My house has a small addition that was added by the previous owners. They ran the HVAC ducts through an old window and some of the block:

HVAC ducts through window

The rectangular section in the window should be easy enough to seal with some plywood, but how about around the ducts and the blocks? My first thought was wood and spray foam, but I have read that spray foam is a no-no near HVAC ducts.

I need to seal this in a way that I can still get access to the crawlspace as this is the only way in.

Two other jobs that I already know about and are related to this problem:

  1. The crawlspace walls and floor need to be sealed properly. This allows bad air and radon to enter the crawlspace area.
  2. The ducts in the crawlspace need to be cleaned, sealed with mastic and insulated. I'm working on this now.
  • Before you get too far, I'd make sure that the ledger over the window opening that they widened is adequately supported.
    – Comintern
    Nov 26, 2015 at 16:04
  • good point. I just checked and they did put a beam over the top of the window that is roughly 4x8"
    – Jay P
    Nov 26, 2015 at 16:23

2 Answers 2


assumptions from what's in your photo:

1) it looks like the right side of the opening is your access, and the ducts are rigidly installed.

2) the crawlspace looks unheated, as the ducts seem to be insulated, and the fact that you mention radon must mean you have an actively vented (fan) crawlspace to evacuate any radon gas (along with a radon alarm in the living space of the basement adjacent). I personally don't have any experience with radon tight building, but certain municipalities in Ontario have this as an issue, so its covered in the provincial building code.

if that's correct here's what you can do.

1) build an airtight, removeable access door and frame assembly to give you access. you can find this on youtube I am sure, or just copy the way an attic access works.

2) on the ductwork runs, build a bigger pipe around them so that you have a 1" gap between the heated duct and the slip pipe. you can wrap a piece of galvanized steel flashing around them, overlapping the ends by approx. 3". use galvanized steel, not aluminum, to prevent galvanic corrosion.

3) stuff fiberglass insulation into the gap between the pipes (not too compactly) and seal the ends with fiberglass cord sized to match the gap (1 in gap needs 1" cord.

4) tape the end of the pipe over with vapour barrier and adhere to the slip and the duct. now you have an airtight insulated joint that can expand and contract and won't cause corrosion on the duct pipe.

5) fill the gaps between the slip pipe and everything else with spray foam. fill the gaps from both the cold side and the warm to trap a dead air space between the foam fields. now you have a vapour proof insulation mass that will keep your heat in.

6) THIS IS VERY IMPORTANT. go through all the pipe insulation on the cold side and make sure that all the insulation is in good shape and that the vapour barrier is completely intact. remember, extra insulation on vent pipes does nothing but save you money and headache when it comes to crawlspace hvac ducting.

now you have a giant section, half access panel, half bulkhead, all of it airtight, insulated and radon-tight.



I'm sure it is a no-no to use the expanding foam over the ductwork insulation wrap or on flexible ductwork, but when the foam is in direct contact with the sheetmetal it should be just fine. Ultimately, you're looking for a solution that prevents the exchange of air between the two spaces, will not condensate, and will not compress the existing ductwork insulation. I can't think of any other product that meets these criteria besides building a form out of wood and filling the cavity with foam. You just need to make sure the insulation is pushed back behind your wood form. Once your foam has cured you can then add/replace the insulation right up to the foam.

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