Insulating a 32 X 48 shop

[J Persons]

My shop is 32'X48' with 12' ceilings. The shop is well ventilated, using powered gable vents on each end gable and an exhaust fan in the compressor room. The roof is constructed using 2X4 engineered trusses, plywood sheathing and composition shingles.

I would like to keep the trusses open, and just insulate under the roof deck. I was told that there needs to be an air gap between the insulation and the underside of the roof deck to keep from overheating the shingles during the summers. I live in central Louisiana, and it gets quite warm here. Since the rafters are 2X4s, R-11 will completely fill the space between the rafters and leave no air space.

My thought is to rip some 2X4s down to 2X2 and fasten the ripped 2X2 to the rafters, to effectively gain some air space between the R-11 batts and the bottom of the roof deck. R-11 batts are about 3.5" thick.

Since I know little about insulation, will this idea work or should I try something else.

I looked into sprayed foam, but the cost is prohibitive.

BTW last summer, the temp on the underside of the roof was 141.5º, recorded with an IR thermometer.

 

[WORKAHOLIC]

I think you could gain a lot by applying foil or foil faced foam to the underside of the rafters. That would reflect a lot of the radiant heat back to the roof deck and contain the hot air to the 3-1/2" space between the foil and the roof. If you have ridge venting , the hot air would then flow upward and exit through them. If you only have the vents in the gable ends you would need to provide a path for the hot air to get to them to exit (maybe add a 2 x 4 collar tie just below the gable vents and cover that). To answer your question I think your idea would work also just be careful not to push the batts up so far that they block air flow.

 

[bczygan]

The first question is why?
Are you trusses designed for a ceiling and insulation? How about for storage?
Do you want to use the space for storage?