Replies

1. 
   webted | Jul 28, 2009 01:56am | #1

   I was little surprised, but the building inspector suggested this on our current remodeling plan. We added ~800 sf of house sitting on a crawlspace. A 2' X 3' cutout connects the old (vented) crawlspace to the new crawlspace. The new crawlspace is connected via ANOTHER 2' X 3' cutout into a 400sf full stemwall crawlspace which sits under a covered deck.

   The BI said that they would be fine with our screening and covering the new cutout (between the two new crawlspaces), and meeting our 5.85 square feet of venting requirements by using this area (in this case, 6 square feet) for the venting calculation.

   Now, the caveats:

   1. I live in a mild Northern heating climate
   2. Who said the BI knows anything about venting science?
   3. I think foundation vents in this climate are BS, but I never would have signed off on this had we not installed a full perimeter drain, waterproofed the new foundation, and had no history of moisture infiltration in the existing crawlspace.

   Your addition is pretty small at ~250 sf. Using our King County Washington requirement, you would need 1.7 square feet of venting space, which would require three typical 0.75 sf vents. I don't remember CBU dimensions off the top of my head, but unless your main structure is "over ventilated", AND you wanted to vent based on the above numbers, you would be knocking out 3 concrete blocks.

   -t

   edit to remove typo

   
   

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   Edited 7/27/2009 7:33 pm by webted

1. 
   jr40 | Jul 28, 2009 03:22am | #2

   Thanks for the note WebTed.  Sounds like you're up there with my son.  We live in the Midwest where it has been much cooler then WA this year.  What kind of a job is it to knock out a concrete block or two or three.  Is there a screen (critter proof) on the market that will fit into such a space (s)?

1. 
   webted | Jul 29, 2009 09:14pm | #3

   Hi Jr:Yeah, we're cooking up real good here today - 100°F - it actually reminds me of my least favorite part of living in the midwest!The carpenter working on my house framed in the 2 X 3 access vent by building a frame out of 2X material and covering both faces with screen. Friction fit it into place with a few concrete anchors to keep it honest. You can caulk around the edges if you want a more finished look. Mine is under an enclosed porch, so it really didn't need to be pretty, just something to keep the mice out.Someone might make prebuilt screens sized to a CBU, maybe someone else here has some insight. I'd call a building supply house (NOT a big box/Home Despot/Lowes) and ask what they have. Building your own as described above is pretty easy.I have no experience with block foundations, so knocking out blocks is not something I'm comfortable telling someone else to do. Normally a guy might drill a couple of holes around the perimeter of the block with a hilti drill and then whack it with a sledge. The problems:1. You might crack adjacent blocks
   2. You might find some rebar, so now you need the sawzall
   3. You might be compromising the structural stability of the wall above. First, I'd call the building department and ask what the venting requirements are for crawlspaces in your area - no sense in putting in more than necessary, and then I'd see if you can find someone to do the work that has experience with block foundations - especially if you don't have a hilti drill and a sawzall.You can also look into cutting vents into the rim, although that can get more involved from a siding/finish point of view.-t

1. 
   jr40 | Jul 30, 2009 05:21am | #4

   Thanks for the added info.  AH!  Its 69deg again tonight.   Little cloudy today with upper 70's, but I miss the Cascades.  I called a block layer to see what he thinks.  I made some plans for double screened vents with 2x6 framing just in case he says go for it.  I will go with two vents unless the block man says three.