[RodFarlee]

On crawlspace ventilation:
- current building codes (since about 1980) require R-18 insulation under the floors. If your home has this, then crawlspace ventilation should have negligible effect on heat loss.
- current building codes require 1.5 sq ft vent area per 100 sq ft floor area to prevent moisture buildup and damage.
- current codes also require a vapor barrier on the ground in the crawlspace area. Since 1980, this is typically 6 mil or thicker plastic, optionally covered with a couple inches of sand to hold it down and protect it. Before then, since 1960 or so, it was a couple inches of cement poured and roughly leveled in the crawlspace after the foundation was built. Before 1950-60, local codes had no vapor barrier requirement.

If the crawlspace area is really dry (has vapor barrier, or is up on a hill or raised and has excellent drainage), then you can block most of the vents without having a problem. But if it's moist, blocking the vents could promote rot. You have to be the judge of this.

You might consider insulating the floors... spray in foam is easiest, fiberglass batt is less expensive but a hassle in older homes with X crossbracing between the floor joists. Either isn't a bad do-it-yourself job, as long as your home has a decently deep crawlspace (more than 2 feet).

First thing to inspect is openings from the crawlspace into the heated area, particularly around bath/kitchen plumbing. Large gaps here can create quite a draft in cold weather. Easy to seal with spray-in foam. Also inspect where wiring goes up from crawlspace into walls - seal any gaps with foam. (In new construction, code now requires these gaps be sealed.)