We need to add ventilation holes to an existing building. The holes will be core-hole drilled through a limestone brick wall. The bricks have crevices. The contractor is acting stupid and obstructive or I would ask them. How would one mount a short (24 cm lond, D= 300mm) piece of round air duct into the core hole? Simply put in? Sikaflex all around? How about risk of moisture creeping in?

My first idea is to let the duct sick out 1 cm on both sides, and to grout the wedge with sikaflex. Criticisms? Better ideas?

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    $\begingroup$ You haven't told us anything useful. How about a blueprint showing what you are doing? $\endgroup$
    – Phil Sweet
    Commented Jun 7, 2019 at 9:40
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    $\begingroup$ A slope to the outside? $\endgroup$
    – Solar Mike
    Commented Jun 7, 2019 at 9:53
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    $\begingroup$ Sounds like you want the DIY stack exchange? $\endgroup$ Commented Jun 7, 2019 at 10:30
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    $\begingroup$ "The contractor is acting stupid and obstructive..." Double check that it's the contractor. Perhaps the contractor has years of experience and doesn't want their name attached to a failure. $\endgroup$
    – TimWescott
    Commented Jun 7, 2019 at 17:51
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    $\begingroup$ I will drill the hole 1/4" larger than the duct to get enough sealant uniformly around the duct to be effective. If you afraid the duct is too flexible or loose, add flanges on both faces. $\endgroup$
    – r13
    Commented Jun 26, 2021 at 21:18

1 Answer 1


In a project which was a mixed use 5 story building including a movie theater in LA in 1988 we needed similar set up.

For under-floor ventilation in the theater we created concrete subfloor channels with service access hatches and sloped to drainage with flex insulated double wall ducts running through opening to vertical registers on the riser of steps on the isles.

I have been there a couple of times since for minor structural changes required by other anchor tenants, but have not heard of any issues with floor ventilation system.

I don't think you need anything around your duct, makes it easy to replace if needed. Just make sure you have drainage for potential condensation.


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