Are you considering spray foaming the sub-floor of your crawl space in order to make your home more comfortable and maybe even save some money? Many contractors and DIYers are using this method but what are the risks involved with spray foam on the sub-floor?
Spray Foam Traps Moisture
If you spray foam your sub-floor and the wood moisture is high, you just trapped the moisture in the wood. Due to the sealing capability of spray foam it is very difficult to dry wood after it is applied. This goes for open cell or closed cell spray foam. Both do a great job at trapping moisture.
Spray Foam Traps Mold
Just like moisture, if mold is present when you apply the spray foam, it gets trapped as well. Spray foam does not kill or remove mold. It encapsulates the mold and moisture together so the mold still has its food source and moisture. With wood to feed on and high moisture levels, there is no remediation process once the mold is encased in spray foam without completely removing the foam.
Spray Foam is Hard to Remove from Sub-floor
In the picture above the mold and moisture are trapped behind the spray foam. One could argue the mold developed after the spray foam was applied but most likely it was used to cover the mold. Some crawl space encapsulation companies that don’t understand mold removal will cover up mold with spray foam or paint versus addressing it properly.
If you made the mistake of hiring such a contractor, your sub-floor is in big trouble. Paints and spray foams are very difficult to remove without grinding. As much as we love soda blasting, it seems to be ineffective at spray foam removal as well. Even if you took the bulk of the spray foam off, it leaves a thin plastic type seal behind. This thin seal still traps moisture and mold.
When to Use Spray Foam in Crawl Space
When Crawl Space Ninja uses spray foam in the crawl space it is to seal large holes and restrict air movement from the crawl space to the living space. We don’t like any type of insulation in the sub-floor when wall or foundation insulation can be installed and your crawl space is encapsulated. If you have a vented crawl space then sub-floor insulation is still recommended. But, I would choose batt insulation over spray foam sub-floor insulation because at least batts can be removed and replaced if necessary.
Do you need sub-floor, foundation, or attic insulation installed in your home? If so, please contact Crawl Space Ninja and we will be glad to help. Also, let us know in the comments below if you installed spray foam in your sub-floor and how did it go.
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25 thoughts on “Is Spray Foam Good for My Crawl Space Sub-floor”
I sure wish you guys were in South Texas. I’ve got a real moisture and mold issue under my 30X80 double-wide.
You mention in prior blog about spray foaming floor joists and subfloors. One foam manufacturer recommends spraying on wood with a moisture content of 19 or below. Why do you think they picked 19?
Thanks,
John Northeast Pa.
Most experts say wood rot fungus begins to grow around 20%. That is probably why they picked that number. But keep in mind a leak from above is difficult to detect if floors are spray foamed and spray foam will lock in moisture once the wood is wet, making it difficult to dry if there is a leak.
I have a bare earth vented crawl space that is below grade and insulated with with batt fiberglass between the joists in my subflooring. In the winter, the room above sometimes gets 20 degrees cooler then the rest of the home. I was thinking of removing the fiberglass insulation, then put down 15mil vapor barrier (single layered not string reenforced) and use closed cell spray foam on the block walls and rim joist only ( leaving the floor joist cavity empty) which will effectively seal up the vent to the outside. Unfortunately, the builder did not leave access to the crawlspace. I have to cut a hole in my subflooring which would eventually be sealed up again under carpet. Since access will once again be sealed I was not planning on putting a dehumidifier in the crawlspace due to lack of access. Is this a bad plan? Will this ventually lead to mold and wood rot? Suggestions, comments or recommended alterations to the plan?
Thank you very much!
I think it could lead to mold because there is no humidity control in the crawl space. Could you install a humidity reader to test it out during hot summer before permanently sealing the subfloor access? Hope that helps.
I’m currently making a sub floor. From main house to a converted garage it’s 9″ step down then step up to get to a laundry room. Using 2×8’s and 3/4″ plywood we have a level walk through, discovering the noise level is high. There’s nothing under the area. Would a filling foam be advisable, or should I pull up the plywood and use sound something else?
Dale, sorry out of my wheelhouse to answer that but I would advise against foam (or get local foam contractor to advise) because it can be difficult to remove if it doesn’t work as hoped.
I wish you were in NJ as I have seen practically all your videos on youtube.
Thanks to your videos I put a dehumidifier and closed up vents in the crawl space. Made a big difference in the cupping of our brand new wood floors from humidity.
I did leave one vent open had put a simple box fan to blow out air from the crawl space.
While we did all that last year, this year we are seeing a that our first floor (just above crawl space) feels very cold – it always feels colder and drafty in the house compared to outside.
Do you recommend we do a spray foam insulation of the crawl space? It has batt insulation right now.
Hi Saurabh, Thank you so much for letting us know our videos are helping and your kind words, I really appreciate that. Who knows, maybe one day we can have a New Jersey Crawl Space Ninja. This may already be done since you have subfloor batt insulation, but are the foundation walls insulated and the rim joists insulated and air sealed as well? If not that could help because it will help keep the outside temperature from affecting the crawl space as much. Also, above the batt insulation, I’d recommend you air seal major penetrations around plumbing, electrical, plumbing. That won’t make floors warmer but should cut down on drafting cold air from the crawl space to inside the home. One last thing. Please keep in mind your floors will never “feel warm”. I feel contractors overstate that claim when it comes to crawl space insulation. No insulation will make the floors feel warm but what I shared could make them a little warmer than they are now. You can only accomplish warm floors with floor heat or a heater in the crawl space because the warmest the floor will ever get is as warm as you set your thermostat inside or as warm as the crawl space is underneath. Since your body temperature is in the high 90s unless you set your thermostat at that temp or above, the floors and other surfaces in your home will always feel colder to you. I hope that helps. Here is an article that may help too https://crawlspaceninja.com/insulation/winterizing-crawl-space/
Hello Michael
Have a three season cottage with a vented crawl space in northern Wisconsin.
Your thoughts, going with foam board on the block walls, with fiberglass batt insulation for the subfloor.
Enjoy your videos, it helps!
Dave
Hi Dave, we just released this video that may help. https://youtu.be/JZfksZZbrW0 Hope you like it and let me know if it answers your question.
Hi. I live in south Louisiana. Very hot and humid in the summer and almost all year around. I have a 100 year old house about 3’ off the ground. All around the bottom of the house is open besides wood lattice. When we purchased the house there was no insulation anywhere. As we remodeled I spayed cellulose in the walls and attic ceiling. (12’ ceiling). I have debated what to do in the floors as there is no insulation now and when it does get cold they are like ice and impossible to keep the house warm unless you climb a ladder. My concern with foam is trapping moisture. ( tongue and groove subfloor with moisture barrier then tongue and groove oak floor. The oak is always cupped. Sometimes more than others pending the weather. Also a lot of the interior walls are open under the house so air can travel up the inside of the walls. Obviously when they built this house there was no ac and they weren’t concerned about drafts)
My second problem is we added an upstairs in the attic. The ceiling is framed below the rafters following the roof. It was insulated with r19 and r13 fiberglass leaving 2” in between the plywood roof and the insulation and has ridge vents to allow air flow from the attic through to the ridge vents. The way the upstairs was built, the walls backside the attic now. The attic has vents in the walls with electric fan pushing air into the attic and out the ridge vents. Attic walls also insulated with r19 and 13. For years we had the upstairs completed but never used it much so we never had the central ac installed. Later on we installed central ac and now have a big moisture problem due to the rooms being 70-74 degrees and the back side of the wall (attic) being around 120 degrees and humid air. We had mold grow which we cleaned and fogged the entire upstairs twice. When I added the electric vent fans it helped but not fixed. I added a dehumidifier and it made a huge difference but it is still gets humid and smells if mold at times even though there is nothing visible. I really am at a loss off what to do or how to seal it being the whole house was built before any climate control was invented and the house built in no way to combat moisture from climate control. If you have any advice I would love to hear it. Thank you
Hi Sam, your instincts are correct about the subfloor insulation trapping moisture. I know it is a difficult task to insulate a crawl space that is open to the elements but subfloor insulation is probably your only option although it could trap moisture. Unless you want to hire a mason to build foundation walls and encapsulate the crawl space. In the attic, make sure the dehumidifier is a heavy pint per day commercial dehumidifier. You are fighting way more outside air, heat and humidity than an average crawl space would face. If you have a 70 ppd dehumidifier, you may want to use 2 or upgrade to a 130ppd. Hope that helps.
Hello, I have a question about this insulation process. In your professional experience, would it be better to have additional electrical outlets done prior to the installation of the foam board insulation, or after?
2nd Question, is there someone or company you could recommend for Central Indiana for this service? We just came through a difficult deep cold spell, but Winter has hardly started yet. We still have 3-4 months to go. Final question, would it be better to have this done now, or wait till warmer weather?
Thank you
Hi Thomas, for us the electrical install doesn’t matter before or after although electricians seem to like after encapsulation because it is better to work in. I don’t know anyone in your area but if you know a business minded person that would like to open a Crawl Space Ninja Franchise, please let us know. Having the work done now may get it done quicker because most people wait until summer after they discover a mold or moisture problem. Our industry sees a small slow down in winter so companies are more likely to get to you faster. Other than that, if your crawl space is wet and moldy, now would be better. If only doing insulation, you probably won’t see that big of an effect on your utility cost between now and summer. Hope that helps.
Michael,
We have a condo in a complex, on the ocean, in Myrtle Beach. It is a wood frame structure and is approximately 40 years old. There are 3 separate buildings in the complex. It is built on pilings and the foundation is completely open, except for the use of lattice around the perimeter to keep larger animals and unwanted people from going under the building. The distance from the ground to the floor joists is anywhere from 18″ to 4 feet.
There is existing bat insulation between the floor joists with sheetrock attached to the bottom of the floor joists to hold the insulation in place and protect it from the elements.
Over the year, contractors, performing work on various units, have pulled the sheetrock and insulation down to access the subfloor. They then failed to replace what they pulled down.
As a result of recent storm damage, the insulation and sheetrock has been completely removed from under one of the buildings. It has been suggested that spray foam be used to reinsulate the floor. It was also stated that sheetrock was not a good choice to use to hold the insulation in place.
After reading your blog, it is apparent, (at least to me), that spray foam is not the way to go. If I am reading your recommendations correctly, we need to reinstall the bat insulation.
I have already forwarded your blog on spray foam to the Board of Directors, but there are some that continue to push for the spray foam.
Given that this is a completely open crawl space, do you still recommend bat insulation?
Also, what type of material should we use to hold the insulation in place and protect it from the elements?
Do you have any other suggestions for a completely open crawl space?
What consideration should be given to the chemical vapors from the spray foam migrating into the living space.
Thanks,
Gary
Hi Gary, unfortunately in your case, I would probably opt to spray foam it and some other options that I explain in the video listed and not use batt insulation or sheetrock. In a perfect world you’d want to dry the subfloor before installing any type of insulation. I did a video on this regarding a sunroom. There are not a lot of options based on an open air crawl space. In the video I discuss using foam board with spray foam and more. I hope this video helps. https://youtu.be/Jc-_9EfkfBs
Hi Michael,
Thank you for your response.
I viewed the video and, given the size of our complex, I feel the methods, noted in the video, would be cost prohibitive.
I have a few additional questions.
First, I found the following website that addresses the air quality inside of a residence after spray home has been applied.
https://www.ecohome.net/guides/3650/is-spray-foam-insulation-good-or-toxic-dangerous-in-homes/
The specific quote I am noting is, “Every year there are numerous legal cases involving uncured foam rendering homes uninhabitable making their way through the courts, including this one with over $2.5m damages awarded for “life-altering serious injuries as a result of exposure to chemicals.”
The last thing we want to do is make someone’s home uninhabitable. Do you have any thoughts on this concern?
Second, if we went with batt insulation, would it be better to apply greenboard/durarock or would it be better to apply an open weave product such as, https://www.mcmaster.com/Plastic-Mesh/plastic-fencing/, to the bottom side of the floor joist?
Hi Gary, unfortunately I cannot speak to the habitability of all spray foams nor the legal ramifications but the one we use has a 1 hour re-entry time as per the manufacturer and is one of the greenest products available in the spray foam arena as per the manufacturer. I have seen batt and blown in insulation installed in crawl spaces using a mesh to support it. I have also removed a lot of batt and blown in that had mesh support. Without controlling moisture/humidity it can become damaged. Our company would not do the drywall/durarock option. Hope that helps.
I need advice, I have a 30 x 30 bungalow in the Catskills mountain of NY. The bungalow is just used in the warm weather months. My crawl space is 2 feet on one side of the house and 6 inches on the other side, the crawl space is also open. Last year I had 15 mil plastic laid over the dirt, with rocks to keep the plastic down. I just received crazy prices to spray foam in the crawl space floor. My subfloor over the crawl space is just 1 inch plywood boards with a rug. I want to rip the rug up and place vinyl laminate flooring down. What’s the best way for me to proceed and keep the moisture out of the bungalow? Lay down another row of plywood going the other way and seal seems? Use a vapor barrier ? Any advice would be very much appreciated..
Check the humidity in the crawl space and it gets above 60%. You may want to install a dehumidifier in the crawl space or install a dehumidifier in the living space that will blow air into the crawl space. If the crawl space is too low for a dehumidifier.
Hi Michael,
We went to contract on a 22 year old house in Tampa that we thought had a minor mold issue in the newly installed floating laminate flooring. Before receiving report, we were told that it was due to original floating laminate flooring not having a moisture barrier & new flooring was installed over original laminate, creating moisture pocket. When we received the scope of work receipt from certified remediation contractor, the floor replacement & air quality reports, its apparent it was a much larger problem affecting the entire sub floor.The seller did a complete remediation, removing 2 layers of laminate flooring, the sub floor, base boards, etc. Scrubbing & treating the joists, walls, etc & having an air quality test done prior to reinstallation of new floor.The sub floor was replaced in about 80% of the house.To prevent the moisture & mold from returning again, a BASF closed cell spray foam was applied to the underside of the sub floor in the crawl space. So…my questions to you are the closed cell spray foam a recommended barrier in a FL crawl space to prevent a repeat issue with mold & moisture. Is it an effective barrier? If it is, are there additional measures we should take to further insure that mold does not reoccur. We have an inspection tomorrow & only have 2 more days to back out out the contract. Are there specific things we should look for in the crawl space during the inspection? We don’t want to worry about recurrent mold issues & wonder whats lurking below out feet. Thanks!
Spray foam is a very effective moisture barrier. But I hope they made sure the floor and sub floor were dry before application because if not it could trap moisture. Another issue with any type of insulation on the sub floor is it can trap moisture if there is a leak from above. I would always recommend a crawl space dehumidifier if there is a way to put one under the floor to keep the flooring from absorbing moisture versus just spraying it with foam or applying insulation especially in a humid environment like Florida. I hope that helps.
Hello. Here’s my thing. 1972 double wide. Floating honeycomb sandwich floor (no floor joists). It lays on top of the MH Chassis. Disintegrating and falling through. Central/North Florida. Dirt crawl space surrounded by solid concrete bricks and small air vents, with one 10’ area being plastic siding (where block walls torn out before my time for new A/C installation access). In the forest. Very hot and humid. When I bought this, the floors were very solid. Over 2-3 years now floors decided to have major problems. I think because I use the air conditioning and prior owners didn’t? Basically what I see is the subfloor is a sawdust glued type sheet (no, not OSB or particle board) more like compressed sawdust. Then a cardboard honeycomb type product about 2-3” high, then another sheet of the compressed sawdust sheet however the underside of that (toward the crawl space) is black in color like painted or something. This is a DYI project, I am going to cut out the subfloor, remove the honeycomb, and most probably also remove that final sawdust sheet to avoid future problems with that. Then going to frame out the sides (inner parts) of the mobile home’s chassis/beams where needed, and then attached some joists to that wood framing (adding jacks of course for the extra weight this causes). Kind of like building a deck. Then screw down ¾” plywood for the subfloor. Now, there are these flat metal A/C ducts (quite thin), that run along the bottom of the subfloor! (Probably major cause of the existing sawdust subfloors falling apart). The ducts were never wrapped! Reading all information on your web, and others, I do not want to encapsulate the crawl space, but I will install larger air vents. 1. Would you use PT wood for framing and joists. 2. Would you use PT plywood for subfloor. 3. Would you use bat insulation, then install 6mil barrier across entire bottom of mobile home. The problem with that, is the A/C ducts being right under the new plywood subfloor and being encapsulated would the condensation from those ducts still wreack havoc? So, 1. Would you weave 6mil barrier under joists then up into the voids and over the A/C vents (between vents and subfloor, and skip insulation etc. Originally that foam spray sounded good, but there is mold and moisture issues already and, if anything happens later I do not want to be the one or paying someone to remove that foam! Pardon my weird way of explaining things, I am female. LOL
Nellie, my apologies but we do not do work on double wides and I don’t want to give you bad information. Your plan seems like a good one to me but you may want to contact a double wide manufacturer for more details. Sorry I couldn’t help.