The following is a conversation between 4 students and Allison A. Bailes III, Ph.D. from our Mastering Building Science course. Dr. Bailes is the founder and owner of Energy Vanguard in Decatur, Georgia. His academic credentials in physics (BS, MS, MST, and Ph.D. all in that field) and his experience with hands-on green building projects (including one high-performance home and many sealed ducts and encapsulated crawlspaces), as well as his service as Regional Manager of the Southeast EarthCraft House, give him a solid foundation in the science that underlies buildings.


  • Climate zones can directly affect the ways in which walls dry, and assessing moisture accumulation properly is key
  • Don’t rely on the look or feel of dirt floor areas… these could be retaining moisture
  • Foundation flaws account for many moisture problems, even in dry climates
  • Materials like fiberglass can trap moisture and cause mold



  1. Is stucco, with air space behind it a different condition with different considerations that normal wood-clad or sided houses, or is it nothing special to worry about?
  2. Does the dew-point always need to be exterior of water/vapor barrier?
  3. What is the difference, or advantage/disadvantage of a wall drying to the outside v. the inside? Does climate zone only effect this?


Student 1, those are good questions that a lot of people ask when they start learning building science. Here are my answers:

  1. The air space is a good thing and helps make the stucco wall more resilient than a wall without a gap. It increases the drying potential of the assembly.
  2. You seem to be referring to the dew point method, which ASHRAE calls the profile method. It calls for modeling the temperature profile through an assembly and finding where the dew point is to determine if the assembly will have moisture problems. Unfortunately it doesn’t really work and ASHRAE doesn’t recommend it anymore. It’s not just about dew point but moisture accumulation, which depends on the time a material remains wet and whether it can dry adequately. A hygrothermal modeling tool like WUFI is a better way to characterize an assembly, but you have to know what you’re doing to use it properly. The GIGO principle (Garbage In, Garbage Out) applies.
  3. Assemblies will dry to the dry side. Sometimes (mainly winter or anytime in dry climates) that’s outdoors. Sometimes (summer in air conditioned homes) it’s indoors. You just want to make sure water doesn’t get trapped.


Regarding Allison’s answer to question 2… I’m trying to understand this better, (I was only taught that method).

So is there no difference where we put the insulation? (interior/exterior) as long as the moisture can get out?


Oh, there can be big differences in what happens among assemblies with insulation in different places. The best place to put the insulation is on the outside because it keeps the structure from going through all the thermal cycling it would otherwise. It can prevent moisture problems as well if you have enough exterior insulation to keep the backside of the insulation from going below the dew point.

The profile method for finding where the dew point will fall in an assembly, though, doesn’t always correspond to reality. Materials have different abilities to store water or let it pass through, and what is often called condensation isn’t a simple matter of water appearing on a surface when the materials are porous, as so many building materials are.


My latest project in a turn of the century building in southern Colorado. The basement is wet and smells of mildew even in our dry climate. The owner would like to use the basement as dry storage and has hired me for a consultation and energy audit.

The current construction is: Foundation Walls 18″-24″ limestone block. Foundation Height 8’6″ (0′ above ground), slab floor in 85% with dirt for the balance. There is an old hydronic boiler in the basement that provides a limited amount of heat from the un-insulated piping. Current condition poor due to decades of neglect, lots of junk and previous water leaks.

Proposed plan of action: Remove all items, clean thoroughly, install de-humidification, and use a mildew / mold paint where needed to treat the floor and timbers.

  1. Is it necessary to install a vapor barrier or pour a concrete slab on the remaining 15% of the floor area that is dirt?
  2. Do the limestone walls significantly add to the moisture content in the space?
  3. I am hesitant to recommend the use of any foam because of the historic nature of the building. The foams are difficult to remove from the old timbers.
  4. Is vinyl fiberglass wall insulation a good option once the area has had a chance to dry out?


Student 3,

As you know, dry climates can have moisture problems, too, and they usually stem from the foundation flaws. Here are my answers to your questions:

  1. If 15% of the floor is dirt and is open to the basement, then yes, it needs to be covered. Even if the dirt looks dry, a lot of moisture can still be evaporating out of it.
  2. Without solid data to go on, I don’t know how much the limestone walls are contributing. If there’s no waterproofing on the outside, though, they could be a significant contributor. Do they look wet? Is there any puddling at the base of the walls?
  3. OK. There are plenty of alternatives to foam.
  4. Yes, I think it could work fine.

Anyone else have thoughts on this?


If Student 3 is talking about drape fiberglass batt with a vinyl covering (in question 4) be careful. If the walls get wet moisture can be trapped in the fiberglass and mold results. Leaving a 2-3″ gap between the insulation and cellar floor may allow the foundation walls to keep dry.


Student 4, thanks for adding that. Draped fiberglass can work fine, as I said, but you need to make sure you don’t have bulk water problems back there that you’re covering up with the insulation.

That said, basement walls do not need to dry to the inside. See the article below for Martin Holladay’s report on Joe Lstiburek’s current thoughts on this. Scroll down a bit to the section titled: “News story #2: Basement walls don’t need to dry to the interior.”


Additional Resources: