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If the area you are in is crawl space construction consider replacing your customer foundation vents with "temp-vents". They are automatic foundation vents that operate on a metal coil like the thermostat in your car. The vents fully open automatically at 70 degrees and close at 40 degrees. Easy to install, they fit standard vent openings. Excellent markup, they cost about $25 each and you typically charge $125 each to install. Usually takes about 20 minutes to install each one. Typical house has 8 vents. I use to put a ton of these things in when I lived in VA. Most crawl space homes also need a vapor barrier installed over the dirt. Poly for the job runs less than $100, takes about an hour, I use to get around $450 for them

Here is a good idea for inside work in the winter months, I might be looking into this myself,  hope this will help!