Moisture Barriers
April 17
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Summary of Article
Throughout our 6 years inspecting properties in the Springfield Missouri area, we have found many homes where we have recommended a moisture barrier be installed in the crawlspace. However, many people don’t realize what a moisture barrier is, what function it performs, or when to use one effectively. These are excellent questions and the answers may not be exactly what you think. We thought you might like to hear an opinion of these issues from a professional who is in the business.
Sections on this page
What is a moisture barrier?
What does a moisture barrier do?
When do I need a moisture barrier?
Photos of a crawlspace that need to have a moisture barrier installed.
What is a moisture barrier?
A moisture barrier (also called a vapor barrier) is a sheet of plastic that is laid over the floor in a crawlspace. Basically, that is all a moisture barrier is, but there is more to the story. First of all, a moisture barrier must cover 100% of the floor surface leaving no gaps exposed. Secondly the plastic is lain in sections, so each section must overlap by roughly a foot. Thirdly, the plastic should be at least 6 mils thick to be durable enough to last over time (sorry, no thin painters plastic).
You can use clear or black, but we prefer clear as this allows visibility under the barrier so you can see how it is functioning, termites, or additional water problems. There is a normal assumption that the sections must be fastened together, or held down with gravel. This is not necessary as the crawlspace generally does not have foot traffic or wind that might disturb the barrier. It is perfectly acceptable to simply lay it in place on the crawlspace floor. There is an exception to this rule for moisture barriers that are also installed for radon mitigation, which is a topic for another article.
Moisture barriers are sheets of 6 mil thick plastic laid on 100% of the crawlspace floor
What does a moisture barrier do?
This is an excellent question deserving consideration as this is the most common question I get as a home inspector. A moisture barrier reduces the relative humidity in the crawlspace, which reduces the potential for organic growths like molds and rot. It doesn’t sound logical that the simple addition of a piece of plastic could prevent mold and rot, but it works!
Here’s how it functions, water vapor from the ground disperses into the air filling the crawlspace which increases its relative humidity. Once the humidity reaches 60% for a sustained period of time, mold can grow by pulling water right out of the air. Many rots can sustain growth with even less humidity. Additionally when there is a temperature differential, the water in the air will condense to form a water droplet when it contacts any surface. This could be the concrete foundation wall, piers or worse – the wooden floor joists. Once the wood becomes wet with surface water, organic growth is ready to start.
The point is, you want to reduce it to around 30% to 40% for the general health of the property. The moisture barrier’s true function is simply to act as a surface for the water vapor to condense onto instead of the wooden members of the structure. By placing the moisture barrier on the ground, water vapor emanates from the ground, condenses onto the underside of the moisture barrier, and drops back into the soil without harming anything. This is very effective at reducing the relative humidity of a crawlspace to levels that do not promote organic growths.
Moisture barriers reduce the relative humidity of crawlspace air by providing a condensation surface close to the ground.
When do I need a moisture barrier?
Our opinion is generally that every crawlspace should have a moisture barrier. There is no crawlspace that could not benefit from this relatively inexpensive feature. However, we highly recommend these barriers to be installed in properties that have signs of past chronic water intrusion, have an organic growth beginning to form on a majority of the wooden structural members, or on properties that exhibit water related problems during the home inspection.
It is important to note that a moisture barrier is completely ineffective at fixing standing water problems in a crawlspace. This is only effective against vapor intrusion from the ground due to wet soils. However, a moisture barrier can be combined with a sump pump installation to both remove standing water and reduce humidity. Read more about that solution in our next article.
Every crawlspace should have a moisture barrier to protect its long term health.
Photos of a crawlspace that needs to have a moisture barrier installed.
I like your writing style. That’s a great description of a crawl space vapor barrier. Would you mind if I ‘re-blogged’ this entry on to my own site? I’ll give you credit for it and I’ll include a link to your site at the end of the blog.
You are welcome to re-blog anything on this site. You can use trackbacks of pings to link back to this site. I appreciate your feedback!
Great detailed info, really opened my eyes more….I am doing an addition on my house, the question I have is…Do I need vapor barrier if my crawlspace is concrete slab instead of dirt. I might as well but just want to know?
Great article. I’ll likely re-post to others as well.
I am planning to build some 8×8or12 storage/workshop buildings which will be on platforms of 2x treated boards covered with particle board. I am considering vapor barriers either on the ground under the 2x frames or on the frames under the sheeting. Your suggestions if you have the time will be much appreciated.
I’ll soon have another web site where I plan to have notes and comments related to realty.
George
George Rowe Realty
Jason,
It depends on if your crawlspace is a conditioned space or not. If you are going to heat and cool the crawlspace, then you should not need a moisture barrier. However, if it is going to remain unconditioned, you might consider adding one. They are very cheap and very effective.
George,
I would not put them between the joists and the decking because that will prevent using construction adhesive to join the decking to the joists (which eliminates floor squeaks) and it will poke a hole in the vapor barrier every time you nail the decking to the joists. I would only recommend these be placed on the ground.
Thank you for your comments.
Sean O’Rear
I AM REQUESTING INFORMATION…i purchased an older home, all of the exterior walls are NOT insulated. i am planning to build new 2×4 walls on the interior side of the existing walls do i use faced or non-faced insulation and do i need a vapor barrier on the new walls? if so, what side of the new wall do i put it? i live in the ozarks it does get humid in the summer and also the home has central air if that makes a difference. thanx to anyone that can shed light on my dilemma.