Renovation & Design

Skylights have improved, but installation costly

Question: A few years ago, I bought a bungalow condominium unit. Because of the east-west orientation of the unit, the living area does not receive any direct natural light. This condominium has a cathedral ceiling, so I am starting to think of a possible skylight above the living room. In 2009, you published an article in which you expressed skepticism about skylights in our climate. Since then have there been advances in skylight technology or installation methods that have caused you to modify your opinion?

Sincerely, Richard Ellis

Answer: Retrofitting a skylight in an existing building does pose challenges for proper air sealing and prevention of leakage, but with modern insulation materials it is much more practical than in the past. The likelihood of leakage is somewhat dependent on the style of the building and roof system, which may be a major determining factor in whether the cost for the roof modifications may be worth the effort.

Skylights, in their most simple terms, are windows that are installed on the roof rather than the vertical walls of a building. As we all have experienced, windows have a high possibility of leakage, no matter where they are installed. Ones that are installed in a vertical plane are the easiest to waterproof, as the top is the most critical area to protect. Installation of a proper drip flashing, and associated other sub-siding membranes, over the top of the window normally will prevent leakage from rain and snow. The sides and bottom are more easily sealed, often with simple exterior caulking, due to the fact that water drains downward due to gravity. With a skylight, all four sides of the unit are at a risk for leakage, dependent on the slope of the roof, and other factors.

Because skylights sit on a semi-horizontal surface, they have several areas that have to be extremely well sealed at the exterior to prevent moisture intrusion. Not only does the glass and surrounding frame have to be resistant to periodic wind-driven rain, like a vertical window, it has to resist constant moisture on the surface. Skylights can have days, weeks, or months where liquid water, or ice and snow, sit on and around the window portion and on the surrounding roof. For this reason, any skylight has a high probability of leakage into the roof system, but the lower the pitch of the roof, the larger the chances of problems. So, if your condo has a moderate to a steep pitched roof, installation may be less problematic. If your building has a flat or low-sloped roof over the living room, I would definitely reconsider your planned renovation.

The second, and much more insidious, moisture issue with skylights often occurs where it cannot initially be seen. Because of the location of these roof-mounted windows, the area around their frames is embedded within an attic or vaulted roof system. These spaces are subject to extreme temperature swings, so they must be very well insulated and air sealed to prevent heat loss during the colder months of the year. The skylight surface temperature can be significantly lower or higher than that of the rest of the roof it resides in, which often leads to condensation on the surface of the glazing. Most skylights have drip troughs, or other mechanisms, to collect the water which may run down the surface, but these are not foolproof. If the frame or ceiling space around the skylight is not very well sealed and insulated, it can also have issues with condensation, which can lead to more significant negative results than in other areas.

The key to proper installation of a new skylight, in a situation like your home, is to completely insulate and air seal the entire exterior of the skylight assembly inside the attic or roof cavity space. In older homes, this was very difficult to accomplish with traditional fibreglass insulation, or even rigid foam products. All of these older types of insulation were near impossible to perfectly seal, leading to gaps where warm air from the living space could enter. In that situation, condensation is a certainty and in bad cases could lead to significant leakage, rot, mould growth, and major damage to building materials.

To answer your question more directly, there have been products developed, often well before 2009, which make the air sealing and insulation much more practical. The primary material used for this purpose is sprayed-on high-density polyurethane foam. While available at the time of this previous article, it was not widely used in our area due to the high cost and a limited number of contractors. Today, it is readily available, with options for several bulk foam installers with mobile trucks to mix and supply the products onsite. Also, commercially produced two-component foams are available at building supply stores in smaller containers, around the size of standard BBQ propane tanks. These smaller units make accessibility much easier, especially in limited areas like attics, but accessibility is the final piece of the puzzle.

Unless your roof system allows good access all the way around the area of the proposed skylight, proper air sealing and insulation may be very difficult. In that case, a much larger portion of the roofing and sheathing may have to be removed to install the foam. This is much more likely in a vaulted ceiling, like yours, so budgeting for a significant roofing repair cost will also be required.

Installation and sealing around a retro-fitted skylight in your condo may be much more practical than in the past, due to better foam insulation availability. Care must still be taken to do the job very well, which may involve much more time and expense to remove a significant portion of the roof, to allow proper foam application.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at




Browse Homes

Browse by Building Type