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In the PNW/Zone 4 climate. The house I am purchasing built in 1942 with an addition added had an inspection performed and it was noted that some of the fiberglass insulation installed under the subfloor between the joists was falling down and/or showing signs of deterioration and/or missing in spots with the sheet vapor barrier missing in a few spots as well and no insulation/wrap on any of the ducting. No A/C just 80% gas furnace in the house with floor vents.

Researching adding insulation options, it seems like the new thought is that vented crawlspaces were done wrong and the new frame of thinking is to encapsulated the crawlspace into the building envelope/conditioned space. I plan to DIY this but a couple of things I had questions on:

Should I remove all the old batting and sheeting if the material is dry if I plan on encapsulating and insulating the walls of the crawlspace and close off the vents?

Mold does not appear to be a significant issue but would fogging the space while its still unoccupied be a good idea?

Is a vapor barrier with 10mil sheeting be sufficient and can I lay this on top of the old sheeting (basically addition underlayment) and up the walls? Is insulation necessary on the ground?

Once the space is encapsulated and insulated does the ducting also need to be insulated? I plan on beefing up the sealing with foil tape and mastic but do I need to wrap as well or would I be better off beefing up the wall/floor insulation?

Once this is all done in the next 5-years my furnace will likely need to be replaced and I would like to move it to the crawlspace with a 90% unit to free up space in the small house would that change much if any of the steps I outlined above in anticipation of this upgrade?

  • You need to ask one question at a time not 4+. I also live in the PNW and have had 4 homes with similar construction and 2 with slab / daylight basements. You can spend a fortune and not have any better insulation than I have in my 1930 farm house with fiberglass batts in the floors and plastic on the ground. No mold , but I have done extensive mold remediation on homes that tried to seal things with plastic. I do both electrical and HVAC and I would not put a furnace in a craw space unless I can stand up in it. – Ed Beal Apr 17 at 1:57
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    I’m voting to close this question because 4 questions is not in our format. – Ed Beal Apr 17 at 1:58
  • And I live in the Mid-Atlantic and have had to have all of the under-the-floor fiberglass batts removed prior to moving in because they were falling down from accumulated moisture. I don't any modern crawlspace insulation techniques that recommend fiberglass batts under the floors. – SteveSh Apr 17 at 12:11
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Should I remove all the old batting and sheeting if the material is dry if I plan on encapsulating and insulating the walls of the crawlspace and close off the vents?

Yes. Any and all fiberglass bat insulation should be removed.

Mold does not appear to be a significant issue but would fogging the space while its still unoccupied be a good idea?

I would have the space inspected by a qualified mold remediation specialist.

Is a vapor barrier with 10mil sheeting be sufficient and can I lay this on top of the old sheeting (basically addition underlayment) and up the walls?

10 mil vapor barrier is fine. As to whether you can lay it on top of the existing vapor barrier, opinions differ. I think that's OK (in fact am thinking about taking that approach myself), but only if there is no possibility of water getting into the crawlspace (pipe leak, from outside, up from the ground) that could then get get between the layers of plastic and just sit there. If you can't ensure that going forward, then you need to remove the existing vapor barrier.

Is insulation necessary on the ground?

No, though I have seen where some installers do that.

Once the space is encapsulated and insulated does the ducting also need to be insulated?

Yes, so as to reduce the amount of heat loss from the ducts into a 50 deg crawlspace.

I plan on beefing up the sealing with foil tape and mastic but do I need to wrap as well or would I be better off beefing up the wall/floor insulation?

The crawlspace walls should be insulated with foam board and a vapor barrier installed. No insulation under the floor.

...if any of the steps I outlined above in anticipation of this upgrade?

You would need to make sure a source of fresh air is provided to the furnace. And, of course, need to vent the furnace exhaust to the outside.

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  • My understanding is that the crawlspace should end up at the same temperature as my living space, and some even suggest adding a duct to feed supply air into the crawlspace so that was my thought on why I wouldn't need to insulate the ducting as well, and that instead I could add more foam board to the walls and joist space to better insulate the crawlspace area to act at the insulation layer compared to insulating the ducting itself. – redlude97 Apr 16 at 23:53
  • The purpose of providing conditioned air into an otherwise sealed crawlspace is to provide a slow amount of air replacement, not to heat or cool the crawlspace to normal house temperature. – SteveSh Apr 17 at 12:08

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