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This article discusses the proper type, location and placement of moisture barriers or vapor barriers in basements to control air and moisture.
Can I use a spray-on basement ceiling barrier?
Does the vapor barrier go "up" towards the warm side or underside of the floor above, or does the vapor barrier go "down" facing basement interior?
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Our horrible crawlspace (above) shows foil-faced fiberglass insulation installed with the vapor barrier facing "down" towards the wet crawlspace floor - this is an example of backwards, incorrect, "cold-side" vapor barrier installation that invites moisture condensation and even mold contamination (see Mold in Fiberglass Insulation) inside the fiberglass insulation. Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
Readers dealing with damp or wet basements or crawl spaces should see BASEMENT WATERPROOFING as well as additional building moisture problem information at see CRAWL SPACE DRYOUT - home. If your basement space has a dirt floor or a damp or wet floor also see our suggestions at CRAWL SPACE GROUND COVERS and MOLD on DIRT FLOORS.
Our page top sketch showing the effects of covering a dirt floor in a crawl space is courtesy of Carson Dunlop Associates. Carson Dunlop's comment that a dirt crawl can contribute several gallons of moisture per day into a home is the best case. If the crawl area is actually wet from surface runoff, roof spillage, ground water, or plumbing leaks, the amount of water pumped into the home can be much larger and more harmful.
The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
How to Install a Vapor Barrier Over an Un-Heated Basement
We want to insulate the ceiling of an unheated basement and have been thinking about a vapor barrier. I wonder if anyone has had automobile undercoating sprayed on their ceiling as a vapor barrier.
How do you feel about that? Would sheet goods such as flooring be better? - Bill Geary, Belfair WA
An insulated floor over an unheated basement (or crawl space) requires an air/vapor barrier4. It belongs on the warm side of the insulation except, perhaps, in a consistently hot, humid climate.
When insulating a floor over a basement, the air/vapor barrier, in addition to its usual job of keeping moisture out of the insulation [thus preserving its insulating R-value], the barrier keeps moist basement air from entering the living area via the stack effect [riding rising warm air currents].
As with other air/vapor barriers, the membrane's air tightness is far more important than its perm rating, since moisture moves through buildings mostly by air leaks.
Placing a Basement Vapor Barrier Over Exposed Bottoms of Overhead Floor Joists Forming the Basement Ceiling
Watch out: But if you are placing insulation between the overhead floor joists that form the basement ceiling, the vapor barrier belongs on the "warm" side not the cool basement room side, so this would be an incorrect installation that risks collecting and trapping moisture between the poly barrier and the underside of the floor above.
Our photo (left) shows basement ceiling insulation installed the "wrong way" with the vapor barrier facing "down" - in this case over a dirt floor basement.
The insulation near the exterior walls had become wet and moldy. We [DF] have found homes where poly was simply stapled up to the underside of the basement ceiling joists only to find that through various penetrations into the plastic air leakage led to so much moisture accumulation that there was both mold and rot in the ceiling.
Basement Ceiling Vapor Barrier over Subfloor Above
A better installation if you are installing a basement ceiling poly vapor barrier during new construction is to place the poly either directly atop the first floor joists (dangerously slippery during installation) or to place the poly barrier atop the subfloor and underneath the next layer of flooring underlayment as the first floor is being built.
A third option is to use the subfloor or underlayment (of the first floor surface) as a vapor barrier itself by sealing all the subfloor joints (or underlayment joints) and also sealing at the wall/floor junction. This is an easier method to accomplish than you may think if you are also using construction adhesive to glue subflooring to floor joists during construction.
Basement Ceiling Vapor Barriers Installed During Insulation Retrofit
In a retrofit job in which you are adding insulation to a basement ceiling by placing it between the joists of the floor above, if you are using foil or kraft-faced insulation, be sure that the insulation is placed with the foil or kraft paper facing "up" towards the heated side of the floor, lest you form another moisture trap.
Or use un-faced insulation in that space, having placed your vapor barrier atop the subfloor above (and below underlayment above) as we just suggested.
Our photo above left) shows un-faced fiberglass insulation that has been added between the joists of a basement ceiling. No vapor barrier was installed facing the basement area.
Our second basement ceiling insulation snafu photo (below right) shows accumulated water trapped above a poly "vapor barrier" that had been stapled to the underside of basement ceiling joists. This image illustrates a failing of the theory that if we can just make a perfect vapor barrier (the hot roof mistake) in a ceiling we don't have to worry about trapped moisture.
Our OPINION is that both due to errors and omissions during construction by normal workers and leaks into or punctures through building surfaces during its life make a perfect vapor barrier difficult to achieve and risk serious hidden leak or mold damage. - DF
Spray-on Basement Ceiling Vapor Barriers
Watch out: as for using automobile undercoating spray as a basement ceiling vapor barrier, our reasoned opinion is "Yuch!". Health reasons aside [the volatiles and solvents in the undercoating spray], do you really want your basement to smell like the underside of a car?
More seriously, tar and asphaltic coatings are likely to outgas unpleasant organic fumes for some time. [Confirmed: In 2010 we sprayed automotive undercoating on a rust repair on the underside of our pickup truck. Even with the truck left outdoors for days to "dry" we observed that the undercoating remained soft and tacky for several weeks and continued to outgas (and smell) - DF.]
There are indeed vapor-barrier paints available that can be used as a moisture-resistant primer paint when renovating the interior of an older building, but these products won't seal holes and air leaks in the basement ceiling. And if the first floor subfloor above the basement is plywood or a similar product, except for its seams, it's already forming a rather good air barrier.
Using Sheet Vinyl Flooring as a Basement Ceiling Vapor Barrier?
We see no reason why sheet vinyl couldn't make a perfectly good vapor barrier as long as you are not creating an "insulation sandwich" with air leaks that will trap moisture in the basement ceiling - that is, as long as you carefully seal at all of the penetrations and at the junction of the wall vapor barrier.
But considering that a principal objective of the basement ceiling barrier is to stop air movement into the building above, it's not clear why sheet vinyl flooring would be a better solution than less costly 6-mil polyethylene plastic. - DF.
Explanation of Basement Moisture and Air Movement
Moisture Movement a Basement into the House Interior
In the summer, however, particularly in air-conditioned homes, moisture may be driven from a basement upwards into the house. Although you could stop this by using a vapor barrier below the under-floor insulation (presumably insulation has been placed under the floor over the crawl area), it is more practical to keep the basement RH level down. If the basement has a dirt floor, this is best done with a polyethylene ground cover (see CRAWL SPACE GROUND COVERS), and by making sure that there are no outside water leaks nor plumbing leaks into the crawl space.
In a finished basement with a concrete slab or tile floor, you may find moisture condensing on the basement floor during humid summer months - more argument for correcting the water sources outside the building and running a dehumidifier indoors during this season.
If you see moisture stains on the basement walls, it is at least sometimes a source of un-wanted building moisture. Even if you think your crawlspace is "dry" it might not be.
EFFLORESCENCE SALTS & WHITE DEPOSITS provides examples of visual clues that moisture is being pumped through foundation walls or up through a dirt floor into the building even when actual crawlspace flooding is not occurring.
Air Movement from Basement to House Interior
Much of the moisture transport from a basement into house is due to the stack effect (or air convection currents) pulling air up through the house. Warm air tends to rise upwards through a building, and that air movement will be even more rapid if upper floor windows are open or attic or upper floor exhaust fans are in operation.
If a basement is damp, wet, moldy, or contaminated by a sewage spill, all of those undesirable ingredients are easily transported from the crawlspace into the occupied space from the crawl area, riding on rising air convection currents. In fact we [DF] have found frequent evidence that moisture from a wet basement or crawl space has produced excessive levels of condensation all the way up in a building's attic.
The best way to prevent un-wanted air movement from a basement or from a crawlspace into a building is to seal off all air leaks between the basement (or crawl area) and the house. See AIR SEALING STRATEGIES for details about sealing building air leaks.
A good ground cover in the crawlspace will also keep the RH of any crawlspace air that does enter the house reasonably low. See CRAWL SPACE GROUND COVERS for details on adding crawlspace ground covers that also pertain to some basements.
The question-and-answer article about use of a plastic barrier on crawl space floors to control crawl space moisture and mold, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
Key building water entry diagnosis and cure articles:
Reader Question: Suffocating air and basement mold - which way to install the basement ceiling vapor barrier?
In a finished basement, if you install a vapor barrier on the ceiling (face hot side) + insulation, will the air in the basement be trapped and cause surface mold?
I ask because I saw two [moldy] basements last week and the air was suffocating, and there was mold in all corners of the living area, massive condensation and mold on the windows.
Both homes had a vapor barrier. In the same basement in another area of each home the was no insulation, no vapor barrier and the air was perfect.
In my opinion the only insulation that you can put on the ceiling of a finished basement is for sound proofing and no vapor barrier. What do you think? - Patrick Massie, pmir inspection, is a professional home inspector and a certified infrared FLIR thermographer [Thermographie Infrarouge] in Quebec.
Note: the vapor barrier illustration above, provided compliments of P.M., is for purposes of discussion and is not necessarily the best moisture barrier placement for all buildings.
Reply: Vapor barrier goes on the warm side in heating climates; but whether in the right or wrong location it won't prevent mold on exposed ceilings and walls if there is basement water entry, condensation, or high interior moisture.
A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem. That said, here are some things to consider:
I'm not sure I've got it right but if by "hot side" you mean that the vapor barrier is tight up under the floor or subfloor - over the basement - followed by insulation between the basement ceiling joists (floor joists of the floor above), that would be SOP.
If you put the barrier facing "down" stapled to the underside of the ceiling joists, you form a potential moisture trap in the ceiling - where moisture finds a hole and leaks in and can't get out.
I know this is sometimes counterintuitive because we think the basement or crawl area is wet and we're trying to keep moisture from moving "up" into the area above.
Like you, I wouldn't use fiberglass over a damp or wet basement or crawl space area anyway as it becomes a mold trap. But solid foam insulation products that are closed-cell and resist water or moisture movement work better in those locations. [Just remember foam insulation should not be left exposed in occupied spaces - a fire and smoke safety hazard.]
Should you Insulate the Basement Ceiling in the First Place?
J.D. Ned Nisson, an energy consultant and writer on that topic, has pointed out that "Basement ceiling insulation looks good on paper for unheated basements. But because of all the obstructions and penetrations in most basement ceilings, the insulation sysem is usually not very effective."  Nisson and other experts prefer to insulate the basement walls.
But people might choose to insulate the ceiling of a finished basement for purposes of sound insulation, as we discuss
at How Effective is Fiberglass as Soundproofing in a Basement Ceiling?and in more detail at SOUND CONTROL in buildings.
Vapor Barriers Keep Moisture Out of Cavities, They Don't Prevent Mold. Keep Out Basement Moisture!
Watch out: But leaving out the vapor barrier does not prevent a basement mold problem. Leaving out the water is what's needed to do that. Paul Eldrenkamp, a Massachusetts remodeler who has written on basement remodeling , J.D. Ned Nisson , and Scott Anderson , , as well as nearly all other building experts join us in pointing out that keeping un-wanted leaks and moisture out of buildings is a first concern.
The presence of visible mold in the basements you inspected is more likely be due to a mix of water entry there and mold-friendly materials - drywall than caused by the placement of moisture barriers in the basement ceilings.
Basement moisture can however, also produce hidden mold, rot, and insect damage problems. If the basements had omitted a moisture barrier, and IF the water was coming in, say at the floor, or just in one area, moisture leaking into the other wall or ceiling cavities at any penetration would be trapped and would be likely to produce additional "hidden" mold there, regardless of which place the vapor barrier had been placed.
Thinking about your specific example, you'd want to trace where water was coming into the structure.
and CRAWL SPACE DRYOUT - home)
Most Severe Moisture Movement into Building Cavities Rides on Air Leaks
Put another way, my view is that the solution is to prevent (or manage with sump pumps) basement leakage, dehumidify as needed, and put the vapor barrier in the correct location to avoid a moisture trap in the floor cavity. Vapor barrier in heating climates (like Quebec) would be on the "warm" side of the wall or floor - in this case up against the underside of the subfloor would be the usual procedure.
It would be ugly to retrofit the vapor barrier if the ceiling (floor above) are already built, which is why people also like to staple the VB to the underside of the floor joists. I wouldn't do it.
Keep in mind that excepting for a direct water leak into a building cavity, most moisture movement into wall or ceiling cavities is through penetrations, not molecular movement through solid drywall.
Steven Bliss comments:
Faced insulating batts can be pushed up between floor joists with the facing against the subfloor, and held in place with metal retaining clips - or strapping, etc. Since a vapor barrier does not need to be continuous (like an air barrier), this would be adequate. If there is a plywood subfloor, roofing felt ("tar paper") under strip wood flooring, or vinyl flooring above, any of those could serve as a vapor retarder, so unfaced insulating batts would also be fine in my opinion, so long as there is not a basement moisture problem.
Where Does the Basement Ceiling Vapor Barrier go? Where is the Moisture! Vapor Barrier Sandwiches: Welcome to the Hotel California!
Mr. Massie, further discussing the construction of a finished, heated basement apartment continued the discussion:
Any vapor-barrier sandwich approach makes me nervous. I've inspected and found joist rot in too many basements and crawl spaces that had a vapor barrier stapled to the underside of the floor joists and facing down to the wet area.
While it sounds correct to put the vapor barrier towards the wet area, dreaming that we were keeping water out of the insulated floor cavity above, what often happens is moisture finds a way to leak into the floor cavity where it is trapped, wetting the insulation, encouraging mold growth, and eventually leading to actual wood rot.
Especially below a kitchen or other room whose floor covering is sheet vinyl or ceramic tile, the floor covering forms a second vapor barrier on the normal "warm-side" of the floor. Like the Eagles Lyrics to Hotel California, moisture entering the vapor barrier sandwich is trapped where
Or over the basement, I'd rather see NO vapor retarder/barrier installed in the basement ceiling than to see a trap of vapor barriers installed on its two sides, risking a one-way moisture leak that accumulates in the ceiling as air and moisture movement cycle.
Even if the resilient flooring above part of the basement forms a warm-side vapor barrier, at least with just one barrier in place, as humidity drops on the other side, moisture has a chance of moving back out of the cavity and into the more dry air.
Is it OK to Use Poly Vapor Barriers to "Hold the Insulation in Place"?
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