I built my own using a 4 X 8 sheet of 1 inch foam insulation. I found a set of plans to do this. Here is a link which has a similar plan. You can use even thicker foam insulation if you want a higher R-value. http://www.jacksonemc.com/Do-it-Yourself-Energy-Projects.43.0.html You may have to modify it a bit to fit your stairs. It was not very difficult to build. I put down weather stripping on the wooden frame. The box sits on the weather stripping which makes a tight seal. I even used the left over pieces on the sides in order to give it extra strength and weight. When I go into the attic I simply push the box so that it stands up on one of the long sides. To close I just lower the box into position onto the weather stripping. For about $25 you can build a custom fit cover. I think the key is the tight fit to stop air infiltration. I am not sure how good a job the store bought versions will do.

have a foam company come and take a look. lots of times we cut thru a wall to allow access. if you only foam part of the roof, you aren't achieving a lot as you will still have air leakage from unfoamed area. if it is truly not acessable then you need to put a wall(..can be felt nailed to studs) between foamed and unfoamed part of attic. foam company can spray the 'wall' to make it air tight to the unvented portion. most companies should be able to figure it out and give you a better idea. shop around. best of luck.

you can diy cellulose. but..sealing the openings from attic to living space below should be done first. hvac supply boxes where they penetrate the attic floor, cuts around bath fans, oversized cut for stove vent and recessed lights. all these openings allow attic air & insulation particles to enter your home. in sealing these areas you improve the air barrier at the attic floor. you would use mastic tape to seal the hvac supply boxes..from inside remove supply grill. use tape to seal sheetrock ceiling to the inside of the supply box's metal lip. hopefully your hvac company mastic sealed the ductwork, but take a look and see. most do not address the sealing of the supply boxes to attic floor. most existing homes have duct leakage of about 30%. air that you pay to heat and cool that never makes it into the house. recessed cans..ice chest method in attic or air tight insert from inside. get brand name & model number to shop online for air tight inserts. bath fans cuts can be big..I use the mastic tape here also. sometimes the stove vent can be sealed with caulk..but mastic tape may be used here also. I use hardcast brand mastic tape, from hvac supply. mastic sealing the ductwork (esp in the attic) is also a good savings. you can use a paint on mastic that is applied with a brush. duct takeoffs @ supply plenum are usually big leakage areas. keep in mind that cellulose is ground newspaper. it creates a fine dust that enters thru all the cracks in the attic. thus the reasons for improving the air barrier. insulation doesn't work when air is moving through it. you will see a savings & improved comfort in adding insulation. more so if you do the work above prior to insulating. I don't understand why the need for closing off the soffits..can you explain a little better? do you have ridge vent & soffits? in an vented attic there needs to be a way for cool air to enter (soffits) and exit (ridge vent). also you should install baffels to keep insulation out of soffits and provide a pathway for air. can you describe the type of ventilation you have? soffit.ridge vent gable end soffit & passive vents (whirly birds that may just be a La description) I can't imagine life without high humidity! it is a killer in my hot humid climate. makes for some long hot hot summers. best of luck

Start with the most important issue and its not the temperature of the attic. If the HVAC equipment and/or ducts are in the attic, you are losing a great deal of conditioned air to the attic space due to leaks and older minimum code insulation and you need to put an end to that waste. You also need to avoid bringing large amounts of hot/humid or cold air into the attic. That is never effective because so much of the heat in an attic is from infra-red radiation from the bottom of the roof deck that passes through air without being cooled. The modern solution is to insulate between the roof rafters and, if you like, add a ventilation space between the insulation and the roof deck so air that enters at the eaves can exit from the ridge and slowly dry the roof deck. Or , alternatively, you can seal the attic - its your choice. The attic space would then be partially conditioned by the HVAC air leaks and thin insulation which will keep it at a moderate temperature, dry and not musty and will reduce the heat and cooling losses from the house below. This is how new houses are built.