What does a vapor barrier actually do?
A vapor barrier, more precisely called a vapor retarder, slows the diffusion of water vapor through a wall assembly — the slow, molecule-by-molecule movement of moisture from a higher-humidity side of a wall toward a lower-humidity side. It is a distinct mechanism from air leakage, which is bulk air movement through gaps, cracks, and unsealed penetrations.
Both mechanisms can move moisture into a wall cavity, but they do not move it at the same rate, and that difference is the whole reason this page exists.
Which one actually moves more moisture, in practice?
Building Science Corporation's finding is that air leakage transports far more moisture into building assemblies than vapor diffusion does. A single unsealed gap — around an outlet box, a top plate, a window rough opening — can move far more water vapor in the air passing through it than the same area of wall moves by diffusion alone, because bulk air movement carries moisture at a completely different rate than diffusion through solid material.
That is the direct reason the same source states that air sealing comes before vapor-retarder adjustments in retrofit work. A wall with a textbook-perfect vapor retarder and an unsealed electrical box is still losing the fight to air leakage; a wall with ordinary air sealing done well is addressing the bigger of the two moisture pathways first.
So what does code actually require for a Longmont wall?
The 2021 IRC's Section R702.7 requires a Class I or II vapor retarder on the interior side of frame walls in Climate Zone 5, which is Longmont's climate zone. Class III vapor retarders — a more vapor-permeable category, like standard interior latex paint — are permitted only under specific cladding and insulation conditions spelled out in the section's tables, not as a general option.
This is a real, specific code requirement, and it applies regardless of how well a wall's air sealing is done. The point of this page is not that vapor retarders don't matter — it is that they are the smaller of two problems, and a retrofit that fixates on vapor-retarder class while leaving obvious air leaks unaddressed is solving the smaller problem first.
What does this mean for a retrofit or remodel project?
On an existing wall, air sealing is usually the more available lever: caulking and sealing penetrations, gaskets on outlet and switch boxes, and closing gaps at top and bottom plates can all be done without opening the wall cavity. Changing a vapor retarder's class, by contrast, often requires removing drywall or exterior cladding — a bigger scope of work.
That is a practical reason air sealing tends to come first in a retrofit sequence, on top of it addressing the larger moisture pathway. Whether a specific wall's existing vapor retarder needs upgrading at all is a question for whoever opens the wall, since it depends on what is actually in there already.
Sources
What the data says
According to Building Science Corporation, “air leakage transports far more moisture into building assemblies than vapor diffusion does — which is why air sealing comes before vapor-retarder adjustments in retrofit work.”
According to the International Residential Code, “the 2021 IRC (Section R702.7) requires a Class I or II vapor retarder on the interior side of frame walls in Climate Zone 5, with Class III permitted only under the specific cladding and insulation conditions listed in the section tables.”
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What happens after you request a quote?
A local contractor can open a small section of wall or inspect accessible areas to see what vapor retarder and air-sealing condition your walls are actually in, rather than guessing from the outside.
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We've got your info and are routing it to a local pro. When they call, here's what to look for in the quote you receive: (1) R-value target — the model energy code for Climate Zone 5 calls for R-49 to R-60 attic insulation; ask what your contractor is quoting to. (2) Air sealing scope — insulation alone does nothing if air leaks aren't sealed first, and Xcel requires a blower door test to rebate it. (3) Rebate handling — ask which programs your specific address qualifies for. Xcel and Efficiency Works are separate programs with separate paperwork, and not every address qualifies for both. (The federal IRA Section 25C tax credit ended December 31, 2025 — no federal insulation credit applies to 2026 work. Xcel Energy's insulation and air-sealing rebate remains active for its gas and electric customers. Longmont Power & Communications electric customers may separately qualify for an Efficiency Works rebate — check your bill to see which utility serves your address. Boulder County EnergySmart pays a third, income-qualified-only rebate as of January 1, 2026.) (4) Removal scope — older homes often need old insulation removed before new install. If a quote skips all four, get another quote.
Frequently asked
Common questions about vapor barriers in Longmont
Do Longmont homes require a vapor barrier by code?
The 2021 IRC's Section R702.7 requires a Class I or II vapor retarder on the interior side of frame walls in Climate Zone 5, which covers Longmont. Class III is allowed only under specific cladding and insulation conditions listed in the code, not as a general substitute.
Is air sealing more important than the vapor barrier?
For moisture control specifically, air leakage transports far more moisture into wall assemblies than vapor diffusion does, per Building Science Corporation, which is why air sealing comes before vapor-retarder adjustments in retrofit work. The vapor retarder still matters and is still a code requirement — it is just addressing the smaller of the two pathways.
Can I use latex paint as my vapor barrier?
Standard latex paint is generally treated as a Class III vapor retarder, which the 2021 IRC permits in Climate Zone 5 only under specific cladding and insulation conditions listed in the code's tables — not automatically. Whether it qualifies for a specific wall assembly depends on those listed conditions.
What counts as air leakage versus vapor diffusion in a wall?
Vapor diffusion is the slow movement of water vapor molecules through solid material from higher to lower humidity. Air leakage is bulk air movement through actual gaps and cracks — around outlet boxes, top plates, and unsealed penetrations. They are different mechanisms, and air leakage moves more moisture in practice.
Should I upgrade my vapor retarder during a remodel?
That depends on what class of vapor retarder the wall already has and what the remodel's scope allows access to, which is a question for whoever opens the wall. Air sealing any exposed penetrations while the wall is open is generally the lower-cost, higher-impact step to prioritize alongside it.
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