damp and moldy. It seems to be in fine shape for now, though. Would it be best to rip it off and glue on some foam board, or just leave what is on there? I think I'll just but some more rolls of 6mil, clean up the space and put down the new plastic. That seems like the best alternative for now. What do you think?