Basement Insulation and Mold

Problem: Joe Lstiburek, forensic engineer and nationally recognized expert on moisture problems in buildings, rolled into Colorado last December and stated that, oh by the way, insulating basements with a vinyl-faced fiberglass drape is a bad idea. The vinyl facing can trap moisture, leading to potential mold problems. Furthermore, he repeated the assertion statewide, only more strenuously, during five seminars co-sponsored by E-Star and Building America during February and March. 

The reaction from builders who have used the product for up to a dozen years? Probably "expletive deleted." 

Most builders in attendance seem to have investigated various courses of action. Possible strategies: 1) don't insulate basements any more, 2) keep insulating basements with the same old product and hope it really isn't a problem; 3) do option #2 while looking for alternatives: 4) shoot the messenger. 

Let's look at these alternatives. 

Eliminate basement insulation?

As more and more building jurisdictions adopt the International Energy Conservation Code (IECC) -- also the threshold for the Built Green program -- this option almost disappears. Installing basement insulation makes compliance much easier.

Most builders elect to comply with the IECC by the code's Chapter 5 "trade-off method" -- a little more insulation in component A allowing them a little less insulation in component B. The tool of choice here is the computerized MECcheck. It is quite impractical to use MECcheck to meet the IECC in Colorado without installing full foundation insulation.

Some builders meet the energy code using the Chapter 4 "performance method" (aka an E-Star home energy rating). When taking this approach and eliminating basement insulation, for all practical purposes you have to tighten your house and ductwork, install a better water heater and either beef up your wall insulation or install a high-efficiency furnace.

The "Hope Option?"

First, what's the potential problem and what's the moisture source'?

Start with a look at some facts. Conventional foundation walls start out wet; typically, they dry to the interior over many months. Then the damp-proofed wall can wick up moisture from the footer by capillary action. Over time, cracks develop that may leak water from the exterior past the damp-proofing layer covering the walls. And if the basement walls get cold enough - which they can when covered on the interior by a layer of insulation - water vapor carried by air movement from the interior can condense against the wall and wet any element in direct contact with that wall. Finally, bulk water from the interior - perhaps an interior drip from an exterior house faucet - can wet the insulation.

Realize that the particular vinyl facing covering basement fiberglass has a permeability rating of 1.0. That barely qualifies it as a vapor barrier. Yet if walls get wet, and vinyl traps moisture, mold could develop behind basement insulation products. Mold can develop wherever there is some food (dirt, sawdust, etc.), condensation, a suitable temperature and oxygen present.

Lstiburek reports he's found mold in Chicago (about as cold as we are during winter, but much more humid in summer), colder Minneapolis and in the Northeast within basement insulation products. He reports that the moisture problem is more a problem during the summer than the winter. Yet after about a dozen years of installing the product in Colorado, insulators such as Allied Insulation and RG report no evidence of moldy vinyl-faced insulation in basements. Why not?

The theory here is that application imperfections may allow drying of the wall to the interior. Water vapor may be moving in either direction past gaps at the bottom and top of the vinyl facing.

[Note to readers: if you have experienced problems with mold behind basement blanket insulation, please contact the author at E-Star: 303-297-7472. Any reported information will be kept strictly confidential. The idea is to expand our current anecdotal information pool. The hope is to formally study the issue more over the next year, perhaps in conjunction with similar efforts by Building Science Corporation in the north-central states.]

So those pursuing the "Hope Option" have the theory against them but the anecdotal evidence in their favor. This isn't a highly desirable position.

Looking for alternatives?

Within a few weeks of Lstiburek's ringing the alarm about the potential for mold formation behind vinyl-faced batts, Scott Zunk's team at Mato -- a Denver-based fiberglass laminator -- developed a very cost-effective modification. They simply perforated the finished blanket. The vinyl facing ends up with tack-sized holes on one-inch squares.

The cost bump is about two cents per square foot, or $15-$20 per average-sized basement. So everyone is doing it already, right`? Well, not exactly. In fact, after an initial flurry of people purchasing the perforated product last April, demand dropped dramatically. During the first three months of its availability, Zunk reports less than 5 percent of his total vinyl-faced blankets have gone out with perforated facings. But the word today is that Allied Insulation will be switching over to nearly 100 percent use of the perforated product as this article goes to print.

There are other alternatives. For example, foam-form foundation systems (ICFs) beat the wetting and drying problem. Use of exterior foam insulation also provides a viable solution. If you want to eliminate the task of applying a finish layer to protect the above-grade concrete, consider Dow Styrofoam's new T -Mass system; it's a one-pour sandwich consisting of 4-inch concrete layers on either side of 2-inch centered foam and spacers.

If you find the above foam approaches too expensive, you can take a Lstiburek recommended variation. First, the insulator attaches a 4-foot-wide band of finch foam (R-7 with a fire-rated foil facing) against the top half of the interior basement surface. When the homeowner wants to finish their basement, they should install 1-inch expanded polystyrene (R-4) over the bottom half of the wall. The theory: while the top half of the wall can dry to the outside through the exposed 6-12 inches above grade, the lower portion will be able to dry to the interior through the expanded polystyrene (perm rating between 2 and 5). In both cases, water vapor from interior air shouldn't migrate toward the wall and condense. But the fly in the ointment with this approach: how likely is the homeowner to follow those instructions'?

Since budget is always a concern, the perforated facing is your most cost-effective alternative. No one has studied the change in permeability achieved by this approach. But on the commonsense level, it passes an initial sniff test.

Lstiburek agrees. "The perforated product probably has a perm rating in the 4 to 5 range. This dramatically reduces the risk, it's a huge improvement and it might-1 emphasize might-be all that's needed in dry Colorado. But the perforated product doesn't work in Minnesota, Iowa and a number of other climates. 1 still recommend a layer of foam to prevent condensation."

Shoot the messenger? 

Nah. Too much jail time.

Steve Andrews consults with builders for E-Star Colorado and writes on energy issues (sbandrews@att.net). E-Star (www.e-star.com), is a nonprofit home energy rating system that works with both new and existing homes statewide.

2008 Built Green Colorado

Home Builders Association of Metro Denver, 9033 E. Easter Place, Suite 200, Centennial, CO 80112
(303) 778-1400 fax: (303) 733-9440  info@builtgreen.org

Last Updated: 10/05/2007