SKY LIGHT INSTALLATION
The most difficult part of installing a sky light is to insulate the light shaft properly. An improperly insulated light shaft will leak and create wood rot. Not something you need in the roof over your head. So right at the planning stage of the light shaft you must figure out how this will be done. The best I can describe this process is that you will be constructing it in the reverse order of an exterior wall. With this technique you should not have to climb in the attic and mess around with the insulation, which is especially useful if you have a low roof over the ceiling. All you need is a 10' step ladder standing inside the light shaft. Very comfortable and secure to stand on.
Having
determined where to install the sky light, cut the hole through the roof and the
ceiling. The two holes are then connect with a
sealed light shaft. I chose a shaft with a 150 bend in it for aesthetics.
It does make drywall finishing a bit more complex though.
Install four 2x2 wood posts from the attic floor to the underside of the roof. Each one is located at 8" radius outside the corners of the light shaft.
Then stretch chicken wire, top to bottom, around the four posts and staple it in place. The purpose of the chicken wire is to hold the fiberglass insulation snug against the outside of the light shaft.
Then construct the light shaft framing from (2x4)" spruce. Use screws as you will likely not have room to swing a hammer.
With the framing done I stuffed the fiberglass insulation between the chicken wire and what will become the outside of the light shaft. Use at least R40 insulation. Tuck it right snug against the roof. No air gaps allowed. This is the messiest part of the job but it sure beats doing it from inside the hot attic. Its a good idea to wear a mask and leave the sky light off to provide fresh air. An assistant could pass you the insulation bats from the roof.
Having satisfied yourself that all the insulation tucked in place correctly, you can install the 6MM vapour barrier on the inside of the light shaft framing. Overlap all joints at least 6" and seal them with acoustic sealant.
Finally you can install the drywall. Fill, tape and paint the drywall. Don't miss a stud with a drywall screw. It will create a hole in the vapour barrier that will cause wood rot down the road.
During the construction of the light shaft I had already built, placed and sealed the (2x4)" dam on the roof to support the glazing. I then laid the glazing over it to close the roof for the predictable evening thunder storms here. When I built the light shaft I simply lifted the glazing off the dam for the welcome ventilation. The skylight is double glazed for extra insulation at 550 N latitude. I used Sikaflex to seal the dam to the shingles. This product is overkill for the job, but if it works on my sailboat it will do a Jim dandy job on the roof.
I would never install a sky light in those parts of the world where you regularly receive deep snow over the winter. In this situation you are likely to develop an ice dam with all of its consequent damage to the roof. The ice dam develops when the relatively warm glazing is submerged under the deep snow. Since the snow is an excellent insulator the heat melts the snow around the glazing and the water flows down hill. The water will freeze when it flows far enough into the cold. You can insulate the light shaft to R100, but you cannot prevent the heat from escaping through the glazing. Only a triple pane unit with inert gas between the panes might limit the heat loss. However, a very effective solution is to install a sheet of acrylic glazing across the bottom of the light shaft, at the ceiling level. Support it on top of wood molding at the perimeter of the light shaft. Acrylic is a good insulator and the temperature inside the light shaft will be very low. Low enough that the temperature differential at the sky light will be minimal to prevent melting. Seal the corner molding to the drywall with silicon seal to block moisture. It wouldn't hurt to install rigid foam insulation along the bottom of the roof at the perimeter of the sky light. This will block more heat to prevent thawing.
NOTE: I developed this technique over several years. I live on the Canadian prairies and winters can be quite different from one year to the next. One year we receive very little snow and the next we get a dump of it. Therefore it takes some time to experience these problems and develop a solution for them. A very difficult thing to solve in the middle of winter. If you have an ice dam, shovel the snow off the roof and the water on top of the shingles will freeze. This will stop the water from leaking through the ceiling till Spring. Spring is the time to solve a problem.
Bob Schimmel