Crawl Space Moisture Barrier Placement Guide
Where should the vapor barrier go in a crawl space?
CRAWL SPACE VAPOR BARRIER LOCATION - CONTENTS: Where to install plastic moisture barriers in a crawl space. Which way does the crawlspace vapor barrier go: up or down? Proper placement of vapor barriers on crawlspace or basement ceilings. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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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 proper crawl space moisture barrier placement make to overall building moisture problems?
Crawl Space Ground Covers to Help Control Crawl Space Moisture
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.
[Click to enlarge any image]
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 question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
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 or Basement 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, or for that matter also on basement walls, it is at least likely to indicate a source of un-wanted building moisture. Even if you think your crawlspace is "dry" it might not be. And the stains may indicate an actual history of water leakage and water entry into the crawl area or basement space.
Our photo at left shows foundation leakage stains that originate high on a concrete block wall of an un-finished basement - a strong suggestion of mis-handling of roof drainage or possibly of in-slope grade of soil outside the buidling in this location.
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 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.
The link to the original Q&A article in PDF form immediately below was preceded above by an expanded/updated online version of this article.
Crawl Space Vapor Barrier Placement - Q&A on the proper location of vapor barriers or moisture barriers in crawlspaces - PDF version, use your browser's back button to return to this page
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.
This article series describes the steps needed to get into, inspect, clean, and then dry out a building crawl space. We give a step by step crawl space entry, inspection, cleanout, dryout and keep dry guide explains how to get into or inspect a crawl space even if there is no ready access, how to assess crawl space conditions, how to stop water that is entering the crawl area, how to dry out the space, how to clean up and if necessary disinfect or sanitize the crawl space, and how to keep out crawl space water and moisture in the future.
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Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
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