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Air sealing/insulating after fresh air intake removal

irene3 | Posted in General Questions on

As some of you may remember from previous discussions, we had a ducted heat pump put in last summer, replacing a gas furnace and making use of the old ductwork. Today the installers came back and took out the fresh air inlet that went from an outside wall to the return duct, on the theory that it was no longer needed and was just making the house more difficult to heat.

They left the outside bit of the inlet in place and capped it off on the inside. I’m assuming this leaves a thermal bridge as well as actual air gaps around the pipe. What’s the best way to seal up/insulate this area? I’m not specially keen on paying to redo any siding, and I don’t mind the look of the vent on the outside (and the inside look doesn’t matter much, as the basement is unfinished — though I think I’d rather not have just a big blob of Great Stuff on the wall).

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  1. Expert Member
    BILL WICHERS | | #1

    Can you provide a pic? My first thought would be to cut a rough circle of some rigid foam like polyiso and then seal that in place in the hole with some great stuff. You may be able to put a plywood backer in there for some structure if you want to make sure critters can’t chew their way through. It’s really hard to be very specific without actually seeing the hole though.


  2. irene3 | | #2

    I probably expressed myself badly. The intake itself is still there but capped over, as if something like the bottom of a paint can were sticking out of the inside wall. (It's something like the one pictured here: [EDIT changing URL to better example]: ) They took away the duct going from the intake to the return duct. So the hole in the wall is mostly plugged up by the capped intake, but there's some gap around it.

  3. GBA Editor
    Martin Holladay | | #3

    I would remove the plastic termination and insert a circular piece of rigid foam, plywood, or OSB in the round opening -- the material choice would depend on the outermost layer of your wall assembly. You can shove a couple of handfuls of fiberglass insulation in the duct if necessary to provide something to keep the circle in place as you secure the circle with canned spray foam or caulk.

    How you finish it depends on your aesthetic sense. I suppose you could just cover it with a nice rectangle of cedar or decorative hardwood (ideally with Z-flashing on top) if you don't want to patch the siding.

    That fresh air intake was a component of your ventilation system. Your ventilation system may have been poorly designed and poorly commissioned, but at least it was a ventilation system. By capping the intake, you no longer have a ventilation system (I'm guessing). That's food for thought.

  4. irene3 | | #4

    I was going by advice such as this:
    "For old buildings, creating the kind of tight construction that would make installing a mechanical ventilation system worthwhile is probably difficult and economically risky. If you live in an old building, the first question to ask is whether a whole-house ventilation system is needed at all. Is your house displaying any symptoms? What are your main comfort concerns? Have you conducted a radon test? If you have limited complaints, it would be most cost-effective to simply take the “first steps”: control sources, exhaust local sources, and keep your filters clean and your ducts tight."

    The older part of the house dates from 1901, and though various updates have been made, including my doing a bunch of air sealing and insulation in the basement, I'm sure it's far from tight. We do have a timer that runs the bathroom fans automatically now and then. (Oh, and I just tried the square of toilet paper test, which they pass, in the course of which I also discovered that the kitchen stepstool has lost a fastening nut. Fortunately I didn't fall down.)

    Just went and looked at the outside of the fresh air intake, which I could have sworn was louvered, and it's not, it's this kind with a hood over it (I'm going to edit the other reply): There is a screen over the mouth of the intake inside the hood. What if I were to spray foam through the screen and/or fill up the hood (which wouldn't show on the outside, which is already painted to match the house, and not conspicuous)?

  5. Expert Member
    Peter Yost | | #5

    Hi Irene -

    I would strongly suggest that you get someone else to take a look at the configuration of your system, what the function of this duct was originally, why it is no longer needed.

    It is true that older leakier homes are less in need of whole house ventilation systems than newer, tighter homes. But the reasoning you state for killing this duct--"it was no longer needed and was just making the house more difficult to heat"--has my radar up.


  6. irene3 | | #6

    Well, if I turn out to be wrong, the duct should be easy to replace, especially if I don't fix the hole in the house. I'm not too worried there. I was told it was necessary for a gas furnace (in addition to the combustion air, I realize that's separate), but not for a heat pump. I think it was also completely uncontrolled (just an open duct between the outside and the return, no baffle or damper), but am not positive about that. Also the outside vent is near the heat pump, so the air around there during the heating season is probably even colder than it would otherwise be.

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