Insulating a Crawl Space

What’s right, what’s wrong & what will cause problems

Improving performance & appearance. A vented crawl space (above) often has falling fiberglass insulation and moisture problems.

Improving performance & appearance. A vented crawl space (above) often has falling fiberglass insulation and moisture problems.

Builders love crawl space foundations because they’re quick and inexpensive to build. You don’t need heavy equipment to excavate a giant hole for a full basement. Instead, you’re erecting short walls to get the first floor framing up off the ground. If soil at the building site is rocky or wet, a crawl space minimizes the risk involved in digging and building a full basement.

The failure of fluffy stuff

The encapsulation process results in a clean, dry interior space, as shown here.

The encapsulation process results in a clean, dry interior space, as shown here.

For many years, the standard way to build a crawl space involved venting the crawl space walls and installing fiberglass batt insulation between joists in the crawl space. The widespread availability of fiberglass insulation, along with its low cost, went along well with the other economies of crawl space construction.

Unfortunately, the same fluffy batt insulation that works well in wood-framed walls performs miserably in crawl spaces. Here’s how energy expert Martin Holladay describes the disastrous results of installing fiberglass batts in a vented crawl space in a recent blog at

“If you’re perverse, and you want to build a damp, moldy, nasty crawl space, just do two things: insulate the crawl space ceiling with fiberglass batts, and vent the crawl space to the exterior. If you live in the Southeast, within a few short years the fiberglass batts will begin to hang down at odd angles like drunken stalactites. Every summer, the open vents will introduce huge amounts of moisture into the crawl space. You’ll end up with a classic moldy crawl space — one that represents a significant source of moisture for the house above.”

The mold that thrives in a damp crawl space won’t just damage wood and other organic materials in the crawl space. Research has shown that up to 40% of the air we breathe upstairs comes from the basement or crawl space, so the potential for hazardous indoor air pollution (from airborne mold spores) is very high. These are serious problems.

The solution: Change the thermal and pressure boundaries

A smarter insulating strategy. Installed against crawl space walls, rigid foam insulation won’t degrade or fall out of place.

A smarter insulating strategy. Installed against crawl space walls, rigid foam insulation won’t degrade or fall out of place.

OK, I know this heading may seem a little cryptic, but stay with me. In the old-style vented crawl space, a layer of poor-performing fiberglass insulation is supposed to provide a thermal barrier directly beneath the first floor of your living space. The pressure (air) barrier in a vented crawl space is also the floor sheathing, even though it’s got plenty of holes in it to bring wiring, plumbing and ductwork up through the crawl space.

What happens if we change these pressure and thermal boundaries? Seal the crawl space vents so that moist exterior air can’t get into the crawl space. If the crawl space has a dirt floor, seal that with a thick plastic vapor barrier. Install rigid foam insulation against crawl space walls, so that we’re insulating the crawl space instead of the first floor.

The result of this crawl space “encapsulation” process is a clean, dry crawl space that isn’t affected by moisture in the soil or in the outside air. Unlike fiberglass crawl space insulation that can degrade into a pink pile on the crawl space floor, rigid foam insulation can’t absorb moisture or be damaged by it, and will never fall out of place. Any moisture that remains in the wood framing after a crawl space is encapsulated can be effectively removed with a dehumidifier. Once this happens, you’ve got a controlled environment where mold can’t grow, and a crawl space that’s more of an asset to your house than a liability.

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About Tim Snyder

A journalist specializing in sustainability, energy efficiency and home improvement topics, Tim is a former executive editor of Fine Homebuilding and American Woodworker magazines. Tim's recent work includes a green remodeling manual for the Environmental Protection Agency, a chapter on “Smart Shelter” for The Whole Green Catalog, and two New Yankee Workshop books, coauthored with Norm Abram. Today Snyder's blogs and web content can be found at,,,,, and other sites.


  1. You are correct in that a rigid style foam insulation is the best form of insulation to use when trying to seal your crawl spaces, as long as each panel is correctly taped together this can be one of the best ways to essentially vapor lock your crawl space, it stops air, moisture, dust, smells, and is truly a great way to eliminate the above from your home, the johns manville foil board is also a good product to use for this.

  2. This is what want to do to my 1920′s craftsman, but the walls are stone masonary and have and uneven suface. If I run the vapor barrier up to the sill and use fiberglass bats will the vapor barrier stop the batts from geting wet? I know I can go the closed cell spray foam way but I worry about wood rot, or insects on the rim joists that I could not see. Spray foam just to the top of the masonary and use ridig fowam bord on the joist so you can still gain acces to inspect?
    Any advice would be great northern NJ.

    • Tim Snyder says:

      Avoid using fiberglass batts in the crawl space. Spray foam offers a fast and effective way insulate the uneven surface of a stone foundation wall, but it can get expensive. A good first step would be simply to insulate the rim joist by friction-fitting rigid foam insulation against the rim joist and between floor joists and then sealing around the foam board with 1-part spray foam. It’s also important to seal gaps where the mudsill rests on the top of the foundation. Termites and carpenter ants are unlikely to go after wood that stays dry, so your risk is minimal if you can keep the rim joist area dry. Just by sealing the rim joist you are likely to notice improved comfort and energy efficiency; then you can decide about insulating the stone wall.

  3. I understand the foam insulation against the walls but what about the portion of the wall above the cinder block/cement wall between the joists? Should I cut up some of the foam board insulation and fit them into the space? What adhesive should i use to place the foam insulation?

    Any tips for sealing the vents?

    • Tim Snyder says:

      Yes, it’s VERY important to seal and insulate the rim joist. See the comment above for details on how it’s done. As for sealing the vents, it’s possible to make vent covers from 3/4-in. exterior plywood cut to fit over the vent opening. A foam gasket should be used to seal the edges of the vent to the wood rough opening. Stainless steel machine bolts, driven into threaded inserts installed around the rough opening, will enable you to remove the cover in the future if necessary. To insulate the vent cover, glue some rigid foam insulation against the inside face of the vent cover.

  4. Shawn Bean says:

    Hi Tim. Thanks for the crawl space insulation tips! I have a few questions for you as I have a slightly unique situation in my house in northeastern PA. The previous owners had expanded the kitchen from the house and installed a crawl space under the new addition. This crawl space is accessible from an existing basement window. For some reason, the builders added crawl space vents. (My understanding is that if the crawl space was opened up to a condition space, – i.e the removed the basement window to open it up to the inside – there really was no need for vents.) I am planning on sealing up the vents and insulating the space per your advice above. If I remove the window and create an open access point the the crawl space, would you recommend installing some sort of vented door hatch or a sealable one? Also…out of curiosity, does foamboard insulation installed in a crawl space require a fire barrier laid over top of it, like drywall? I know it’s required for living spaces like a finished basement, but just curious if building codes have addressed this.

    Your wisdom is appreciated!

    • Tim Snyder says:

      Assuming that you’ll be able to access the crawl space from the basement window, all vents in the crawl space can be sealed permanently as part of the encapsulation process. Rigid foam board insulation should not require a fire barrier when used in a crawl space. Once the crawl space has been insulated (make sure to air-seal and insulate the rim joist throughout the crawl space, as well as the crawl space walls), there should be no need for an insulated door between basement and crawl space. You may want a door for other reasons –to keep out pests, for example. Good luck.

  5. Hi Tim,
    I have enjoyed your comments. I have a situation with a 1930 home in Savannah, Ga. It sits on a above ground 4 ft crawlspace. It has approx 2′x2′ brick pillars spaced around the perimeter with lattice in between. It is not encapsulated, free movement of air. The house is in a flood zone, but the 1st floor sits about 5′ above the base flood elevation. It is not insulated at this time. How would you insulate the floor? I have worked storm claims / flood claims for insurance companies and I have seen my share of wet insulation after a flood. It is important to be able to quickly remove it from the floor to prevent more damage, thus my concern in doing closed cell foam insulation. Thank you for your time and advice.

    • Tim Snyder says:

      Savannah is a beautiful city and it sounds like you have an historic house that is worth caring for. The first step in your case would be air-sealing the first floor (crawl space ceiling) with 1-part spray foam (cans of GREAT STUFF). All penetrations for wiring, plumbing and ductwork can be foamed to form an air barrier that will also help keep out insects and other pests. Pier-supported buildings have no foundation walls, so encapsulation isn’t an option. Having 2-part, closed-cell spray foam sprayed between floor joists would accomplish air-sealing and insulation in one step, but (as you mention) it would also trap moisture if a storm surge brings water above the first floor level. A viable alternative would be to fasten sheets of rigid foam (1.5 or 2 in. thick) along the bottom edges of the joists, using screws and large washers. Don’t use EPS (white Expanded Polystyrene) rigid foam; it’s less durable than extruded polystyrene and polyiso foam board and with lower R-value as well. Sealing the seams between panels with tape will improve the air barrier. For odd-sized areas you may need to fill voids around the rigid foam with GREAT STUFF. This treatment will greatly improve comfort and energy efficiency, and it can be taken down if necessary. Good luck.

      • Tim,
        Thank you so much for your advice. You are correct this is a historic home. Couple of questions/clarifications. If I understand your advice correctly–you are saying first use a 1 part spray foam Great Stuff (is this specifically called 1 part?) around all wiring, plumbing, any cracks. Since the house is in a flood zone you aren’t recommending spraying closed cell foam insulation between the joists but as an alternative screw rigid foam insulation to the bottom of the joists taping at the seams. Last week the pipes froze in the crawlspace, would you put the foam tubes over all these pipes as well? Usually freezing isn’t an issue in Savannah, but it was last week with 19 degrees. Thank you again for your advice. Kathy

      • Tim Snyder says:

        Up north where frozen pipes are an issue every winter, beach houses built on piers and crawl space houses have a way to protect water lines that extend beneath the house. The rigid foam application (to the bottom of the crawl space joists) should protect water lines that run in the joist space. For the lines that extend lower, into the ground, it’s common practice to build an insulated box that extends from the floor to the ground. Access to the box interior will be necessary –either through a hatch in the first floor, or through an insulated, weatherstripped door in the box. Rigid foam is the preferred insulation to use around the box. As an extra precaution, the water lines can be wrapped with electrical heat tape that will turn on automatically if temperatures drop below freezing.

      • Tim, Kathy again. Another question: if we use rigid foam insulation screwed to the joists like you suggested, is this going to create a moisture problem between the floor and the foam in this air space? I don’t know if the floors have a sub floor, the floors are the original heart of pine in the house. Thanks.

      • Tim Snyder says:

        The potential for a moisture problem will occur on humid summer days. If warm, moist outside air gets past the foam insulation and into the joist space, condensation is likely to occur when this air contacts the underside of the flooring, which will be cooler. To avoid this moisture issue, I suggest doing a good job of sealing the rim joist and the joints between foam insulation boards.

      • Tim
        Thank you so much for all the information.


  6. Tim,
    This information has been very helpful but I was wondering is it relatively easy for anyone to do? A few months back my husband and I had several companies give us estimates on encapsulation of our crawlspace. The cheapest was $6,700 that was with a sub pump and the stabling of main bean. Do you think we should attempt this project ourselves or have a professional do the work? We really would like to save money but we want it done right so we’re not paying more in the long run anyway. Neither of us have done any real home improvements outside of painting and laying floor tiles.

    • Tim Snyder says:

      Making structural repairs (like stabilizing a beam) and installing a sump pump are definitely jobs for pros. Installing the rigid insulation and air-sealing involve techniques that DIYers can accomplish. Because you’re working in a space that’s dark, cramped, dirty and damp, it’s hard work. The good news is that if it’s done right, it only needs to be done once. Good luck.

  7. John Barry says:

    I have a 1925 log house in Finland ( that has an earth floor underneath. The conventional wisdom here is that wooden houses need to breath and sealing is not good in the long run. My problem is the floor is typically 50 degrees and room is 68 and our energy consumption keeps creeping up despite the outside temperature being constant (around 6 degrees). I believe as the foundation freezes it’s taking more and more heat to keep the house warm. If I were to condition the crawl space do I put in a vapor barrier by default and if not what are the conditions to consider before putting in a vapor barrier?

    • Tim Snyder says:

      It looks to me like your house crawl space needs a full encapsulation treatment with extra rigid foam insulation installed against the foundation walls. It’s worth studying the aspects of encapsulation so that you can be sure the process is completed correctly. You’ll find good information at

  8. Roy Schlaudt says:

    Hi Tim,
    My situation is similar to Kathy’s. I live in Greenville, SC, and my home is built on piers with an exterior brick facade. I have hardwood floors with 15lb felt between the subfloor and hardwood. The batt insulation underneath is old and falling out and the duct work needs replacing as well. My thought is to do what you suggested for Kathy after I have new ducts installed. So, do I have to remove old insulation? Do you think I could be creating a moisture problem that would effect the hardwood flooring?

    • Tim Snyder says:

      Hi Roy,
      I would definitely remove the old fiberglass insulation. Some of it will probably have to come out anyway to facilitate the new ductwork installation; the rest is often a favorite habitat for mice. Make sure to air-seal the penetrations where duct boots go up through the floor, and seal the new ductwork so that you don’t have crawl space air being pulled up into the house. Installing the rigid foam insulation against the joists in the crawl space shouldn’t create a moisture problem. Good luck.

  9. I have a ranch with half finished basement and half crawl space. The crawl space is not vented to outside but actually opens to the inside – to the finished basement. My heating pipes run through the crawl space. Is it better to insulate the crawl space and leave the pipes uninsulated? Or better to close off the crawl space completely (close opening into finished basement) and insulate the pipes? There is no moisture at all as is now but since there’s no insulation, it obviously is losing a lot of heat.

    • Tim Snyder says:

      PROBLEM: Crawl space opens into basement.
      SOLUTION: In most cases, the best treatment will be to fully encapsulate the crawl space. Do not attempt to simply insulate crawl space plumbing and the crawl space ceiling. Make sure that air-sealing the rim joist and insulating the rim joist and crawl space walls are included in the encapsulation process. Once this has been done, you can isolate the crawl space from the basement by installing an insulated door over each opening that connects the basement to the crawl space. 3/4-in. rigid foam sandwiched between two “skins” of 1/4-in. or 1/2-in. plywood will make a good door. Adhesive-backed foam weatherstripping will create a good air seal between the crawl space door and the door frame. If you screw the door in place, you’ll be able to remove it if access is needed in the future.
      The basement also needs insulation and air-sealing treatment, but that’s a topic for future discussion. Good luck.

      • Thanks so much for the advice. So from what youre saying, if the crawl space is encapuslated by insulating the walls and rim joint, I dont need to insulate the plumbing that runs through it?

  10. Karen Merchant says:

    Hi Tim –
    I just bought an 1890s farmhouse that’s had a few additions over the years. Under the main part of the house is a basement with about 5 1/2 ft ceilings with cemented stone walls and small windows (4 around the perimeter). There’s no cement floor, just a skim of very thin concrete. There’s a small sump pump, but I don’t think there is any perimeter drainage. The floor (basement ceiling) is completely uninsulated as are the walls and windows of the basement. In addition, there are two crawl spaces (unvented) under two additions that adjoin the basement without any door/window or other seal (i.e. the crawl spaces open up to the basements directly). I’m trying to figure out the best solution for this basement and whether or not I can do it myself. If I encapsulate the crawl spaces, what do I do about the fact that they open up directly to the uninsulated basement? What should I do about the basement? Put in a perimeter drainage system and sump pump and then insulate the windows? What about insulating the floor (basement ceiling) and walls? Help!

    • Tim Snyder says:

      Congratulations on the historic house purchase. Old houses are lucky when they get caring owners. See my replies to other comments regarding recommendations for treating crawl spaces that open into basements. Regarding your basement with a low ceiling, stone foundation walls, a sump pump and skim-coat floor: These are challenging conditions. I’m going to assume that you won’t be using this as living space. The first thing to do is to have an experienced stone mason evaluate the stone foundation walls and make repointing and mortar replacement as necessary to ensure soundness. Second task: install an interior French drain system and connect it to a new high-quality sump pump. Third, I suggest you consider either replacing old basement windows or simply cover them with insulated, airtight covers. By your description I’m going to guess that the window openings are small and your basement won’t need natural light if it’s not going to be used frequently. Finally, air-seal and insulate the rim joist. At this stage, you might just see how the house performs for a year. If too much cold is coming up through the basement, you can insulate the stone walls by applying 2-3 in. of 2-part, closed-cell spray foam. Make sure the spray foam treatment doesn’t impede drainage into your French drain system.

  11. Tim, thanks for the great information. I have a unique situation in that I have a home with radiant floor heat via pex tubing and aluminum fins running through the joists about an inch from the subfloor. There is a thin layer of foil reflecting insulation under that followed by R-30 unfaced and string tied to complete the insulation. I am wondering about securing rigid foam to the joists encapsulate the joist bays and help focus the heat upwards. Is there a downside to such actions?

    • Tim Snyder says:

      Chris: I don’t see a downside. In fact, if you tape the seams of the rigid foam sheets, you’ll create an air barrier that will help prevent convective heat loss that occurs when air moves through the batt insulation. I would just take care to make your rigid insulation installation reversible by attaching the foam with screws driven through oversize washers. If you can’t source washers, it’s possible to make your own from 2-in. squares of 1/4-in. (or thinner) plywood. With this type of installation, you’ll be able to remove a section of rigid foam without destroying the foam or making a huge mess. Good luck.

  12. Thanks for the article. Saved me from making the fiberglass in the floor mistake. Two Questions. My crawlspace has a cement floor with about 3-4 inch space to the wall, around the entire perimeter used as a french drain to sump pump. The walls have poorly applied 1/2 inch rigid Styrofoam, attached to the concrete with fasteners. Q1. Should I attached new rigid polystyrene board over the existing Styrofoam or remove all the existing? Q2. To properly encapsulate the crawl space do I cover the cement floor with plastic attached to the rigid board, and if I do, should I run perforated pipe in the french drain space before attaching the floor covering to the wall polystyrene?

    • Tim Snyder says:

      The 1-in.-thick Styrofoam isn’t providing much R-value. If it’s not in good shape, you’re probably better off replacing it with 2-in.-thick extruded polystyrene. The major benefits of covering the concrete floor with a vapor barrier would be lowering the concentration of radon that comes up through the soil (Have you had your house tested for radon? It’s a good idea to do so, especially if you have children in the house.) and lowering humidity in the crawl space. Yes, I’d recommend putting perforated pipe in the perimeter drain system before coverage with the plastic sheet.

  13. Jary Mitchell says:

    Is there anything to be gained by placing rigid foam on the dirt floor then sealing poly vapor barrier over it?

    • Tim Snyder says:

      If you live in a colder climate where the ground freezes during winter months, installing rigid foam insulation over a dirt crawl space floor is a smart idea. The floor insulation will help to keep more heat within your home’s building envelope. If the crawl space’s dirt floor is often damp, it’s smart to lay down a layer of drainage matting beneath the rigid foam. This stippled plastic sheet material is available from most Basement Systems contractors. Perimeter French drains and a sump pump are recommended if the crawl space frequently has standing water.

  14. Hello Tim,

    I recently found your write up and have learned more about my CS. I put down a 6 mill barrier in my dirt floor CS 4-5 years ago but did not foam board the walls. Now I know I should do the walls and will, but I’m a little confused about the ceiling and insulating it.

    Right now the ceiling has nasty decades old pink batt insulation that I did not know to remove when I sealed the dirt ground and walls. The batt was put there when the addition was built, again, decades ago. Now I know to take all that batt out ASAP, but do I put foam board in its place, between the joists, or just leave the ceiling bare?

    My new plan is to remove the ceiling batt, foam board the walls, seal off and spray foam the rim joist, and re-install my 6 mill plastic barrier on the floor and walls. But what about the ceiling?

    Thanks a lot!


    • Tim Snyder says:

      You’re on the right track, Todd. Once you’ve removed the fiberglass batt insulation from between crawl space ceiling joists, just leave the ceiling as it is. Isulate the crawl space walls with rigid foam. Insulate and air-seal the rim joist. These two treatments bring the crawl space within your home’s building envelope, making crawl space ceiling insulation redundant. You didn’t mention sealing the crawl space vents; this should also be done to complete the encapsulation process. One vent can have a removable door or cover to provide crawls space access. Good luck.

  15. I live in the Chicago land NW area- my house is a old 1955 cottage converted into a 2,600 home.
    The crawl is vented- it’s about 22” to 34” throughout it. I live on the lake.
    It does flood every year with the thaw-and high waters from WI coming down to us “Chain of Lakes”.
    It’s a dirt or mainly mud space always damp-always- it has gotten 20 deg down there but only when it gets 10 or lower for several days which that seems to keep happening—I have insulated my water pipes but the heat duct are un insulated and only cold air seems to come out when the heat kicks on—I understand the reason is nothing is insulated under the house-
    I can’t use the plastic vapor barrio due to the water just pours in from the ground and above the ground – it just comes in everywhere until it recedes- I would like to insulate the concrete walls with the spray foam but don’t want to create a rotting wood effect on the wooden ceil plate- I would love to create a vapor barrier from the musty smell but don’t want to create molding or rotting wood on my floor boards—if it reached that point- last year we were 1 inch away from it overtaking the 1st floor-
    What would you suggest for insulating walls or some kind of insulation with no rotting wood effect?
    And some type of vapor barrier? But keep in mind it will flood.
    I’m trying not to spend $12,000.

    • Tim Snyder says:

      You’ve got very challenging conditions to deal with. I would suggest contacting a Basement Systems contractor in your area (go to to locate the closest contractor) as a first step. This specialist will be able to tell you whether or not it’s possible to waterproof your crawl space by installing French drains and a sump pump. This should be your first option because it will keep the crawl space area dry. Add a heavy-duty dehumidifier and you’ll seriously dry out the space, creating conditions that are hostile to mold growth. If you find that “Plan A” isn’t feasible, Plan B would be to insulate the crawl space ceiling with rigid foam insulation as described elsewhere in these comments. In this case you would keep the crawl space vented and even try to increase drainage through the space to provide an escape route for water that gets into the crawl space. This strategy is often used in seaside homes that experience flooding from storms and high tides. Good luck.

      • Tim,
        having quite a bit of experience with crawl spaces now, I would also like to add that Kris should consider wrapping his heat ducts with insulation (fiberglass) and then encapsulating this by overwrapping them in foil bubble insulation (4′ x 125′) and sealing those seams with a high quality foil tape (not duct or duck tape as it will break down in a couple of years). This is a little pricey, but somewhat doable for the do-it-yourselfer, effective, and looks great as an end product. This will get him back to having hot air rather than cool/warmish air trying to heat his home.

      • Tim Snyder says:

        Good advice, Chris. Leaky ducts can diminish HVAC efficiency by 20%. If the crawl space walls and rim joist are insulated, insulating the ductwork in the crawl space is less important than sealing duct leaks. Aluminum tape made (and approved) for duct sealing is the material to use, as you suggest.

  16. Richard Ray says:

    I have a ranch style house built in 1990 and it has a crawlspace with vents. I am encapsulating it with a 12mil vapor barrier, closing off the vents, and want to install the 2″ polystyrene rigid board. The house is brick and around the exterior walls (brick) there are block support piers about every 8 foot, so they stick out either 6″ or 8″ from the brick. How is the best way to fasten the foam board, as well as seal the rim joist. Also should the vapor barrier go on the inside of foam board against brick or on the outside of foam board?

    • Tim Snyder says:

      The vapor barrier can be installed over the foam board. As for installing the foam board against the foundation walls, many installers find that a combination of masonry fasteners (like Tapcon or barbed plastic fasteners) and panel adhesive is the best solution. The fasteners are mainly to hold the panels in place until the adhesive cures. Make sure that the panel adhesive is compatible with the foam board. Yes, you’ll need to cover the block supports with rigid foam to get the full benefit of perimeter insulation. About sealing & insulating the rim joist: Do this task first, when the rim joist area is most accessible.

  17. Tim,

    I have a tall crawlspace (4′ to 7′) with a good foil faced asphalt roofing membrane as a vapor barrier on the dirt. I already have r19 unfaced batts against the floor. It is 8 years old, in good shape, with no signs of moisture issues. What do you think about leaving the floor insulation and then sealing the brick crawlspace walls with rigid. My pipes never freeze. They are insulated and the south Arkansas climate isn’t that cold. I would hate to remove the batted insulation if it isn’t necessary. I am just trying to reduce my heating bills. I have electric heat pumps, and my winter heat cost me significantly more than summer cooling.


    Arkadelphia, AR

    • Tim Snyder says:

      Hi Chris,
      It should be fine to leave the batts in place, especially if they are dry, fully fluffed and in good condition. Definitely air-seal the crawl space walls, paying special attention to the rim joist. Insulating the crawl space walls will help reduce your heating bill, but in your case the air sealing may be even more important. Read the blogs on stack effect; this will explain how negative pressure created by escaping heated air upstairs tends to suck outside air into a crawl space or basement. Fiberglass batts won’t stop air movement, so any cold air that gets into your crawl space has a good chance of moving upstairs. If you air-seal the crawl space and air-seal the attic/living space pressure boundary, you can short-circuit the stack effect and score good savings during heating season. Good luck.

  18. Hi Tim,

    I’ve been reading everyone’s posts and find them similar to mine, but I didnt notice any of them mentioning dehumidifiers or sir systems. I have a 12 X 10 vented crawl space with a concrete floor; bought the house last year and noticed that the ceiling has batt insulation, which as you guessed it is/ was hanging down. There are two vents and an access panel to the outside, which consists of two 1/2″ sheets of plywood on rusty hinges as you can imagine. The area opens to the basement via the rim joist on one side, which I currently have insulation jammed into. The space in the summer has a lot of moisture and I’m looking to insulate and seal to keep the above kitchen floors warm and protect against mold.

    I had a company come in to get an estimate on encapsulating, which consisted of the following:
    1. Vapor barrier to the floor and walls.
    2. rigid foam board on floor, walls, and in rim joists
    3. dehumidifier
    4. covers for vents
    5. Sealed access panels.

    Now for my questions, are all these necessary? Is the dehumidifier a necessity when the rim joists on the back side of the space open to the basement for air passage? Thanks in advance for your time in addressing my questions.


    • Tim Snyder says:

      Mark: The ingredients in your encapsulation proposal sound good, but there could be some confusion about the dehumidifier. When CS encapsulation is done correctly, a dehumidifier probably will only be needed on a short-term basis –to dry out a space that has been absorbing moisture for years. Once you lower the humidity level, the moisture barriers and vent covers should keep the humidity low. However, if your crawl space is open on one side to the basement, it’s going to take longer for the CS dehumidification to take place. Assuming that the company you’re dealing with is experienced in CS encapsulation, I’d query them on a)where the dehumidifier will be installed, and b)why they wish to maintain the connection between CS & basement. An associated issue: how you will you empty the dehumidifier if it’s installed in the crawl space? Some CS dehumidifiers can be installed in a basement area and ducted to an adjacent crawl space. Resolve these issues and you’ll be on your way. Good luck.

      • Thanks Tim for the prompt response!

        If they seal the rim joists leading into the basement thereby creating a total encapsulation of the area, is it my understanding that the dehumidifier will be used on a short term basis to initially dry the area and then seal it? With proper monitoring, over time, I would need to watch the humidity in the CS and add or remove a dehumidifier?

        Thanks again,

      • Hi Tim,

        I had another company give me an estimate on the same Cleanspace encapsulation, but this company included as an optional upgrade 4″ rigid foam board for the CS walls instead of the 2″. Is this upgrade worth it or a waste of money and I should stay with the 2″.

        Thanks again,

  19. Tim Snyder says:

    2-in.-thick or 4-in.-thick rigid foam insulation for crawl space walls: I’d opt for the thicker insulation if I were planning to stay in the house for more than two years. The severely cold temperatures we’ve experienced this winter reinforce the value of additional insulation. I think this falls into the category of “do it once, do it right, so you don’t have to do it again.”

  20. Tim, Thanks for sharing your knowledge with everyone and keeping this topic open for late comers…My house is in Northern Michigan, built on a crawl (40′ x 100′). The crawl is presently vented and the house is heated by electric baseboard. Since the crawl has no heat runs the only heat source is from hot water plumbing pipes (I will insulate them to reduce heat loss), so I’m thinking that the soil temp in the winter is 45 degrees which should basically be the same in the crawl since it has no heat runs. How do I protect the conditioned space (floors) which have R-19 insulation (apparently the recommendation for floor over unheated spaces is R-30) (house is 40 years old and I have lived there 22 of those years). One of the issues I have is the access point which is just beg enough for a person to enter through an access under the washing machine (yes, we do not go in the crawl at all). Thus, getting materials into the crawl will be an issue. Also, is there anyone pipe insulation material that is better to use over another? Thanks again! Craig

    • Tim Snyder says:

      OK, so what we have here (I think) is a large vented crawl space with a difficult entry point, in an area where winters can get very cold. One option worth considering is having a spray foam contractor insulate your crawl space walls with SPF (spray polyurethane foam). If the contractor can get the two hoses through the access opening and into the crawl space, he’ll be able to coat the interior crawl space walls with 2-3 in. of SPF, while also sealing the rim joist. This will be a lot faster than moving small pieces of rigid foam insulation through the access door and then piecing them together against the crawl space walls. SPF is more expensive, but it’s very effective insulation (R-6 or more per in.) that also acts as an air and moisture barrier. A once-and-done expense. Good luck.

  21. Tim, I have a weird situation—partial basement with crawl space adjoining it. They put in two small window like structures with screens to give air to the crawl space,I guess. Today a basement repair company said that I needed to put the plastic on the sides of the crawl space and then to seal it off from the basement. He would make a panel that could be removed if someone needed to get to the crawl space. He wants to install an EZ breathe drainage system around the basement inside and seal the sump pump. I can’t afford anything else at this time such as sumps in the crawl space. It was inspected a couple of years ago and he said it was dry. But the contractor seems to think that it’s unsafe and unhealthy to have this nasty crawl space open to the basement. Does what he says make sense? I am clueless as to the options. My basement is damp according to him because it has water spots on some of the walls. He is going to seal the obvious leaks which resulted from roofing and draining which will also be fixed later. It just makes no sense to me to have the crawl space sealed off and yet it makes no sense to me to have all that nasty vapors floating into the basement area and eventually upstairs. I would appreciate thoughts.

    • Tim Snyder says:

      Becky: You’re facing a difficult situation that often occurs when an older home with a basement foundation gets a crawl space addition. Let me say up front that there’s not an easy fix for your problem. The crawl space should be encapsulated (sealed), with plastic barriers on floor and walls. I’d also recommend insulating crawl space walls with rigid foam, as discussed elsewhere on this site. When the crawl space is encapsulated, there will be no “nasty” crawl space air coming into the basement, so it won’t be necessary to seal the openings. If your basement has only dampness and water seepage (rather than standing water), I’d recommend sealing the crawl space first, then paying to have interior drains and a sump pump installed in the basement. In the meantime, do what you can outside the house to direct roof runoff and ground water away from the foundation. Good luck.

  22. I was the one asking about the crawl space adjoining the basement. I guess what they proposed to do was the encapulation thing you were talking about. They gave me an estimate of $15000 for doing the crawl space with plastic everywhere and the ezbreathe drainage system around interior of basement. He was also going to put a proper sealed top on my sump. Thanks again.

  23. I have a house in Alabama built in the 1880′s. The crawlspace is vented, and fiberglass batting was installed between the floor joists a few years ago. Recently we noticed problems with mold growth in one of the front rooms. I don’t have much money to spend, so I won’t be able to install spay or rigid foam underneath. What would you suggest? Based on what I read here, I would assume that it would be best to remove the fiberglass batting, put a moisture barrier plastic on the ground, and seal the outside vents. Would you agree?

    • Tim Snyder says:

      I think the first step would be to complete a thorough assessment of your crawl space to see if there’s serious moisture damage (mold, wood rot) on any framing or sheathing materials. Yes, you’ll need to remove the fiberglass insulation to make this inspection. Make repairs if necessary, and contact a mold abatement specialist if there’s widespread mold. You’ll definitely want to fully encapsulate the crawl space, with moisture barriers on the floor and walls, well sealed to each other. Seal the rim joist and any wall penetrations as well. Vents can get airtight (weatherstripped) covers; same goes for the crawl space access door, if there is one. Once the space is isolated, my guess is that you will need to run a dehumidifier in order to dry out wood that has accumulated a lot of moisture over the years. Lowering the humidity of the space (and drying out the wood) is the only way to avoid additional mold problems.

      • A dehumidifier has been installed in the room where mold was found, and it seems to have taken care of that problem. Should all of the fiberglass insulation be removed permanently? If so, can we expect our heating & cooling bills to jump?

      • Removing fiberglass insulation installed between crawl space ceiling joists. We’ve had a couple of questions about this so I’ll try to clear up some confusion. If the fiberglass insulation is in good condition (not compressed, wet or inhabited by mice), it’s usually OK to leave it in place. But because fiberglass only attains its advertised R-value when installed in an enclosed, airtight space, it’s critical to air-seal the crawl space. Pay attention to vents, crawl space access doors and the rim joist. Consider replacing old, single-pane crawl space windows with vinyl, insulated glass windows. Complete the encapsulation process with moisture barriers on floor & walls. If the crawl space was damp for a long time before encapsulation began, dry out the space with a dehumidifier to create a mold-hostile environment.

  24. I’m very much in the same boat as Kathy from Savannah. Same town and same open pier crawlspace. I would like to add crawlspace walls to the perimeter. I need to do this for critter control. They are tearing up my flex ducts. Long term, I would like to encapsulate. Would it be better to go with hollow concrete block or frame it out with hardi-board panels. Either way, I was planning on pouring footers. Do I need to vent the walls? So far everyone is telling me I should. As of now, ground vapor barrier only only covers 90% and the joists are mostly clear of batt insulation. Previous owner tried to install unsuccessfully in several locations.

    • Tim Snyder says:

      If it were my house, I’d skip the block walls because I don’t like the appearance and because I’m a carpenter. So I’d use p-t lumber to frame crawl space walls, and finish with Hardiboard panels as you suggest. I’d frame & finish an access door with beefy stop molding that would accept equally beefy weatherstripping for maintaining a good airseal. No vents. Assuming there’s no groundwater problem (soil stays dry), install a good plastic moisture barrier over the ground. I’d probably opt for mineral wool (Roxul) insulation between joists in the crawl space walls, then fasten 1-in.-thick foil-faced rigid foam insulation against the inside edges of the studs, taping the seams to seal air leaks. It would be smart to make provisions for running a dehumidifier in the crawl space even after it’s sealed. Wood stores a lot of moisture, and it will need a way to dry out after the crawl space is sealed. This treatment will take more time, trouble and money than treating your first floor framing as the thermal & pressure boundary, but it will eliminate the pest problem, help your HVAC performance, and put a better-looking foundation on your house. Good luck.

  25. Hi Tim, question for ya. I have a three level split with crawl space under middle level (in mn). i am going to insulate the wall and rim joists and lay down new vapor barrier like you’ve suggested. the crawl space has an HVAC vent in it so it is heated in winter and cooled in summer. there is a regular interior door that accesses a closet that is open to the crawl space. after I encapsulate the crawl space, will it still be ok to have just an interior door closing off the crawl space from the downstaris? how do I run the vapor barrier against the downstairs wall (studded and no insulation?) Also, i would really like to use it for storage since its conditioned, but i dont want to ruin a new vapor barrier. Could i throw down some 4×4′s and screw decking boards to them force platform for staorage? will that be ok to sit on top of the poly. thanks

    • Tim Snyder says:

      Once the crawl space is encapsulated, it will be within your home’s building envelope, so the interior door should still be fine. Don’t attempt to run a plastic vapor barrier up along a bare studded wall. Instead, install foil-faced rigid foam insulation against the stud wall, and tape all insulation seams with aluminum tape. The foil facing will act as a vapor barrier as long as it’s continuous; use sealant to adhere the plastic vapor barrier to the foil facing. It’s fine to use the crawl space floor for storage, but (as you point out) care must be taken not to tear or perforate the floor vapor barrier. One strategy would be to build wood-frame platforms as you suggest, but glue some rigid foam insulation to the underside of the framed platforms so that sharp wood edges don’t tear the plastic. It’s also smart to remove sharp rocks from a dirt crawl space floor before laying down the vapor barrier.
      Good luck.


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