Crawl Space Insulation

If your floors are cold, your house smells musty, or you have had a pipe freeze in the crawl space, the fix is to bring that crawl space inside the heated envelope of the house. That means sealing the ground, insulating the walls with closed cell foam, and closing the vents. Not stuffing more fiberglass between the floor joists.

Call (218) 514-6594 and we will get under there.

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Why vented crawl spaces fail in northern Minnesota

Crawl space venting is a southern idea. The theory was that outdoor air moving through the space would carry moisture away. In a hot humid climate it partly works, though even there it is now heavily questioned. In Climate Zone 7 it does not work at all, for two reasons.

In winter, the vents make the space outdoor temperature. Your floor becomes the only thing between your living room and 10 degree air. Any plumbing down there is exposed. Any ductwork down there is losing heat straight to the outside. The floor joists are cold, so the subfloor is cold, so the floor above it is cold no matter how hard the furnace works.

In summer, the vents bring humidity in. Warm humid July air enters a crawl space where the ground and the walls are still cool from winter. That air hits a cool surface and condenses. You get wet joists, wet ductwork, and the conditions mold likes.

So the vented crawl space is cold when you want it warm and wet when you want it dry. That is a bad trade in both directions.

The batts between the joists do almost nothing

This is the standard old fix and it is why people are skeptical when we recommend anything. Fiberglass batts pushed up between the floor joists, sometimes held with wire rods, usually falling down within a decade.

Fiberglass slows conduction. It does nothing about air movement, and in a vented crawl space there is plenty of that. Cold air washes through and around the batts and reaches the underside of the subfloor anyway. The batts also absorb moisture from the humid crawl space air, get heavy, sag, and end up on the ground.

You have probably seen it. Half the batts hanging down like a cave ceiling. That assembly failed.

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What we actually do

An encapsulated, insulated, conditioned crawl space. The whole space comes inside the house.

1. Clear it out. Old fallen batts, debris, rodent nesting, and anything sitting on the dirt. Standing water gets addressed before anything else, and if that requires grading, a drain, or a sump, that is a separate trade and we will say so.

2. Ground vapor barrier. Heavy poly, generally 10 to 20 mil, run across the entire ground surface, lapped and sealed at the seams, and run up the foundation walls and mechanically fastened. This is the single biggest moisture change in the space. Bare soil in a Duluth crawl space evaporates a meaningful amount of water into that space every day.

3. Closed cell foam on the walls. Two and a half to three inches of closed cell sprayed on the interior face of the foundation wall, run down onto the footing and up to seal against the sill. That lands around R-16 to R-20. Closed cell rather than open cell, always, because it does not absorb water and it is a vapor retarder in its own right.

4. Rim joist bays. Same pass, same material. The rim joist above a crawl space is often the leakiest part of the whole assembly. See the rim joist page.

5. Seal the vents. Foam blocks, sealed and insulated. The space is no longer vented on purpose.

6. Give the space a moisture strategy. A supply register off the HVAC system, a small continuous exhaust, or a dedicated dehumidifier. Which one depends on the space and the heating system.

7. Cover the foam where required. Depending on the AHJ and the product, exposed foam in a crawl space may need an ignition barrier or an approved intumescent coating. Some products carry testing that allows exposure in a crawl space with limited access. That determination goes in the written scope, not a shrug.

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Insulate the walls, not the floor

This is the decision people ask about most, so here is the reasoning in one place.

Insulating the floor above keeps the crawl space cold. That means:

• The plumbing down there is still at risk of freezing
• Ductwork and the water heater, if they are down there, are losing heat to a cold space
• Every pipe and duct penetration through the floor is a thermal and air leak you now have to seal individually
• The space is still humid in summer

Insulating the walls brings the crawl space to roughly house temperature. That means:

• Pipes are protected without heat tape
• Ductwork is inside the envelope where it belongs
• You have one continuous air and thermal boundary at the foundation wall instead of a boundary with a hundred holes in it at the floor
• The space is dry enough to store things in

The wall area of a crawl space is also usually smaller than the floor area above it, so you are insulating less square footage. It is a better result for less material.

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What it costs around Duluth

Job	Typical range
Ground vapor barrier only, sealed and fastened	$1,500 to $3,500
Closed cell on walls and rim, average crawl space	$2,500 to $4,500
Full encapsulation, vapor barrier plus foam plus vent sealing	$4,000 to $9,000
Add a dedicated crawl space dehumidifier	$1,200 to $2,500
Debris and old insulation removal	$500 to $2,000

Ranges, not quotes. What moves the number: how much headroom there is, whether a crew can move around or has to belly crawl, how much comes out first, the perimeter length, and whether there is water to deal with.

Headroom matters more than square footage. An 800 square foot crawl space with four feet of clearance is a straightforward day. The same 800 square feet with 22 inches of clearance is slow, miserable work and it costs more. That is not a markup, it is time.

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When the crawl space jumps the line ahead of the attic

We tell most people to do the attic first. The crawl space goes first when it is causing damage rather than just costing money.

Call us before the attic if you have:

• Standing water or wet soil. That is a drainage problem and it has to be solved before anything gets sealed in.
• Soft or discolored floor joists or subfloor. Rot is a repair, not an efficiency project.
• Visible mold on the framing. Same reasoning.
• Pipes that have frozen or come close. One burst supply line in January costs more than the whole crawl space job.
• A persistent musty smell upstairs. That is crawl space air in your living space and it is worth solving on its own terms.

If none of those apply and the crawl space is just cold, the attic is the better first dollar. See the attic page.

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Two things we will not do

We will not seal a crawl space that has an active water problem. Encapsulating over standing water traps it. The water gets solved first, whether that is exterior grading, a downspout extension, a drain tile, or a sump. Sometimes it is as simple as a downspout dumping next to the foundation, and we will point that out for free.

We will not put open cell foam in a crawl space. It absorbs water. In a space in contact with soil, that is the wrong material regardless of what it saves you.

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Get under there and find out

Call (218) 514-6594 or send the form. Tell us roughly how much headroom there is and whether the crawl space is dirt or concrete, and we will know what we are walking into.

We work in Duluth, Superior, Hermantown, Proctor, Cloquet, Two Harbors, and the surrounding communities. Full list on the service area page.