Skylight Shafts: The Hidden Weak Spot in Your Building Envelope

Skylights are a great way to bring daylight deep into a home’s interior, but they also introduce a complex thermal and moisture dynamic that’s often misunderstood—or completely overlooked. A well-insulated skylight shaft performs like an exterior wall. A poorly insulated one acts more like a chimney for heat loss and condensation.

So if you're planning to add a skylight—or wondering why your existing one is sweating in winter—read on.

A Quick Anatomy of the Problem

Most skylights are installed in framed openings that connect the ceiling plane to the roof deck through a vertical shaft. This shaft cuts straight through the thermal boundary of the home, extending into what is typically an unconditioned attic. That means every square inch of that shaft is now part of the building envelope—and needs to be insulated and air-sealed accordingly.

Here’s the problem: many skylight shafts are framed and drywalled like interior walls, with little regard for thermal bridging or air leakage. If warm, humid interior air makes its way into the shaft, it will contact the cooler surfaces near the attic and condense. This often looks like a roof leak—but it’s not. It’s building physics in action.

Thermal Bridging and the Stack Effect

The shaft creates an opportunity for thermal bridging, where framing members conduct heat from the conditioned interior to the unconditioned attic. This not only lowers energy efficiency but also contributes to cold surface temperatures inside the shaft.

Add the stack effect into the mix—where warm air rises and is drawn up through any holes in the building envelope—and you’ve got a recipe for continuous moisture cycling inside the skylight cavity.

Three Ways to Insulate (Correctly)

1. Spray Foam (Best All-Around Performer)
Closed-cell spray polyurethane foam (ccSPF) is ideal because it insulates, air-seals, and adds structural rigidity in a single step. It has a high R-value per inch and virtually eliminates convective looping inside the cavity. Open-cell foam can also work in milder climates but may require a vapor retarder on the interior side.

2. Rigid Foam with Taped Seams
A DIY-friendly method involves cutting rigid foam panels (polyiso or XPS) to fit snugly between framing members, then taping all seams and sealing edges with canned foam. This approach minimizes air infiltration and offers good thermal resistance if installed meticulously.

3. Fiberglass or Mineral Wool—Only If Fully Encapsulated
Batt insulation can work, but only if it’s fully enclosed on all six sides with an air barrier (like drywall or rigid foam). Leaving insulation exposed to the attic air is a guarantee of reduced performance—often by 50% or more—due to air movement and convective loss.

Don’t Forget: The Air Barrier Is Just as Important as the Insulation

Insulation without a continuous air barrier is like wearing a down jacket full of holes. A successful skylight shaft needs to be detailed like any other part of the building envelope:

• No gaps around framing

• No unsealed penetrations

• Vapor control on the warm-in-winter side (typically the ceiling side)

• Airtight transitions to the ceiling and roof planes

Bonus Consideration: Ventilated Attics vs. Conditioned Roofs

In vented attics, the temperature delta between the skylight shaft and the attic air can be extreme—especially in cold climates. That’s why insulation and air sealing are non-negotiable. But in homes with conditioned roof assemblies (like those using spray foam on the underside of the roof deck), the shaft is surrounded by semi-conditioned space, reducing risk. Still, it should be detailed correctly, especially for dew point control.

In Summary

Insulating a skylight shaft isn’t optional—it’s critical. Treated properly, a skylight can be a durable, high-performance architectural feature. Treated casually, it becomes a thermal liability and moisture trap.

If you're seeing water stains, peeling paint, or just want to improve comfort and efficiency, it’s worth revisiting how your skylight shaft was built. And if you're installing a new one, make sure it’s detailed like the exterior wall that it is.

At Peak Property Services, we bring building science and craftsmanship together to deliver results that perform—not just look good. If you have questions about your skylight, we’re happy to take a look.