Hello. I’m helping a friend get his house reasonably efficient and I need some advise as I’m outside of my climate zone. I’m used to Northern Ontario building but I’m helping my friend in MO. 639xx. I think he is climate zone 4.

We want to seal the house up, add insulation and close the vented crawlspace. The house has had a few additions which complicates things in the attic and the crawlspace.

Most of it has a crawlspace with 24-36″ of clearance but there is a section (15% of the home) where they converted a carport into living space. In this area, the floor joists appear to be 1-2 inches above a slab but I am not 100% sure about this. The only access to this area is a single missing CMU that has a 6″ insulated flex duct running through it. I’m planning on just ignoring this area of the crawlspace as I don’t think it’s possible to do anything with it.

The rest of the crawlspace has a dirt floor. It had faced fiberglass batts hung from the ceiling but many had fallen down. I’ve removed the rest of them. There is a fair bit of mold where the batts where still intact. I also found some termites so that has become the highest priority. Once the termites are handled, we plan to lay 6 mil poly over the dirt floor. They never backfilled the footing so there is a trough around the house that resembles a crude interior french drain. There is very little risk of flooding here due to the elevation so I was thinking of raking the “drain” full with the existing dirt under the house. I’m not sure if I’m missing out on significant insulating opportunities in doing so. The trough is about 6-8″ deep in most places.

All the supply ducts are in the crawlspace. I plan to seal them with mastic.
Then I plan to run poly up to the mud plate and tape/seal it to the block or wood with the product that you guys recommend. If it’s better to leave 3″ of bare block for termites I’ll do that instead.
Then I plan to run 4 inches of foil faced poly iso from the ground level to within 3″ of the top of the top course of blocks.
I’m unsure if I should bend some flashing to make a ledge that the termites would have to crawl over or not. I haven’t been able to determine if it is a barrier or if it just facilitates identifying existing termite infestations.
The vents will be sealed shut, either with the poly and poly iso, or maybe with concrete.
I plan to insulate the rim joist with 4-6″ of foil faced poly iso sealing the perimeter with canned foam.
The access door will be upgraded to be sealed and insulated.
I will continue to monitor the humidity levels and will add a dehumidifier if needed. Currently it’s been 70F@ 50% RH while the living space has been at 70F@ 70% RH. I need to swap the meters around to verify accuracy. There is an electric water heater in the crawlspace.

In the attic, I’m planning on pulling up the existing 4″ of insulation and air sealing the attic floor. I’m then planning to cobble in 4-6″ of foil faced poly iso with taped or spray foamed seams then putting the existing 4″ of batts/blown in back on top. I’m hoping that creating an air barrier with the poly iso in this application is not critical.
All the return ducts are in the attic. I am planning on sealing them, and then covering the trunk line with foil faced poly iso and then burying the flex lines with blown in insulation. The trunk line is internally insulated, the flex lines look to be R4 or R6.
If I’m understanding ducts in an attic correctly, I need to be mindful of the dew point. It appears that the dew point here is around 70F in the summer. Since the ducts are already insulated (R4 maybe?) and they are only return ducts, I think I’m reasonably safe burying them. Am I understanding this issue correctly?

Any flaws or areas that I can improve in my plan?

I’m going with foil faced poly iso because I found a place that supplies 4’x4’x8′ loads of it for $50. It’s a mixed bag of thicknesses and almost all of it is damaged (cracks, missing corners etc).

Thanks for any input you can offer.