Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
WATER ENTRY IN BUILDINGS
AGE of MOLD - Old is the Mold?
BASEMENT CEILING VAPOR BARRIER
BASEMENT MOLD WATER IMPACT
BRICK WALL DRAINAGE WEEP HOLES
BUCKLED FOUNDATIONS due to INSULATION?
BUILDING DAMAGE ASSESSMENT & REPAIR
CONDENSATION on WINDOWS & SKYLIGHTS
DEW POINT TABLE - CONDENSATION POINT GUIDE
EFFLORESCENCE, Salts & White / Brown Deposits
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FLOOD DAMAGED FOUNDATIONS
FLOOD VENTS & FLOOD PORTS
FLOODS IN BUILDINGS-mold
FLOOR DAMAGE DIAGNOSIS
FOOTING & FOUNDATION DRAINS
FOUNDATION BULGE or LEAN MEASUREMENTS
FOUNDATION CRACKS & DAMAGE GUIDE
FREEZE-PROOF A BUILDING
FROST HEAVES, FOUNDATION, SLAB
HUMIDITY LEVEL TARGET
ICE DAM PREVENTION
MOISTURE CONTROL in BUILDINGS
MOLD INFORMATION CENTER
NOISE / SOUND DIAGNOSIS & CURE
ODORS & SMELLS DIAGNOSIS & CURE
SEWAGE BACKUP, WHAT TO DO
SEWAGE BACKUP TEST & CLEANUP
SEWAGE BACKUP PREVENTION
SEWAGE PUMP CLOG DAMAGE
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIN DIAGNOSIS on BUILDING INTERIORS
SWEATING (CONDENSATION) on PIPES, TANKS
TOILETS, INSPECT, INSTALL, REPAIR
TRAPS on PLUMBING FIXTURES
VAPOR BARRIERS & CONDENSATION in BUILDINGS
VENTILATION in BUILDINGS
WATER ENTRY in BUILDINGS
WINTERIZE A BUILDING
Which way does the insulation vapor barrier or kraft paper go over a basement or crawl space: facing up (towards the floor above) or facing down (towards the crawl space or basement floor)? This article discusses the proper location and placement of moisture barriers or vapor barriers in crawl spaces to crawl space moisture. Does the vapor barrier go "up" towards the warm side or underside of the floor above, or does the vapor barrier go "down" facing the crawl space or basement interior? And what difference does it make to building moisture problems?
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved.
Readers dealing with damp or wet crawl spaces should start reading at CRAWL SPACE DRYOUT PROCEDURES. If your crawl space has a dirt floor or a damp or wet floor also see CRAWL SPACE GROUND COVERS and MOLD on DIRT FLOORS. Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
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 photo at left shows what can happen when a vapor barrier is placed on the cold side of a building floor - in this case over a damp or wet crawl space or basement. Wet insulation means lost insulation value, risk of mold contamianation, and in some cases structural damage from rot or wood destroying insects.
I have observed extreme strucutral damage from rot in a clean looking crawl space sporting a nice concrete rat slab but where a poly vapor barrier stapled to the joist bottoms trapped and accumulated moisture, finally water, in the floor structure. I could reach out and literally grab and crush rotted floor joist bottoms by the handful.
The link to the original Q&A article in PDF form immediately below is followed by an expanded/updated online version of this article.
How Best to Control Crawl Space Moisture
If you place a vapor barrier on the warm side of the insulation between a heated area and an unheated crawlspace, will moisture from the crawlspace migrate into the insulation and condense?
In the winter, when the house is hot and dry, it seems the vapor barrier should be on the cold side of the insulation. John Mascaro, Mascaro Construction, Philadelphia PA
How Moisture Moves in Buildings
Moisture migration and relative humidity remain two of the least-understood topics builders face. First, remember that moisture moves from a point of high vapor pressure to a point of low vapor pressure.
Since warm air holds more moisture than cold air, warmer air generally has the higher vapor pressure.
For example, air at 68 degF. at or above 33-percent relative humidity (RH) will have a higher vapor pressure than crawlspace air at 40 degF and 90-pecent relative humidity (RH), and moisture will tend to move, if at all, from the warmer house to the cooler crawlspace.
Using a cold-side vapor barrier would almost guarantee moisture condensation in the insulation in winter - a very undesirable condition that reduces the effectiveness of the insulation and is just asking for a mold contamination problem.
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.
It is easier to retrofit-add crawlspace insulation this way since we are tempted to just push the insulation up between the floor joists and use the vapor barrier or kraft facing to staple the insulation in place. Notice that the crawlspace insulation is falling down? Stapling insulation up through the foil facing to the underside of the floor joists in this location didn't work.
The insulation in the crawlspace in our photo should have been installed with the kraft or foil facing "up" towards the warm area or against the underside of the subfloor overhead. Using that method, insulation is supported in place by spring-wire that is simply placed at intervals between facing sides of floor joists.
Moisture Movement from Crawlspace into House Interior
In the summer, however, particularly in air-conditioned homes, moisture may be driven from the crawlspace 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 crawlspace RH level down. 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.
If you see moisture stains on the crawlspace 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 / Brown 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 Crawlspace to House Interior
Much of the moisture transport from crawlspace 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 crawlspace 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 crawlspace into a building is to seal off all air leaks between the crawlspace 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.
CRAWL SPACE REINSPECTION: Inspect the crawl space periodically to make sure your crawlspace dryout measures have been effective. How often do you need to inspect the area? It depends ... on site conditions and building history. At least once a year you should look for any new leaks such as a leaky plumbing drain or an outside water entry problem. If you have been having trouble keeping water out of the crawl area, you should check more often until your confidence is restored.
Separately our series of basement dry out, clean up and leak prevention articles begins at BASEMENT LEAKS, INSPECT FOR. Also see MOLD CLEANUP - MISTAKES to AVOID for a master list of the principal ways that people foul up mold cleanup projects.
Key building water entry diagnosis and cure articles:
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.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Use the search box below to ask a question or to search the InspectApedia.com website.
Ask a Question or Enter Search Terms in the InspectApedia search box just below.
Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.