Basement and Crawl Space Insulation Done Right

Below-grade insulation is where enthusiasm causes damage. The energy case is real — a lot of heat leaves through the basement, and cold floors over a crawl space are a genuine comfort complaint — but the assemblies down here are moisture assemblies first and thermal assemblies second.

Get the order right: water, then air, then insulation. And start with the rim joist, which is the best-value job in the whole building.

Approach by space type

The mistake that causes real damage is fiberglass against a concrete foundation wall: it holds moisture against the concrete and grows mould. Rigid or closed-cell foam belongs there, with a ground vapour barrier in any crawl space regardless of approach.

What is the rim joist and why insulate it first?

The rim joist (or band joist) is the band of framing sitting on top of the foundation wall, closing off the ends of the floor joists. In most houses it's uninsulated, thin, and riddled with gaps where the framing meets the foundation.

That makes it three bad things at once: cold, leaky, and low. Because of the stack effect — warm air escaping high, cold air pulled in low — the rim joist is where a lot of your replacement air comes in. See how to air seal a house.

It's also usually accessible, which is what makes it such good value.

The standard fix:

1. Cut rigid foam board to fit each bay between joists.
2. Push it tight against the rim joist.
3. Seal all four edges with canned foam or caulk — this is what makes it an air barrier.

Or have closed-cell spray foam applied, which does the insulating and sealing in one pass and handles irregular geometry better.

Don't just stuff fibreglass batts in. Batts don't stop air movement, and warm moist indoor air reaching the cold rim joist behind them is exactly how you get condensation and rot.

Should you vent or encapsulate a crawl space?

The single most important decision, and mixing approaches is what causes failures.

Vented crawl space

The traditional assembly. Vents to outside, and the crawl space is treated as outside the thermal envelope.

• Insulation goes in the floor above, between the joists.
• Ground gets a vapor barrier regardless — bare soil evaporates a surprising amount of water.
• Vents stay open per code requirements.
• Pipes and ducts in the crawl space are in unconditioned space and need their own insulation and freeze protection.

The practical problem: floor insulation between joists performs poorly in reality. Batts sag away from the subfloor, leaving an air gap that defeats them. They absorb moisture. Pests nest in them. If you go this route, the material must be held tight against the subfloor with proper supports, and the crawl space must be genuinely dry.

Also, in humid climates, venting a crawl space in summer brings warm moist air into contact with cool surfaces — which is the opposite of drying it. This is why the sealed approach has become common in those climates.

Sealed (encapsulated) crawl space

The crawl space is brought inside the thermal envelope.

• Vents are sealed.
• A heavy vapor barrier covers the ground and is lapped up the walls and sealed.
• Insulation goes on the perimeter walls, typically rigid foam.
• The space gets some conditioning or dehumidification so moisture doesn't accumulate.
• Floor insulation is not used — the floor is now an interior surface.

Better performance, more expensive, more detail-sensitive. Requirements for sealed crawl spaces — vapor barrier specification, conditioning or exhaust, ignition barriers over foam — are set by code and vary. Check yours.

What you must not do: seal the vents without a ground vapor barrier and without addressing moisture. That creates a damp, unventilated space, which is a rot and mould problem.

How do you insulate basement walls?

Concrete and block foundations are damp. They wick ground moisture and dry inward, into the basement. Any insulation assembly has to accommodate that.

The assembly that fails: fibreglass batts in a stud wall built a few inches off the concrete, with drywall over the top. Warm humid indoor air reaches the cold concrete through the cavity, condenses, and the batts hold the water against the framing. This is a common and damaging mistake.

The assemblies that work put rigid foam directly against the concrete, sealed at the seams and edges so no interior air can reach the cold surface:

• Rigid foam board adhered or mechanically fastened to the wall, seams taped, edges sealed. A stud wall can then be built in front of it for finishing, with or without additional cavity insulation depending on your climate.
• Closed-cell spray foam applied directly to the concrete — insulation, air barrier and vapor retarder in one.

Requirements to check: most codes require a thermal or ignition barrier over foam plastic in occupied and many unoccupied spaces. Don't leave foam exposed without confirming what's required.

Why fix water before insulating a basement?

Nothing below grade should be insulated while bulk water is getting in. Insulation doesn't stop water; it hides it.

Before any of the above:

• Grade slopes away from the foundation
• Gutters and downspouts discharge well clear of the wall
• Cracks and active seepage addressed
• Sump pump working, with a sealed lid
• Interior humidity under control — see home humidity control

If you have visible water, efflorescence on the walls, or a musty smell, solve that before spending anything on insulation.

Combustion appliances and sealing

Basements and crawl spaces often house furnaces and water heaters. If any of them is atmospherically vented — drawing combustion air from the room and venting up a natural-draft flue — then sealing the space affects the air it depends on.

Two implications:

• Don't seal a space containing a naturally drafting appliance without checking combustion air requirements and having draft verified. Backdrafting is a carbon monoxide risk, not just an efficiency issue.
• Sealed-combustion and direct-vent appliances draw air from outside through their own intake and aren't affected the same way.

Have combustion safety checked after significant sealing work. And make sure CO alarms are installed and current — see smoke and CO alarm placement.

If you're removing gas appliances from the space entirely, that changes the picture — see orphaned water heater venting.

Pipes and ducts

While you're down there:

• Insulate hot water pipes — cheap, and it reduces standby loss and wait time. See hot water recirculation pumps.
• Protect against freezing any pipes in a vented crawl space.
• Seal and insulate ducts running through unconditioned space. Leaking ducts in a crawl space or unconditioned basement can waste a large share of what your system produces — see ductwork problems and cost.

If you seal the crawl space, ducts inside it are now in conditioned space, which improves their effective performance considerably.

Priority order

For most houses:

1. Fix bulk water
2. Ground vapor barrier in any crawl space
3. Rim joist — seal and insulate. Best value in the building
4. Duct sealing in unconditioned space
5. Basement walls or crawl space assembly, whichever applies
6. Pipe insulation

Estimate what it does to your bill with the electricity cost calculator, and get a whole-house picture from the home energy score calculator.

The bottom line

The rim joist is the highest-return insulation job in most houses and it's usually easy to reach — rigid foam sealed at every edge, not batts. Below that, choose one crawl space strategy and execute it fully: vented with floor insulation and a ground vapor barrier, or sealed with wall insulation and moisture control. On basement walls, put rigid foam directly against the concrete so interior air never reaches a cold surface. And fix water before insulating anything, because insulation over a moisture problem just conceals the damage.

Next: how to air seal a house.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• IRC R402 — foundation and slab insulation requirements
• IRC R408 — crawl space ventilation and vapour retarder provisions

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.