PS: As for what to use when cutting-in crawlspace vents, I'm not sure if you can get away with just 'carefully bustin' a block out. You may have to rent a saw to cut the rough-out of the block. Do measure the block size and take it to a Home Center or Lumber Yard and they'll tell you what the 'rough size cut-out' should be for the vent. Then ask for something to secure it to your cut-out whether it be some type of adhesive around the 'lip' of the vent or concrete block screws. Those places will tell you if you need a saw for cutting too. One last thing: usually it's one vent per 250 sq. ft. for unheated spaces. Go on the 'high side'. So, 15 x 25 = 375, get 2 vents. Place them 'across' from each other in the crawlspace.