A couple of annotations regarding the most recent post. While I would not recommend the spray-in insulation, if you do go that route a few things to remember:
1) I would advise, if you do go with spray-in insulation, do NOT let them coat the top of the joists - in case they are exposed to moisture on the house side due to poor or holed vapor barrier, you want the joists to have a free surface to evaporate moisture from to reduce the risk of dry rot.
2) If you take the suggestion about spraying ducts, bear in mind 3 things - makes it a much harder job to work on them if they need work in the future, on horizontal runs make sure they are supported every few feet or the weight of the wet foam can crumple them before it sets up (or use thin coats), and be especially careful about the weight of the wet foam (before it gains strength) pulling flexible duct apart or loose of connections. Also test a short piece first - some spray foam chemicals (when still wet) dissolve the plastic coating on some flexible duct materials - I have seen it on a spiral-reinforced bathroom duct hose and on A/C flex duct, where it ate right through the plastic and half-filled the duct with foam. Also,, check all first to be sure there are no holes, splits, or gaps where the foam could fill the duct up.
3) Keep at least 18 inches clear of mechanical equipment like fans or HVAC units to leave work space and to provide cooling air, and do not "box" any in so heat cannot rise off the unit or fresh air flow to it at base level.
4) Do NOT spray insulate can type recessed ceiling lights - they have to either have free air flow to them, or have a specified open air space around them, depending on rating marked on or in the can. If yours are the type that cannot be insulated, this would be a good time to change those out, because they provide a continuous year-around free flow of moist air to the attic from the house, and hot summer air down into the house during high pressure times. Fully encapsulate in foam and you are asking for a fire. Even if rated for full insulation wrapping, wrap in a minimum 3 inch layer of fiberglass before spraying, because the cans get hot enough to melt and set fire to sprayed foam.
5) Put a thin layer of fiberglass over each electrical box exposed from the ceiling below - makes it much easier to work on in the future, and keeps the expanding foam from filling the box and risking overheating of the wires in the box, not to mention making it impossible to work on in the future.
6) It is illegal to encapsulate electric wires in the foam - they are rated for some air movement, such as occurs in fiberglass, and not only will their amperage capacity be significantly degraded by encapsulation, but the foam melts many types of electric wire insulation before it sets up. This is commonly ignored and has been the cause of electrical fires, particularly in houses where most runs go to the attic and then down into the walls rather than from basement up. In this type of house wiring, most of the wires are run through the middle of the attic floor joists, so it is pretty much impossible to spray foam insulate without embedding them, so foam insualtion is out of the picture for that case. Going ahead an encapsulting them, in addition to making future work on them almost impossible, is as dangerous as running normal romex through conduit - it needs the ability to radiate heat around the wires or they can overheat.
7) As stated in the other recent comment, you have to be sure to keep your eave openings open with eave baffles or air chutes, not only from foam but also if you overstack with the old fiberglass, you need a good clearance at least the depth of the rafters, and preferably a couple of inches more, so air can move around as needed to vent heat and moisture from the attic.
8) Be sure penetrations like bathroom and kitchen fans, lights, etc are well sealed to the vapor barrier and covered with a thin layer of fiberglass, because they commonly have openings around them through the ceiling under the trim ring around them or snap-on cover - I have seen a brand new kitchen sprayed all over with attic spray foam because it sprayed through the air gaps around the recessed ceiling light fixtures that had not had trim rings put on yet - was about $30,000 in damage in about 2 minutes of spraying, because the applicator was trying real hard to get those gaps filled well !
9) Ditto to eaves - they need to be temporarily blocked from the inside with paper during spraying, or the overspray will either block the bug screening on the eave openings, plug up the eave cover vents, or spray out the eave openings all over the underside of the eaves, side of the house and deck or patio outside.
10) It is illegal to cover over any type of electrical junction or splice box without providing open access to it, so that needs to be boxed out for access or raised above the joist level. You can box it out with wood in a taper opening away from the box and a couple of layers of unstapled visqueen and a "handle" of wood or rope sticking up and then spray the inside of the boxout, creating a pull-out foam block when you pull the rope or handle that gives access to the electrical box. If this is done, requires a yellow notice on the front that there is a splice or junction box below, the circuit number(s), and the voltage.
11) It is illegal to spray foam on or have foam within, if I remember correctly, 6 inches (but don't hold me to that) of a chimney flue or furnace/water heater chimney / exhaust vent pipe, or exhaust of any other combustion device. If you have an attic mounted furnace or hot water heater the safe distance is greater, depending on model of furnace - it should have safe distance information on a label. I would not have exposed spray foam anywhere near a furnace - I would have drywall or at least 4 inches of fibreglass as the closest material.
12) Be sure to have any attic ventilation fans OFF during the spraying operation, as the fine spray mist will stick to the blades and motor and unbalance them and cause overheating. In fact, any electrical equipment in the attic should be turned OFF (and tagged out) and wrapped in plastic sheeting during the insulation removal/moving process and during the spraying process. Just don't forget to remove the covering before operating them afterward. If the contractor gets lazy and does not seal off equipment until the spraying process starts, all equipment should be well vacuumed FIRST, and then blown out with compressed air after all the work is done, else you risk motor failure from dust and fiberglass contamination in the motors. This can really shorten a motor's life, so proper protection (garbage bags tied around the base with duct tape work well) before any dust is stirred up is the best prevention. Do not ignore the lockout / switch covering, because a minute or tewo running under plastic coering will total most electric motors.
13) if you have an HVAC unit in the attic, you should check (and probably change out) the filter about a week or so after the job is done and residual dust has had a chance to clear out. If your filters are expensive, it might be an idea to put a used but usable one in before the job, then change out for a new one after the job is done. It definitely should NOT be run when insulation or foam is being handled and dust stirred around.
14) Be sure contractor's personnel understand that the trusses and sheathing above the joist level is NOT to be sprayed - I have seen operators think they were doing a favor by shooting a layer of foam on the underside of the sheathing without extra charge, as free insulation. This is destructive to the roof materials and should not be done, as Todd Shell pointed out.
15) As you can see, any fool can jump in and spray foam an attic, but if not properly done, you can cuase a lot more problems, and more serious ones, than you are solving.
16) If there is any living space in the attic, or thoughts of that in the future, the foam has to be fire rated. In all probability, you have 1/2" sheetrock for ceilings, which also is not thick enough for the required 1 hour fire rating against non-rated foam, so that needs consideration.