New home owner here - I’m working on sealing the furnace ducts that run through our crawlspace. Previous owner used silver duct tape and not the proper mastic or UL approves aluminum tape. Long story short - I realized there are ducts in our crawl that actually blow heated air into the crawlspace. We don’t have AC, just a furnace (located in Pacific Northwest).

Is this normal? None of the ducts have insulation either, that was going to be my next task but now I’m questioning if it needs to be done? No insulation under the flooring or on the concrete slab. (Single story ranch home - approx 1500sq ft with a 3.5 ft tall crawlspace)

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3 Answers 3


I have seen this done in our area to keep the pipes from freezing and the floors from being cold. On the homes that do this they usually plug the foundation vents for the winter. Many years ago electric heat was super cheap so this option was less expensive than insulating everything. Most places have gone back and insulated the floors and pipes then closed off the duct to save on heating cost

  • Keeping the pipes warm makes sense. It does seem like a pretty inefficient setup but it’s not that surprising - most of the things in this house are that way. I guess it’s just another thing to add to the list to improve on.
    – nstclair13
    Oct 6, 2018 at 2:32
  • Yes but power was very cheap because of all the power generated on the Columbia that's why Alcoa aluminum smelters were here until power started getting expensive. I have seen lots of houses with no insulation at all. Our highs and our lows don't last as long as other areas of the country usually only a few weeks at most close to 100 and 1 or 2 weeks in the teens where back east they may have 3 months for there extremes.
    – Ed Beal
    Oct 6, 2018 at 3:02

That looks like a black ABS water drain pipe (sewer pipe) right underneath the air duct. There's probably other water supply pipes under there too, right?

I'm assuming it gets below freezing in the winter, so if your crawl space had no heat the water pipes could freeze and burst if there were no heat under there. Even if it's just the sewer pipe, if it froze it could plug your sewer line, and water wouldn't drain (and it could burst too).

Insulating and sealing the pipe directly in that space wouldn't be necessary, since the end is open and you want to keep the whole space warm anyway.

Insulating the outside walls of the crawl space is an option, but you'd have to be careful to avoid creating any condensation that could lead to mould & water problems.

  • Yes, the photo is specifically of one of the open vents down there. You are correct in our plumbing is completely under there as wel (newly installed PEX). Is insulating the ductwork that vents up into our living space worth doing? And ignoring the ones that are venting into the crawl? In general, the air coming into the house never gets that hot even though we have a brand new, properly sized furnace. The non sealed ducts are obviously an issue which I’m fixing currently but wondering if I need to do anything additional.
    – nstclair13
    Oct 6, 2018 at 0:33
  • Apparently newer furnaces aren't supposed to get nearly as hot as the old ones - I remember old furnace vents used to get so hot you couldn't touch the metal vent for more than a second, so maybe "warm" is just how it's supposed to be. But if an outlet very close to the furnace (or at it, maybe you can feel the air at the humidifier opening?) is much warmer than a far room outlet... insulating the ducts would help some, or if a room's cold then re-balancing the air going to different ducts might help too (& maybe cheaper/easier?)
    – Xen2050
    Oct 6, 2018 at 0:42
  • I don’t think I should have the much air being discharged below. When I had a crawlspace the heat was just waste heat from the ducts being in the crawlspace. Enough nothing froze but cold. I suspect that got unhooked in a remodel and forgotten.
    – Tyson
    Oct 6, 2018 at 1:38
  • Hmm, It’s definetly not leftover from any renovations. It’s a 1992 house and we bought it from the original owners and it’s never been changed. There are three in the crawl in total, one in each end of the house so it looks like it was planned that way.
    – nstclair13
    Oct 6, 2018 at 2:13

If the level of the soil on the other side of the foundation perimeter walls is significantly higher than inside, and if the soil has a high moisture content, then this introduction of warm dry air into the crawl space might be necessary to keep it dry. I would definitely NOT shut off the air completely and if you reduce it to less than 1/3 of the current amount you should monitor the humidity and temperature in the crawl space.

It seems to me that the main problem with having significant air flow into the basement from the air handler is that the intake for the air handler is in the living space and so this amounts to a significant net pumping of air out of the living space. This would be especially the case if there is significant installed vent area to the outside from the crawl space. If there are no purpose built vents to the outside, then air pumped into the crawl space will cause exfiltration to the outside and back into the living space (the latter of which would be desirable, except for contaminants, if any).

So you could try to reduce to a minimum the airflow into the crawl space from the main air handler. Heat flow into the crawl space through the un-insulated floor of the heated living space above might keep the pipes from freezing. PEX is not destroyed by freezing but obviously it is not acceptable to allow it to freeze, especially since the couplings could be damaged by freezing. Insulating the PEX and the ABS drains could be done, but is not normally done and would interfere with detecting leaks.

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