Crawl Space Insulation — Walls, Vapor Barrier and Encapsulation
The floor over the crawl space is cold, the batts under it are hanging in the mice's way and the vents blow winter through it; sealing and insulating the walls turns it into part of the house.
JMB Builders does crawl space insulation across the Twin Cities metro, and in Minnesota that means insulating the crawl space walls, not the floor above. We seal the vents and the rim joist, put rigid foam on the walls, lay and seal a heavy vapor barrier over the dirt and up the walls, and make the crawl space part of the heated house. Cold floors and frozen pipes stop. Priced per project, quoted from photos within one business day.
- Priced per project: walls, rim and floor barrier
- Walls insulated, vents sealed, vapor barrier down
- Quote from crawl space photos in 1 business day
- Licensed MN contractor BC809005
Most Twin Cities crawl spaces were built vented with fiberglass batts between the floor joists. That is the worst of both. The vents let winter air in, the batts sag and soak up damp from the dirt floor, mice live in them, and the pipes and ducts under the floor are out in the cold. The floor above is cold no matter how many batts are stuffed up there.
The approach that works here is to treat the crawl space like a short basement. Close the vents. Foam the rim joist. Put rigid foam board on the inside of the foundation walls, taped at the seams. Lay a heavy vapor barrier over the dirt, overlapped and taped, run it up the wall and seal it to the foam. Then the crawl space is dry, the air in it is house air, and the floor, pipes and ducts are on the warm side. Old batts come out with the debris.
Some things get looked at first. Standing water or a spring in the crawl space is a drainage problem to fix before anything gets sealed. Heavy mold is a remediation contractor's job. A furnace or water heater in the crawl space needs its combustion air checked by the HVAC tech before the vents close. Radon mitigation, if a test says so, ties into the same vapor barrier and is done by a radon contractor. We flag any of those from the photos.
What you supply
- Photos from the crawl space access: the walls, the floor, the rim joist and whatever insulation is there now
- Whether the floor is dirt, gravel or concrete, and whether it has ever had water in it
- Whether a furnace, water heater or ducts are in the crawl space
- Access to the hatch; we bag and haul what comes out
What JMB supplies
- Old batts, debris and fallen insulation bagged and hauled out
- Vents sealed with rigid foam and spray foam from inside
- Rim joist air-sealed and insulated with rigid foam and spray foam
- Rigid foam board on the foundation walls, seams taped, held with adhesive and fasteners
- Heavy vapor barrier over the floor, overlapped, taped and run up the wall, sealed to the foam
- Insulated, weatherstripped access hatch or door
- A conditioned air supply or a small dehumidifier when the space needs it, sized by the HVAC tech
- Before and after photos in your job folder
How it is priced: per project
Quoted in writing from your photos within one business day. What moves the number:
- Square footage of the floor and the height of the walls
- How much old insulation and debris has to come out
- Crawl height: room to kneel vs. a belly crawl
- Number and size of vents to seal
- Foam thickness on the walls to reach the code range for a Minnesota foundation wall
- A rim joist that is already done vs. one that needs sealing
- Drainage, mold or radon work by another trade before we start
- Whether the access hatch or door has to be rebuilt and insulated
Photos from the hatch of the walls, the floor and the rim, with the rough size of the crawl space, are enough to quote. Tell us if it has ever had water.
How long it takes
A typical crawl space under part of a house: two to three days including removing the old insulation. A full-house crawl space with low clearance: about a week.
How your project works
- 1
Send photos, get a written quote
Send three photos of the space and a sentence about what you want. You get a clear, itemized quote within one business day. We only come out to measure first when the job needs a tape measure — no pressure, no games.
- 2
Pick materials, or bring your own
We help you choose profiles and materials that fit the house and the budget, or match your existing millwork exactly — or you buy the door or trim yourself and we install it. Everything is confirmed before the crew is scheduled.
- 3
Clean, precise installation
The crew shows up on time, protects floors and furniture, and installs with tight joints and consistent reveals. Photos of the finished work go in your job folder.
- 4
Walkthrough and invoice
We walk the finished work with you. If anything is not right, we fix it before we call the job done. Then one invoice, paid by check or direct deposit — no card fees, no payment links.
Situations and options
Wall insulation and vapor barrier
The standard job. Vents closed, rim foamed, rigid foam on the walls, vapor barrier on the floor sealed to the walls. The crawl space becomes a conditioned, dry space under the house.
Full encapsulation with a dehumidifier
Everything above plus a reinforced liner up the walls to the sill, taped at every seam and pier, and a dehumidifier or a conditioned air supply. For crawl spaces that have been damp or musty.
Rim joist only
When the walls were done before or the space is too tight for a full job: the rim joist and the vents sealed and foamed from inside. See the rim joist page for the full version of that.
Floor insulation, when the walls cannot be done
In a crawl space that must stay vented or has water problems that will not be solved, we foam board or spray foam the underside of the floor and seal the pipes and ducts inside it. Second choice in Minnesota and we say so.
Crawl Space Insulation — Walls, Vapor Barrier and Encapsulation: questions, answered
Should I insulate the crawl space walls or the floor above?
In Minnesota, the walls. Insulating the walls and sealing the vents puts the crawl space, its pipes and ducts on the warm side, and a vapor barrier on the floor keeps the ground moisture out of the house. Floor batts in a vented crawl space get wet, sag and feed mice, and the pipes under the floor still freeze. Floor insulation is the fallback when the crawl space cannot be sealed.
Why is the floor over my crawl space so cold?
The crawl space is vented to outside, so the air under the floor is winter air, and the batts between the joists have fallen or have gaps that let that air reach the subfloor. Heat ducts in the crawl space lose heat to it too. Sealing the vents and rim and insulating the walls fixes the floor without touching the joists.
What is crawl space encapsulation?
Sealing the crawl space off from the ground and the outside: vents closed, a heavy vapor barrier over the dirt and up the walls sealed at every seam, insulation on the walls and rim, and a way to keep the air dry, either a supply from the furnace or a dehumidifier. It is the same job as insulating the walls done more completely, for spaces that have been damp.
Is it safe to close the crawl space vents in Minnesota?
Yes, when the space is sealed and insulated at the same time and the moisture is handled. The vents were meant to dry the crawl space in summer and they mostly let humid air in instead. Closed vents, a vapor barrier and a little conditioned air or a dehumidifier keep it dry. A furnace or water heater in the crawl space needs its combustion air checked by the HVAC tech before the vents close.
Do you remove the old insulation in the crawl space?
Yes. Fallen, wet or mouse-damaged batts get bagged and hauled out before anything else goes in, and it is included in the quote. Clean, dry batts that are still up can stay, but once the walls are insulated they are doing little and most people have them taken out.
What if there is water or mold in the crawl space?
Standing water or a wet floor is a drainage problem to solve first: grading, downspouts, a sump or drain tile, depending on where it comes from. Heavy mold is a remediation contractor's job before we seal anything in. We look for both in your photos and tell you what has to happen before the insulation, and we can do the grading and downspout work ourselves.
Get a written quote for this job
Three photos and a sentence is enough. You hear back within one business day. Or text the photos to 763-204-2099.
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Read morePage reviewed September 24, 2026 · JMB Builders LLC, MN license BC809005 · All insulation pages
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