Best Practices for Installing a Crawl Space Vapor Barrier

[nobes]

Hello everyone,

I'm looking for advice on installing a vapor barrier in my crawl space. I'm planning to use a 6-mil polyethylene sheet, but I'm unsure about the proper technique for installation.

Should the barrier cover the entire floor and overlap up the walls?

How should I seal the seams and edges to ensure no moisture gets through?

Also, is it necessary to add a drainage layer underneath the vapor barrier?

Any tips or recommendations based on your experiences would be greatly appreciated!

 

[nobes]

Hello everyone,

I'm looking for advice on installing a vapor barrier in my crawl space. I'm planning to use a 6-mil polyethylene sheet, but I'm unsure about the proper technique for installation.

Should the barrier cover the entire floor and overlap up the walls?

How should I seal the seams and edges to ensure no moisture gets through?

Also, is it necessary to add a drainage layer underneath the crawl space vapor barrier?

Any tips or recommendations based on your experiences would be greatly appreciated!

thanks in advance for any help

 

[Sparky617]

Previous conventional wisdom was to cover 80-90% of the floor. That has largely been changed. Are you planning on doing a sealed, conditioned crawlspace (best practice but not widely done yet) or continue to have it open to the outside via foundation vents?

Ideally, I'd go conditioned. Insulate the walls and completely cover the ground with 6 mil plastic with overlapping taped seams. I'd have it go up the walls taped to the rigid insulation on the walls. That's how I'm seeing professionals do it today. Then I would block off and insulate the foundation vents and have either a dehumidifier installed or have supply and return air ducts feeding the crawlspace to use your HVAC to condition the space. A lot depends on where you are in the USA as we have a wide diversity of climates across the fruited plain. Once your walls are insulated and your conditioning the space the insulation under your floors can be removed. It tends to not be well installed and large gaps form as the supports slip and the insulation falls down.

This is a good article on the subject. The site has a wealth of information with solutions tailored to the various climates around the USA.

https://buildingscience.com/documents/information-sheets/crawlspace-insulation

 

[PJB12]

Hello Nobes,

Part of our house is over a crawl. I chose conditioned for our craw space so that I could seal off the vents to outside. (Michigan climate- cold in winter & humid in summer.)

Under the guidance from the professional, institutional insulation company we used at work, I did the following:

Made sure no sharp rocks were on the floor & leveled it.

For the floor, I laid 6-mil, although I think today's standard is thicker. I laid it to cover the floor, overlapping seams 12". (Not taped).

Next I laid perforated radiant barrier with scrim down on the poly sheeting. (Perforated is critical.)

On the radiant barrier, I put 1 x 3 furring strip grid to give the 19mm radiant gap. (Radiant barriers need 19mm or larger gap on one side to work properly.)

On the furring, I attached 12 mm OSB.

On the walls I draped basement mass insulation with a vapor barrier and let if lay over the floor 18" all around. (The coldest part of the floor is the 1st 18" past the walls.) The batts got tucked between joists all the way to the subfloor above.
Seams were taped with vapor barrier tape.

Over the batt insulation, I put perforated radiant barrier.

The rooms above are far, far warmer in winter and cooler in summer than they were. As an added bonus, since the crawl is conditioned (and stays dry), I can store stuff in there. And the light sure is brighter with the radiant barrier reflecting it!

I hope your project goes well!
Paul