My house has 2x4 walls with R11 insulation. My roof is made without trusses just 2x4 rafters. I would like to put 3 inches of foam sheathing on my walls from the bottom of the foundation up right tight to the roof doing away with my soffit vents. My cieling has R40 insulation in the midle but because it has 2x4 rafters the insulation tapers down to 3 1/2 inches at the sides that is where I am looseing most of my heat. My question is if I put 3 1/2 inch fiberglass insulation under the roof between the rafters and plug off my ridge vent would I get moisture build up between the fiberglass and the roof? Thanks

A coupla questions (well, maybe more than a couple... ;-) )

What climate zone do you live in,

what type of foam sheathing to you envision using,

what material is the floor of your attic space insulated with,

how well sealed is your attic from the space below,

do you have a full basement/slab-on-grade/crawlspace,

is any part of your heating/cooling/hot-water heating system in the attic/basement/crawlspace,

is your foundation insulated on the inside/underneath,

Could go on....

But for attic moisture trapping issues, start by reading this:

http://www.buildingscienceconsulting.com/resources/4-Understanding_Attic_Ventilation.pdf

Odds are you might be better off putting foam on the underside of your rafters or spray-foaming between them and sealing off the vents, making the roof deck the pressure-boundary of the building envelope.

3" of foam sheathing could have moisture-trapping effects in your walls too, if not done right, and "right" is often different in hot humid AC-dominated climates than it is in cool heating-dominated climates. The foam board that goes below grade may not be the right stuff for what goes on the outside of the framed wall.

R11 low-density batting was the standard for much of the '50s & '60s, but it has fairly poor performance that only gets worse when the difference between indoor & outdoor temps are more than 50F degrees (like when you need it the MOST.) If you're taking the trouble to strip the siding, it's probably worth ripping out the R11 (if you can), and dense-packing cellulose in there (which can often be done even without removing low-density fiberglass insulation.) Whether you use highly water-vapor-permeable foam on the outside (like fiber-face iso board or EPS) vs vapor-retardent foam (like XPS or foil-faced iso board) is a function of climate zone, and the permeability of interior paints/wallpapers. If you create a vapor trap, the wood framing inside the wall will be susceptible to mold growth. (Cellulose insulation can lower that risk in reasonable wall-assemblies, but can't eliminate it in a true moisture-trap stackup like vinyl-wallpaper interior combined with foil-faced iso exterior.)

I live in cold climate upper Michigan,plan on useing owens corning foam sheathing,the attic has no floor just fiberglass insulation on top of drywall,house just has crawlspace not insulated inside. The furnace cold air return is in the attic under 16 inches of fiberglass insulation.

I know there's a foam board sold which comes w/dovetails on the back of it which would help drain the moisture out, combined w/ a rain screen no moisture will get trapped in behind. A sure shot would be rain screen, 1x3 and then foam. Either way will accomplish your goal. Be sure to vent at the bottom and top to allow air passage.
Good Luck.

Spraying 4" of half-pound foam or 2.5-3" of 2lb foam to the underside of the roof deck between the rafters to insulate and seal the soffit/ridge vents is probably the right thing to do.  Unless you have an extremely tight attic floor (spray-foam seal under that fiberglass) and mastic-sealed ultra-tight ducts a significant amount of conditioned air is ending up in the attic only to condense/frost on the wood or in the fiberglass before exiting out the attic vents.  By sealing and insulating the attic you will be eliminating that air migration out of the house and slowing (but not eliminating) the communication between the conditioned space & attic, reducing  condensation potential in the attic somewhat. (It might still need a de-humidifier up there- depends.  Monitor the relative humidity to be sure.)

Owens Corning makes a variety of XPS foam sheathing- go with the one with the highest permeabilty (Foamular ? which is still pretty low), but definitiely NOT the low-perm ProPink, and don't use any low-permeability sheeting or sheathing over that so that at least SOME outward drying can occur.

At R5 per inch you're looking at a true R15 clear-wall value on the outside of ~R7-R9 studwall.  The average winter humidity & temperature profile through the wall probably puts the dew-point inside the foam, and keeps the studs in the dry zone, which is good.  Be sure to use low permeability interior finishes to allow the wall to dry inward though. (No foil or vinyl wallpapers or low-permebility paints.)

An alternative would be to use 3" fiber-faced isocynaurate insulation (often sold as roofing insulation for flat or low-slope roofs) for the walls above the foundation (still use XPS on the foundation though), held in place with a high-permeability sheathing like Stedi-R.  In that case the wall structure would be fully outward-drying (good, in cold climates), and would add about R21-23 to the existing structure for about the same cost & thickness as going with XPS sheathing.  Then using vapor-retardent interior paints or wallpapers would be a GOOD thing, especially if combined with good caulking/air-sealing around all kick-boards, plumbing & electrical penetrations, crown moldings, etc to keep room air out of the wall cavities.

By sealing up the upper house & foundation with foam paneling from the outside crawlspace vapor pressures become an issue.  Be sure to block any crawlspace vents, use concrete-sealer on the interior walls, and put a 6-10-mil full-span poly vapor-retarder on the floor, sealed to the concrete at least a foot above the dirt with mastic, mechanically held to the wall  with furring strips.  Any seams/splits on between sheets of poly need a 1' overlap, sealed with mastic.  Insulating the floor with 2" of XPS foam might be your next insulating step too.

http://www.fpl.fs.fed.us/documnts/techline/crawl-space-ventilation.pdf

Foam sheathing plus converting to unvented attic/crawspace may tighten up the place to the point where you'll need/want active ventilation with either a heat-recovery ventilation (HRV) system, or a small bathroom exhaust vent running off a timer, humidistat, or occupancy sensor etc.  In a tight house you also have to be mindful of potential backdrafting/combustion air supply problems for the furnace when it's running, particularly when you have kitchen/bath exhaust ventilation & clothes-dryers running. (Carbon monoxide detectors are must anyway, tight house or not- get them if you don't have them!)  If possible, convert the furnace to run "sealed combustion", drawing it's combustion air from outdoors. (Woodburning appliances, same story.)  Also, consider that by tripling the clear-wall value of your walls (20-fold on the foundation!) your heat loss will probably be cut by more than half, and the furnace is now oversized for the load (which it might already be in the first place), which means it won't run anywhere near it's rated efficiency.  Do a careful heat-loss calculation (Manual-J or similar) and if the existing unit turns out to be 400%+  oversized (which is surprisingly typical) a "right sized" replacement is probably cost-effective.

Further reading on the subject:

http://www.buildingscienceconsulting.com/resources/walls/insulation_sheathings.pdf