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Making a new penetration in Sealed Crawlspace?? Help

ncgamedog's picture

I am working on a kitchen remodel that will require the addtion of a MUA system to accomodate a High Power Range Hood.

The best opportunity to add the outside air intake is through one of the crawlspace vent holes. The vents are no longer functional as the crawl has been sealed using a 20mil vapor barrier and closed cell sprayfoam on the inner walls.

The idea is to fix a duct to one of the old vents (16"x8") to feed the MUA. Unfortunately this will require cutting away the foam around the vent. I can break away the mortar holding in the vent grate from the outside to remove it. Then I will carefully try to cut a rectangular hole in the foam to fit the duct. I feel like I will need to seal the duct to the foundation wall somehow on the inside, but that would require me to remove a wider section of foam to access the masonary on the inner wall.

Either way, how can I reestablish the integrity of the seal around the duct once it is installed??

I also need to add a few penetrations for some electrical conduits (1.5" - 2") they would need resealing too.


Only Idea I have is to use a can of Great Stuff or one of the higher end Great Stuff type kits that uses a gun. Is there potential that Great Stuff, or the like, would react with the existing closed cell foam in a damaging way?

I do not have a way to take the MUA intake through the ceiling/roof for acess reasons over the kitchen (2nd story). The attic and roof deck is sealed with foam too, so the same challenge would apply.


Thanks for support!


There's a better way (post #213129, reply #1 of 3)

I assume you have access to within the crawl space.

Hire a concrete core dutting firm to drill a round hole in the stem wall, sized to match your (round) duct. Once under the house, use the appropriate transition piece to change over to a rectangular duct. Seal around the duct with ordinary caulking. Seal your duct joints with mastic- NOT tape.

Be sure there is a bug screen on the duct end. Thdere should also be some sort of shield to prevent leaves, and other debris, from blocking the opening. Wrap the duct with heavy plastic tape where it might be in contact with the concrete. Pitch the duct slightly, so it drains to the outside.

I appreciate the feedback. (post #213129, reply #2 of 3)

I appreciate the feedback. I'm not worried about making a hole in the foundation wall. The hole is already there (via the foundation vent) 

The masonry wall is not poured concrete anyway. It's just block w a brick veneer. I'd rather use the existing vent hole which is 16" x 8". This has an equivalent diameter of over 10", which will provide over 1000cfm. I will have my HVAC guy make a transition to fit the existing vent hole in the masonry wall and transition to 8" round duct. 


My question pertains to reestablishing the seal around the duct by the closed cell spray foam that currently lines the inside of the crawl space walls. Maybe I should have made it clear the sealed crawl space was a retrofit. When the house was built it was a standard vented crawl space. Recently the vents were covered inside and closed cell spray foam was applied to the walls. Reopening one of the vents to apply the duct will require me cutting out the foam, hence breaking the "seal". I want to be sure I don't compromise the sealed crawl. The crawl space is now part of the building envelope so I want to maintain the R value and moisture barrier.

Maybe I just need to contact the insulation company...

I think you're obsessing. (post #213129, reply #3 of 3)

I think you're obsessing.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville