GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Mold and cold – Updating an old home

chadwik66 | Posted in General Questions on

We moved into a 1960s colonial reproduction about a year ago and have been on a near constant upgrade cycle since. With spring upon us, we’re now attacking the moisture and resulting mold problems head on. A bit about the house:
-Southern NH, on the border of 5A/6A climate zone
-Slab on grade with one small addition over a crawl space
-Spray foam recently installed in attic and on crawl space walls
-Old oil heater pulled out, air source heat pumps in all but 3 rooms. Upstairs is new ducting, addition has mini splits.
-Remaining 3 rooms are heated by the propane backup heater via old ducts in floor.
-No known vapor or other barriers in place
-Dehumidifiers running as needed

Our current goal is to find a more permanent means to control moisture. The thought is to update the remaining few rooms with mini splits and get the combined heating, cooling and dehumidification benefits.

The question at hand – what else should should we consider to minimize moisture? Are there ways to add a vapor barrier to an old house slab on grade foundation? Should we consider something other than new mini splits to help?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    You mentioned that you applied spray foam to your crawlspace walls, but did you fully encapsulate the crawlspace too (close any vents to the outdoors, put down a poly liner on the floor, if it’s a dirt floor)?

    Moisture problems can be from ground moisture from a crawlspace, but they can also be from air leakage from the outdoors. You mentioned that you have a crawlspace, but you also say you have a slab on grade foundation so that’s a little confusing.


  2. chadwik66 | | #2

    Thanks Bill. The floor is concrete and we did fill in all 6 of the troublesome vents. We also added a larger dehumidifier to help with any moisture that sneaks in. Between that and the current mini splits we're finding the addition section of the house is much better moisture-wise than the rest of the house.

    The bigger concern is reducing the moisture in the section that is slab on grade.

  3. GBA Editor
    Martin Holladay | | #3

    We need a lot more information. The first question in any investigation of a moisture problem is, "What is the source of the moisture?" Candidates include moisture sources generated indoors (from houseplants, tropical fish tanks, or drying firewood, for example), a damp crawl space, moisture intrusion through poorly flashed walls or roofs, and plumbing leaks.

    If you have no idea what the source of the moisture is, you need to describe your perceived problems.

  4. chadwik66 | | #4

    Hi Martin,

    The source of the moisture is a slab foundation on an incredibly wet lot. The lot is adjacent to a pond and has a large amount of fill. The result is a slab that appears to be letting moisture rise up, resulting in high humidity levels and mold showing up every couple of months. We have sealed off the addition crawl space with luck, but have not done anything to the slab under the main house. I'd love to lay down a vapor barrier on the slab but don't know if that's an option without significant work.


  5. tommay | | #5

    What is the detail of how the house is set or attached to the slab? Can I assume main water, waste or drain lines are set into the slab?

    1. chadwik66 | | #6

      Hi Tom,

      I'm sorry - I don't know how it is attached other than resting on the slab. You are correct that all water lines are set into the slab and connected to well, septic, etc.


      1. tommay | | #7

        So all your first floor flooring is basically concrete with perhaps carpeting, tile or wood laid directly on top? No additional framing to raise the floor above the slab? Two baths stacked or first floor half bath with laundry? Have you checked the septic tank water level? If the land is wet, it may not be leeching so well especially in the spring, which can lead to backed up drain lines which may leak out first floor toilets into the slab. Any noticeable water damage on floors or lower walls.? Do you see a drop in water pressure on your well when water is not in use?

        1. chadwik66 | | #8

          Hi Tom,

          In most places our first floor flooring is pine on top of wooden joists sitting on the slab. There are a few spots of brick and laminate of which only the brick shows moisture coming through. There is no obvious damage to the wood.

          There are no pressure drops that we notice and no noticeable leaks - just mold that grows in a few consistent locations, mostly low along interior walls. We constructed a new leach field and installed a whole new septic system last fall, but are seeing mold in the same places as before.

          I'll take a look at where the lines run and see if that may be the issue. I'll also try to better pinpoint any patterns in the mold - thank you for continuing to help!

          One question - is there a common way to add a vapor barrier between the joists/wood floor and the slab? It seems like a large project, but I want to make sure I'm not missing anything obvious.


          1. tommay | | #13

            If the mold patterns are consistent, then there is probably something in those areas that comes and goes with certain indoor conditions. Have you tried bleaching those areas? Perhaps a test. Remove a few inches of the wallboard near the floor in the areas where the mold grows. Put the piece you cut out someplace where you can observe it. Leave the area you cut out open and put some type of test material, eg another piece of wall board or staple paper towels or brown paper bags in place and inspect over a period of time. If the piece of wallboard you removed continues to grow mold, it is probably embedded in the wall board and will grow when conditions are ripe. If the test material show signs of moisture then it is coming from the framing and the mold is embedded there. Bleach will kill the existing mold but now you have to find where the moisture is coming from. It's obviously coming up through the gap between the flooring and the wall getting into the wallboard, so if individual bays can be singled out, it may narrow things down.
            One quick question, How high above grade is the top of your slab?

  6. GBA Editor
    Martin Holladay | | #9

    Most slabs are poured over a layer of polyethylene, which prevents vapor diffusion or wicking. Do you suspect that the builders of your home forgot the polyethylene?

    It's certainly possible to install a layer of polyethylene above your slab, but you would have to remove the floor framing and finish flooring to do that.

    If I were you, I would probably start with exterior solutions: changing the exterior grade, and installing more perforated drainage pipes at the perimeter of your house -- with the pipes leading to daylight, to a distant drywell, or to your local municipal sewer system (if such connections are legal in your town).

    1. chadwik66 | | #11

      Thank you Martin. The home is ~60 years old and we've seen no evidence of a vapor barrier, but that doesn't mean it's not there. We have some grading and drainage work planned in the near future, so hopefully that helps. Thank you for all the help!

  7. Expert Member
    BILL WICHERS | | #10

    It might also be worth checking that any existing drain tile is clear and functional. A lot of older homes used terra-cotta drain tile, and that type of perimeter drain can be damaged by tree roots or excavation (it’s fairly brittle stuff). It may be possible to repair what you already have and see an improvement that way.


    1. chadwik66 | | #12

      Thank you Bill. We'll take a look at the perimeter and see what that situation is.

  8. Expert Member
    Peter Yost | | #14

    Hi Chad -

    This is a tough one. You don't "see" the moisture as vapor coming out of your slab, but that does not mean that is not the source, or a primary one.

    The chances of your slab being installed over a moisture barrier is very low, given its age.

    First, eliminate all other sources of moisture - see this blog on sources of interior moisture:

    I routinely tests slabs for their moisture content - see this blog:

    The problem with stopping the vapor with a Class I retarder/barrier at the face of the slab is that with complete coverage, the vapor can condense on the back side of the membrane, accumulate, and run to rot bottom plates of interior partitions and exterior walls.

    But Martin and others are dead-on: you can't develop a solution or set of possible solutions until you identify the source(s) of moisture.


    1. chadwik66 | | #15

      Thanks Peter. It's always nice to be reassured that we do have a tough problem and aren't just missing obvious solutions! We received some small hygrometers yesterday and are already starting to see variations in humidity around the house that will help us pinpoint the sources of the moisture. Thanks to everyone who has helped getting to this point.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |