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Community and Q&A

Insulating Floor Joists Over Exposed Ground

dabinco | Posted in GBA Pro Help on

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.


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.


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  1. Expert Member
  2. dabinco | | #2

    DCContrarian: if I followed the article correctly, spray foam between the joists, but not encapsulating the joists, along with an open crawlspace with piers supporting the house may work; however, as I have no sub-floor, unless I install some sort of separation sheet or similar product under the floor (between the joists), I will risk having spray foam expand through the T&G and/or any places where the T&G may be broken or where holes exist.

    The polyiso under the joists seems to be the "winner winner, chicken dinner" with the hybrid system of the polyiso beneath the joists and fiberglass batts between the joists with an airspace between the underside of the floor and the batts, being the runner-up, or maybe preferred method.

    I have read, but do not know for sure, that the air space may be contrary to current, or most recently published building codes, and that the insulation should touch against the bottom of the floor.

    Do I seem to be interpreting correctly, the article that you sent?

  3. Expert Member
    NICK KEENAN | | #3

    Sounds right. Lstiburek seems to prefer the polyiso under the joists. How practical that is depends a lot on how "busy" the underside is -- if the framing is complicated or there are lots of mechanicals it can be hard to get a continuous layer of foam, and that technique really depends on a continuous layer. Spray foam has the advantage that it doesn't care how complicated the surface is. You can staple up rosin paper to keep the foam out of the cracks. How complicated that is depends on how busy the underside is too...

    I seem to recall another Lstiburek article where he mentioned the requirement that insulation be against the bottom of the floor, railed against it, and said he had lobbied to get the requirement removed from the model code. I don't know if he was successful.

  4. dabinco | | #4

    DCContrarian....I failed to say "thank you" in my first response. Thank you and I appreciate your input.

    Rosin paper is an excellent idea. Any my underside is not bad, when comparing to most that I have fat figure has been under the house many times and can get under there to do this if I go the rigid polyiso route. I'm a bit of a fanatic about neat work, so I know it will be done properly if I do it....just need to start!

  5. dabinco | | #5
  6. dabinco | | #6

    If I utilized the approach with both the rigid polyisocyanurate board along the bottoms of the joists and completely sealed the joints, penetrations, etc. AND also used fiberglass batts between the joists:

    1) Should the fiberglass batts be faced or unfaced?...or does it matter if the polyiso is properly installed (meticulously sealed)?

    2) Could I completely fill, without creating undesirable issues, the cavities between the joists with the fiberglass batts (I would need R-30 batts which is overkill for the New Orleans area).

    3) If I used only R-13 or R-19, and therefore did not completely fill the cavities between the joists, and whether I left an air gap between the batt and the bottom of the floor, or between the batt and the rigid polyiso under the joists, is there a consensus as to facer placement (assuming faced batts are used)? I'm assuming the facer would be facing the floor rather than downward towards the polyiso board.

  7. GBA Editor
    Kiley Jacques | | #7

    Take a look at these two articles: Insulating an Exposed Floor and How to Insulate a Cold Floor. The most relevant-to-your-situation bit of information I found: It’s always a good idea to install a continuous layer of rigid foam on the underside of the floor joists, especially if the joist bays are insulated with fiberglass batts or cellulose. Rigid foam stops thermal bridging through the floor joists and helps with air-sealing, especially if the perimeter of each piece of foam is sealed with caulk or high-quality tape. One of the best tapes for sealing the seams of rigid foam is 3M All Weather Flashing Tape 8067. The rigid foam should be protected by a layer of plywood, OSB, or drywall (for a garage ceiling).

  8. dabinco | | #8

    Kiley: Thanks.

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