Sealing existing crawlspaces

We have a house with two 1970-ish wings which have dirt ventilated crawlspaces, and I'm wondering if I should seal and insulate the walls of the crawlspaces as per current science.

One is ~600 sq ft, 0 to 3' exposure. 1.5-2" of some sort of fiberboard nailed on perhaps 50-75% of the walls (not tight though). Floor insulation has been compromised by humidity and damage; probably 70-80% insulated. Carrier 2-stage air handler (2.5 ton) and first floor ductwork runs through it (R5.x solid plus R6 fle (short runs). Vents (closable mostly) plus an old fan unit (dead). Floor had been damp on uphill side, now space should be less damp (6mil poly recently installed).

The other is perhaps 700-800+ sq ft, similar construction. Older system (Trane 10 SEER) in the crawl; evil internally insulated hard duct plus longer flex runs. Very short return (floor). Ditto on floor, though this was the wettest by a long shot; we put in uphill-side drainage at the foundation bottom plus poly. wall -4 feet (ground 4ft above top of crawl) to 4 feet.

I could
(a) leave it alone, fix floor insulation (redo in first section, probably repair in 2nd.)
(b) seal and insulate crawls, plus dehumidify if needed in the summer. Perhaps remove (some) floor insulation.

Suggestions? Is it worth it to retrofit if the current floor insulation is iffy or in need of replacement?

What about termite inspections? Do I need to leave an inspection gap? Local insulation guy (generally good, does energy audits, etc) was kinda confused by the whole idea; they can seal/insulate (spray foam) the walls, but hadn't given any thought to termite inspections.

I am in the termite business. Be sure your termite company has 3-5 inches of visible clearance from ground to insulation should you insulate the crawl walls. Should you ever need another termite treatment, the hollow voids of the block wall will have to be drilled out (to spray inside).

The majority of moisture in your situation is probably coming from thr uncovered earth. Cover the earth with 6 mil poly and close the vents now, before it gets humid. make sure all under floor insulation is properly fitted.

read at BUILDINGSCIENCE.com --

We had 6 mil poly put down in both this fall, after some work to reduce intrusion on the uphill side (on the side that sometimes had water running through it after multi-inch downpours, we dug to the foundation and installed drainpipe and gravel; on the other side (which stayed much drier, just not dry) we just improved the surface drainage.

jared - 3-5" gap - thanks for the advice. As I said, the local energy audit/insulation guy basically said "sure, I can spray the crawlspace walls with foam".

I've read the buildingscience.com stuff before; that was where I got the idea from. It mostly targets new construction though; the suggestions aren't really geared towards what to do with an existing house.

Close the outside vents to stop high dew point outside air from infiltrating the crawlspace after covering all the exposed earth with overlapped plastic. Monitor the %RH with remote %RH meter. Radio Shack has a good remote monitor for fifty bucks. Keep the %RH <60%. During wet outside weather you may need 50 pints of supplemental dehumidification per 1,000 sq.ft. of basement/crawlspace. EPA Energy star web site list size and eff. Get the biggest best dehu you can afford like Santa Fe.

Thanks - I hadn't thought of remote-monitoring it. I have had plastic put down, and have closed the vents already. It sounds like it makes most sense from here to repair/replace the floor insulation (sealing as I go) and dehumidify instead of trying to retrofit to modern (buildingscience.com) conditioned crawlspace. I take you're suggesting not insulating and totally sealing the crawl then.

You say close the vents - both crawls have old non-functional exhaust fans on timers, probably 18" diameter. I assume I'll need to seal those and also seal around penetrations (HP lines, some electric, etc).

I have two crawlspaces (ok, 3, but the other one stays bone-dry year-round, and is combined with a basement area that's alerady dehumidified). One is 550ish sq ft, the other is probably closer to 800. Both also have the HP air handlers and primary ducting. (One older installation; one rigid duct running the length with internal insulation (yes, evil) with flex runs to the vents at the outside walls, the other a new installation (Carrier dual-stage) with R5.x insulated rigid feeding R6 flex in the crawl, and feeding hard ducts in interior walls to the second floor. Hard-ducted returns.

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Originally posted by jesup
Thanks - IIt sounds like it makes most sense from here to repair/replace the floor insulation (sealing as I go) and dehumidify instead of trying to retrofit to modern (buildingscience.com) conditioned crawlspace.

You say close the vents - both crawls have old non-functional exhaust fans on timers, probably 18" diameter. I assume I'll need to seal those and also seal around penetrations (HP lines, some electric, etc).

I have two crawlspaces (ok, 3, but the other one stays bone-dry year-round, and is combined with a basement area that's alerady dehumidified). One is 550ish sq ft, the other is probably closer to 800. Both also have the HP air handlers and primary ducting. (One older installation; one rigid duct running the length with internal insulation (yes, evil) with flex runs to the vents at the outside walls, the other a new installation (Carrier dual-stage) with R5.x insulated rigid feeding R6 flex in the crawl, and feeding hard ducts in interior walls to the second floor. Hard-ducted returns.

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Seal all the openings to the outside in all the crawlspace/basements after all exposed earth is covered. Even dry earth dumps a lot moisture in the space. This will change the overall humidity load on your home. Insulation above a sealed crawlspace agrevaites the humidity level between the insulation and subfloor because the subfloor is colder with the insulation. Lowering the temperature of subfloor 5^F raises the %RH 15%. Wood responds to %RH.

old bsmt have same characteristics & physicals as do new ones -- just have dirt against bsmt walls outside. speaking of which, best is to have insulation outside to keep cold away from masonry. try 2ft down, 2ft out, better would be below frostline.