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Q. My house was built long ago before all the energy concerns. In the attic it had three inches of loose blown-in insulation. Several years back I put rolls of four inch paper backed fiberglass insulation on top of that. I did not staple it to the rafters. Everything is still OK up there. I know I need more insulation. Can I put some R-19, or better-- not paper backed, on top of that? Also, should the eaves in the attic be covered? Mine are.
A. Think of your living space as a box that ends just at the other side of the floor, walls and ceilings. That is the space in which you want to keep conditioned or warmed air. In the construction world that’s called the building envelope. The attic air space is above the building envelope and is merely an enclosed area that a happens to be the size it is as a consequence of the way the roof is framed. The roof’s only job is to keep rain and snow off you. It doesn’t need or want heating or cooling. It’s known as a “cold” roof and when you see snow or frost melting off unevenly that’s a signal that the house is losing heat.
Insulation is the stuff we use to try to keep as much of that hard-earned heat inside the house where we can benefit from it. Your original insulation has probably settled flat on the top of the ceiling drywall and has a marginal insulation quality left in it. You were wise going over it with even the nominal four inches of rolled fiberglass. That insulation was intended as wall insulation and has an R-value of 11. Unless you were able to place it side by side almost air tight, your coverage was probably only in the high 80 percent range and although better than no insulation at all, was still pretty heat leaky.
The kraft paper backing on the four-inch rolled insulation has traditionally been called a vapor barrier but it’s really not-- all it ever did was provide a surface upon which the batt insulation could be glued and made the product much easier to install . Foil backing does act as a barrier-- a whole other subject.
Some might tell you that you’ll have to remove the four-inch layer of insulation before adding more insulation over because the paper backing could be a condensation plane and cause you problems-- if it’s kraft paper only and not foil then I wouldn’t worry about it.
I assume you are planning to add the additional layer of insulation yourself. You also say you are going to use unfaced batts. That means you’ll buy bags of R-19 fiberglass insulation that when opened sections of six-inch thick insulation without any paper on it will pop out and you can handle them in about three-foot sections. They are normally 14-1/2 inches wide and are designed to fit tightly between the ceiling joists. You may be able to do this with just what you have up there but if not then lay them perpendicular to the joists and tight side by side and end to end. Work from the outside edge in towards the access to the attic so you don’t paint yourself into a corner. If the ventilation of the attic is by louvers at the attic ends then it really doesn’t make that much difference if some overhangs the eaves but it does make a difference if any touches the underside of the roof as it slants down towards the eave. Insulation in contact with the under roof can cause a condensation place and roof rot can occur there. If you have had roof ridge ventilation installed sometime over the life of this house then the eaves must be kept clear. They manufacture foam plastic baffles that you install at the roof edge to keep the eave space open and prevent the insulation from coming in contact with the underside of the roof.
Now is the time to do this job because although there is no time during which working in an attic is fun-- it’s a whole lot more tolerable when it’s cool up there and not 130º degrees in July. Also, when handling fiberglass insulation wear old long sleeve shirts and wrap your wrists with duct tape and wear goggles, a good respirator and a hat as the stuff launches small glass fibers that itch and annoy. Take the clothes off when you are done and pitch them out because if you run them through the clothes washer they can leave itchy glass bits in the machine only to be deposited on other clothes during the next load.