I've found that rooms over garages are very hard to condition. there are a couple of ways to address this, walls shared with attic space are called kneewalls. on one side is living space (conditioned) on the other side attic space (extreme heat/cold). Southface has great info, check page 5 for visual of air sealing these walls. http://www.southface.org/default-interior/Documents/airsealingkeypoints.pdf with these attic rooms surrounded on 6 sides by extreme attic temps there are two ways to address them. one is pictured in link. convnetional insulation in stud bays, foam sheathing, caulked nailed & taped in place from top plate to bottom plate of kneewall. the detail on link also shows how to seal for air movement under floors of these rooms by using foam board, and placing it between floor joists & sealing air tight. I personally like to cut the foam board to fit tightly & air seal with caulk. I see people using great stuff..but am not a fan of the stuff myself. too much expansion to get a good seal, and over time, it shrinks. this is a process that is time consuming & detail oriented. easiest in new construction prior to any insulation being added to attic floor. more cost effective than spray foaming the walls and between floor area. but labor intensive. so its a easy fast process (spray foam) or a longer lower cost higher labor install with foam board. the benefit of creating an unvented semi conditioned attic space works great in my hot humid climate where in the summer attic temps reach 130+ degrees in attics. as we often put our ducts in the attic, the foam creates a less extreme temp for less heat gain. install of foam is crucial. temp mix has to be correct chemical mix has to be correct, temp of area sprayed is a factor. foam has to meet code requirements for your area. if code says R-30 in attic, enough inches of foam to reach that R-value has to be installed. seal has to be complete in an unvented attic. roof connection to attic floor has to be sealed. install should be level, no belly in or out between rafters. faces of rafters should be covered. none of that average fill stuff which means the spray can be less in some areas & more in others so that it 'averages' out to xx inches. low spots & voids will cause problems. testing for leakage, marking leakage sites with spray paint and having foam company spray to seal is the way I do my unvented attics. this all has to be established, in writing prior to any work being done. shop companies carefully. avoid 'average' fills, Kvalues Qvalues. R-values that is what code looks for. there is No exception for spray foam. I'm with Manhatten all the way on spray foam in walls. long payback when a better option is to put foam sheathing on exterior of walls & pay close attention to air sealing details. search southface site for air sealing pdfs..great stuff there! I've never recommended spray foam in walls after running the numbers for my climate & seeing 25+ year paybacks. and I've done these numbers lots of times, not just for one house. (btw hey Manhatten..long time no see) if you chose to use these details of air sealing for the tradeoff for this better insulation is SMALLER hvac system. it only makes sense people if you weight the benefits of upgrades against each other. get a load calc for the house including all types of insulation, window info, air tightness tight not leaky or average & the size needed lessens. bigger is not better, it is just lack of correct education in the trades & misinformatin to the homeowner to sell them more equipment than needed. you don't size the system for the 10 days a year of 50 people in the house & then pay for that the other 355 days. two stage units, variable speed air handlers are great things that vary with the needs of the house as they change. these are good investments. if you are truly interested in making your house efficient, hire an energy rater. specific to you information with inspections, testing & verification. unbiased working for you. resnet.us is a good place to start. new construction, existing homes we do it all. best of uck.

love this forum that you can do a little diy advising! ok the mold in the supply boxes and on the grills probably isn't from the duct takeoffs @ the plenum. it takes a long time for mold to travel that far..and if there isn't moisture & food source it won't spread to that area. it isn't always mold..sometimes it is just dust. so mold/dust buildup at the ducts @ plenum is lack of proper air seal. mold in supply boxes & on grills is lack of proper air seal at the cut in the sheetrock ceiling where supply grill is attached to the box. nine times out of ten..you can solve the issue by sealing supply boxes to sheetrock. see attached picture. hardcast brand #1402 mastic tape is used to seal supply box to sheetrock. this tape is 3" wide. I split the 3" into 1 1/2" lengths. stick to sheetrock and fold into supply box. as the tape adheres to many surfaces you need to be careful to keep it within the area covered by the supply grill. once it sticks to the sheetrock..you'll lose paper if you try to move it. surfaces have to be clean & dry for tape to adhere. and once in place you'll want to press it in place as it is somewhat pressure sensitive. the tape is strong enough to keep the box tight to attic floor on the attic side when installing the supply grill. wash the dust off the grills, wipe inside the supply box & into the duct. reinstall the supply grill. oh and when you remove the supply grills..take a sharpie and mark what direction they faced (throw to window..dirveway side whatever note you need to put them back in same position) this way you'll put them back as they were. I've tried several other brands of mastic tape. hardcast 1402 is the best. the others are 2" width and mastic is thinner, resulting in less strength and useful life. why would I chose spray foam for my floors? I've sealed floors several ways, foam is the easiest albeit more costly, once prep work is done. there isn't enough room to physically install foam sheathing to the bottom of the floor joists. I'll seal any big holes..like under tub then do 3" foam. I've looked at froth packs like tiger foam..but having worked with so many foam companies locally, will just have them do my floor when they are in my area. the software is pricy. we lease it yearly. once upon a time we bought the cd..but too many people abused it. so now it is a download & upgrades to original install. a lot of mastic isn't always an indication of a good seal. it doesn't take a lot if you put it in the actual leaks. you can feel around the duct take offs on the plenum while unit is running and feel some of the leakage. also checking plenum connections to the equipment at the same time. I use mastic tape to connect plenums (after screwing them in place) much easier to seal under the plenum to equipment with tape than painting mastic upside down.

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.

Normally on a cabinet mount microwave/hood combo,you have two venting choices.Direct vent out of the back of the unit, through the wall,(exterior),with a diffuser. The other venting option is up through the cabinet, into the attic and up venting out the roof.Both options require a boot that rectangularly fits on your microwave and adapts to a round vent pipe usually 6".With either of these methods, the only place you would woory about sealing is where the vent pipe goes through the drywall on either the wall or the cieling. I generally just seal these protrusions using joint compound and fiberglass mesh tape and finishing off if the vent pipe is exposed.On cabinets that are lower than the cieling where the vent pipe is exposed, i generally build a chase to cover the vent pipe,but still seal where the vent pipe goes through the cieling.On your situation, the way im picturing it is that you have a vent that is protruding out through the wall. You might run this by the hvac forum as maybe there is a gasket that could be placed between the unit and the hole, giving you a seal.IT doesn't sound like you are vented properly by stating that there is no seal/connection between the vent on the wall and the microwave. I think you should leave this to a pro and call an hvac contractor and get their opinion of your situation, as this might be a little more involved than just spraying foam trying to form a seal that isn't there to begin with.An hvac or an appliance outfit should be able to rectify this for you as we can only try to visualize your situation and end up giving advise that might not be safe.