Floor Joists or Perimeter Walls? Where Crawlspace Insulation Belongs

One decision, most of the outcome

If you change only one thing about a Winnipeg crawlspace, change where the insulation sits.

Almost every older crawlspace in this city has fibreglass batts pushed up between the floor joists, with the crawlspace itself vented and unheated. The intent was reasonable: insulate the floor, keep the house warm, let the crawlspace look after itself.

In our climate it fails, and it fails in four separate ways at once.

Why joist insulation fails here

It leaves the crawlspace cold. The space runs near outdoor temperature, which puts any plumbing in it at risk and makes the floor above feel cold regardless of how much insulation is in the joist bays.

It leaves the rim joist exposed. That band where the floor structure meets the foundation is thin, poorly sealed and directly exposed to outdoor temperature. It is one of the largest air leakage paths in a typical house, and batts between joists do nothing for it.

It traps moisture against the subfloor. Fibreglass is moisture-absorbent and air-permeable. Warm indoor air moving down through the floor condenses on the cold sheathing, and the batt holds that moisture there instead of letting it dry.

It falls apart. Batts sag, compress and drop out of the bays, particularly once damp. We routinely find half the insulation lying on the crawlspace floor, which means the assembly has been performing at a fraction of its design value for years.

Why perimeter insulation works

Insulating the crawlspace foundation walls and rim joist, with continuous air sealing, changes the fundamentals:

The crawlspace comes inside the heated envelope and stays well above freezing
Interior surfaces run warmer, which keeps them above the dew point and stops condensation
Insulation is continuous rather than interrupted by joists, with no thermal bridging
Plumbing, well lines and drains in the space are protected
Nothing sags, because the insulation is fixed to a wall rather than wedged into a cavity

The floor above stops feeling cold not because there is more insulation under it, but because the space beneath it is no longer outdoors.

Rigid board or spray foam?

Both are used, and the choice is mostly about wall condition and access.

Closed-cell spray foam air seals and insulates in one operation, conforms to irregular surfaces, and handles older rubble or uneven walls well. It is the easier choice in awkward spaces and gives excellent continuity. It is also less inspectable after the fact and generally costs more.

Rigid foam board is often more economical on sound, flat concrete. Joints need to be sealed carefully, and the assembly needs to be detailed so air cannot bypass behind the board.

Both require code-required ignition and thermal barrier treatment depending on the situation, which is worth confirming for your specific project rather than assuming.

Do not insulate over a wet wall

This matters. Insulating a foundation wall that is actively damp traps moisture against it, and foam applied to a wet surface may not adhere properly.

The order is: manage water, dry the space, seal the soil floor, then insulate. If a contractor proposes insulating a wall with efflorescence and visible moisture on it, ask why.

What about the old batts?

In most projects they come out. Once the perimeter is insulated and the space is conditioned, batts in the joists serve no purpose, and if they are damp or mould-affected they are actively harmful.

Removing them also exposes the joists, sill plates and subfloor for proper inspection — which is frequently when we discover decay that nobody knew about.

R-value is not the whole story

Homeowners often ask what R-value they need, and the honest answer is that continuity and air sealing deliver more real-world performance than nominal R-value alone. A continuous R-10 wall assembly with no air leakage will outperform a nominal R-20 joist assembly with gaps, sagging batts and an unsealed rim joist.

Code minimums apply and we confirm them for each project. But the design goal is a continuous, air-sealed, moisture-tolerant envelope — not a number on a label.

Expected results

After this work, in roughly this order:

The floor above stops being cold, usually noticed within a day
Frost on the subfloor does not reappear next winter
Framing moisture content drops over the following weeks
Any musty smell upstairs diminishes as the source dries
Heating costs fall, by an amount that depends on how bad the starting condition was

We measure framing moisture and humidity before and after, so the first three are documented rather than impressions.