I know I need more insulation
26 November 2005
A. There are some basics we need to go over before you decide what and how to proceed with your insulation plans.
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 thatís called the building envelope. The attic air sp ace is above the building envelope and is merely an enclosed area that a happens to be the size that it is as a consequence of the the way the roof is framed. The roofís only job is to keep rain and snow off you. It doesnít need or want heating or cooli ng.
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 three inches of blown insulation from 1959 has probably settled flat on the rear of the ceiling drywall and has a marginal insulation quality left in it, so you were wise in going over it with the nominal four inches of rolled fibreglass. That insulation was intended as wall insulation and has an R-value of 11. But unless you were able to place it side by side al most 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 bar rier but itís 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 youíll 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 itís kraft paper only and not foil then I wouldnít wor ry about it.
I presume you are planning to add the additional layer of insulation yourself. You also say you are going to use unfaced batts. That means youíll buy bags of R-19 fiberglass insulation that when opened sections of six-inch 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 the m perpendicular to the joists and tight side by side and end to end. Obviously, work from the outside in towards the access to the attic so you donít paint vourself into a corner. If the ventilation of the attic is by louvers at the attic ends then it really doesnít 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 pr event 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-- itís a whole lot more tolerable when itís cool up there and not 130ļ degrees. Also, when handling fiberglass insulation wear old long sleeve shirts and wrap your wrists with 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 the y can leave glass bits in the machine only to be deposited on other clothes during the next load.