The Stack Effect in North Idaho Homes: How Conditioned Crawl SpacesImprove Indoor Air Quality

The Stack Effect in North Idaho Homes: How Conditioned Crawl Spaces

Improve Indoor Air Quality

Stack effect is one of those invisible forces that quietly runs your house whether you like it

or not—like gravity, but draftie. And in a cold-dominant, shoulder-season-heavy place

like North Idaho, the stack effect becomes even more important because we have long periods of cool outdoor temps, high indoor-outdoor deltas, and lots of houses with vented or poorly detailed crawl spaces. Add in: wood foundations, older lake cabins turned “year-round,” and suddenly the way air

moves through your house matters a lot for indoor air quality, moisture, and even mold concerns.

Here’s a quick walk-through of what’s going on.

What is the stack effect?

The stack effect (sometimes called chimney effect) is the natural movement of air in a building caused by temperature differences between indoors and outdoors. Warm air is lighter and wants to rise. Cold air is heavier and wants to fall. In winter, your house is basically a very short chimney:

- Warm indoor air rises and tries to escape at the top (attic leaks, can lights, wall tops,

leaky hatches, second-story windows).

- When air leaves at the top, replacement air has to come from somewhere—so the

house pulls in air from lower levels: basements, crawl spaces, attached garages, rim joists,

even through floor penetrations.

So in heating season the air pathway is usually:

Crawl space → main floor → upper floor → attic → outside.

That means: whatever is in your crawl space air can and will migrate into your breathing zone.

In summer, the stack effect can reverse or weaken, but in North Idaho we spend way more time with indoor warmer than outdoor than the reverse—especially nights, shoulder seasons (Sept–Nov, March–May), and pretty much all winter. So the winter stack effect is the one to design for.

Why crawl spaces are such a big deal here

Plenty of North Idaho homes have:

- Vented crawl spaces (based on the outdated thinking: “let it breathe, it’ll dry itself”)

- Exposed soil or poorly sealed poly

- Cold, damp concrete stem walls

- Plumbing and mechanicals down there

- Occasional bulk water or seasonal high water (spring melt, wet falls)

Now… pair that with winter stack effect. If the crawl space is cold, damp, and vented to outside, it is also connected—often sloppily—to the framing above: unsealed subfloor penetrations, tub drains, plumbing chases, HVAC boots, electrical holes. So every time warm air leaks out the top of the house, your house replaces it with crawl space air. That crawl space air can have:

- Higher relative humidity

- Soil gases

- Musty odors / fungal spores from wood or rim joist growth

- Radon (parts of North Idaho do have radon pockets)

- Cold air that drops your floor temp and comfort

That’s why building-science people keep saying: “Your house breathes from the bottom

up.” We like to imagine “fresh mountain air” is coming in. Often it’s “crawl-space air.”

Enter the conditioned (or encapsulated) crawl space

A conditioned or encapsulated crawl space is basically treating the crawl like a

short basement:

1. Ground sealed with durable, sealed vapor barrier (ideally 10–20 mil, seams taped,

mechanically fastened).

2. Foundation walls insulated (inside is common).

3. Vents closed and air sealed.

4. Crawl air connected to the conditioned space (small supply register + return path, or

a dehumidifier + passive transfer) so conditions down there stay near house conditions.

5. Bulk water kept out (drainage, gutters, slope, sometimes perimeter drain).

When you do that, you turn the crawl from a source of pollutants and cold/damp air into a neutral or even positive zone. And now, when the stack effect pulls air from below, it’s pulling conditioned, dry, relatively clean air—not cold, damp, spore-y air.

Why this matters more in North Idaho’s climate

North Idaho has some quirks:

- Cold winters (prolonged temperature deltas = stronger stack effect).

- Shoulder-season dampness (Oct–Nov and March–April can be chilly + wet).

- Snowmelt and perched water around foundations (especially if downspouts just dump

at the stem wall).

- A lot of mixed-age housing—1970s lake houses, 1990s custom homes, and recent

energy-code builds, sometimes all on the same road.

- Forest/soil organic loads: even if the crawl is vented, you’re not getting cute alpine air;

you’re often getting humid, cool air that condenses on cold framing.

In a climate like Phoenix, a vented crawl might be less dramatic. In Sandpoint in January? A vented crawl is like hooking your house to a mini walk-in fridge full of damp plywood. Because it stays cold for long stretches, materials in the crawl stay at or near dew point. That means wood moisture content can creep up. Once wood moisture stays in the 16–20% band, surface mold can start. It doesn’t have to be a horror-movie bloom; even light fungal growth on joists is enough to create musty odors. And stack effect will lift those right through the floor. So: colder climate = stronger stack effect + slower drying = higher benefit from conditioning the crawl.

Indoor air quality (IAQ) link

Let’s connect the dots.

1. Source: Crawl has damp wood, maybe a little fungal growth, maybe occasional mouse activity, maybe soil gases, maybe stored stuff (paint, lawn chemicals—don’t do that, but people do).

2. Pathway: Unsealed floor penetrations, plumbing chases, rim joist gaps, un-gasketed crawl hatches, duct leakage in the crawl.

3. Driving force: Stack effect (warm air up → replacement air from crawl).

= Result: Crawl becomes a permanent low-grade IAQ pollutant source.

When we encapsulate and condition:

- You remove or reduce the source (drier materials → less moldy odor).

- You reduce the pathway (air sealing).

- You weaken the driving force at the bottom (pressure is more even).

Therefore, IAQ improves. And this is the part people often forget: IAQ is a building-plus-people system. If someone in the home has asthma, mold sensitivity, or you have concerns (like we talked about with mold, gut health, and differing human sensitivity), then reducing any low-level, chronic indoor source is worth it. Conditioned crawls are low-hanging fruit.

But don’t we need vents to “let it dry”?

That’s old-school prescriptive code thinking from milder climates. In a cold/mixed climate like North Idaho, venting often does not dry the crawl. It often introduces cold, damp air that condenses on colder surfaces. In summer, warm humid air can also hit cooler crawl surfaces and condense. So the venting-dries-everything idea doesn’t hold up well here.

Modern building-science guidance (and the stuff you were reading yesterday about tying crawl spaces into conditioned space) tends to say: Seal it, keep water out, insulate the walls, control humidity, connect it to the house. That’s how we get a crawl that follows indoor conditions instead of fighting them.

How the stack effect changes once you condition the crawl

Let’s do a simple before/after.

Before (vented, cold crawl)

- Crawl temp: 35–50°F in winter

- RH: high

- Air: musty, sometimes soil-smelling

- Pressure: lower than house → air gets pulled up

- Result: house pulls in cold, dirty, damp air

After (encapsulated, conditioned crawl)

- Crawl temp: maybe 60–68°F (tracks house within ~5–10°F)

- RH: controlled (50–60% target)

- Air: similar to house air

- Pressure: closer to neutral

- Result: house pulls in "okay" air (because it’s now part of the house)

You haven’t eliminated stack effect (physics will physics), but you’ve changed what air is

available to be drawn upward. That’s the key.

Design notes specific to North Idaho

If you were doing this on your Sandpoint property or a tiny house/cottage you’re building,

here’s what I’d insist on:

1. Drainage first. Conditioned crawls don’t work if they’re wet. Make sure:

- Gutters with downspouts extended 6–10 ft

- Grade slopes away

- Perimeter drain / sump if site is wet

- No dryer vent exhausting into crawl (yep, people do that)

2. Ground vapor barrier:

- 10–20 mil poly or reinforced liner

- Run it up the wall 6–12"

- Tape seams, mechanically fasten to wall

- Seal around piers

3. Insulate the walls, not the floor:

- R-10 to R-15 on interior of stem wall works in our climate band

- Protect foam if it’s exposed

4. Air seal the band/rim joist:

- This is where crawl air often gets into the house

- Foam + flashing or rigid + sealant

5. Condition the space:

- Easiest: small supply from the main system plus a return path (transfer grille or

undercut)

- Or: dedicated crawlspace dehumidifier (nice in our damp shoulder seasons)

6. Monitor:

- Hygrometer in crawl

- Check seasonally (spring melt, fall rains, January cold snap)

That’s a realistic, not-crazy-expensive way to turn a liability crawl into a neutral space.

What about energy use?

Yes, a conditioned crawl can increase conditioned volume, which sounds like more

energy. But in a cold climate, two things offset that:

1. Warmer floors = better comfort = lower thermostat setpoint. People often turn the

heat up just because floors are cold.

2. Less cold air infiltration from below. When the crawl is 65°F instead of 40°F, the air

you’re accidentally pulling up isn’t tanking your room temps.

So in practice many houses feel more efficient after encapsulation, and some actually

are.

Quick FAQ

Q: Can I just run a dehumidifier in the crawl and call it good?

A: Only if it’s sealed. A dehu in a vented, leaky crawl will just dehu northern Idaho. Seal

first, then control.

Q: Do I still need a radon system?

A: If you’re in a radon area, yes—encapsulation + passive/active radon works great

together.

Q: What if my HVAC is in the crawl?

A: Even more reason to condition it. You do not want ducts and air handlers sitting in a

cold, damp, mold-friendly space.

Bottom line

- Stack effect is real and stronger in cold North Idaho winters.

- Houses breathe from the bottom up in heating season.

- If the bottom is a nasty crawl, that’s what you’re breathing.

- Conditioned/encapsulated crawls flip the script: they make the air at the “bottom” of

the stack clean, dry, and temperature-controlled.

- In our climate, where drying potential is limited for months, encapsulation is extra

valuable.

- And if there are sensitive occupants (kids, people with mold/humidity reactivity), this

is one of the highest-impact building fixes you can make.