Best way to insulate a bonus room over the garage?
We live in a 7 year old ranch house near Raleigh, NC. There is a 350 sq ft unfinished room over the 2 car garage and an adjacent large unfinished second floor attic area. An open stairwell leads from the first floor (has a door at the first floor level) to the room over the garage. The unfinished room has plywood flooring and batt insulation separating it from the garage. The rest of the attic has blown in cellulose insulation but is not floored (except for a small storage area). The room over the garage is about 10’ wide and 35’ long with 5’ kneewalls and a cathedral ceiling. The roof rafters are 2” x 8” and 16” apart. The rafters are connected with collar ties that are 8’ from the floor. The roof has a ridge vent and there are soffit vents on both sides behind the kneewalls. The floor behind each kneewall is not insulated to the garage below. One end of the room has a single window. The other end has the stairwell on one side and a connection to the rest of the attic on the other. The roof is plywood that is sheathed and shingled. There is no shade over the roof. The main part of the house is heated/cooled with a heat pump and there is no duct work in the attic (all ducts are in the crawlspace).
We would like to finish off the room over the garage for use as an occasional guest bedroom, and playroom for grandkids. We plan on putting a ½ bath in the main attic space that is nearest to the room over the garage. The bath will be framed off from the rest of the attic and have an 8 ft ceiling. The plan is to use a high SEER mini-split ductless system for heating/cooling the new room and bath, with the indoor unit being placed above the window.
The questions I have involve insulating the new room and bath:
1) Each rafter would have to be extended to be able to accommodate the code required R30 insulation (plus a ventilation gap) if we used batt insulation. We don’t want to lose any more head space, so the plan is to use open cell spray foam insulation between the ceiling rafters. What is the best way (energy cost/insulation cost) to accomplish this? Should the spray be applied all the way from the ridge vent to the soffit vent? If so, how should the rest of the area behind the kneewall be insulated? Should the uninsulated floor above the garage that is behind the kneewall also be sprayed, or should batt insulation be laid down? In either case, should there be any additional insulation in the kneewall itself? A second option would be to spray between the rafters from the ridge vent to only just behind the kneewall. If this is done, how should the kneewall itself be insulated? Spray or R19 batt insulation? Code says that there can be no plastic sheathing between the wall batt insulation and the sheetrock (paper faced insulation is ok). Is there any value to putting plastic sheathing on the back side of the batt insulation if the area behind the kneewall remains uninsulated and open to the soffit vent?
2) Should I use spray insulation on the gable (window) end of the room, or would it be ok to use regular R13?
3) What is the best way to insulate the ½ bath? One wall of the bath is part of the gable end of the main attic space, and the other 2 walls and the ceiling are framed off from the unheated part of the attic.
Any opinions/suggestions would be greatly appreciated!
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