Skylight Shaft Walls

When skylights are installed, special attention must be paid to insulating the vertical shafts extending from the roof to the ceiling. Unsealed attics are often insulated along the attic floor with blown fiberglass or cellulose insulation. A different type of insulation will need to be installed along the skylight shaft walls, one that will maintain full alignment (or contact) with the back side of the sheathing for the height of the skylight shaft walls and that will maintain at least the minimum required depth of insulation along the full height. Skylight shaft walls should be insulated to the same R-value as other exterior walls if the attic they pass through is not insulated.

There are several options for insulating skylight shafts. One option to consider, especially if several skylights are being installed, is to insulate the entire attic, either by spray foaming along the underside of the roofline or installing rigid foam above the roof deck, or by constructing the roof with structural insulated panels (SIPs). This insulated attic would also provide a conditioned space for any HVAC equipment installed in the attic.

If the skylight shafts are installed in a vented, uninsulated attic, then the sides of the skylight shaft should be insulated. This can be accomplished using spray foam, or rigid foam, or by constructing the shaft walls with SIPs. Fiberglass batt insulation is not recommended for insulating the skylight shaft walls because it can settle or pull away from the wall. If batt insulation is used, it should be covered with a rigid material such as rigid foam, which will provide an interior air barrier and ensure that the batts will not slide down or fall away from the wall. These materials may be installed by insulators, framers, or subcontractors or vendors hired specifically to install the skylight. This task should be included in the contract for the appropriate trade depending on the workflow at the specific job site.

How to Insulate a Skylight Shaft

1. Select the skylight size and location to avoid cutting roof or ceiling rafters, if possible. (House designs that use 24-in on-center fully aligned framing members are less likely to require cutting of studs for skylight installation.) Follow the manufacturer’s instructions to install the skylight on the roof deck, properly flashing and integrating the skylight frame with roofing materials to minimize the risk of water intrusion. The roof opening is framed with headers, with framing lumber run horizontally across the opening and securely nailed to the rafters to support the structure.
2. Install headers across the ceiling opening as you did for the roof opening, using a single header at each end if no ceiling joists are cut or double headers if a ceiling joist has to be cut. If the light shaft will be flared, cut the joist at the angle of the flare.
3. Cut 2x4 studs (or 2x6 to allow for more cavity insulation) for the corners of the light shaft, angled on each end to fit flush against the rafters and ceiling joists. Next, place studs 24-inch on-center around the opening. Nail 2x2 cleats to the inside edges of the corner studs or use drywall clips to serve as backing for the drywall.

Or-

Frame the shaft with structural insulated panels (SIPs) following the manufacturer’s instructions for air sealing the corners, seams, and top and bottom edges.

1. Fill the stud cavities with spray foam, batt insulation, or rigid foam. Next, cover them with a layer of rigid foam to achieve the R-value level required for your climate zone.

Or, nail rigid foam in place over the studs and fill the cavities with blown cellulose or blown fiberglass.

1. Caulk drywall and rigid foam to framing at all edges. Seal any seams in the rigid foam with caulk, tape, or foam.
2. Paint drywall with latex paint to serve as the vapor barrier.

Figure 1- Frame, insulate, and air seal the walls of the skylight shaft as you would an exterior wall 

Figure 2 - Non-rigid cavity insulation should be covered with a rigid air barrier that is sealed at the edges

How to Insulate a Solar Tube

1. Install the tube according to the manufacturer’s instructions.
2. Insulate the solar tube with plastic-covered flex duct insulation.
3. Or - Wrap the tube with unfaced fiberglass batt insulation. Secure the insulation in place with ties. Cover the unfaced batt insulation with flexible foil-faced insulation and secure it with ties and metal tape.
4. Use spray foam or caulk to air seal the solar tube to the ceiling from above.
5. Caulk under the decorative ring on room side of the ceiling.