How To Install a Vapor Barrier in a Crawl Space
How To Install a Crawl Space Vapor Barrier To Control Moisture and Odor
Like many homes, our cabin is built over a soil crawl space. While crawl spaces offer simple construction and access to the underside of your home, they provide little protection from the environment, and can lead to structural and air quality problems within your home.
Standing water, high moisture levels, soil gas, dust and an abundance of rodents and other critters are common in unsealed dirt crawl spaces.
Our crawl space had all of these, including several inches of water after heavy rains. In addition to water, we also noticed odors and fine dust in our home that seemed to be coming from our heating and air conditioning ducts from the crawl space. We needed to do something.
After some reading, I decided to seal the space with a crawl space vapor barrier. After a few bids running in the $5,000 – $8,000 range (just for the membrane install), I decided to do some research and do it myself. Read on for the juicy details.
Crawl Space Encapsulation
As most of you know, crawl spaces are shallow, typically unfinished basement spaces. Many have dirt floors and readily allow entry of moisture, mold, soil gasses, radon, insects and vermin, which can compromise the structural integrity and air quality of the home.
In addition, moist crawl spaces promote rot of structural components, and floor joists contained within the space.
Traditional thinking (and building codes) promoted ventilation of crawl spaces to circulate outside air into the crawl space and control moisture.
Unfortunately, ventilation of crawl spaces often does more harm than good. In many climates, ventilation can make moisture problems worse by introducing an unending supply of relatively humid outside air into the crawl space that can form water once in contact with the cool surfaces within a crawl space.
Current recommendations (and newer building codes) recognize the moisture problems associated with ventilated crawl spaces and call for sealed crawl spaces without foundation vents.
Unsealed soil or concrete floors within the crawl space can allow the passage of moisture and liquid water into a crawl space. Building codes typically call for the addition of a vapor barrier to a concrete slab or over an unfinished dirt floor of a crawl space to help mitigate the passage of moisture into the space.
Unsealed soil or concrete crawl space floors also allow the entry of various gasses into the crawl space. Some of the gasses are known to be dangerous (radon, methane) while others simply compromise the air quality within the home by way of unwanted odors.
If water and unwanted gasses are not bad enough, particulates like silt and dust can easily move from a crawl space into living spaces. This is especially true if heating and air conditioning ducts are located within the crawl space.
Natural movements of air through the home during heating periods due to “stack effect” can also easily transport particulates with air movement through the home.
As you can see in the photo above, we do have HVAC ducting within the crawl space. We suspect that much of the dust and dirt we noticed in our cabin was entering our home from the crawl space by way of this ductwork.
So how do you tame the nasty beast that is the dirt crawl space? Simple: seal it up. The ideal crawlspace should be an extension of the living space within your home.
It should be clean, dry, insulated along the exterior walls (foundation walls) and sealed with a weatherproof barrier to exclude moisture, gas and other contaminants from the soil and atmosphere.
Prior to installing this vapor barrier, care should be taken to eliminate other sources of water entry into your crawl space. In our crawl space, we first sealed old foundation vents and installed a drainage tile and sump pump system.
For additional information see my article detailing Sump Pump Installation.
The Addition of an Active Membrane Depressurization System in Your Crawl Space
Adding an active depressurization system to your encapsulation project can significantly improve the performance of the system – especially for the removal of moisture and soil gas.
An Active Depressurization system incorporates a radon pump to create a continuous vacuum under the vapor barrier membrane.
This vacuum actively sucks out moisture, gasses, and fine particulates from under the membrane. These are the same systems used to evacuate radon gas from basements and crawl spaces of homes.
The benefit of adding this active depressurization system is the better performance of the barrier system; the downside is more work, complexity, and cost.
Despite the added cost, many professional installers strongly recommend the addition of a depressurization system with vapor barrier installation. The barrier alone may only concentrate moisture and gasses that later leak into the crawl space through imperfections in the barrier system.
Active depressurization systems are identical to those used to mitigate Radon gas. The principals of such systems are identical and the concepts the same for these systems.
In fact, I used a publication for radon system design to create the system for my crawl space. The publication is the excellent book by Douglas Kladder, Protecting Your Home From Radon: A Step By Step Manual for Radon Reduction (Second Edition)
The book is a terrific reference, and I highly recommend reading it if you plan to install an active system.
Additional Crawl Space Information and Reading
- CrawlSpaceVaporBarrier.net – High quality U.S. made membranes, tapes and supplies and excellent how-to videos for crawl space encapsulation. (Highly Recommended)
- Protecting Your Home From Radon. A Step-By-Step Manual For Radon Reduction. 2nd edition. Colorado Vintage Companies, Inc. July 2003. (Highly recommended)
- Crawl Spaces. GreenBuildingAdvisor.com
- Research Report RR-0401 Conditioned Crawlspace Construction, Performance and Codes. Joseph Lstiburek. Nov 27, 2006. BuildingScience.com.
- BSI 009: New Light in Crawlspaces. Joseph Lstiburek. Oct 16, 2008. BuildingScience.com.
- Building an Unvented Crawl Space. Martin Halladay. May 13, 2011. GreenBuildingAdvisor.com
A word of caution before you embark on this project – crawl spaces can be nasty places! Rodents, mold, dust, debris and tight working conditions can make this a very unpleasant and possibly dangerous place to work. Contact with mice is associated with Hantavirus infection – which can be fatal. Contact with mold can also have adverse health effects. If you are unsure about the condition of your crawl space contact a professional for further advice before proceeding.
Vapor Barrier Installation / Sub-Membrane Depressurization System Installation
Preparation and Materials
- Level: Moderate
- Time: Days to Weeks
- Cost: $500 – $2000+
Project Big Picture
- Clean crawl space
- Measure and diagram crawl space
- Plan membrane layout
- Plan Sub-Membrane Depressurization System
- Order Materials
- Prep crawl space floor and foundation walls
- Lay under-layment fabric
- Lay drain pipe for depressurization system
- Lay vapor barrier
- Connect Sub-Membrane Depressurization System
Materials – Crawl Space Vapor Barrier Cost (materials)
|Item||What I used|
|Vapor Barrier Membrane|
|Vapor Barrier Seam Tape|
|Vapor Barrier Fasteners|
|Vapor Barrier Underlayment Fabric|
|Spray Foam Kit|
Materials – Sub-Membrane Depressurization System (if using)
Crawl Space Vapor Barrier – Tools / Supplies
- Rake, hoe or similar tools to prep and level crawl space
- Tape measure, and similar to diagram crawl space
- Utility knife, scissors to cut materials
- Caulk gun, rags
- Drill, hammer for installing foundation pin fasteners
- Tools, supplies for PVC circuit install
- Tools, supplies for radon fan install
- Protective gear – eye wear, mask/respirator, coveralls
- Portable small vacuum, wool duster
- Flat plastic storage bin as tool tote
Vapor Barrier / Active Depressurization System Installation – Step by Step
- Clean Crawl Space and Level Surface.
Thoroughly clean crawl space and remove trash, old wood, metal, sharp rocks and other debris that could puncture the membrane. Use a lawn rake to remove larger debris and level the crawl space floor.
- Measure Crawl Space, Obtain Materials and Plan material layout.
Measure and diagram your crawl space including the height of foundation walls and supports. Create a diagram of your space. Once your diagram is complete, estimate the amount of membrane and underlayment (if using) to order.
Assume 6 – 12-inch seal overlaps and foundation membrane heights of 1 – 2 feet or more. If using an underlayment fabric, you will obviously not need to include foundation walls in your calculations for the amount of underlayment material.
Next, obtain your materials. I ordered my vapor barrier membrane, underlayment fabric (if using), seal tape, foundation wall mounting tape, polyurethane caulk and mechanical mounting supplies (plastic pins, masonry screws or nails, etc) online and had it delivered to my home.
When looking for materials, I highly recommend considering the highest quality materials you can afford. The difference between thin, low quality, recycled plastic membranes from your local hardware store or home center and high quality, reinforced membranes is enormous.
Crawl space encapsulation is a nasty job, and that offers terrific benefits if done right — use quality materials and do it right the first time!
I ordered my material from CrawlSpaceVaporBarrier.com (Crawl Space Repair) and highly recommend their products! I used their 20 mil SilverBack Vapor barrier, Felt 550 underlayment fabric, white seal tape, foundation tape and the plastic foundation fastening pins.
All of the products I purchased and installed from Crawl Space Repair were top notch – I could feel and see the quality compared to other membranes I have seen. Ordering from their site was easy, and everything arrived quickly, well packaged and intact.
When deciding what membrane thickness to use and whether or not to use an underlayment fabric, consider your crawl space conditions and your desired use of the space once finished.
Thicker, higher quality membranes are indicated for high traffic crawl spaces and those with uneven surfaces and surface characteristics that increase the risk of puncture of the membrane. Crawl spaces with a greater risk of membrane puncture (coarse soil with rocks, concrete debris, etc.) may also benefit from a protective underlayment fabric — like the Felt 550 that I used for my project.
Low use crawl spaces with good quality, even surfaces that are lower risk for membrane puncture typically can use thinner membranes. I still would recommend using a minimum of 12-mil thickness and stick to quality, US made products. Most building codes call for 6-mil or greater membrane thickness.
- Cut and Label Material For Install.
Plan your desired material layout, and then cut the pieces of membrane for the installation. I recommend doing this outside of the crawl space. Measure, mark and cut membrane sections first, outside of the crawl space, then pass them back into the crawl space to install.
Most crawl spaces are cramped, dirty and dark. Measuring and cutting rolls of thick plastic is much easier outside of the space. I used my lawn to spread out the material, cut it and roll it back up into labeled pieces. I then passed the rolls into the crawl space and unrolled them in the space for the install.
Many choose to install the membrane along the foundation walls first. Hang it like wallpaper and leaving enough material to overlap at the floor by 1 to 2 feet. With the wall pieces installed, install the floor membrane.
For me, it made more sense to install a portion of my L-shaped crawl space as above, foundation walls first followed by the floor. For the remainder of the crawl space, I started the membrane at the top of one foundation wall, continued to the floor and finished at the top of the adjacent foundation wall. It makes little difference how you orient the material as long as you install it flat with well overlapped properly sealed seams.
When laying the membrane do not pull the membrane tight – leave enough slack in the material to allow movement and not to stress the taped seams and wall attachments. Be sure to leave sufficient slack over any components of a depressurization system that will suck the membrane to the contours of the floor and walls once activated.
- Install Underlayment Fabric.
I started my project with Felt 550 underlayment to help cover the rocky, debris-filled dirt floor of the crawl space and to help provide a slight airspace for the sub-membrane depressurization system.
One of the hidden advantages of using the underlayment fabric is the improvement of the working environment the material provides. My crawl space floor is a very fine silty soil that made it a very nasty place to be.
Once the underlayment felt was down, conditions improved dramatically, making the work much more enjoyable (and likely safer).
For an active depressurization (suction) circuit below the vapor barrier, you will need to layout the drain tile pipe for this system on top of the crawl space floor (on top of an underlayment if using) before rolling out the vapor membrane.
- Layout and Install Sub-membrane Depressurization Drain Tile Pipe (if using).
The corrugated drain tile I used is the typical 4″ perforated plastic drain tile found at most home improvement stores. I used the type with a synthetic filter fabric sleeve to help prevent plugging of the perforations over time. The underlayment fabric should also help to prevent the entry of sand, clay and other obstructing debris.
The use of a perforated pipe provides a vacuum circuit for the removal of accumulated moisture, moisture and gas and odors below the vapor membrane. The use of this perforated drain pipe can also act as a conduit for water movement (into a sump basin for example) if installed in a properly sloped space. However, it is not necessary to consider slope when using the drain pipe solely to provide suction under the membrane.
To layout the drain tile, extend the corrugated pipe by pulling it to full length. Then loosely layout drain pipe throughout the crawl space. You do not have to pipe on both sides of the space for most average size spaces — a single pass of pipe down the middle of the space is sufficient.
Plan the layout to allow for easy connection to a PVC riser to connect to a radon fan. Plan the passage of this riser PVC pipe (usually 4″ schedule 40 PVC) through the living space in a concealed spot (closet, wall cavity or similar) or to the outside of the home. For my project, I ran my PVC pipe through the back of a small broom closet.
For my sub-membrane corrugated drain tile layout, I ran the pipe along the foundation walls of the entire space. I planned for the riser connection, just under a closet where I planned to route the riser PVC pipe to the attic.
To connect the drain pipe to the PVC riser, use a system of 2 rubber roof boots (4″) is used. Sandwich the vapor barrier membrane between a bottom and top roofing boot through which, a piece of thin wall (DVW) 4″ PVC is passed and attached to a DVW Tee fitting. This Tee fitting connects to the corrugated vent pipe network.
If you plan to run 4″ schedule-40 PVC for the evacuation circuit to the attic, you will need a 4″ thin wall (DVW) to 4″ schedule-40 PVC coupling. Use this coupling to connect the riser boot (connected to the corrugated pipe) stub out to the schedule-40 evacuation circuit out of the crawl space.
To connect my riser apparatus to the long run of 4″ PVC heading to the attic, I used a 4″ rubber plumbing coupling to allow for service later if needed and help reduce any vibrations in the system.
Although I only used one suction point riser, you may consider multiple suction points for larger or non-continuous spaces. My crawl space is rather small (~600 SQ FT) so I used just one suction point.
When planning the location of the suction point riser, you will need to add a 3-way Tee fitting to the corrugated drain pipe run. The top joint of the Tee will allow you to connect the corrugated piping run.
Once you have laid the corrugated pipe, connected a Tee fitting at your planned location for the suction riser. Later, you will be ready to create the suction riser by cutting through the membrane to access the Tee fitting.
To seal this riser, you will use two 4″ plastic roof boots that are typically used to seal 4″ PVC pipe penetrations through the roofing. One will sit on top of the corrugated drain pipe Tee, under the vapor barrier membrane, and the other will sit on top of the vapor barrier.
A piece of 4″ thin wall PVC pipe will then pass through both roof boots and connect to the corrugated drain pipe Tee. The two roof boots are then sandwiched together with caulk and sheet metal screws to create an airtight seal between the suction riser and the vapor barrier membrane.
Prior to sealing the membrane, you will need to have one roof boot under the membrane. Even if you forget to install the first roof boot prior to sealing the membrane, you can typically roll the plastic roof boot enough to pass it through the hole you will cut in the membrane for the suction riser pipe.
So, once you have the corrugated pipe installed with the 3-way Tee connection with one of the roof boots aligned over the top Tee connection, locate the top of this Tee under the membrane and cut a circular hole. The hole should be large enough to allow the crown of the first (under membrane) roofing boot to stick through.
Once the hole is cut, align the under-membrane roofing boot and the top connection of the corrugated 3-way Tee opening.
Then pass a piece of 4″ thin wall PVC through the roofing boot that is sticking out of the membrane hole and into the corrugated Tee connection. Seal this connection if the fit is not tight and there is a risk of the components separating.
Because this connection is under the membrane, it does not have to be “airtight”, but should be mechanically tight. Now pass the second roofing boot over the thin wall PVC riser and push down to sit on top of the membrane. Under this boot apply a generous bead of polyurethane caulk.
After caulking, push the roof boot down firmly on top of the membrane. Secure the top roofing boot to the boot under the membrane using stainless steel sheet metal screws. The screws should draw the two boots together and along with the caulk form an airtight seal (see photo).
Naturally, you will not complete the assembly of the suction riser until the vapor barrier is installed and is discussed here to aid in the required planning for the riser assembly after the membrane is laid.
- Layout and Secure Vapor Barrier Membrane.
Now that you have your underlayment fabric (if using) and corrugated drain pipe (if using) in place, it’s time to begin to install your vapor barrier membrane.
Depending on the size of your crawl space, you will likely choose to install your membrane in sections. Many install the foundation wall sections of the space followed by floor pieces. The specifics of your crawl space and material size will determine the best install plan for your project.
To securely hang the vapor barrier membrane, you will need to use a system of fastening the membrane to the foundation walls. Many use double-sided tape (foundation wall mounting tape) to provide an initial stick and seal and complete the attachment with mechanical fasteners spaced every few feet along the foundation walls.
In addition, to double-sided mounting tape, polyurethane caulk can be added to help secure and seal the membrane to the foundation walls. The addition of caulk is especially useful for irregular, wet, or uneven foundation wall surfaces (older, irregular wall block, stone, walls with spray foam insulation).
To complete the mounting of the membrane, most use a mechanical fastener such as a plastic trim pins or similar to hold the membrane securely in place on the foundation walls.
Prior to applying the foundation tape and caulk (if using), clean the foundation block with a broom, brush, vacuum or similar for improved adhesion. Take some time to rub the tape into the block with pressure to enhance the tape contact with the foundation.
After the membrane is attached to the double sided tape and caulk, secure the membrane to the wall with a mechanical fastener spaced every 1 – 2 feet.
To use plastic trim pins (similar to those used to hold on auto body trim pieces), first pre-drill holes through the membrane into the block. Then, tap the pins into the holes with a hammer. Pass the pins through the double-sided tape or caulk bead or higher to avoid leaks.
This system of double-sided tape/caulk plus plastic trim pins works well with flat foundation walls. However, my foundation walls had a very irregular layer of spray foam that made the above, standard method of fastening the membrane difficult.
Because the foundation tape did not stick very well to the spray foam surface, I used a very thick (3/8″+) bead of polyurethane caulk with the double stick tape. In addition, I use powder-actuated concrete nails as mechanical fasteners to secure the membrane to the wall.
When I was thinking of using a powder-actuated nailer (Ramset Cobra) to fasten the membrane, I had to consider the risks of such a technique. A powder-actuated faster system can easily damage your foundation walls.
Prior to using this system, I trialed the nailer using a few spare foundation blocks I had in my boathouse to test whether the nailer would crack the blocks.
My tests did not reveal any evidence of block damage when using the Ramset system, so I proceeded with caution and only used the Ramset nails were I could not use the traditional plastic trim pins.
Other options to consider; concrete screws through pre-drilled pilot holes, and pre-drilling pilot holes for the Ramset concrete nails.
As you install the membrane on the foundation walls, take some time to rub the membrane over the area of the double sided tape and caulk to promote adhesion and a good seal.
Once the foundation wall pieces are in place, begin laying out your floor pieces. Overlap the foundation pieces and floor pieces of the membrane and any seams by 6 – 8″ or more.
As you install the vapor barrier membrane, overlap the seams by at least 4″ – 8″ or more. Under these seams, run a bead of polyurethane caulk and finish with seam tape.
Take the time to rub these seams flat to spread the caulk and promote adhesion of the seam tape.
- Seal membrane around pipes, columns and other penetrations.
When you encounter pipes, support columns, or other objects that will penetrate the membrane, you will need to cut the membrane and seal the membrane to these penetrations.
For pipes and similar items, cut a hole in the main membrane around the penetration. Then, use a separate, oversized, overlapping patch piece of the membrane to conform to and seal to the penetration. Seal this patch to the penetration and main membrane using polyurethane caulk and seal tape.
Support columns should be finished in the same fashion as foundation walls, running the membrane up the support several feet. At the foot of the column, overlap the main membrane by at least 6 – 12″.
Seal the membrane to the support block in the same manner as the foundation walls (double-sided tape, caulk, mechanical fasteners) and seal the overlapping membrane with urethane caulk and seam tape.
Once all the seams and penetrations have been sealed and taped go back and check your seams to ensure you haven’t missed any. Seams in areas of foot traffic or those at risk of higher stress or wear can be reinforced with a second layer of seam tape.
While a vast major of vapor barrier installs should be sealed, secure and complete at this point, I took my vapor barrier project a step further.
For my project, I added a band of spray foam at the top seam of the barrier attachment to the foundation walls. My walls had an existing irregular layer of spray foam in place that made connecting to and making a seal to this surface difficult. The spray foam was my secret weapon to help complete the job.
The other huge benefit of the spray foam is that it allowed me to secure and seal several sections of the membrane in areas that I could not completely reach due to very low clearances.
In some areas, I could just barely touch the foundation wall with my outstretched arm, making it nearly impossible to fasten and seal the membrane to these areas using standard tape-caulk-fastener methods.
The foam also helped me seal the membrane around a section of very irregularly shaped walls that were the foundation of a fireplace and built with fieldstone.
DIY foam kits make this easy to do. You can order kits online that are ready to use for this purpose.
Be sure to read the safety and use documentation from the spray foam manufacturer carefully. You will need a quality respirator with cartridges suitable for use with spray foam. Additionally, a disposable coverall, hat, gloves, and goggles is recommended when using spray foam systems.
- Plan and pipe the remaining components of the evacuation system (Optional).
If you plan to use a sub-membrane depressurization system with the membrane, it is now time to run the piping for the system and install the vacuum pump (radon pump). I will assume you already installed the flexible corrugated drain tile pipe under the vapor barrier membrane or have an existing drain tile or similar system you can connect to.
As illustrated above, you should have access to the drain tile system by way of one or more roof boot sealed PVC riser connections through the membrane.
The riser pipe used to connect to the flexible corrugated drain pipe tee fitting needs to be thin wall type PVC (waste pipe) and thus this thin wall PVC riser pipe will need a thin wall to standard (schedule 40) PVC adapter. To this adapter, connect the PVC pipe run to the vacuum fan (radon pump) you will be using.
Most system designers and fan manufacturers recommend placing the fan outside of the living space of your home — typically in the attic, along the outside of your home or in the garage. The main reason for this help prevent high levels of exhaust gasses (radon) from spilling into the living space if a leak should develop.
My system is not targeting radon since our levels are low, but the consideration still makes sense, and I planned to install my system fan in the attic. Decide where you will route the PVC pipe run to the pump.
I chose the back of a closet and planned the placement of the membrane riser connection just below it to minimize pipe runs and elbows. You will need to cut a hole through the floor(s), and ceiling(s) involved in your pipe run. Use a slightly over-sized hole saw on a heavy duty (1/2″) drill for easy, clean cuts.
Once the pipe is run through the floor and ceiling holes, seal the pipe hole with caulk.
- Connect and install the radon fan in the depressurization system.
Once the PVC piping from the membrane riser connection (in the crawl space) is in place, then the next task is to connect a radon fan (pump) to the system.
As previously mentioned, radon fans are typically installed outside of the living space or crawl space. I plan to install mine in the attic of the cabin.
With the PVC pipe to the attic, you simply need to complete the PVC circuit to the radon fan.
The fan will need a source of 120 AC power, typically by way of a standard grounded plug. I did not have an outlet near the planned fan installation, so I installed one prior to connecting the pump.
I used the fresh outlet install as an opportunity to “upgrade” my fan system by adding a heavy-duty fan speed switch to the electrical circuit I installed for the radon fan.
This fan speed switch will allow me to adjust the speed of the radon pump and control the amount of suction applied to the membrane. The adjustable fan speed switch will allow me to tweak the fan speed for considerations of noise, power use, and desired vacuum.
For radon installations, the use of a fan speed controller is typically not recommended, but since my system is just to control moisture and odor, I welcome the flexibility a speed control allows.
When connecting the radon fan, I used rubber couplings to connect both ends of the fan. The couplings limit the transmission of vibration and allow for easy fan replacement if needed.
- Connect the exhaust circuit to the outside of the home.
Complete the system by piping the exhaust end of the fan to the outside of the home, in my case through the roof. If you chose the through the roof option to exhaust your system, be sure you complete the proper roof work to ensure a leak-proof install.
The exhaust circuit can also be passed through an exterior wall, avoiding an additional penetration through your roof. If you decide to use an exterior wall to exhaust the system, consider the system’s proximity to windows, decks, and other areas were exhausting gasses may be undesirable.
To cut the exhaust circuit hole through the roof, I used a drill with a hole saw bit. Next, I passed a length of PVC pipe from the exhaust end of the radon fan through this hole and out the roof. Use care to align the pipe so that it is plumb and straight as it leaves the attic.
Other than the cap that comes with the roof flashing boot, there is no need to add a cover or tee fitting to the end of the exhaust pipe.
The system is designed to get wet and in cold environments, the addition of a cap to the end of the pipe tends to accumulate ice from the moisture in the exhausted air.
- Turn on the system and check for leaks.
With the depressurization system connected, and the membrane laid and complete it is now time to power the radon fan and inspect the membrane for leaks.
The radon fan will provide substantial negative pressure below the installed vapor barrier membrane.
Depending on the characteristics of your membrane install and how porous your soil and foundation walls are, the system should be able to develop a vacuum of several inches of water pressure.
The system will perform better if the membrane is well sealed to the foundation walls and all seams in the membrane are tight.
With the depressurization fan on, the vapor barrier membrane should draw down on the soil floor and against the foundation walls. This drawing down of the membrane highlights the importance of leaving slack in the membrane material when installing. Membranes installed without slack can pull apart at the seams when the vacuum pulls the membrane flat.
Once you have checked the system and repaired any leaks, your system is ready to run.