I found my situation to be somewhat similar to an old post that I found titled “Why does my house SUCK” posted by Cole Sumner in mechanicals on September 18, 2016.

I have an old home, built around 1881. The home is located in New Orleans, within the City limits, and is not along the bank of any bayou, lake, river, etc.

The home is on brick piers and the bottoms of the floor joists are about 20″ – 24″ off of the dirt under the house.

Dirt exists under 100% of the house. There is no slab, no rock, just dirt, without a vapor barrier.

As the home is on piers, the right, left, and rear sides are open between the piers and the front of the home has a solid brick “wall” across the underside of the front porch.

At times, the interior smells musty, what I describe as a mold or mildew smell. Mold / mildew has not been noticed on the floor joists or the underside of the wood floor.

The home has two central air and heating units with natural gas heat. All ducts are in the attic and, as such, all air conditioning / heating registers, as well as return air grills, are in the ceiling. The only “equipment” under the house is a limited amount of new wiring, some natural gas lines, and some water lines. There is about 70′ of 1″ cold supply water line under the house which I have seen sweating, as it is not insulated. I have not seen condensation on the smaller 3/4″ cold water lines or on the limited amount of hot water piping under the house, but suspect it occurs.. I just have not seen it. The water lines are strapped to the bottom of the joists, rather than being routed through the floor joists.

The floor of the home is original tongue and groove pine, installed when the home was constructed in or near 1881. There is no subfloor. The pine floor was installed directly over, and obviously perpendicular to the wood floor joists, which are 2 x 10 or 2 x 12. The wood floor has a water-based polyurethane finish on the top surface.

At some point in the past, the owner had what appears to be cellulose blown under the house, between the floor joists. This was removed by that Owner after Hurricane Katrina in 2005, although a small amount still exists as a light “skin” in places on the underside of the wood floor. The home did not flood with Katrina, but the water reached to within a couple of inches of the bottoms of the floor joists.

The crawl space under the home is damp and I have placed some sensors both in and under the house which give me Temp, DP, Air Pressure, and Humidity readings. It is not uncommon to have the humidity level under the house over 90%.

Recent readings under the house (all temps in F) taken January 2, 2022…much cooler here than in the summer when it is in the 90’s with very high RH and the two central air units running a lot:

Temp 65 (actual 64.75)
DP 64 (actual 63.99)
RH 97

Readings in the house, both directly at floor level and at standard table height). These were taken at same time as the readings above:

Temp 69
DP 55
RH 61

I don’t have readings available for conditions “outside” only those for under the house and within the house.

I suspect the close temperature and DP readings under the house are a big part of the problem coupled with the central air and heat pulling, I assume, that moist likely “dirty” air from the crawl space under the house, through the old floor.

My research suggests that the best solution is to add between 1″ and 2″ of foil-faced polyisocyanurate board to the underside of the floor joists, foil facing down, with all joints taped, while also ensuring there are no air leaks around sill plates, pipes, etc.

Questions:

Am I on the right path with this line of reasoning?

Is the a/c pulling “bad” air from under the house?

Are the RH levels under the house, coupled with the close Temp and DP numbers the source of or contributing to the problem (the mold / mildew smell in the house, on occasion)?

Should the polyiso under the floor joists, if installed properly, correct the problem?

Would a vapor barrier over the dirt in the crawl space be recommended in addition to the polyiso?

Other notes:

Some water from blowing rains gets under the house, along the perimeter of the house, but this doesn’t seem to run or pool under the house, although the soil under the house stays damp for a long time, especially in the summer when it rains a lot.

This is a one-story, wood-framed home (old balloon framing) with plaster interior walls. The walls have been insulated with fiberglass batts, and wood blocking was installed above the sills to prevent airflow through the wall framing and to prevent rats and other critters from getting into the walls. The exterior of the walls has 1/2″ plywood over the joists, building paper or similar and cementitious siding.

If I run two Home depot / Lowe’s type inexpensive dehumidifiers inside of the house, constantly, the problem is not noticeable. Wood floors are not cupping and have not cupped, although one could argue that they aren’t as flat as they were the day after the last coat of water-based urethane was installed. House is 2,600 square feet, all wood floors, except carpet over padding and plywood in one bedroom where the old wood was removed to patch other areas.

Typically, the RH in the house is between 48% and 60%, but rises over 60% especially in summer if the dehumidifiers aren’t emptied to keep them running.  Running the dehumidifiers over the wood floor with no subfloor and no insulation seems as though I am trying to dehumidify the City…it helps, but seems to be a fruitless effort without under floor insulation.

Any advice / input will be greatly appreciated.

Thanks.