Should I Insulate the Floor Over My Dirt Crawl Space?
People ask me all the time if insulation is necessary in the crawl space. If you are thinking of insulating the floor over a dirt crawl space, we recommend you think about the long term of what you will be doing with the crawl space.
If you plan to encapsulate the crawl space your insulation procedure may be different than if you are planning to keep it vented or open to the outside.
Have You Ever Heard, “Don’t Insulate Your Crawl Space”…?
To us it is comical that there are crawl space repair companies out there telling people not to insulate their crawl space.
Why would they say this?
The fact is international residential building code requires crawl spaces to be insulated.
So when you pay someone to encapsulate your crawl space and they do not install crawl space insulation, that’s the same as paying an electrician to install wires not to code.
Remember doing something to code is considered the minimum of what should be done. So many crawl space companies are not even doing the minimum requirements to making your crawl space healthy and energy efficient.
Insulating a Vented Crawl Space
If you would like to keep your crawl space vented because you live in a low humidity environment we recommend insulating the crawl space floor with fiberglass or rock-wool mixed with spray foam for air sealing. This will give you the thermal barrier between the crawl space and your living space as well as help keep the air from getting from the crawl space into the living space.
Remember up to 50% of the crawl space air can transfer into the living space.
Insulating During Crawl Space Encapsulation
You can also insulate the floor of the crawl space and the sub-floor of the crawl space if you live in a cold environment. Insulation on the dirt floor may be useful but in Tennessee we find this not to be so. If we are performing crawl space encapsulation and we feel the insulation needs to be removed due to water or mold damage, then we recommend insulating the walls of the crawl space to keep the cold out of the crawl space in the winter and the warmth out in the summer.
This will make the crawl space better conditioned and more energy efficient.
Insulating the crawl space and air sealing the sub-floor along with rim joist insulation and air sealing is a great way to make sure your entire home is energy efficient.
Schedule a Consultation to Encapsulate Your Crawl Space
An unfinished crawl space can lead to several issues with your home. Damp conditions are a breeding ground for mold and pests. Secure your family’s health by improving your home’s air quality with an encapsulation. Please contact us if you would like a free crawl space inspection and see how Crawl Space Ninja can help you!
38 thoughts on “Insulating Floor Over Dirt Crawl Space”
I have a 20 year old, 2 story log home, with a huge basement, The basement has a back entry, 2 levels and a what I call a shelf toward the front of the home.The basement floor is dirt, so I initially covered with 6 mil black plastic, but have planned to cover the walls with either eps with foil backing, or polyiso with foil backing, minimum of 2 inches thick, I live in 4-5 climate zone. Floor is covered with 6 mil but still need to cover the concrete block walls. There is no mechanical in the basement, other then a newly installed 200 amp electrical box and generator transfer switch. 4 over head, single, low wattage bulbs, I do not plan on using this space except for storing clay and porcelain slip. Can you offer any suggestions?
It sounds like all you are doing is great. The BuildingScience.com gurus would recommend rim joist insulation as I would too as well as air sealing major penetrations in the sub-floor, like plumbing, electrical, hvac etc. Hope that helps and thank you for writing us.
Thanks for your suggestions, I had read so many articles related to eps, XPS, and polyiso, that it seemed like I was going in circles, as many contractors and air quality experts were contradicting each other’s science, it was quite confusing. What made me decide on EPS, that XP’s gases off much over time, and looses its r value, and polyiso, the one I almost went for, I found it’s r value is 13, but it’s efficiency drops if not warmed, so I was going with EPS, it’s the middle one, at 7.7, but found another one, which is gps, (eps/ graphite) raises the r value to 10. Foil faced on one side and a vapor barrier on the other, going to begin my project soon, there are no codes in my area, no mechanicals to start fires, but hopefully, I will work on getting a concrete floor, and finally studded walls,.
I have a 30 year old home in southern middle Tn with vented crawl space and Terminex said current insulation was not good anymore and needed to be removed and replaced cost est of almost 20k of which I didn’t have. Husband got under there soon after and removed the insulation with intentions of replacing it himself and then covid hit and price skyrocketed ugh, so it’s been couple years now with no insulation and was considering a product that is a reflective insulation roll with closed cell foam core radiant barrier sandwiched between engineered foil with an r value of 15 do you feel this would be good enough to use? Seems it would be much easier for diy and wouldn’t have to worry about irritation to skin and breathing concerns while installing THANKS SO MUCH FOR YOUR INFO AND POSTS❤️
Hi April, I understand the logic of installing a product in the subfloor that does not have fiberglass, but I would caution you on any material that could trap moisture between the joists or hide water leaks from above. Both could cause an unseen mold issue. Have you considered installing that radiant barrier insulation product on the foundation walls and sealing the crawl space? Most likely you would use less materials because wall square footage tends to be less than subfloor square footage, unless your foundation walls are really tall. Also, sealing the vents and controlling humidity with a dehumidifier can cut down on drafts and moisture because it keeps outside air from entering the crawl space. Have you seen the Essential Guide to Crawl Space Encapsulation video https://youtu.be/oy68gtEi_bg? If not, it may have some good information to help you decide on whether to seal the crawl space or keep it vented. Hope that helps and thank you so much for reading our posts :).
Hey Michael,
I live in the SF Bay Area and thinking about insulating our crawl space. The crawl space is very low in height… maybe 3 feet. First question, is it necessary to insulate the crawl space? If the answer is yes, what kind of insulation do you recommend? Second question, a vapor barrier is a must if insulating, correct?
Thanks!
Hello Richard. We always recommend insulating the crawl space when it can be done. Short crawl spaces pose many challenges. Would it be possible to install insulation between the joists if the crawl space is vented or not vented or spray foam or foamboard walls if it is not vented? If it is 2-3 feet we would do walls with foam board if not vented. Also, the vapor barrier is important when used with a dehumidifier at controlling humidity in humid environments.
I own a home built in 1923 here in Tennessee. It has nonsub floors. I have recently purchased incapsulation materials most of then from your diy store to close off crawl — 12 mil incapusulation vapor barrier / insulation for walls/ dehumidifier/ and stuff to seal off vents and door access to crawl, but what do i do with the air leaky, not to mention just dirt you sweep up in the home and anything you spill all getting into the crawl space due to no sub floors. Im doin this because im 2 feet above the ground and the air quaility is horrible. Thought about putting plywood over the floors and then vinyl planked hard wood with what ever elese may be needed for vinyl flooring installation?? Please need your advise to no what will be my best option.
It sounds like plywood and new flooring would help but I would get some local flooring companies opinion just to be sure. This way your original floor would b the subfloor and build on top of that. Hope that helps.
Hi Michael,
We live in Southern Middle Tennessee and have a 1800 sqft single story home on a vented crawl space. We have 6 mil plastic on the ground in the crawl space. We are wanting to get the floors warmer, A Lot Warmer. We where going to Install insulation between the floor joist and where told not to because our water pipes would freeze because the heat inside the home is what keeps pipes from freezing during the winter. Our floors run around 5 to 6 degrees cooler than what our thermostat on the wall shows. What can we do ?
Hello, a great place to start would be sealing the vents, insulating the foundation wall with minimum R10 foamboard and air sealing major penetrations in the sub-floor. Thank you and hope that helps.
Hi Michael, I live in Charlotte NC and looking to install Reflectix type insulation in my crawlspace …nail it to the beams of the subfloors (already have fiberglass insulation that I am going to leave in there ) to reduce the level moisture of the wood and also help my wood floors. My crawlspace is vented and not encapsulated. I am also thinking to replace the origin 6 mm liner with a 10 or higher. I would love your feedback on this. Thank you
We don’t recommend nailing anything to sub-floors that could potentially trap humidity (open crawl spaces tend to be more humid) or a plumbing leak from above. The 10 mil or higher vapor barrier is great for encapsulation (won’t tear when fastened to walls) while 6 mil tends to be a loose laid plastic installed in vented crawl spaces on the ground only with little to no traffic or storage. Hope that helps.
Hi Michael,
We live in Baltimore Maryland and are looking to insulate the area under our 300 sqft addition. The challenge is that the addition only sits about 18-20″ above the ground. We are thinking of installing some mineral wool batted insulation so that we can avoid having to deal with installing a the vapor barrier. Not sure if there are additional considerations for spacers that are so close to ground level
Keep in mind mineral wool may still absorb moisture and the vapor barrier may or may not stop the moisture. Plus both are required by code. As far as prioritizing what is more important insulation for warmth or vapor barrier for code? Living in Maryland I would guess insulation but up to you to decide. Sorry I know that’s not a direct answer but sounds like you can only choose one or the other.
Hi Michael, thanks for all the info,I Live in New Jersey supposedly the Jersey shore area even though I’m about 15 miles inland from the ocean, do you know of anybody that does floor joist insulation along with some spray foam for gaps and cracks and what not in the Ocean and Monmouth County areas thanks so much, Todd..
Todd, I am so sorry but I don’t know anyone in that area.
Hello
1921 house on a raised foundation here in Southern California. Vented crawl space with dirt floor. No vapor barriers or any insulation
Floors are quite cold in winter, I wanted to insulate between the joists.
What type of insulation would you recommend in this case ?
Many thanks
For vented crawl space I’d recommend air seal major gaps with one component spray foam (https://diy.crawlspaceninja.com/attic-products-duct-sealing-spray-foam/) then install fiberglass or rock wool insulation. Hope that helps.
Hi Michael, we are located in SE Wyoming at an elevation of 6516 ft. It can get really cold and I am told that humidity is generally low. We have built a two story 1850 square foot stick built cabin. We have a metal roof and had a insulation contractor spray foam the interior roof with spray foam and batts for a value of R 50.That same contractor also spray foamed our exterior walls, some being 2×4 and some 2×6.
We are on a raised foundation and have two non vented crawl spaces.The floor in the crawl space is dirt, one side being covered in 6 mil plastic. There is about 2 feet of clearance between the sub floor and ground. We have a HVAC system as well as a pellet stove.
The subfloor is framed with 2×10 and is not insulated.
Now for my questions. The floor is pretty cold compared to the rest of the house. What would be best, spray foam or batts? I’m concerned about the useful life of each product and moisture and mold over time.
Someone recommended an insulation product that could be put on top of the dirt. I believe it was similar to the insulation used in metal building walls. What are your thoughts?
Thank you for your time. Enjoy your You Tube channel.
Hello Michael, please keep in mind any vented crawl space with sub-floor insulation may trap moisture if environment is humid or you get a leak from above. We try to insulate foundation walls in closed crawl space and sub-floor in vented crawl space. I’d recommend air seal major gaps with one component spray foam (https://diy.crawlspaceninja.com/attic-products-duct-sealing-spray-foam/) then install fiberglass or rock wool insulation for vented crawl space. Hope that helps.
Hello Michael,
I live in St Augustine Florida and bought a double wide mobile home. It sits on an acre of land and and we’ve literally torn it down to the platform and built a stick frame home back up on top of it. We paid cash for the purchase of the property and the renovation. My plan was to encapsulate the bottom and install a dehu underneath. We would like to make the dwelling mortgage ready if we were to sell, in order to do that code requires that the space be kept vented, what would you recommend for sealing the bottom joists and insulation?
If you are required to keep it vented a fiberglass or rockwool R-19 (check code for r-value) installed in sub-floor should be sufficient. You can also use some spray foam to seal major gaps in sub-floor.
HI Michael,
I live in upstate New York and have two vented crawl spaces. Each about two feet high with dirt floors and concrete block walls. The central portion of the basement is about 7 feet high with poured concrete wall and floor and contains all the mechanicals. The basement gets quite humid/damp. The floor over the crawl spaces can get quite cold in the winter. What would you recommend?
If possible, control humidity per recommendations in this article: https://crawlspaceninja.com/faq/should-i-dehumidify-my-crawl-space/. Insulate foundation wall of crawl space if possible and keep sub-floor insulation if present and no signs of water damage or mold. You could insulate basement walls too if you like but I’d do crawl space walls and sub-floor first. Hope that helps. Make sure to check local codes against my suggestions.
Hi Michael
I live in middle Tennessee about an hour SE of Nashville. We have a 1900 farmhouse that has zero insulation. I want to start on the attic and the subfloors for the most benefit. The attic is a no brainer with R-30 batts or more but the subfloor above the crawlspace is a challenge. The house is built off a hill with 12″ clearance on one end and about 30″ on the other end. We will be pulling up the floors from the inside to level and repair joists. Whay would be the best way to insulate….sprayfoam between the joists, batts & rigid foam between the joists?
Thanks
Hi Gary if you are unable to insulate foundation walls and seal the crawl space, I would recommend a combination of spray foam and batt insulation. I would use one component spray foam to air seal major penetrations in the subfloor then put batt insulation up. This way you will cut down on drafts entering your home and give you the thermal barrier between the crawl space and living space. Hope that helps and thanks for reading our blog. Recommended spray foam: https://diy.crawlspaceninja.com/attic-products-duct-sealing-spray-foam/
Hi we have a home by a lake in east texas that has a large open craw space under it. we are noticing that the floor is very cold and we would like to warm them up some. we were thinking of installing the foam board but im not sure thats a good idea with what im reading. the ground is dirt below the home and not sealed. living by a lake it can be very humid. what would be the best course of action to insulate the underside of the home?
Advanced Energy research shows a sealed crawl space with humidity control (dehumidifier) and wall insulation is a great way to control moisture and insulate in warmer climates. In cold climates subfloor insulation and wall insulation could be effective but subfloor insulation can trap moisture. Hope that helps.
Hey there Michael! Great site.
I’m in Portland, OR. I’m made a small 7’x10′ office by claiming 4′ feet of my garage and added a 3′ bump-out the back giving me two situations: 1) A bump-out floor framed with 2×6 PT that is about 5-6″ off the soil and ledgered to the small garage foundation. 2) 2×6 floor over slab ledgered to other side of the foundation.
I was planning on doing the following for both:
Top
— 3/4″ T&G subfloor
— 3 1/2″ R15 rockwool (compressed to 2 1/2″)
— 3″ Polyiso w fiberglass facing on both sides (edges foamed in for air sealing)
— 3/4″ rigid foam under all joists
— 1/2″ plywood for protection and to secure the rigid foam
Bottom
Most articles talk about keeping the foam up against the subfloor so I could switch the foam and rockwool but wasn’t sure.
The mini “crawl space” will have airflow and the floor over slab doesn’t.
Thanks!
Justin
I live in Humboldt County (extremely humid and cold). We have a 1948 house with a 3 foot crawl space. The house was built with no insulation in the entire house. I sprayed some cellulose insulation in the attic.
My crawl space is ventilated with screens on all sides. It looks like there is nothing between the bottom of our hardwood floors and the crawl space, except a cavity.
Should I insulate down there? Also, what would you use? Thank you, CrawlSpaceNinja.
-JP
Hello JP, we did a video about 6 places to insulate a vented crawl space. Please take a look, hopefully it will help you. https://youtu.be/Vtj9Or17_rI
Hi Michael
I have a small cabin, 400 sqft, built on pole foundation with a 30” dirt crawl space. The crawl space is skirted with 2×6 treated t&g lumber. The floor joists are not insulated and the crawl space is vented. I’m considering insulating the floor joists with batt insulation, paper side up. I also was considering put plywood on the bottom of the floor joists to prevent mice from getting into the insulation.
Thanks for your input.
Rocky
I see this done a lot around the Gulf coast where they have plywood covering the sub floor and keeping the insulation in place and it is very good at keeping rodents and other pest out of the home but not so great at keeping humidity out of the home.
Hi Michael, I’ve watched several of your videos and learned a lot, but hoping a direct message can help clear up some of my confusion.
I live in Utah which is quite dry and has cold winters. We have a vented dirt floor crawl space and not looking to encapsulate it. I am putting down a 10 mil vapor barrier on the ground, but also looking at the subfloor insulation. It currently has R-19 unfaced fiberglass batts, but I’ve noticed some of them have what appears to be mouse droppings and possibly urine stains, tho I have no other indication the mice are still there. I’m wondering if I need to replace them tho. If so, should I use faced or unfaced? And if faced, what orientation? If not faced, I’d like to put something up to cover the fiberglass – would polyiso be okay? Or would I be better off with something like tyvek and/or hardware cloth? There are electrical wires, HVAC ducts, and plumbing (water pipes and a gas line to our fireplace) that all run under the joists in the crawlspace. I have already air sealed all of the holes with spray foam. Please let me know your thoughts on this.
Thank you in advance!
Hi Josh, the faced is basically a moisture barrier and if you install it, it goes paper touching the subfloor according to the manufacturer. I am not a fan of covering insulation but since your climate is dry, there is a possibility the insulation will not get wet from the crawl space side but keep in mind, could get wet from the living space side. If you decided to cover the insulation, a foamboard would give you more R value vs a Tyvek material but but could hide subfloor issues. That being said, R 19 can also hide subfloor issues, which is why I am not a fan but realize you must insulate due to the crawl space being vented. Great job on air sealing penetrations with spray foam. I don’t like to give direct advice, especially in climates I am not familiar with but the paper backing on R 19 will probably not serve a purpose except to trap a moisture leak even more than no paper from above. If your winters are extremely cold, I’d see how the home does with air sealed penetrations and R 19 only. You can always add a layer of foamboard later but I am not sure that will make a huge difference. I hope that helps.
Hi Michael,
I’m hoping you could give me some guidance on a topic I can’t find much guidance about. We recently bought our first home build in 1947 and I discovered that we don’t have any subfloor, only hardwood directly onto the floor joists. I am currently working on a French drain, Vapor barrier, and rim joist insulation, but not sure what do do about the subfloor. My main concern is indoor air quality, and I assume my current efforts will help a good deal with that, but as it’s an old home the floor panels have occasional gaps between them which are obvious air gaps. Should I fill in the air gaps with something? I know not to spray foam everything, but we can’t afford a full encapsulation, so how to I close the air gaps while letting it breath at the same time? Thanks for your help!
I am sorry but I don’t have a lot of good advice on this one. If you install wood under the hardwood, it may take forever to install and be expensive. You could caulk gaps but that could look bad from living space. Could you wedge foamboard between the joists up to the hard wood, again that could trap moisture if humidity is not controlled. Have any local contractors been able to offer advice?